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Default Geography One - Quakes, Foldings and Faultings

EARTHQUAKES

INTRODUCTION
An earthquake is a movement or tremor of the earth’s crust and it originates naturally and below the earth’s surface. It is generally accompanied by permanent change of level at the surface; but mostly no permanent effect is visible at the surface except the damage done by the shaking. Owing to the lowering or raising of parts of the earth’s surface relative to sea level, portions of the sea floor covered with sediments may be raised up to form dry land whereas other parts of the sea floor may be slightly raised so that the sea may become much shallower.

CAUSES OF EARTHQUAKES

According to modern science, the causes of earthquakes are outlined under three major headings:
i. Earthquakes due to volcanoes;
ii. Earthquakes due to tectonic movement of the earth; and,
iii. Earthquakes due to isostatic adjustments.
Besides these three major causes, which produce greater intensity in the tremor, there are some localized and minor producing minor or local earthquakes.

Major Causes


Volcanoes

In the volcanic belts earthquakes are frequent. When volcanic eruptions take place, lava, steam, gases as well as volcanic bombs and ashes, etc., come out with so enormous force that they exert great impact on the sides of the volcanic vent. As a result of this, shaking of the earth’s crust takes place. This shaking or quake continues till the force of eruption slows down. The velocity of the earthquake waves depends upon the intensity and magnitude of the volcanic eruptions. Generally, volcanic earthquakes have their severity felt upto a distance of 100 or 150 miles. But there are cases when the shock was experienced at a very long distance. Such was the case, with the earthquake in Krakatoa Island in 1883. The whole island itself was blown off and huge sea waves washed away many villages from the nearby islands of Sumatra and Java. Its impact was experienced in Cape Horn in South America at a distance of 8,000 miles.

Tectonic Movement of Earth

Tectonic is a term derived from a Greek word “Tekton” meaning “Builder”, applied to all “internal forces”, which build-up the features of the earth’s crust. The term includes both Diastrophism and Volcanicity. Diastrophic forces are those forces which have disturbed or deformed the earth’s crust, including folding, faulting, uplift (rejuvenation) and depression. Diastrophic forces may be of continent and plateau building (Epeirogenic), or of mountain building (Orogenic). Diastrophic forces generally cause earth movements, meaning movements of the earth caused by the internal forces (compressive, tensional, uplifting, folding and faulting), on both a major and minor scale, and both rapid (earthquakes) and slow, which affect the crust of the earth. All tectonic forces are due to some substantial change in the substratum under the crust. As a result, tensional forces (causing faults) and compressional forces (causing folds) are produced. These forces are greatly responsible for earthquakes. The earthquake in Assam which affected mainly north Lakhimpur and Sibasagar districts in 1950 and the Bihar earthquake in 1934 are two good examples of earthquakes due to tectonic movements.

Isostatic Adjustments
It is known to the modern earth-scientists that there exists a State of Balance between the Sialic (Silicon-Aluminum) Crust and the Simatic (Silicon-Magnesium) Substratum. The mountain chains or the highlands are not just surface features but have sufficient downward projection, so that the various relief features are arranged with a state of balance. This concept is known as isostasy. If due to some cause or other, this state of balance between the Sial and Sima (or the crust and the substratum) is disturbed then the forces are developed beneath the surface to cause violent earthquakes as a result of an isostatic rise or fall of landmass. This rise or fall of landmass takes place to restore the isostatic balance between the Sialic and Simatic masses. Earthquakes caused by the isostatic readjustments produce tremor on the surface and generally are not so much destructive as the volcanic and tectonic earthquakes are. The earthquake on the 4th March, 1949 in the Hindukush region is an example of Isostatic Earthquakes. Its shock was experienced in Lahore and the surrounding areas.

Minor Causes

The minor earthquakes are localized in nature and are not destructive, because the forces which cause these are not of greater intensity as they are produced locally. There are several causes which produce minor earthquakes. The following are some of them:

Falling of the Roof of Cavern (Deep Cave)
In limestone topography, water percolates through sinkholes or fissures (gaps, cracks) and reaches the underground caverns (hole in the rock). In course of time, the roof of the cavern falls with great force and thereby causes minor earthquakes.

