Friday, March 29, 2024
03:03 AM (GMT +5)

Go Back   CSS Forums > CSS Compulsory Subjects > General Science & Ability > General Science Notes

Reply Share Thread: Submit Thread to Facebook Facebook     Submit Thread to Twitter Twitter     Submit Thread to Google+ Google+    
 
LinkBack Thread Tools Search this Thread
  #1  
Old Sunday, May 04, 2008
Princess Royal's Avatar
Super Moderator
Medal of Appreciation: Awarded to appreciate member's contribution on forum. (Academic and professional achievements do not make you eligible for this medal) - Issue reason: Best Moderator Award: Awarded for censoring all swearing and keeping posts in order. - Issue reason: Best Mod 2008
 
Join Date: Sep 2007
Location: K.S.A.
Posts: 2,115
Thanks: 869
Thanked 1,764 Times in 818 Posts
Princess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to behold
Default Earthquakes.

WHAT ARE EARTHQUAKES?

An earthquake is the shaking of the ground caused by an abrupt shift of rock along a fracture in the Earth, called a fault (Figure 1). Within seconds, an earthquake releases stress that has slowly accumulated within the rock, sometimes over hundreds of years.




FIGURE 1
Quote:

Block diagrams of fault types. (a) An earthquake is caused by the sudden fracturing of rock along part of a fault surface, shown here as a plane. If the fault reaches the surface, a visible ground fracture is created. The focus or hypocenter is the point on the fault plane where fracturing begins. The epicenter is the point on the ground surface directly over the focus. If the fault plane is inclined, the position of the epicenter will not coincide with the ground fracture. Simple fault motions are shown in (b), (c), and (d); directions of compressive stress are indicated. In a normal fault, (b), adjacent blocks of rock behave as if they were being pulled apart; the upper block slides downward along the fault relative to the other. In a thrust fault, (c), the blocks behave as if they were being pushed together; the upper block rides up the fault plane. In a strike-slip fault (d), one block moves horizontally past the other. Oblique motion of the blocks (not illustrated) combines thrust or normal fault motion with strike-slip motion. From an analysis of the seismic waves generated by an earthquake, called a fault-plane solution, scientists can determine the type of fault motion that occurred.

It is also possible for the accumulated stress to be released more gradually, by continuous slippage along a fault; this movement may amount to only a few millimeters a year. Such faults are said to undergo aseismic fault creep because the stress release occurs without earthquakes.

Faults are a record of past earth movements, just as fossils are a record of plants and animals that once inhabited the Earth. However, like volcanoes, faults may be extinct or active. Some faults are continuously active, while others may have occasional earthquakes and long periods of quiescence. Thousands of "extinct faults" have been mapped in Washington. A few active faults have also been mapped; these active faults are said to be active because they have experienced surface movement in the last 10,000 years. However, in the last 100 years earthquakes in Washington have not been associated with known active faults.

The earthquake process can be compared to the bending of a stick until it snaps. Stress accumulated during bending is suddenly released when the stick breaks. Vibrations are produced as the stick springs back to its pre-stressed position. In the Earth, seismic waves (Figure 2) are the vibrations caused by the sudden release of stress built up in rocks on either side of a fault. The rupturing of a fault may release all or only some of the stress. Any residual stress is often released by later minor readjustments along the fault causing smaller earthquakes called aftershocks.



FIGURE 2


Quote:
Diagrams of near-surface ground motions produced by seismic waves. The P and S waves, (a) and (b) respectively, travel through the earth in all directions from the focus of the earthquake; the first wave to reach an observer during an earthquake is the P wave. Two types of surface waves shown in (c) and (d), travel along the ground surface, somewhat like water waves, and arrive after the S waves. The direction the wave travels is indicated by the arrow below each diagram; the direction of ground movement caused by each wave is indicated by the solid arrows on the diagrams. P and S waves cause the ground to vibrate in mutually perpendicular directions.

Earthquakes generate several kinds of seismic waves that vibrate the ground (Figure 2). These seismic waves travel through the Earth at speeds of several kilometers per second, and they cause ground motions that can be detected by seismographs (or by accelerographs) far from the epicenter of the earthquake. In 1987, the University of Washington was operating more than 100 seismograph stations in Washington and northern Oregon (Figure 3). Several thousand seismographs are operated throughout the world by other groups of seismologists.


FIGURE 3

Quote:
Active seismograph stations in the Pacific Northwest in 1987. Stations operated by the University of Washington are shown as triangles, Canadian stations as squares. Seismic signals from the University's stations are received in Seattle. Signals from stations shown as solid triangles are also transmitted to the National Earthquake Information Service in Golden, Colorado. Station LON, at Mount Rainier, is part of an international recording system known as the World Wide Standard Seismograph Station Network (WWSSN). At LON six seismometers measure the various kinds of seismic waves shown in Figure 2.



FIGURE 4

Quote:
Components and dimensions of a typical remote seismograph station, similar to stations located on Fig. 3. The seismometer (a) converts small ground motions into an electric signal that has varying voltage. An amplifier and a voltage-controlled oscillator amplify this signal and convert it to a frequency-modulated (FM) tone. The radio transmitter (c) and the antenna (d) transmit the tone signal to the recording site.

Typical components of a modern seismograph station are shown inFigure 4 The signals produced by the seismographs in response to ground vibrations from an earthquake are commonly recorded on paper and magnetic tape. The display of ground motion versus time on paper record is called a seismogram (Figure 5).Seismographs can detect ground motions caused by sources other than earthquakes, such as explosions, volcanic eruptions, sonic booms, helicopters, and cars. Each of these sources can generally be identified from their characteristic signals recorded on seismograms.



FIGURE 5

Quote:

Seismograms (a) through (e) were recorded by stations in Washington and Oregon and illustrate the range of ground motion frequencies commonly recorded. The seismometers that recorded these motions are similar, and all had natural periods of 1.0 second. The seismograms for (a), (b), and (c) are expanded as (A), (B), and (C) in the lower part of the figure. P and S waves are marked on all seismograms. (a) Seismogram of a small (magnitude 1.2) volcanic earthquake at Mount St. Helens on November 23, 1987. The focus was less than I km below the surface, and the epicenter was less than I km from the station. (b) and (c) Seismograms of a magnitude 0.9 earthquake in the Cascade Range on November 18, 1987. The focus was at a depth of 17 km, and the epicenter was 13 km from the station that recorded (b) and 47 km from the station that recorded (c). (d) and (e) Seismograms from a magnitude 6.3 earthquake in the Imperial Valley of California on November 24, 1987 (d) shows the P wave as recorded at a station in northern Oregon, 1427 km from the epicenter. (e) shows the surface waves, which have lower frequencies, recorded at the same station. The surface waves arrived about 5-1/2 minutes after the P waves.
__________________
Regards,
P.R.

Last edited by Last Island; Sunday, May 04, 2008 at 03:58 PM.
Reply With Quote
  #2  
Old Sunday, May 04, 2008
Princess Royal's Avatar
Super Moderator
Medal of Appreciation: Awarded to appreciate member's contribution on forum. (Academic and professional achievements do not make you eligible for this medal) - Issue reason: Best Moderator Award: Awarded for censoring all swearing and keeping posts in order. - Issue reason: Best Mod 2008
 
Join Date: Sep 2007
Location: K.S.A.
Posts: 2,115
Thanks: 869
Thanked 1,764 Times in 818 Posts
Princess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to behold
Default

HOW ARE EARTHQUAKES MEASURED?

The size of an earthquake is indicated by a number called its magnitude. Magnitude is calculated from a measurement of either the amplitude or the duration of specific types of recorded seismic waves. Magnitude is determined from measurements made from seismograms and not on reports of shaking or interpretations of building damage. In general, the different magnitude scales (for example, local or Richter magnitude and surface wave magnitude) give similar numerical estimates of the size of an earthquake, and all display a logarithmic relation to recorded ground motion. That means each unit increase in magnitude represents an increase in the size of the recorded signal by a factor of 10. Therefore, a magnitude 7 earthquake would have a maximum signal amplitude 10 times greater than that of a magnitude 6 earthquake and 100 times greater than that of a magnitude 5 earthquake. Seismologists sometimes refer to the size of an earthquake as moderate (magnitude 5), large (magnitude 6), major (magnitude 7), or great (magnitude 8).Figure 6 shows how the Richter magnitude of an earthquake is calculated by measuring the amplitude of the maximum wave motion recorded on the seismogram.




FIGURE 6

Quote:
A method for calculating the epicentral distance and magnitude of a local earthquake (ML) ftom the wave amplitude recorded on a seismogram. The seismograph that recorded this seismogram is a standard Wood-Anderson seismograph. In this ex- ample, the ruler below the seismogram in (a) indicates the time in seconds between the arrivals of the P and S waves; here, S - P = 24 seconds. This difference between arrival times can be used to calculate the distance between the epicenter and the recording sta- tion. The arrival-time difference is shown on the vertical scale (b) and corresponds to an epicentral distance of about 214 km. The amplitude of the seismic waves, 23 mm, is measured on the vertical scale in (a); this measurement is noted on the vertical scale (c, on the right). The magnitude is determined by drawing a line that connects the points on vertical scales (b) and (c). This line passes through 5 on vertical scale (d), cor- responding to a magnitude of 5.0
The intensity of an earthquake is a measure of the amount of ground shaking at a particular site, and it is determined from reports of human reaction to shaking, damage done to structures, and other effects. The Modified Mercalli Intensity Scale (Table 1) is now the scale most commonly used to rank earthquakes felt in the United States. If magnitude is compared to the power output of a radio broadcasting station, then the intensity of an earthquake is the signal strength at a particular radio receiver. In practice, an earthquake is assigned one magnitude, but it may give rise to reports of intensities at many different levels. The magnitude 6.5 April 29, 1965, Seattle-Tacoma earthquake produced intensity VII to VIII damage near its epicenter, intensity V damage 150 kilometers away, and intensity I and 11 (barely felt) 300 to 500 kilometers from the epicenter (Figure 7). Although the greatest damage, and thus highest intensity, is usually near the earthquake's origin, damage to buildings depends on many factors, such as the type of construction, distance from the epicenter, and type of soil beneath the building. (See Structural Failure of Buildings, in the section titled What Causes Damage?) Therefore, maps of earthquake intensity commonly show complex patterns.



