ergy (Lg waves) in the more stable crust of the central and eastern United States. Regional studies that deploy seismometers more densely will be needed to clarify these explanations and to understand how the vertical and lateral structure of the crust controls ground motions.

Subsidence and Uplift

Large thrust earthquakes in subduction zones can cause sudden, permanent elevation changes with damaging effects to coastal areas. Uplift and subsidence related to fault slippage on shallow thrusts have been documented in New Zealand, Japan, Chile, and southeast Alaska (17). During the great 1964 Alaska earthquake (Box 2.3), the shorelines of Prince William Sound rose in some places by several meters, draining small-craft harbors, while they dropped in others, causing the streets of coastal towns to flood at high tide. Submerged marshlands in several estuaries along the coasts of Washington, Oregon, and northern California indicate that similar episodes of sudden subsidence have resulted from large thrust events in the Cascadia subduction zone (described in Section 3.2). The pattern of uplift and subsidence during an earthquake can be predicted from elastic dislocation models if the area of the fault plane and the distribution of slip within that plane are known (18). Anticipating the damage from elevation changes in future events can thus be approached by combining theoretical studies with seismic, paleoseismic, and geodetic observations.

Secondary Ground Failures

The secondary hazards caused by seismic shaking include forms of mass wasting—such as landslides, rockfalls, and slumps—as well as soil failures associated with compaction, liquefaction, and lateral spreading (19). In some instances, these failures cause more damage than the ground shaking itself. An M 8.6 earthquake in China’s Gansu Province in 1920 triggered an extensive debris flow, which covered a region larger than 100 square kilometers and resulted in roughly 200,000 deaths. An immense rock and snow avalanche (60 million cubic meters) triggered by the 1970 Peru earthquake (M 8.0) buried the mountain towns of Yungay and Ranrahirca, killing 66,000 people (Figure 3.3). Many of those killed in the January 13, 2001, El Salvador earthquake were buried by a muddy landslide loosened from a slope in the capital’s suburbs.

Liquefaction is the temporary conversion of water-saturated, unconsolidated soils into a medium that behaves like a fluid. It occurs when saturated sand or silty sand is shaken hard enough to mobilize individual grains. If the water cannot escape the granular soil matrix fast enough to



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