Ejection of Underground Steam

Sometimes water percolates through fissures and reaches the surface of hot material in the interior. This results in the formation of clouds of steam which try to come out of the surface. This also causes minor earthquakes.

Landslide

Landslide is another cause of minor earthquakes. Earthquakes due to landslide are experienced in the Himalayan Region, particularly in the rainy season.

Displacement of Huge Ice Blocks

In highlands, covered with snow, sometimes huge blocks of ice are displaced and fall into the valley with great force. This causes minor earthquakes.

Local Isostatic Adjustments
It is an important and general cause of minor earthquakes. This happens when in an elevated area, the balance between crust and substratum is disturbed.

Falling of Huge Rocks

Sometimes huge rocks from a cliff, near the sea, fall on the crust, thereby creating force to cause earthquakes of minor nature.

EFFECTS OF THE EARTHQUAKES
The nature of damage caused by earthquake is multifarious (diverse). In addition to man-made structures such as buildings, bridges, towers etc., that collapse due to them, there are a number of other effects produced in nature. Roads are severely damaged due to subsidence (ebb) of ground and enormous fissures appear on the ground. Where the ground consists of water-soaked alluvium, the underground water gushes out at many places and deposits masses of sand in the form of craters (valleys). Extensive landslides occur in the hilly regions and rocky debris comes down to block the path of streams, passing the valley below. These rocks impound large quantities of water and form artificial lakes which after a few days or weeks may burst due to the pressure of accumulated water. This results in extensive devastation. These floods also deposit large quantities of silt in the bed of the streams and upset the natural drainage of water and the hydrologic regime of the area.
The destruction is at the highest when the shock has originated at some place below the sea. In that case, huge sea waves are formed which cause enormous damage when they strike the shore. On the other hand, when the earthquakes are accompanied by volcanic activity, the destruction brought by them is neither very enormous nor is there intensity very violent. Again the greatest destruction brought about by the earthquake is never at the epicenter because at that point the waves only produce an up and down movement. The destruction is the greatest at that point which is so situated that the waves reach it in an oblique manner and produce a side to side shacking. But this point must not be at a great distance from the focus or the place or origin of the earthquake.

CHANGES BROUGHT ABOUT BY EARTHQUAKES
The earthquakes seldom cause any significance change in the shape and disposition of the landforms but they, no doubt, bring a great disaster and their influence is most felt through the damage caused by them. Changes brought about by the earthquakes may be studies under the following headings:

Expansion of the Crust

Expansion of the crust results in faults and fissures. The tension produced by the earthquake displaces the earth’s crust along some fault plane. First fissures, or cracks, or fault planes are formed and then the crust is displaced over these planes either in such a way that one side goes up and the other goes down, or along the fault plane the blocks move sideways. This displacement is either vertical or lateral. Sometimes the displacement is buried deep under the mantle and is not visible to the eye. Sometimes an earth mold, like a mole hill, is formed above the fault so buried.

Contraction of the Earth

Contraction of the earth is more common than expansion. It is seen in the doubling, buckling, or bending of railway lines, pipe line, and the collapse of river bridges. All these are the manifestations of the ends of the joints coming closer to each other.

Derangement of Drainage System
Expansion and contraction also take place at the bed of water bodies and the manifest result is that the whole drainage system is deranged (unbalanced). If at certain places, lakes and swamps are drained off, at others, fountains of water gush forth from below the earth. The gushing force of water has often left the place flooded and a semi-permanent lake is formed on that piece of land, which was once dry and inhabited. During one of the recent earthquakes, the Lungarno Pacnotti in Pisa, Italy, dropped 3 meters from its previous level and crevices (fissure) opened on the road following the earth movement and the nearby buildings had to be evacuated.

Crater or Cone Elevation

During an earthquake, the jets of water, that may be formed, often gather sand or mud at their mouths. Gradually these accumulations begin to rise high and with their elevation craterlets or cones may be formed. These are also known as Mud Volcanoes and continue to dot the area, even though sprouting of the water through the fountains has terminated.

GEOGRAPHIC DISTRIBUTION OF EARTHQUAKES

The occurrence of earthquakes is restricted largely to two long narrow zones. One zone surrounds the Pacific Ocean and the other extends from the Azores Islands eastward to southeastern Asia. There is also a minor zone which runs down the center of the Atlantic Ocean along the Mid-Atlantic Ridge.