FIGURE 7

Quote:
Isoseismal map for the Seattle-Tacoma earthquake of April 29, 1965. The lines enclose areas of equal intensity as designated on the Modified Mercalli Intensity Scale (Table 1)
__________________
Regards,
P.R.

Last edited by Last Island; Sunday, May 04, 2008 at 04:02 PM.
Reply With Quote
The Following User Says Thank You to Princess Royal For This Useful Post:
Jahanzebmemon (Thursday, October 22, 2009)
  #3  
Old Sunday, May 04, 2008
Princess Royal's Avatar
Super Moderator
Medal of Appreciation: Awarded to appreciate member's contribution on forum. (Academic and professional achievements do not make you eligible for this medal) - Issue reason: Best Moderator Award: Awarded for censoring all swearing and keeping posts in order. - Issue reason: Best Mod 2008
 
Join Date: Sep 2007
Location: K.S.A.
Posts: 2,115
Thanks: 869
Thanked 1,764 Times in 818 Posts
Princess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to behold
Default

WHAT CAUSES EARTHQUAKES?

Plate Tectonics Theory
The plate tectonics theory is a starting point for understanding the forces within the Earth that cause earthquakes. Plates are thick slabs of rock that make up the outermost 100 kilometers or so of the Earth.(Figure 8) Geologists use the term tectonics to describe deformation of the Earth's crust, the forces producing such deformation, and the geologic and structural features that result.





Figure 8

Quote:
Figure 8. Relation between major tectonic plates and earthquakes. The Earth's surface is made up of 10 major plates and several smaller plates. Most earthquakes occur along plate margins. Small dots represent earthquake epicenters; large dots indicate locations of volcanoes. An enlargement (bottom) shows tectonic plates along the Pacific coast of North America. Arrows show motions of the Pacific and Juan de Fuca plates relative to North America. (World plate map from "Earthquakes" by Bruce A. Bolt. Copyright @1978, 1988 W. H. Freeman and Company. Reprinted with permission; the explanation has been modified)
Earthquakes occur only in the outer, brittle portions of these plates, where temperatures in the rock are relatively low. Deep in the Earth's interior, convection of the rocks, caused by temperature variations in the Earth, induces stresses that result in movement of the overlying plates (Figure 9). The rates of plate movements range from about 2 to 12 centimeters per year and can now be measured by precise surveying techniques. The stresses from convection can also deform the brittle portions of overlying plates, thereby Storing tremendous energy within the plates. If the accumulating stress exceeds the strength of the rocks comprising these brittle zones, the rocks can break suddenly, releasing the stored elastic energy as an earthquake.



Figure 9

Quote:
Figure 9. Cutaway view of the Earth showing the rocky mantle and iron core. The outermost layer consists of tectonic plates that are commonly about 100 km thick. Earthquakes occur within or at the boundaries of these plates. Although the mantle is solid, the rocks that comprise it act like a very viscous liquid and may move a few centimeters a year in great convection cells driven by temperature differences in the Earth. The plates move slowly with these currents. Spreading plate boundaries are thought to lie above areas of upwelling currents, and converging plate boundaries above areas where the currents move towards the center of the Earth. (See also Figure 10.)
Three major types of plate boundaries are recognized (Figure 10). These are called spreading, convergent, or transform, depending on whether the plates move away from, toward, or laterally past one another, respectively. Subduction occurs where one plate converges toward another plate, moves beneath it, and plunges as much as several hundred kilometers into the Earth's interior. The Juan de Fuca plate off the coasts of Washington and Oregon is subducting beneath North America (Figure 11).



Figure 10

Quote:
Figure 10. Three types of plate boundaries. A spreading boundary (a) marks the divergence of two plates. Material welling up from the mantle creates a rise or ri dge bordering the rift between separating plates. A convergent boundary (b) occurs where one plate moves t owards another. If one of these plates slides beneath the other, the motion is called subduction. A tra nsform boundary (c) occurs where relative plate motion is neither divergent or convergent, but is parallel to the plate edges. The geometry of plates off the coast of Washington is schematically shown in this figur e; plate locations are shown in Fig. 8.


Figure 11

Quote:
Figure 11. Cross sections of Washington showing plate convergence (top fig ure) and earthquake hypocenter locations. Some major topographic features and underl ying geologic structures of Washington are shown diagrammatically in the upper figure. In th e lower figure, selected hypocenters of earthquakes that occurred in 1982 through 1986 between latitudes 47' and 48'N are projected onto a vertical plane that generally corresponds to the diagram in the upper figure. Because of the great number of shallow earthquakes that occurred between 1982 and 1986, only hy pocenters of those having magnitudes equal to or greater than 1.8 are shown in the lower figure. Belo w 30 km, hypocenters of all earthquakes having magnitudes of 1.0 or greater that occurred during this pe riod are shown. The distribution of deep earthquakes indicates the slope of the zone of subduction. In th e lower figure there is a vertical exaggeration of 2 to I below sea level; this creates the illusion that the subducting Juan de Fuca plate dips more steeply than it actually does. Topography indicated on the lower figure has a vertical exaggeration of 12 to 1.
Ninety percent of the world's earthquakes occur along plate boundaries (Figure 8) where the rocks are usually weaker and yield more readily to stress than do the rocks within a plate. The remaining 10 percent occur in areas away from present plate boundaries-like the great New Madrid, Missouri, earthquakes of 1811 and 1812, felt over at least 3.2 million square kilometers, which occurred in a region of southeast Missouri that continues to show seismic activity today (Schnell and Herd, 1984).



Figure 12

Quote:
Figure 12. Epicenters of earthquakes in the Pacific Northwest since 1960. Only the largest earthquakes near Mount St. Helens are indicated. Note the position of the Ca scadia subduction zone relative to Washington's coast and that epicentral locations mark plate boun daries shown in Figure 8. (Data from the National Oceanic and Atmospheric Administration and the Univ ersity of Washington.)
Plate Tectonics and Earthquakes in the Northwestern United States
The Cascadia subduction zone off the coasts of Washington, Oregon, and northern California is a convergent boundary between the large North America plate and the small Juan de Fuca plate to the west (Figures 11, 12). The Juan de Fuca plate moves northeastward and then plunges (subducts) obliquely beneath the North America plate at a rate of 3 to 4 centimeters per year (Chase and others, 1975; Adams, 1984; Riddihough, 1984).

Washington has features typical of convergent boundaries in other parts of the world. These are illustrated in Figure 11:
  • (1) A zone of deep earthquakes near the probable boundary between the Juan de Fuca plate and North America plate (Crosson, 1983; Taber and Smith, 1985; Weaver and Baker, 1988). The 1949 magnitude 7.1 Olympia earthquake and the 1965 magnitude 6.5 Seattle-Tacoma earthquake occurred within this deep zone.
  • (2) The active or recently active volcanoes of the Cascade range created by the upward migration of magma (molten rock) above the Juan de Fuca plate. Rock in the subducting plate may melt at depths of 100 kilometers or more in the Earth. Because melted rock is lighter, it can sometimes rise to the surface through weakened areas in the overlying materials.
  • (3) Young, highly deformed mountains composed of formerly oceanic rocks scraped off the Juan de Fuca plate during subduction and piled up on the Olympic peninsula (Tabor and Cady, 1978).
  • (4) Deformed young sediments offshore in the Pacific Ocean where the converging plates meet (Barnard, 1978).
In sum, the subduction of the Juan de Fuca plate beneath the North America plate is believed to directly or indirectly cause most of the earthquakes and young geologic features in Washington and Oregon.

The major plate boundaries in the Pacific Northwest are graphically delineated by the locations of recent earthquakes (Figure 12). Narrow zones of shallow offshore earthquakes result from the movement of the Juan de Fuca plate relative to the Pacific plate, particularly along transform boundaries such as the Blanco Fracture Zone off the coast of Oregon. As expected, a few shallow offshore earthquakes occur along the Juan de Fuca Ridge, a spreading boundary between the Juan de Fuca and Pacific plates. Scattered earthquakes occur to the east in Washington, Oregon, and northern California, both in the subducting Juan de Fuca plate and in the overlying North America plate.

The world's greatest earthquakes occur on subduction-zone boundaries. These magnitude 8+ thrust-type earthquakes, sometimes called subduction earthquakes, occur from time to time as the two converging plates jerk past one another. There are no reports of such earthquakes in Washington since the first written records of permanent occupation by Europeans in 1833 when the Hudson Bay Trading Company post was established at Fort Nisqually (Hawkins and Crosson, 1975). And, since the installation in 1969 of a multistation seismograph network in Washington, there has been no evidence of even small thrust-type earthquakes between the plates in Washington and Oregon and offshore.

