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.


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.