Circum Pacific Zone

The earthquake zone around the Pacific Ocean follows the west coast of South America, splitting in Central America into two branches, one of which follows the West Indian Island area, the other broadening in the United States to include the Western Rocky Mountain area. The zone follows the Aleutian Island across to Kamchatka and passes southeast through the Japanese Islands, the Philippines, New Guinea, the Islands of the Southwestern Pacific to New Zealand and the Antarctica.

Azores Islands Zone

This zone runs east from the Azores Islands through the Alps and across Turkey. It becomes very broad across southern Asia and then narrows and turns south through Burma, Sumatra, and Java to join the Circum Pacific Zone near New Guinea.

Other Areas

There are several other areas where earthquakes are fairly common but not as frequent as in the zones described above. The Mid Atlantic Region has been mentioned. Others are Eastern Africa and the Indian Ocean, in North America, two such regions are the Valley of St. Lawrence River and northeastern United States. It has been demonstrated many times that large earthquakes may occur outside of these seismically active regions.

DISTRIBUTION OF EARTHQUAKES WITH REFERENCE TO FOLDING AND FAULTING

Faulting
Transformed boundaries that cut through the continental lithosphere are sites of intense seismic activity, with moderate to strong earthquakes. The familiar examples are;

San Andreas Fault

The most familiar example is the 965 km long San Andreas Fault (through San Francisco Bay extending into southern California), which forms the transform boundary between the American Plate and the Pacific Plate in California. 1906 earthquake in San Francisco is an example.

Mid Atlantic Ridge
Most of the spreading boundaries are identified with mid oceanic ridge and its branches. Mid Atlantic Ridge is the most suitable example of this activity. Earthquakes are generated both along the ridge axis and on the transformed faults that connect offset ends of the ridge.

Indian Ocean Carlsberg Ridge
Mid Atlantic Ridge also extends to the center of Indian Ocean. There occur shallow earthquakes.

Folding
The zones of major seismic activity in the earth correspond closely to belts of young mountains. Surrounding the ancient shield areas of the continents and the large stable block of the Pacific Ocean Basin (excluding the vicinity of the Hawaiian Islands) are situated the following main earthquake zones:

Pacific Ocean Belt
The borders of the Pacific Ocean, with many complex branches, including a branching loop through the Caribbean Sea, the islands of which are structurally like the Circum Pacific Belt. In this belt occur about 80% of all shallow quakes, 90% of the intermediate ones and 99% of the deep ones. The largest of the intermediate and deep quakes are assigned to the Islands of Japan. Near Japan and the Philippines Islands, extreme northwestern side of Pacific Plate is sub-ducting into the Continental Eurasian Plate, making extraordinary deep trenches like Mariana and intense seismic activity. More deep quakes take place in the triangle of Fiji, Tonga, and Kermadec Islands (east of Australia) than anywhere else. Earthquakes in 1923 at Kwanto, Japan and in 1995 Kobe, Japan are examples of such seismic activity.

The Mediterranean-Trans-Asiatic (Alpide) Belt
This whole belt is merging with the arc that runs through the East Indies. Here two kinds of activities are involved. One is the transformed fault between African and Eurasian plates, forming Atlas Mountains of the North Africa. This is of relatively shallow earthquake activity area. Another activity area is where Austral-Indian Plate is sub-ducting into the Eurasian plate, making an intense seismic area with epicenter at Himalayas, Hindukush, Karakoram and Chittagong ranges. Slippage in these two plates are causing the formation of Himalayas Mountains and also resulting in the seismic activity, like that of the earthquake of 1950 at Assam and Tibet. The recent earthquake Harnai, Pakistan in 1997 is due to slippage in transform boundary at Owen Fault Zone. Earthquakes in 1962 and 1968 with their epicenter at Zagros Mountain Range in Iran are other examples of folding of folding activity.
Earthquakes of intense activity happened when movement occurred along with the structural axis of Eurasian Plate. Examples are; in 1939 in Caucasus Range and Anatolia sub-plateau [Atlas 30-G6]; in 1960 in Atlas Mountain Range at Agadir, Morocco [Atlas 36-C2]; in 1963 in the tail of Alps Mountain Range at Skoplje, Yugoslavia [Atlas 22-F4]. The whole city was destroyed.