In fact, few earthquakes of any kind or size have been recorded along the coastal region of the Pacific Northwest. However, parts of subduction zones in Japan and Chile also appear to have had very low levels of seismicity prior to great subduction earthquakes (Heaton and Kanamori, 1984; Heaton and Hartzell, 1986). Therefore the seismic quiescence observed historically along coastal region of Washington and Oregon does not refute the possibility that an earthquake having a magnitude of greater than 8 could occur there. Heaton and Hartzell (1986) note the problem of incomplete seismic data when comparing one subduction zone with another, but they still conclude that available data support the finding that low levels of seismicity may exist in subduction zones prior to a magnitude 8 earthquake.

The convergence of the Juan de Fuca and North America plates is quite slow, so great subduction earthquakes may be rare. Savage and others (I 98 1) interpret geodetic strain measurements near Seattle as indicating that compressional strain is accumulating parallel to the direction of convergence between the

Juan de Fuca and North America plates, as would be expected prior to a great, thrust earthquake off the coast of Washington and British Columbia.
Atwater (1987) has found geologic evidence that he believes shows that the last great subduction earthquake in Washington occur ed as recently as 300 years ago.

Historically, many earthquakes have occurred in the subducting Juan de Fuca plate deep beneath Puget Sound and at shallow depths in many places in Washington, Oregon, and British Columbia in the overlying North America plate. It is reasonable to expect future earthquakes in these areas to have magnitudes comparable to the magnitudes of past earthquakes. The biggest historical earthquakes include the shallow magnitude 7.4 earthquake in the North Cascades in 1872 and the deep magnitude 7.1 earthquake in the southern Puget Sound area in 1949 (Rasmussen, 1967; U.S. Geological Survey, 1975; Malone and Bor, 1979). Therefore, even without the occurrence of great subduction-style earthquakes in the Pacific Northwest, Washington is still earthquake country.
__________________
Regards,
P.R.
Reply With Quote
The Following User Says Thank You to Princess Royal For This Useful Post:
Jahanzebmemon (Thursday, October 22, 2009)
  #4  
Old Sunday, May 04, 2008
Engr.Aftab's Avatar
Senior Member
 
Join Date: Nov 2007
Posts: 206
Thanks: 240
Thanked 239 Times in 108 Posts
Engr.Aftab will become famous soon enough
Default

Many people think that an earthquake is just a simple rattle in the earth the destruction from this type of natural disaster can be devastating. Seismologists have been studying earthquakes for many years to help us to figure out where and when they are going to happen. An earthquake is the trembling of the earth’s surface caused by rapid movement of the earth’s rocky outer layer. They occur when energy stored in the earth, usually in the form of strain in rocks, suddenly releases. This energy is transmitted to the surface of the earth by earthquake waves (known as primary and secondary waves).
The destruction an earthquake causes mainly depends on its magnitude and duration. However, a buildings structural design as well as the materials used in the construction process also effects the amount of damage done when an earthquake hits. Some earthquakes are small and we don’t even notice them, while others can be felt over thousands of kilometers.
__________________
Engr.Aftab
Reply With Quote
  #5  
Old Monday, May 05, 2008
Princess Royal's Avatar
Super Moderator
Medal of Appreciation: Awarded to appreciate member's contribution on forum. (Academic and professional achievements do not make you eligible for this medal) - Issue reason: Best Moderator Award: Awarded for censoring all swearing and keeping posts in order. - Issue reason: Best Mod 2008
 
Join Date: Sep 2007
Location: K.S.A.
Posts: 2,115
Thanks: 869
Thanked 1,764 Times in 818 Posts
Princess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to behold
Default

WHAT CAUSES DAMAGE?

Direct Causes

Ground Shaking
Factors Affecting Ground Shaking
Most earthquake damage is caused by ground shaking. The magnitude of an earthquake, distance to the earthquake focus, type of faulting, depth, and type of material are important factors in determining the amount of ground shaking that might be produced at a particular site. Where there is an extensive history of earthquake activity, these parameters can often be estimated; however, in many areas of Washington they are still poorly defined.

The magnitude of an earthquake influences ground shaking in several ways. Large earthquakes usually produce ground motions with large amplitudes and long durations. In addition, large earthquakes produce strong shaking over much larger areas than do smaller earthquakes. The 1949 magnitude 7.1 Olympia earthquake produced ground shaking lasting 30 seconds and was felt over an area of 550,000 square kilometers. In contrast, the 1964 magnitude 8.3 Alaska earthquake produced ground shaking for about 300 seconds and was felt over an area more than five times larger.

The distance of a site from an earthquake affects the amplitude of ground shaking. In general, the amplitude of ground motion decreases with increasing distance from the focus of an earthquake. The considerable depth of the 1949 and 1965 earthquakes put even the closest sites, those directly over the earthquake focus, at least 50 to 65 kilometers from the source of the ground shaking, a factor that contributed to the lower intensity experienced near the epicenter.

The frequency content of the shaking also changes with distance. Close to the epicenter, both high (rapid)and low (slow)-frequency motions are present. Farther away, low-frequency motions are dominant, a natural consequence of wave attenuation in rock. The frequency of ground motion is an important factor in determining the severity of damage to structures and which structures are affected. (See Structural Failure of Buildings - below)
Analyses of earthquake damage in Washington and elsewhere suggest that the severity of shaking depends on several factors besides the distance and magnitude of an earthquake. (See Structural Failure of Buildings - below) These factors include the kinds and thicknesses of geologic materials exposed at the surface and the subsurface geologic structure (Rasmussen and others, 1974; Newman and Hall, 1982). Natural and artificial unconsolidated materials, such as sediments in river deltas and materials used as landfill, commonly amplify ground motions relative to motion in consolidated sediments or bedrock. Such areas, in general, have had higher levels of ground shaking in past Washington earthquakes. The thickness of unconsolidated material may also affect the amount of ground shaking produced. Certain frequencies of ground shaking may generate disproportionately large motions because of wave resonance in sedimentary basins. Just as the pitch of sound from an organ pipe depends on the length of the pipe and the density and compressibility of air, the various frequencies at which a sedimentary basin will resonate when shaken by seismic waves depend on the thickness, density, and stiffness of the sedimentary layers.
Subsurface structures, such as sedimentary layers that vary in thickness or degree of consolidation, may increase ground motion by focusing seismic wave energy at a particular site. The curved surfaces of buried bedrock topography may also focus waves. Langston and Lee (1983) suggested focusing as a mechanism to explain why the severity of damage observed in West Seattle during the 1965 Seattle-Tacoma earthquake seemed unrelated to surface geology in many places (Mullineaux and others, 1967; Yount, 1983). The depth to bedrock changes from very near the surface in the West Seattle area to significantly deeper just a short distance away in downtown Seattle.

Estimating Future Ground Shaking
Studies of the 1949 and 1965 earthquakes have provided most of the data used to estimate future ground shaking in Washington (Langston, 1981; Langston and Lee, 1983; Ihnen and Hadley, 1986). The depths of these two earthquakes (54 and 63 kilometers below Puget Sound in the subducting Juan de Fuca plate), their magnitudes, and the reports of damage at sites in Washington having a variety of geologic materials have led to estimates of future ground shaking for similar events (U.S. Geological Survey, 1975). For example, the intensity of ground shaking in the epicentral area of a future large Puget Sound earthquake if that earthquake occurred at a depth comparable to those of the 1949 and 1965 earthquakes would be lower than the intensity that would be expected for a shallow earthquake of the same magnitude. The reduced intensity would be related to the effect of depth to the focus and the possible attenuation of ground shaking in some areas identified during past earthquakes caused by the nature of the geologic materials between the focus and the site.

A magnitude 8 subduction earthquake along the coast of Washington or a large shallow earthquake in the Puget Sound area or in the Cascade Mountains would not be expected to produce the same distribution of ground shaking observed during the large deep Puget Sound earthquakes. However, the expected motion can be estimated. For example, Heaton and Hartzell (1986) have estimated ground motions for a hypothetical magnitude 9 earthquake along coastal Washington.

Surface Faulting
The consequences of major fault rupture at the surface can be extreme. Buildings may be torn apart, gas lines severed, and roads made impassible. Damage by faults is more localized than the widespread damage caused by ground shaking. Nevertheless, the identification of active surface faults is an important part of estimating future earthquake losses.
Faults that have so far been identified as active or possibly active within the last 10,000 years are shown on Figure 15.

Many maps of surface faults in Washington have been published (for example, McLucas, 1980, and Gower and others, 1985). Most of the faults on these maps are presently inactive. Geologic evidence indicating active fault movement within the last 10,000 years has been reported for only a few small faults in Washington. The best documented active surface faults in the state are located near Lake Cushman in westem Washington ( Figure 16). The most recent time of movement of many faults is unknown because, in many places, the faults are not covered by young geologic materials. Such material, if found to be disturbed, would provide geologic evidence of the time of movement.

Seismicity, another indication of active faulting, has only rarely been associated with recognized surface faults in Washington. However, seismic activity has been used to define faults that do not currently rupture the surface, such as the St. Helens Seismic Zone shown on Figure 15 .

