cally induced landslide originating 130 km from the earthquake claimed at least 18,000 and possibly as many as 25,000 lives (Gere and Shah, 1984). Splashes or waves caused by landslides may also have extensive effects; the 1958 earthquake induced a landslide into Lituya Bay, which caused a giant wave reaching 1720 ft above sea level (Miller, 1960).

Liquefaction is a phenomenon in which near-surface water-saturated sediments are shaken, lowering their rigid strength and behaving as a semiliquid material. Structures such as buildings and pipelines built on ground that liquifies can tilt or sink or may be moved as the ground flows. This phenomenon has been noted in many earthquakes including classic effects of large apartment houses tilting in Niigata, Japan, during the June 1964 earthquake and spectacular effects during the March 1964 earthquake at Turnagain Heights and Valdez Harbor, Alaska. Liquefaction effects can be predicted in a gross way using simple linear diagrams (Youd, 1978) or more precisely with sophisticated computer models for specific sites (Youd et al., 1978).

A seiche is a wave set up in a body of water in response to earthquake waves and also can occur great distances from the earthquake source. The danger of seiche is temporary flooding of areas near lakes and reservoirs and overtopping of dams. During the 1954 Dixie Valley-Fairview Park, Nevada, earthquakes, a seiche was set up in a covered water reservoir in Sacramento, California, 300 km away, damaging support pillars, concrete walls, and gunite panels (Steinbugge and Moran, 1957).

Large ocean waves created by uplift or downdropping of the seafloor during an earthquake are called tsunamis. Tsunamis can move hundreds of kilometers per hour and destroy facilities and structures along the coast, thousands of kilometers from the earthquake. Warnings are issued when there is a large earthquake in oceanic areas, which allows coastal residents to go to safety on higher ground until the tsunami danger has passed.

Detailed observations and investigations of secondary effects have led to the understanding, prediction, and mitigation of these effects in many areas. Today, secondary effects can often be identified and risks assessed thanks to geologic, geophysical, and engineering laboratory and field studies.

TECTONIC SETTING

Plate-Tectonic Relationships

Plate-tectonic concepts are accepted by most earth scientists as a working model of the crustal behavior of the Earth. This model suggests that the Earth’s surface is composed of several large plates and numerous smaller plates that are slowly moving and rotating with respect to each other. Since this behavior is dynamic, faulting, earthquake activity, and rates of fault slip and folding are closely related to the rate of movement between plates. Most seismic activity occurs along plate boundaries, areas known as interplate areas. Intraplate areas are areas within plates and have less seismic activity and lower rates of tectonic activity than interplate areas. Closer inspection of interplate regions reveals smaller “microplate” tectonic domains characterized by a particular faulting style, such as the Basin and Range province in the western United States.

Interplate Regions

These regions have many active faults with a potential for future displacements and associated earthquake activity. The methods described in this chapter are especially appropriate for many faults in these regions. There is a great range in rates of fault activity with recurrence intervals that range from decades to hundreds of thousand years. Additionally, the plate and microplate boundaries vary from sharp and narrow to broad (“soft”) zones that may extend hundreds to thousands of kilometers from the main boundary, including most of the western United States, although there are “islands” of inactive subplates within this region and variations in rate of activity within active provinces. Wallace and Whitney (1984) described an example of variable rates of tectonic activity within the Great Basin province. Complete evaluations of a fault are needed to assess the following characteristics: seismogenic character, segmentation, recurrence interval and slip rate, recency of fault activity, and the relation to the site or area being evaluated.

The maximum magnitude of earthquakes varies with seismotectonic province or fault and includes a range in values for each type of plate boundary. The maximum historical values include Mw (moment magnitude) values of over 9 for some subduction zones, MS (surface wave) magnitudes of up to about 8.3 for strike-slip faults, and MS magnitudes of up to about 7.5 or 8 for extensional zones.

Intraplate Regions

Seismic hazard evaluation of intraplate regions has evolved rapidly in the last few years, although processes will become more refined and better understood. Rates of activity are generally orders of magnitude lower within intraplate areas as compared with interplate areas. The low rates of faulting and warping and sub-



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