Pamir-Baikal Belt

Pamir-Baikal Belt of Central Asia [Atlas 30-L6/P4] is noted for large shallow earthquakes. Here, in earlier times (at the time of separation of Pangaea – a super continent), European and Asian Plates collided with each other and collision resulted in the formation of Ural Mountains [Atlas 30-J4]. For many centuries, location of Ural Range in the mid of continent remained a mystery. After the Theory of Plate Tectonics devised, this mystery was resolved. It is an orogeny (periods of mountain making) of continental type when two plates collide with each other. Whenever there is any subduction, it results in the formation of Ural Mountain Range and seismic activity.

Andes Belt
Here, Nazca Sub-Plate is sub-ducting under the South American plate and forming the lateral range of Andes Mountains in South America [Atlas 19]. This is Cordilleran Type Orogeny and this is the area of extreme seismic activity. 1906 and 1922 earthquakes in Chile were destructive enough to cause the loss of thousands of life and destruction of ports. 1906 earthquake in Columbia and Ecuador was of same activity.

Appalachian Belt

Earthquake in the Appalachian Mountain Range [Atlas 14-K3] in the North American plate is an example of isostatic kind of earthquake. When the state of balance between the Sialic crust and the substratum is disturbed, tremors of shallow intensity are reported, like that of Charleston, South Carolina, in 1886.

FOLDING

INTRODUCTION
Compressional stresses are especially associated with mountain building. Rocks react in various ways to such stresses, depending on the composition and thickness of the beds and the intensity of the stresses. When the horizontal strata of the earth’s crust is compressed from side to side, the beds are bent into a series of bands or folds. Folding means contraction in area and hence a member a number of troughs and arches are built. The arches of the fold are known as Anticlines (upfolds) while the troughs are known as Synclines (downfolds). Because of these anticlines and synclines, the whole crust appears like a wave. Generally the push or compression is quite intense and from all sides. As a result, folds of various shapes, sizes and inclinations are formed [Strahler 493, 494].

CLASSIFICATION OF FOLDS

Open Folds
If the push or compression is from only one side, simple arches and troughs may be formed. The folds are low and broad. These are known as open folds or folds of the Jura type.

Narrow Folds

If the push is of sufficient intensity and from both the sides, the folds which may be formed will be both high and narrow. Such types of folds are known as narrow folds. These folds may continue to close up until their limbs or sides are both inclined toward one side. Such narrow folds of parallel sides produce a fan-like structure as seen in the Alps Mountain System.

TYPES OF FOLDS
Every fold, whether it is open or narrow, consists of two limbs or sides and it is rarely that the two limbs of all the folds may have the same inclination. Depending on the intensity of the movement and the rock structure, different types of folds are formed:

Symmetrical Folds
These are generally open and upright. Their axis is vertical and their sides are inclined in the same manner.

Asymmetrical Folds

These are formed when one side of the fold is long and gradual in slope while other is shorter and steep. In this the plane is inclined to the vertical.

One Limb Vertical Folds
If the short and steep side of a fold is so placed that it is vertical or perpendicular, a third type is formed with one limb vertical.

Isoclinal or Overfold
It is formed when the fold is pushed over on one side so that both the sides of the fold are inclined in the same direction. The short and steep side also dips in the same direction as a flatter or gradual sloping side. This is also known as an Overturned Fold [Gupta 271]

Nappe or Recumbent Fold
When the overturning of the fold has gone onto such an extent that there is no symmetry at all, the plane of axis forms a flat angle, and one side of the fold lies parallel over the other. This is called the recumbent fold. The imposed strata or the overthrust limb of the fold is known as the Nappe. [Gupta 272]

LANDFORMS ASSOCIATED WITH FOLDING
The differential erosion of sedimentary rock strata that have been warped (curved) by compressional stresses produce many distinctive types of landforms.