Subsidence and Uplift
Sudden elevation changes during earthquakes can have severe long-term economic impact on coastal development. Some parts of Prince William Sound were uplifted by several meters during the 1964 Alaska earthquake; the amount of rise was as much as I I meters on Montague Island. Conversely, parts of the Kenai Peninsula and Kodiak Island subsided as much as 2 meters during that earthquake (Plafker, 1969). Some raised harbors on Prince William Sound could no longer be used by boats. In other areas streets and buildings subsided so much that they were flooded at high tide (Plafker and others, 1969). Major subsidence or uplift of large regions often occurs as a result of great subduction-style, thrust earthquakes. Such elevation changes have been reported after earthquakes in New Zealand, Japan, Chile, and southeast Alaska (Plafker, 1969). Submerged marshlands in several estuaries along Washington's coast suggest that similar episodes of sudden subsidence have also occurred in the Pacific Northwest ( Figure 17). Preliminary dating indicates that many of the subsidence events at different sites in Washington occurred at the same time ( Figure 18). For this reason, Atwater (1987) and Hull (1987) have attributed these subsidence events to the occurrence of large subduction earthquakes.

Secondary Causes of Earthquake Damage

While earthquakes may produce ground shaking, surface faulting, and vertical movements that cause direct damage to buildings and land, damage and personal injury may also be caused by several additional factors.
Earthquakes may trigger ground failures such as landslides, differential compaction of soil, and liquefaction of water-saturated deposits like landfills, sandy soils, and river deposits. Such ground failures may cause more damage to structures than the shaking itself. Earthquakes may also cause destructive water waves such as tsunamis and seiches. Non-structural building components like ceiling panels, windows, and furniture can cause severe injury if shaking causes them to shift or break. Broken or impaired lifelines (gas, water, or electric lines and transportation and communication networks) can produce hazardous situations and distress to a community. A reservoir can be a hazard, should shaking cause the dam to fail.

Ground Failure
Major property damage, death, and injury have resulted from ground failures triggered by earthquakes in many parts of the world. More than $200 million in property losses and a substantial number of deaths in the 1964 Alaska earthquake were caused by earthquake-induced ground failures. A 1970 earthquake off the coast of Peru triggered an ice and rock avalanche in the Andes that killed more than 18,000 people when it buried the city of Yungay. Earthquakes in the Puget Sound region have induced ground failures responsible for substantial damage to buildings, bridges, highways, railroads, water distribution systems, and marine facilities (Keefer, 1983; Grant, 1986). Ground failures induced by the 1949 Olympia earthquake occurred at scattered sites over an area of 30,000 square kilometers, and ground failures induced by the 1965 Seattle-Tacoma earthquake occurred over 20,000 square kilometers (Keefer, 1983).

In reviewing records of the 1949 and 1965 Puget Sound earthquakes, Keefer (1983) noted that geologic environments in the Puget Sound region having high susceptibilities to ground failure include areas of poorly compacted artificial fill, postglacial stream, lake, or beach sediments, river deltas, and areas having slopes steeper than 35 degrees. The types of ground failures associated with past Washington earthquakes and expected to accompany future earthquakes include landslides, soil liquefaction, and differential compaction. Such failures commonly occur in combination-for example, liquefaction may cause a landslide or accompany compaction.

Landslides
Washington has many sites susceptible to landslides, including steep bluffs of eroded glacial deposits in the Puget Sound region, steep rocky slopes along the Columbia River Gorge, and rugged terrain in the Cascade Mountains. Fourteen earthquakes, from 1872 to 1980, are known to have triggered landslides in Washington (Townley and Allen, 1939; Coffman and others, 1982; Bradford and Waters, 1934; Meyer and others, 1986).
Dozens of ancient landslides have been identified in the bluffs along Puget Sound, indicating their susceptibility to ground failure. The landslides may also be susceptible to further failure if the headwall or toe areas are steepened by erosion or excavation (Keefer, 1983; Harp and others, 1981). Ground shaking produced by recent large Puget Sound earthquakes generated 20 landslides, some as far as 180 kilometers from the epicenter of the 1949 Olympia earthquake, and 21 landslides as far as 100 kilometers from the epicenter of the 1965 SeattleTacoma earthquake (Keefer, 1983, 1984).
Figure 19 shows damage that occurred in 1965 to a railroad line between Olympia and Tumwater.

Washington's five stratovolcanoes (Mount Baker, Glacier Peak, Mount Rainier, Mount St. Helens, and Mount Adams) offer many sites for rock and ice avalanches, rock falls, and debris flows on their steep slopes (Beget, 1983). The slopes of volcanoes are particularly vulnerable to landslides because of the layered and jointed volcanic rocks lying parallel to the mountain slopes, weakened by the effects of steam and hot ground water, and oversteepened by erosion. In addition, ice falls from glaciers can trigger landslides, and snow and ice add to the mobility of such slides (Dreidger and Kennard, 1986).
Landslides on Mount Rainier were reported for earthquakes in 1894, 1903, and 1917 (Townley and Allen, 1939). Crandell (1973) suggests that valley floors within a few kilometers of the base of Mount Rainier could be buried by rockfall avalanches triggered by a strong earthquake. The massive 2.8-cubic-kilometer rockslide/debris avalanche on the north side of Mount St. Helens during the catastrophic eruption of May 18, 1980, was triggered by a moderate (magnitude 5) earthquake that followed 8 weeks of intense earthquake activity beneath the volcano.

The impact of landslides on stream drainages and reservoirs also can pose significant danger to populations and developments downstream (Beget, 1983). Water ponded behind landslide-debris dams can cause severe floods when these natural dams are suddenly breached. Such outburst floods are most likely near volcanic centers active within the past 2 million years (Evans, 1986, p. 128). The Toutle River was blocked by a debris flow triggered by an earthquake during the 1980 eruption of Mount St. Helens. The debris flow dam raised the level of Spirit Lake by 60 meters. The U.S. Army Corps of Engineers constructed a tunnel through bedrock in order to lower the lake level and thereby reduce the danger of flooding from a sudden release of water and lessen the risk to persons living downstream.
Landslides or debris flows into reservoirs or lakes may displace enough water to cause severe downstream flooding (Crandell, 1973; Crandell and Mullineaux, 1976, 1978; Hyde and Crandell, 1978). Communities and developments located downstream of reservoirs and lakes along drainages from Mounts Baker, Adams, and St. Helens must all be considered at some risk from earthquake-induced landslides.

The sudden displacement of water by landslides can also generate destructive water waves. A 300-foot bluff along the Tacoma Narrows, thought to have been weakened by the 1949 earthquake, collapsed into Puget Sound 3 days after the 1949 earthquake (U.S. Army Corps of Engineers, 1949). Figure 20 shows the slide area. Minor wave damage occurred to houses adjacent to the slide; a slide-generated wave was directed against the opposite shore, but no property damage occurred because that shore was undeveloped at the time.

Future earthquakes in Washington are expected to generate more landslides and greater losses than reported for past earthquakes. Earthquakes with shallow focal depths or a longer duration of shaking will trigger more landslides than reported for the 1949 or 1965 earthquakes. In addition, a review of weather data indicates that precipitation during the rainy seasons preceding both the 1949 and 1965 events was near or below average throughout most of the Puget Sound area and may have been responsible for there having been fewer landslides than would have been expected in unusually wet weather. Continued population growth and development in areas of steep slopes further increase the possibility of substantial property damage and loss of life from landslides in Washington.

Liquefaction
Liquefaction occurs when saturated sand or silt is shaken violently enough to rearrange its individual grains. Such rearrangement has a tendency to compact the deposit. If the intragranular water cannot escape fast enough to permit compaction, the load of overlying material and structures may be temporarily transferred from the grains of sand or silt to the water, and the saturated deposit becomes "quicksand". The liquefied material may then cause lateral-spread landslides or loss of bearing strength under foundations or roadways, depending on the depth and thickness of the liquefied zone and local topography ( Figure 21).

If the liquefied layer is near the surface it may break through overlying "dry" deposits, forming geysers or curtains of muddy water that may leave sand blows as evidence ( Figure 22). Retaining walls may tilt or break from the fluid pressure of the liquefied zone. Shallow liquefaction zones can also cause severe damage to structures whose foundation support has suddenly become fluid. Liquefaction caused basement floors to break and be pushed upward in Seattle and Puyallup during the 1949 earthquake (Murphy and Ulrich, 1951). Other basements cracked open and completely filled with water and silt. Lighter structures may float in liquefied soil. Buried fuel tanks, if sufficiently empty, may pop to the surface, breaking connecting pipes in the process. Pilings without loads may also float upwards. Heavy structures may tilt in response to the loss of bearing strength by underlying soil. During the 1964 Niigata, Japan, earthquake, four-story apartment buildings tilted on liquefied soils, one as much as 60 degrees!

If a thick section of unconsolidated deposits liquefies near the surface, it will tend to flow into and fill topographic depressions. For example, a stream channel may be narrowed as saturated and liquefied deposits on both sides of the stream flow into it. Compression resulting from such flow buckled or skewed spans and damaged abutments on more than 250 bridges during the 1964 Alaska earthquake (National Research Council, 1985). This form of liquefaction failure was so widespread that McCulloch and Bonilla (1970) coined the term "land spreading" to distinguish it from the more widely recognized lateral-spread landslides that tend to occur on slopes due to failure along a particular subsurface layer. Land spreading may have been responsible for the disabling of three drawbridges across the Duwamish Waterway in Seattle during the 1949 earthquake. The distance between the piers in the main span of the Spokane Street bridge was shortened by 6 to 8 inches, causing the bridge to jam in the closed position until the concrete and steel edges could be trimmed off sufficiently to permit reopening (Gonen and Hawkins, 1974). These and other drawspans over the Duwamish were also jammed by the 1965 earthquake (U.S. Coast and Geodetic Survey, 1967).

Earthquakes may trigger a phenomenon in certain clays that produces effects similar to liquefaction in water-saturated sand. When vibrated, these "quick" or "sensitive" clays undergo a drastic loss of shear strength. For example, a relatively thin sensitive zone in the Bootlegger Cove Clay, located about 25 meters below the surface, was blamed for the spectacular lateral-spread landslides that destroyed parts of Anchorage in 1964 (Hansen, 1966). The sensitive layer responsible for these landslides had been deposited in a marine environment, in contrast to the underlying and overlying fresh-water clays. Later leaching of the salt from the marine clay by fresh ground water may have increased the clay's sensitivity to vibration-induced loss of shear strength by shaking (Hansen, 1966). Glacial clays are present in the northern Puget Lowland, and Armstrong (1984) mentions one instance of a slide in such material that was apparently triggered by the vibration of a passing train. However, it is currently unknown whether all marine clays of the Puget Lowland have a significant susceptibility to such vibration-induced failure.

Differential Compaction
Structural damage commonly occurs to buildings underlain by foundation materials that have different physical properties. Materials such as tide flat sediments, glacial outwash sands, dredging muck, sawdust, and building rubble will settle by different amounts when shaken. These materials are prevalent under parts of the downtown and waterfront areas of Seattle, Tacoma, Olympia, and Aberdeen-Hoquiam. Dozens of water and/or gas line breaks occurred in these cities as a result of differential compaction during the 1949 earthquake (U.S. Army Corps of Engineers, Seattle District, unpub. report, May 12, 1949), and virtually every building along the Seattle waterfront was damaged by settling during the 1965 earthquake. Many waterfront areas around Puget Sound are underlain by material susceptible to differential compaction and are thus vulnerable to damage in future earthquakes.

Water Waves
Tsunamis
Tsunamis are long-wavelength, long-period sea waves generated by an abrupt movement of large volumes of water. In the open ocean, the distance between wave crests can be greater than 100 kilometers, and the wave periods can vary from 5 minutes to I hour. Such tsunamis travel 600-800 kilometers per hour, depending on water depth. Large subduction earthquakes causing vertical displacement of the sea floor and having magnitudes greater than 7.5 are the most common cause of destructive tsunamis. Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across open ocean and wreak destruction on far shores hours after the earthquake that generated them.

Tsunami wave heights at sea are usually less than I meter, and the waves are not frequently noticed by people in ships. As tsunami waves approach the shallow water of the coast, their heights increase and sometimes exceed 20 meters. Table 3 summarizes the heights of recent tsunamis at Neah Bay, Washington, and some other sites in westem North America. Figure 23 shows the effect of a tsunami on the water levels recorded at four selected Pacific Northwest tide gage stations; this tsunami was caused by the March 27, 1964, Alaska earthquake.

Historically, tsunamis originating in the northern Pacific and in South America have caused more damage on the west coast of the United States than tsunamis originating in Japan and the South Pacific. The tsunami generated by the 1964 Alaska earthquake caused $85 million damage in Alaska, $10 million damage in Canada, $115,000 damage in Washington, $754,000 damage in Oregon, and $11 million damage in California (Wilson and Torum, 1972). Figure 24 summarizes the effects of the tsunami from the 1964 Alaska earthquake along the Washington coastline. In places, wave heights reached 4.5 meters. The wave heights varied considerably, depending on the local water depth and the shape of inlets. The 1964 tsunami destroyed a small bridge across the Copalis River (Grays Harbor County) by hurling log debris against supporting piles. The tsunami was also detected on the Columbia River as far as 160 kilometers from the ocean (Wilson and Torum, 1972). Besides causing property damage, the 1964 tsunami killed 103 people in Alaska, 4 in Oregon, and 12 in California. Newspaper accounts tell of narrow escapes along the Washington coast, but there were no fatalities there.
The regional variations in damage caused by a tsunami from a particular source region can be estimated for future earthquakes. The basis for such estimates, particularly the influence of near-shore bottom topography and irregular coastline on the height of an arriving tsunami wave, is described by Wiegel (1970) and Wilson and Torum (1972). Table 3 shows that tsunami waves at some locations are consistently higher, even from sources in opposite directions.

Past tsunamis have caused only minor damage in Washington. The damage caused in the state by the tsunami triggered by the 1964 Alaska earthquake occurred along small estuaries north of Grays Harbor. In some places south of Grays Harbor sand dunes protected developed areas from damage (Hogan and others, 1964). Parts of these dunes have since been cleared to enhance the view ( Figure 25); some homes behind the dune area may be exposed to greater risk from tsunami damage in the future.

What kind of tsunami might coastal Washington experience in the future? We can certainly expect another tsunami from a great earthquake in Alaska or other seismically active areas in the Pacific. One likely source area in the next two decades is the Shumagin Islands region of the Aleutians (Davies and others, 1981; Kowalik and Murty, 1984). A tsunami from the Shumagin Islands would reach Washington in about 3 hours. Preuss (1986) has estimated the impact this tsunami would have on the coastal community of Aberdeen, Washington.

In addition to a tsunami generated by a distant earthquake, a magnitude 8 or greater subduction earthquake between the Juan de Fuca and North America plates might create a large local tsunami on the coast of Washington. Atwater (1987) and Reinhart and Bourgeois (1987) have found evidence they believe indicates that a tsunami from a nearby great subduction earthquake did affect the coast of Washington about 300 years ago. In general, local tsunamis are much more destructive than tsunamis generated from a distant source. In addition, they may occur within minutes of the earthquake or landslide that produces them, allowing little time for evacuation. Estimates of the effect of a local tsunami in Washington are speculative because we have no written record of a large, shallow earthquake near the coast. However, the sudden submergence of coastal areas that may accompany great earthquakes might increase the amount of land in Washington susceptible to tsunami damage.

Seiches
A seiche is a standing wave in an enclosed or partly enclosed body of water and is analogous to the sloshing of water that occurs when an adult suddenly sits down in a bathtub. Earthquakes may induce seiches in lakes, bays, and rivers. More commonly, seiches are caused by wind-driven currents or tides. Water from a seiche in Hebgen Reservoir caused by the 1959 earthquake near Yellowstone National Park repeatedly overtopped the dam, causing considerable damage to the dam and its spillway (Stermitz, 1964). The 1964 Alaska earthquake created a 0.3-meter-high seiche on the reservoir behind Grand Coulee Dam, and similar seiches were detected on 14 other bodies of water in Washington (McGarr and Vorhis, 1968). Several pleasure craft, houseboats, and floats sustained minor damage when a seiche caused some mooring lines to break on Lake Union in Seattle (Wilson and Torum, 1972). Seiches generated by the 1949 Queen Charlotte Islands earthquake were reported on Lake Union and Lake Washington in Seattle and on Commencement Bay in Tacoma. They separated boats from their moorings and stranded fish on the shore at Clear Lake in eastern Washington (Murphy and Ulrich, 1951). So far, no significant damage has been reported from seismic seiches in Washington caused by local or distant earthquakes.

Structural Failure of Buildings
A building's structure may be damaged if its vibratory response to ground motion exceeds design limits. The response depends on the interaction between structural elements of the building and the direction, frequency, and duration of ground motion. These factors must be considered to produce a building design that prevents structural failure during earthquakes. In the absence of proper design, a building is exposed to greater risk of earthquake damage, particularly if the building has been subjected to prior strong earthquakes. The cumulative damage caused by prior earthquakes was stressed by Edwards (1951) in his analysis of structural damage by the 1949 Puget Sound earthquake.

Importance of Type of Construction to Building Damage
Usually, buildings can better withstand the vertical component of the earthquake-induced ground motion because they are designed to resist the large vertical loads generated by their own weight. Many are, however, vulnerable to large horizontal motions. Resistance to horizontal motion is usually accomplished by using lateral bracing and strong connections to hold structural elements together. Horizontal elements like floors can then distribute the building's weight to the building's strong vertical elements (Yanev, 1974). <A href="http://www.geophys.washington.edu/SEIS/PNSN/INFO_GENERAL/NQT/f26.html">Figure 26 illustrates the basic structural components of any building.

Construction that provides a continuous path to transfer the lateral load from roof to foundation is more resistant to ground shaking than construction in which that path can be easily broken. For example, a well-nailed wood frame house resists ground shaking better than an unreinforced brick house because, once the brick cracks, the path along which the lateral load is transferred is broken. During both the 1949 and 1965 Washington earthquakes, buildings having unreinforced brick walls with sand-lime mortar suffered more damage than any other type of construction (Murphy and Ulrich, 1951, reprinted in Thorsen, 1986; U.S. Coast and Geodetic Survey, 1967). That damage was compounded by the lack of proper ties between the floors and walls ( Figure 27). Examples of structural damage in 1949 included: (1) Centralia-many walls collapsed, two schools were permanently closed, and one church was condemned; (2) Buckley-part of the high school collapsed; (3) Castle Rock-bricks and masonry from a gable over the main entrance of a Castle Rock high school collapsed, killing one student; (4) Chehalis-extensive damage to downtown buildings, schools, and churches (Murphy and Ulrich, 1951, reprinted in Tborsen, 1986); and (5) Seattle-1,900 brick walls that collapsed, fractured, or bulged were condemned and removed (Gonen and Hawkins, 1974). Other examples of structural damage include the collapse of unreinforced brick walls from the sixth story of the Fisher Flouring Mills in Seattle ( Figure 28) and the severe cracking of unreinforced masonry walls in Issaquah school buildings.