Cuestas
These are the most common landforms associated with folding. They are asymmetrical ridges that have a steep slope on one side and a gentle one on the other, the latter confirming to the slope order of the rock strata. Cuesta escarpments (elevations) with cliffed edges comprise major surface landforms in many parts of the world. [Strahler 474]

Domes
Although most folds are caused by compressional stresses that squeeze and crumb strata, some folds are a consequence of vertical displacement. When up-warping produces a circular or elongated structure, the feature is called a Dome. [Strahler 472]

Basins
Down-warped structures having similar structure as that of domes are termed basins. Because large basins contain sedimentary beds sloping at low angles, they are usually identified by the age of the rocks composing them. The youngest rocks are found near the center and the oldest rocks at the flanks (lateral edges, sides). [Strahler 472]

FAULTING

INTRODUCTION
Faults are fractures in the earth’s crust along which appreciable movement has taken place. Compression at one place involves tension at another and sometimes when the compression may be quite intense, it results in the fracture of the earth’s crust. The forces of compression produce folds and folded mountains. The tension produces faults and joints.
A fault is a break in the brittle surficial rocks of the earth’s crust as a result of unequal stresses. Faulting is accompanied by a slippage or displacement along the plane of breakage. Faults are often of great horizontal extent, so that the fault line can be traced along the ground for many miles. Sometimes even 100 miles or more. Little is known of what happens to faults to at depth, but in all probability most extend down for atleast several thousands of feet.

TYPES OF TENSION
The tension is usually of two types:

Local Tension

This type of tension may be due to intense compression and produce overthrust faults.

Regional Tensional

This type of tension, however, alternates with compression and produces faults.

TYPES OF FAULTS
Faults are categorized on the basis of the relative movement between the blocks on both sides of the fault planes. The movement can be horizontal, vertical, or oblique.

Dip-Slip Faults
Faults having primarily vertical movement are called dip-slip faults, since the displacement is in the direction of the inclination, or dip of the fault plane.

Normal Faults
A normal fault has a steep or nearly vertical fault plane. Movement is predominantly in a vertical direction, so that one side is raised or upthrown relative to the other, which is downthrown. A normal fault results in a steep, straight fault scarp (very steep slope), whose height is an approximate measure of the vertical element of displacement.

Reverse Faults
In a reverse fault the inclination of the fault plane is such that one side rise up over the other and a crustal shortening occurs. Reverse faults produce fault scarps similar to those of normal faults, but the possibility of landsliding is greater because an overhanging scarp tends to be formed [Strahler 495].

Transcurrent Fault
A transcurrent is unique in that the movement is predominantly in a horizontal direction. Hence no scarp results, or a very low one at most. Instead, only a thin line traceable across the surface. Sometimes a narrow trench, or rift, marks the fault line.

Thrust or Low-Angle Overthrust Faults
Reverse faults having a very low angle to the horizontal are referred to as thrust faults. In mountainous regions, such as the Alps and the Appalachians, thrust faults have displaced rock as far as 50 km over adjacent strata. Thrust faults of this type result from strong compressional stresses.

Strike-Slip Faults

Faults in which the dominant displacement is along the trend of strike of the fault are called strike-slip faults.

Transform Faults
Many large strike faults are associated with plate boundaries and are called transform faults. Transform faults have nearly vertical dips and serve to connect large structures such as segments of oceanic crust. The San Andreas Fault in California is a well known transformed fault in which the displacement has been in the order of several hundred kilometers.

Oblique-Slip Faults
When faults have vertical and horizontal movement, they are called oblique-slip faults.

LANDFORMS ASSOCIATED WITH FAULTING
Several distinctive landforms are related to faulting, or the slipping of blocks of the earth’s crust along great fractures.

Rift Valley or Graben
It is a very important relief feature formed by the down-throwing of a block of country in between two parallel faults. The lakes of Africa, Dead Sea of Israel, the Red Sea, and the Rhine Valley in Germany are very important examples of rift valley.
The rift valley is usually long and narrow. It is seldom that only two parallel faults are sufficient for its formation. In most of these cases, more than one fault occurs on each side of the rift and the whole structure is quite complex in detail, although the outline is very simple. [Strahler 496]

Horst
It is formed by the up-throwing or raising of a block of country that lies between two parallel faults. In this case the center block is not only up-thrown but the side blocks are also relatively down-thrown so that the whole central mass appears like a dome. [Strahler 496]

Block Mountain
It may be formed by the up-throwing of a block of country on one side of a pair of parallel faults. Thus the up-thrown block stands like a mountain and is known as a Monocline or Block Mountain. These block mountains are bounded by a fault scarp on one side. They alternate with the asymmetrical fault (normal fault) valleys. These mountains have a fault scarp face and are quite common in western North America and the Northern Pennine Chain Mountains. [Atlas 22-D3] [Strahler 496]

TERMINOLOGY


Fault Plane

The line of breaking in a fracture is known as the fault plane and the fault so caused may be as narrow as a few inches or as wide as several feet.