Proper ties between the foundation and the structure and between the various elements of the structure are essential for good earthquake resistance. Buildings or other structures that are poorly attached or unattached to their foundations may shift off the foundation during an earthquake. In 1965, two 2,000-barrel aging tanks at the Rainier Brewing Company in Seattle fell off their foundations; one split and released its contents. Mobile homes merely resting on blocks have been especially vulnerable to damage during earthquakes (Yanev, 1974). Floors poorly attached to walls can pull away, permitting collapse of the wall or roof or a failure of the floor ( Figure 29). Because of the lack of proper ties, the third floor of the Seattle Union Pacific Railroad Station sagged after the 1965 earthquake.

Importance of Frequency of Ground Shaking to Building Damage
Building damage commonly depends on the frequency of ground motion. Damage can be particularly severe if the frequency of ground motion matches the natural vibration frequencies of the structure. In this case, the shaking response of the structure is enhanced, and the phenomenon is called resonance. Tall buildings, bridges, and other large structures respond most to low-frequency ground shaking, and small structures respond most to high-frequency shaking. Tall buildings in Seattle like the Smith Tower responded strongly to the low-frequency ground motions produced by the 1946 Queen Charlotte earthquake located on Vancouver Island, 330 kilometers away. Other large earthquakes beyond the state borders have also caused damage in Washington-for example, earthquakes in British Columbia (Dec. 6, 1918, M=7.0; Aug. 21, 1949, M=8.1), in Montana (Aug. 17, 1959, M=7.5), and in Idaho (Oct. 23, 1983, M=7.5).

Tall buildings in sedimentary basins often suffer disproportionate damage because wave resonance in the basin amplifies low-frequency ground vibrations. During the September 19, 1985, Mexico earthquake (magnitude 8.1), 7to 15-story buildings on unconsolidated sediments in Mexico City, 320 kilometers from the epicenter, collapsed because the low-frequency ground vibrations were enhanced by the sediments in a frequency range that matched the natural vibration frequency of the buildings (Rosenblueth, 1986).

Importance of Building Shape to Damage
The shape of a building can influence the severity of damage during earthquakes. Buildings that are L or U shaped in plan view (as seen from the air) may sustain more damage than a symmetrical building. This damage occurs because large stresses develop at the intersection between the building's segments, which respond differently to ground vibrations of different frequencies and different directions of motion. A building with sections that differ in height or width may develop large stresses at certain points because each section will vibrate at its own natural frequency in response to ground shaking. Separate buildings that vibrate at different frequencies can damage each other if they are built close together. During the 1985 Mexico earthquake, tall buildings in Mexico City swayed more slowly than shorter buildings, causing them to hit each other. This "hammering" of buildings on each other caused considerable damage and may have been responsible for the total collapse of some, Hammering was reported in both the 1949 and 1965 Puget Sound earthquakes (Edwards, 1951; U.S. Coast and Geodetic Survey, 1967).

Importance of Past Earthquakes to Building Damage
The history of a building and its exposure to prior earthquakes are also important in estimating the amount of damage it may sustain in future earthquakes. People often assume that a building that has survived an earthquake with no visible damage will likely not be damaged in subsequent earthquakes. However, ground shaking can weaken a building by damaging walls internally. Failure to detect and strengthen concealed damage can lead to complete destruction in a subsequent earthquake. For example, a 7-story reinforced-concrete refrigeration warehouse in Seattle had been damaged by previous earthquakes; a 20-foot-high concrete water tank platform atop this building collapsed during the 1949 earthquake (Edwards, 1951). The influence of the 1965 earthquake on buildings was difficult to evaluate due to previous structural damage caused by the 1949 earthquake (Gonen and Hawkins, 1974).

Importance of Building Remodeling to Damage
A building also may be weakened by structural alterations since its initial construction. For example, doors or other openings may have been cut through bearing walls, thereby increasing the risk of damage in future earthquakes.

Hazards of Non-structural Building Components
Non-structural Hazards
The non-structural elements of a building include parapets, architectural decorations (such as terra cotta cornices and ornamentation), chimneys, partition walls, ceiling panels, windows, light fixtures, and building contents. Displacement or distortion of these elements during ground shaking can be a major hazard to building occupants and result in extensive building damage. Damage to the non-structural elements of a building can include the destruction of costly equipment, such as computer systems, and the loss or extensive disorganization of important company records.

Displacement of non-structural elements occurs when they are unattached or poorly attached to the surrounding structure. The 1949 and 1965 Puget Sound earthquakes have provided several examples of damage and injury due to the displacement of non-structural parts of buildings. Many parapets collapsed, covering sidewalks with bricks ( Figure 30). A worker in the Fisher Flouring Mills was killed when a wooden water reservoir located on top of the building collapsed and fell in pieces to the ground. Cornices on Seattle's Franklin High School broke and dropped to the school yard below. Chimneys cracked and twisted, showering bricks on sidewalks, porches, school yards, and streets in many areas in the Puget Sound region ( Figure 31). In Chehalis, 75 percent of the town's chimneys were destroyed.

Light fixtures fell in many schools ( Figure 32). When a building shakes, objects like furniture may slide around violently. Many examples of shifted file cabinets, refrigerators, and overturned bookshelves were reported during the 1949 and 1965 earthquakes ( Figure 33). Objects on shelves also pose hazards if knocked to the floor or across a room. Books spilled from the shelves into the aisles of the Seattle Public Library, and most of the liquor bottles in the North Bend State Liquor Store fell to the floor. Magnetic computer tapes spilled from tape racks at the Boeing Company during the 1965 earthquake ( Figure 34).

Distortion of the non-structural elements occurs when the building flexes, putting extreme stress on rigid items like windows, panels, and built-in furniture (Bay Area Regional Earthquake Preparedness Project, 1985). In 1965 many windows in the Schoenfeld furniture store in Tacoma were shattered when glass was broken by the flexing of the window frames ( Figure 35). Other examples of this type of damage are noted in Thorsen (1986).
Economic loss during an earthquake is not confined to damaged building elements, equipment, and products. Loss of important company records, including inventory and customer lists, sales records, information about suppliers, and accounting, can contribute to disastrous business interruption costs ( Figure 36).

Damaged Lifelines
Lifelines include the utilities (power, water, gas), communication networks, and transportation systems that crisscross and link our communities. Damage to these lifelines by earthquakes can create dangerous situations. Broken gas and power lines are serious threats to safety, largely because of risk of fire. Cracked water mains reduce the amount of water available for fire suppression. (See discussion of the 1949 and 1965 Puget Sound earthquakes.) Lack of communication isolates people from help and needed information ( Figure 37). Blocked or damaged transportation routes interfere with the ability of emergency personnel to respond promptly to requests for assistance.

Other Hazards: Fires, Hazardous Spills, Dam Failures
Earthquakes may trigger many other hazards in a community. Damage to a dam caused by ground shaking could result in flooding downstream. Following the magnitude 6.4 San Fernando, California, earthquake in 1971, between 75,000 and 80,000 people were evacuated for 3 days from below the Lower Van Norman Dam. Damage occurred to both the Upper and Lower Van Norman Dams ( Figure 38), and authorities felt that a small amount of additional shaking would have caused both to fail (Subcommittee on Water and Sewerage Facilities, 1973).

The greater use and storage of hazardous materials in recent years increases the potential for loss of life and injury resulting from damage to storage or transport containers during an earthquake. A chemical spill in a general hospital at Santa Rosa, California, was caused by ground shaking produced by two earthquakes (magnitudes 5.6 and 5.7) that occurred nearby on October 1, 1969. A fire resulting from the spill spread to the surgery facility on the next floor (Reitherman, 1986). Bottles of chemicals stored on open shelves in a Coalinga, California, high school were shattered on the floor during the 1983 Coalinga earthquake. Sulfuric acid ate through from the second floor to the first floor and the mixing of other chemicals released toxic fumes throughout the building (Bulman, 1983).
Fires are a common problem during earthquakes. The devastating 1906 San Francisco earthquake is often called the San Francisco fire because of the tremendous damage caused by fires started during the ground shaking. Once fires are started, fire suppression may be hampered by damaged water distribution systems. Response to fires may be slow because of blocked transportation routes and damage to communication networks.
__________________
Regards,
P.R.

Last edited by Princess Royal; Tuesday, May 06, 2008 at 08:30 AM.
Reply With Quote
  #6  
Old Tuesday, May 06, 2008
Princess Royal's Avatar
Super Moderator
Medal of Appreciation: Awarded to appreciate member's contribution on forum. (Academic and professional achievements do not make you eligible for this medal) - Issue reason: Best Moderator Award: Awarded for censoring all swearing and keeping posts in order. - Issue reason: Best Mod 2008
 
Join Date: Sep 2007
Location: K.S.A.
Posts: 2,115
Thanks: 869
Thanked 1,764 Times in 818 Posts
Princess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to beholdPrincess Royal is a splendid one to behold
Default

GLOSSARY

This glossary includes words commonly used to describe the nature of earthquakes, how they occur, and their effects, as well as a discussion of the instruments used to record earthquake motion. Each word or phrase that is in bold print in the text is explained in this glossary.

Accelerograph A seismograph whose output is proportional to ground acceleration (in comparison to the usual seismograph whose output is proportional to ground velocity). Accelerographs are typically used as instruments designed to record very strong ground motion useful in engineering design; seismographs commonly record off scale in these circumstances. Normally, strong motion instruments do not record unless triggered by strong ground motion.

Aftershock One of many earthquakes that often occur during the days to months after some larger earthquake (mainshock) has occurred. Aftershocks occur in the same general region as the mainshock and are believed to be the result of minor readjustments of stress at places in the fault zone.

Amplitude The amplitude of a seismic wave is the amount the ground moves as the wave passes by. (As an illustration, the amplitude of an ocean wave is one-half the distance between the peak and trough of the wave. The amplitude of a seismic wave can be measured from the signal recorded on a seismogram.)

Aseismic creep Movement along a fracture in the Earth that occurs without causing earthquakes. This movement is so slow that it is not recorded by ordinary seismographs.

Collision A term sometimes applied to the convergence of two plates in which neither plate subducts. Instead, the edges of the plates crumple and are severely deformed.

Convection The motion of a liquid driven by gravity and temperature differences in the material. In the Earth, where pressure and temperature are high, rocks can act like viscous fluids on a time scale of millions of years. Thus, scientists believe that convection is an important process in the rocks that make up the Earth.

Convergent boundary The boundary between two plates that approach one another. The convergence may result in subduction if one plate yields by diving deep into the Earth, obduction if one plate is thrust over the other, or collision if the plates simply ram into each other and are deformed.

Core The Earth's central region, believed to be composed mostly of iron. The core has a radius of 3,477 kilometers and is surrounded by the Earth's mantle. At the center of the molten outer core is a solid inner core with a radius of 1,213 kilometers. (See figure 9)



Figure 9

Quote:
Cutaway view of the Earth showing the rocky mantle and iron core. The outermost layer consists of tectonic plates that are commonly about 100 km thick. Earthquakes occur within or at the boundaries of these plates. Although the mantle is solid, the rocks that comprise it act like a very viscous liquid and may move a few centimeters a year in great convection cells driven by temperature differences in the Earth. The plates move slowly with these currents. Spreading plate boundaries are thought to lie above areas of upwelling currents, and converging plate boundaries above areas where the currents move towards the center of the Earth. (See also Figure 10.)
Earthquake The release of stored clastic energy caused by sudden fracture and movement of rocks inside the Earth. Part of the energy released produces seismic waves, like P, S, and surface waves, that travel outward in all directions from the point of initial rupture. These waves shake the ground as they pass by. An earthquake is felt if the shaking is strong enough to cause ground accelerations exceeding approximately 1.0 centimeter/second' (Richter, 1958).

Epicenter The location on the surface of the Earth directly above the focus, or place where an earthquake originates. An earthquake caused by a fault that offsets features on the Earth's surface may have an epicenter that does not lie on the trace of that fault on the surface. This occurs if the fault plane is not vertical and the earthquake occurs below the Earth's surface. (See figure 1).



Figure 1

Quote:
Block diagrams of fault types. (a) An earthquake is caused by the sudden fracturing of rock along part of a fault surface, shown here as a plane. If the fault reaches the surface, a visible ground fracture is created. The focus or hypocenter is the point on the fault plane where fracturing begins. The epicenter is the point on the ground surface directly over the focus. If the fault plane is inclined, the position of the epicenter will not coincide with the ground fracture. Simple fault motions are shown in (b), (c), and (d); directions of compressive stress are indicated. In a normal fault, (b), adjacent blocks of rock behave as if they were being pulled apart; the upper block slides downward along the fault relative to the other. In a thrust fault, (c), the blocks behave as if they were being pushed together; the upper block rides up the fault plane. In a strike-slip fault (d), one block moves horizontally past the other. Oblique motion of the blocks (not illustrated) combines thrust or normal fault motion with strike-slip motion. From an analysis of the seismic waves generated by an earthquake, called a fault-plane solution, scientists can determine the type of fault motion that occurred.
Fault A break in the Earth along which movement occurs. Sudden movement along a fault produces earthquakes. Slow movement produces aseismic creep.

Fault plane solution The calculation of the orientation, dip, and slip direction of a fault that produced the ground motion recorded at seismograph stations. Sometimes called a focal mechanism solution.

Focus The place in the Earth where rock first breaks or slips at the time of an earthquake; also called the hypocenter. The focus is a single point on the surface of a ruptured fault. During a great earthquake, which might rupture a fault for hundreds of kilometers, one could be standing on the rupturing fault, yet be hundreds of kilometers from the focus.

Hypocenter See Focus.

Intensity A measure of the severity of shaking at a particular site. It is usually estimated from descriptions of damage to buildings and terrain. The intensity is often greatest near the earthquake epicenter. Today, the Modified Mercalli Scale is commonly used to rank the intensity from I to XII according to the kind and amount of damage produced. Before 1931 earthquake intensifies were often reported using the Rossi-Forel scale (Richter, 1958).

Kilometers and other metric units of measure: Conversion formulae Millimeters x 0.039 = inches
Centimeters x 0.394 = inches
Meters x 3.28 = feet
Kilometers x 0.621 = statute miles
Square kilometers x 0.386 = square miles
Cubic kilometers x 0.240 = cubic miles

Liquifaction A process, in which, during ground shaking, some sandy, water-saturated soils can behave like liquids rather than solids.

Magnitude A quantity characteristic of the total energy released by an earthquake, as contrasted with intensity, which describes its effects at a particular place. A number of earthquake magnitude scales exist, including local (or Richter) magnitude (ML), body wave magnitude (Mb), surface wave magnitude (Ms), moment magnitude (Mw), and coda magnitude (Mc). As a general rule, an increase of one magnitude unit corresponds to ten times greater ground motion, an increase of two magnitude units corresponds to 100 times greater ground motion, and so on in a logarithmic series. Commonly, earthquakes are recorded with magnitudes from 0 to 8, although occasionally large ones (M = 9) and very small ones (M = -I or -2) are also recorded. Nearby earthquakes with magnitudes as small as 2 to 3 are frequently felt. The actual ground motion for, say, a magnitude 5 earthquake is about 0.04 millimeters at a distance of 100 kilometers from the epicenter; it is 1.1 millimeters at a distance of 10 kilometers from the epicenter.

Mainshock The largest in a series of earthquakes occurring closely in time and space. The mainshock may be preceded by foreshocks or followed by aftershocks.

Mantle A rock layer, about 2,894 kilometers thick, between the Earth's crust and core. Like the crust, the upper part of the mantle is relatively brittle. Together, the upper brittle part of the mantle and the crust form tectonic plates.

Modified Mercalli Intensity Scale A scale for measuring ground shaking at a site, and whose values range from I (not felt) to XII (extreme damage to buildings and land surfaces). (See intensity and table 1).

Quote:
I. Not felt except by a very few under especially favorable circumstances.

II. Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.

III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of truck. Duration estimated.

IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably.

V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop.

VI. Felt by all; many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.

VII. Damage negligible in building of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. Noticed by persons driving motor cars.

VIII. Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.

IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.

X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

XI. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.

XII. Damage total. Lines of sight and level distorted. Objects thrown into the air.
Table 1

NEHRP The federal National Earthquake Hazard Reduction Program, enacted in 1977, to reduce potential losses from earthquakes by funding research in earthquake prediction and hazards and to guide the implementation of earthquake loss reduction programs.

Normal Fault A normal fault can result from vertical motion of two adjacent blocks under horizontal tension. (It also occurs in rocks under compression if stress is unequal in different directions. In this case, the minimum and maximum compressive stresses must be applied horizontally and vertically respectively.) In a normal fault, the upper of the two adjacent blocks of rock slips relatively downward. (See reverse (thrust) fault and figure 1).

P (Primary) waves Also called compressional or longitudinal waves, P waves are the fastest seismic waves produced by an earthquake. (See seismic waves and figure 2.) They oscillate the ground back and forth along the direction of wave travel, in much the same way as sound waves, which are also compressional, move the air back and forth as the waves travel from the sound source to a sound receiver.



Figure 2

Quote:
Diagrams of near-surface ground motions produced by seismic waves. The P and S waves, (a) and (b) respectively, travel through the earth in all directions from the focus of the earthquake; the first wave to reach an observer during an earthquake is the P wave. Two types of surface waves shown in (c) and (d), travel along the ground surface, somewhat like water waves, and arrive after the S waves. The direction the wave travels is indicated by the arrow below each diagram; the direction of ground movement caused by each wave is indicated by the solid arrows on the diagrams. P and S waves cause the ground to vibrate in mutually perpendicular directions. (Modified from "Earthquakes" by Bruce A. Bolt. Copyright @1978, 1988 W.H. Freeman Company. Reprinted with permission) .
Plates Pieces of crust and brittle uppermost mantle, perhaps 100 kilometers thick and hundreds or thousands of kilometers wide, that cover the Earth's surface. The plates move very slowly over, or possibly with, a viscous layer in the mantle at rates of a few centimeters per year. (See figure 8).




Figure 8

Quote:
Relation between major tectonic plates and earthquakes. The Earth's surface is made up of 10 major plates and several smaller plates. Most earthquakes occur along plate margins. Small dots represent earthquake epicenters; large dots indicate locations of volcanoes. An enlargement (bottom) shows tectonic plates along the Pacific coast of North America. Arrows show motions of the Pacific and Juan de Fuca plates relative to North America. (World plate map from "Earthquakes" by Bruce A. Bolt. Copyright @1978, 1988 W. H. Freeman and Company. Reprinted with permission; the explanation has been modified)
Plate boundaries The edges of plates or the junction between plates. See also plates, convergent (both collision and subduction), spreading, and transform boundaries.

Plate tectonics A widely accepted theory that relates most of the geologic features near the Earth's surface to the movement and interaction of relatively thin rock plates. The theory predicts that most earthquakes occur when plates move past each other.(See also mantle.)