Fault Zone
If the break in the crust is distributed over a number of planes generally parallel to each other, it is known as a fault zone.

IMPORTANCE OF FAULT MOTION

Fault motions provide the geologists with a method of determining the nature of the forces at work within the earth. Normal faults indicate the existence of tensional stresses that pull the crust apart. Since the blocks involved in reverse and thrust faulting are displaced toward one anther, geologists conclude that compressional forces are at work.
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The Herd of Elephants Theory
Have you ever felt the ground shake as a herd of elephants stampeded by? Has your mom ever told you that you sound (and feel!) like a herd of elephants because you are making the whole house shake? Is that an earthquake? Maybe its just a housequake. Have you ever felt your house shake when a truck drives by? Well, that is a very local earthquake. In all these cases, the earth shakes in response to a local shock. These shakes would show up on a seismograph. In fact, they do show up on seismographs and scientists have to know how to tell the difference between a big truck going by outside (or a herd of elephants stampeding down the hall) and a large earthquake halfway around the world.
The Nuclear Explosion Theory
On a larger scale an explosion can cause the earth to shake for a considerable distance. Scientists use seismographs to monitor nuclear tests. People in Las Vegas could feel the shaking caused by underground nuclear tests in the desert miles away. The government analyzes the shock waves (earthquakes) produced by nuclear explosions to study the effects of nuclear tests and to monitor tests elsewhere in the world.
The Extraterrestrial, or Meteor, Theory
Every day tiny meteors hit the earth, as we move through space. The vast majority of them burn up in the atmosphere, leaving no more trace than a shooting star across the sky. Once in a while, a meteorite will reach the surface of the earth. Very rarely a great meteorite will hit, causing the ground to shake and creating a large crater. The Meteor Crater in Arizona is an excellent example of this type of crater. Imagine how the ground shook for miles around when it was formed!
The moon is full of meteor craters that we can see because they have not eroded away. The earth also has been struck many times over its history. Erosion by wind and rain wear down the craters so we can't see most of them anymore. Scientists studying the earth have found traces of many meteor impacts around the world. Each impact creates an earthquake.
Volcanoes
Earthquakes are one of the indicators of increased volcanic activity leading up to an eruption. As magma forces its way up into a volcano, it pushes aside the rocks in its way, causing bulges in the ground and a flurry of earthquakes. Scientists studying volcanoes watch for an increase in earthquakes to tell them that an eruption may be on the way. Using this and other measures of volcanic activity, they have been able to warn residents to evacuate before eruptions. Although they still cannot predict eruptions with absolute certainty, they are learning more all the time about what to look for to make better predictions. Much of this knowledge comes from studying volcanic earthquakes.
Plate Tectonics
Most Earthquakes are caused by Plate Tectonics. The earth's crust consists of a number of sections or plates that float on the molten rock of the mantle. These plates move on convection currents caused by heat rising from the center of the earth. The hot magma rises and spreads out on the surface, creating new crust. The crust spreads out forming a new plate until it meets another plate. One of the plates will be pushed down into the interior of the earth and reabsorbed into the mantle. Plates can also be compressed to push up mountains when they collide or move sideways along transform faults.
The plates are the Earth's crust that float on the molten rock in the center of the Earth. Most of the inside of the Earth is so hot that the rock melts. Just as a pot of hot chocolate on the stove will bubble as it is heated; the molten rock, or magma, very slowly bubbles up in great currents under the surface of the Earth. The crust that floats on the magma moves with it, like the skin that might form on the hot chocolate. The Plates are just pieces of the crust. The part that makes it hard to understand is that it all moves so slowly. Even though the magma is very hot it is also very thick and under tremendous pressure in the middle of the Earth. So it moves only a few centimeters a year. Over millions of years that adds up to a lot of movement.
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