Return times Sometimes called the recurrence time or recurrence interval. The return time, or more properly the average return time, of an earthquake is the number of years between occurrences of an earthquake of a given magnitude in a particular area. For example, if the average time of an earthquake having magnitude greater than or equal to 7 is 100 years, then, on the average, such earthquakes will occur every 100 years. If such earthquakes occur randomly in time, there is always the chance that the actual time interval between the events will be less or greater than 100 years. Return time is best described in terms of probabilities. In the case of an earthquake having a 100-year average return time, there is about an 18 percent chance that such an earthquake will occur in the next 20 years and a 63 percent chance than it will occur in the next 100 years. On the other hand, there is a 14 percent chance that it will not occur in the next 200 years.

Reverse Fault A rupture that results from vertical motion of two adjacent blocks caused by horizontal compression. Sometimes called a thrust fault. In a reverse fault, the upper of the two adjacent blocks moves relatively upward. (See figure 1 and normal fault.)

Richter Magnigtude Scale An earthquake magnitude scale, more properly called local magnitude scale, based on measurements of the amplitude of earthquake waves recorded on a standard Wood-Anderson type seismograph at a distance of less than 600 kilometers from the epicenter (Richter, 1958). (See magnitude and Figure 6. ).

S (Secondary or shear) waves S waves oscillate the ground perpendicular to the direction of wave travel. They travel about 1.7 times slower than P waves. Because liquids will not sustain shear stresses, S waves will not travel through liquids like water, molten rock, or the Earth's outer core. (See seismic waves and figure 2).

Seiche A standing wave in a closed body of water such as a lake or bay. It can be characterized as the sloshing of water in the enclosing basin. Seiches can be produced by seismic waves from earthquakes. The permanent tilting of lake basins caused by nearby fault motions has produced very energetic seiches.

Seismic waves A vibrational disturbance in the Earth that travels at speeds of several kilometers per second. There are three main types of seismic waves in the earth: P (fastest), S (slower), and surface waves (slowest). Seismic waves are produced by earthquakes.

Seismogram A graph showing the motion of the ground versus time. (See figure 5).



Figure 5

Quote:
Seismograms (a) through (e) were recorded by stations in Washington and Oregon and illustrate the range of ground motion frequencies commonly recorded. The seismometers that recorded these motions are similar, and all had natural periods of 1.0 second. The seismograms for (a), (b), and (c) are expanded as (A), (B), and (C) in the lower part of the figure. P and S waves are marked on all seismograms. (a) Seismogram of a small (magnitude 1.2) volcanic earthquake at Mount St. Helens on November 23, 1987. The focus was less than I km below the surface, and the epicenter was less than I km from the station. (b) and (c) Seismograms of a magnitude 0.9 earthquake in the Cascade Range on November 18, 1987. The focus was at a depth of 17 km, and the epicenter was 13 km from the station that recorded (b) and 47 km from the station that recorded (c). (d) and (e) Seismograms from a magnitude 6.3 earthquake in the Imperial Valley of California on November 24, 1987 (d) shows the P wave as recorded at a station in northern Oregon, 1427 km from the epicenter. (e) shows the surface waves, which have lower frequencies, recorded at the same station. The surface waves arrived about 5-1/2 minutes after the P waves.
Seismograph A sensitive instrument that can detect, amplify, and record ground vibrations too small to be perceived by human beings. (See also accelerograph.)

Site response Local vibratory response to seismic waves. Some sites experience more or less violent shaking than others, depending on factors such as the nature and thickness of unconsolidated sediments and/or the configuration of the underlying bedrock.

Strike-slip fault Horizontal motion of one block relative to another along a fault plane. If one stands on one side of the fault and observes that an object on the other side moves to the right during an earthquake, the fault is called a right-lateral strike-slip fault (like California's San Andreas fault). If the object moves to the left, the fault is called a left-lateral strike-slip fault.

Subduction zone boundary The region between converging plates, one of which dives beneath the other. The Cascadia subduction zone boundary ( Figure 12 ) is an example.

Subduction earthquake A thrust-type earthquake caused by slip between converging plates in a subduction zone. Such earthquakes usually occur on the shallow part of the boundary and can exceed magnitude 8.

Surface waves Seismic waves, slower than P or S waves, that propagate along the Earth's surface rather than through the deep interior. Two principal types of surface waves, Love and Rayleigh waves, are generated during an earthquake. Rayleigh waves cause both vertical and horizontal ground motion, and Love waves cause horizontal motion only. They both produce ground shaking at the Earth's surface but very little motion deep in the Earth. Because the amplitude of surface waves diminishes less rapidly with distance than the amplitude of P or S waves, surface waves are often the most important component of ground shaking far from the earthquake source. (See seismic waves.)

Thrust fault See reverse fault and figure 1.

Transform boundary A boundary between plates where the relative motion is horizontal. The San Andreas fault is a transform boundary between the North America plate and the Pacific plate. The Blanco fracture zone (Figure 12 ) is a transform boundary between the Juan de Fuca and the Pacific plates.

Tsunami A tsunami is a series of very long wavelength ocean waves caused by the sudden displacement of water by earthquakes, landslides, or submarine slumps. Ordinarily, tsunamis are produced only by earthquakes exceeding magnitude 7.5. In the open ocean, tsunami waves travel at speeds of 600-800 kilometers/hour, but their wave heights are usually only a few centimeters. As they approach shallow water near a coast, tsunami waves travel more slowly, but their wave heights may increase to many meters, and thus they can become very destructive.

World-wide Standard Seismograph Network A network of about 110 similarly calibrated seismograph stations that are distributed throughout the world. The network was originally established in the early 1960s, and its operation is now coordinated by the U.S. Geological Survey. Each station has six seismometers that measure vertical and horizontal ground motion in two frequency ranges.




Source: http://www.geophys.washington.edu/SE...T/welcome.html
.
__________________
Regards,
P.R.

Last edited by Princess Royal; Wednesday, May 07, 2008 at 09:05 AM.
Reply With Quote
The Following User Says Thank You to Princess Royal For This Useful Post:
Qaiserks (Wednesday, April 01, 2009)
  #7  
Old Thursday, May 08, 2008
Engr.Aftab's Avatar
Senior Member
 
Join Date: Nov 2007
Posts: 206
Thanks: 240
Thanked 239 Times in 108 Posts
Engr.Aftab will become famous soon enough
Default

Tectonics

The earth, from its deep majestic oceans to its breathtaking mountains, is our home. But what goes undetected to humans is the violent cycle that is going on underneath our earth at this very moment. Volcanoes are spouting hot magma. Earthquakes are destroying cities and the continents we live on are moving around the ocean. But why do these phenomenon occur and what causes them to happen?

From space the earth may look like one large floating mass. But underneath all of the oceans and continents there lies many large. These plates were once all formed together in one large continent called Pangaea. Over the millions of centuries the plates have shifted and formed the continents that we now see today. Scientists first discovered this theory when they noticed that the Northeastern part of South America seemed to fit into the Southwestern part of Africa. These plates have been very instrumental in the forming of our planet. They form the high mountains and the deep oceans. They form volcanoes and cause earthquakes. The plates underneath our earth are very important to us and I believe we need to pay more attention to them.

The plates themselves are very large, the largest being the Pacific and Antarctic plates. There are two types of plates on the earth. The first, oceanic is made up of a heavy rock called basalt. This causes the plate to sink deeper into the earth’s mantle causing our oceans to appear. The other type of plate is continental. These plates are made up of granite, which is much lighter than the basalt that makes up the oceanic plates. Oceanic plates are not as thick as continental plates. Oceanic plates average 5 kilometers in thickness while continental plates can be up to 100 kilometers thick. This allows the formations of mountains to occur.

The plates were first outlined by charting where the major earthquakes were occurring and drawing a line, more or less down the center of them to show a line of best fit. This was first done in 1961 and it outlined the main plates of the world.

500 million years ago the earth was not made up of the seven continents we see today. It was made up of one giant continent called Pangaea(see figure C). Around 200 million years ago the continent Pangaea broke into smaller continents called Gondwanaland and Laurasia. Over time these two continents slowly broke apart to form the continents we see today the reason we see the continents as they are today is because of continental drift. Scientists believe that this may not be the only time there has been a “Pangaea”. Continental drift has caused the separation of many of the “Pangaea’s” in the past before the one that existed 200 million years ago. In 250 million years scientists believe once again that the continents will come together forming what they call “Pangaea Ultima” and eventually Pangaea Ultima will once again break apart forming new continents.
__________________
Engr.Aftab
Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Solved Everyday Science Papers Dilrauf General Science & Ability 4 Friday, April 08, 2011 06:10 PM
Geography One - Quakes, Foldings and Faultings Bhalla Changa Geography 1 Sunday, February 28, 2010 01:39 PM
Types of Earth Quakes Qurratulain General Science & Ability 1 Wednesday, August 23, 2006 12:30 PM
Can Animal Sense Disasters or maybe Earthquakes Satan Humorous, Inspirational and General Stuff 2 Friday, October 28, 2005 03:18 AM
Islamic perspective on earthquakes Chilli General Knowledge, Quizzes, IQ Tests 0 Sunday, October 16, 2005 08:59 PM


CSS Forum on Facebook Follow CSS Forum on Twitter

Disclaimer: All messages made available as part of this discussion group (including any bulletin boards and chat rooms) and any opinions, advice, statements or other information contained in any messages posted or transmitted by any third party are the responsibility of the author of that message and not of CSSForum.com.pk (unless CSSForum.com.pk is specifically identified as the author of the message). The fact that a particular message is posted on or transmitted using this web site does not mean that CSSForum has endorsed that message in any way or verified the accuracy, completeness or usefulness of any message. We encourage visitors to the forum to report any objectionable message in site feedback. This forum is not monitored 24/7.

Sponsors: ArgusVision   vBulletin, Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.