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Scenario Earthquakes for Urban Areas Along the Atlantic Seaboard of the United States
Seismic Hazard Factors
Before we describe plausible scenario events, let us briefly summarize at least qualitatively some of the basic characteristics of eastern seismic hazard, and how they differ from their western U.S. equivalents. Seismic hazard differs in the eastern U.S. from that in the west because of:
- 10 to 100 times lower seismicity rates (longer recurrence periods for the same magnitudes);
- general lack of surface faulting (i.e. potential loci of future quakes are not well known);
- less rapid attenuation of ground motions with distance, i.e.: larger damage areas (Figure 4) for the same magnitudes of earthquakes;
- higher high-frequency content of seismic ground motions to large distances;
- higher contrast of shaking on soft soils vs. hard rock (high site amplification);
- liquefaction of non-cohesive soils appears to reach out to larger distances;
- the ratio of ground motions between 2,500- and 500-year recurrence events is higher in the east than for the west. Given this trend, an eastern engineered structure is likely to experience more severe damage from a 2,500- year event than a western structure, provided both are properly designed for 500-year events.
Also there is greater uncertainty in quantitative hazard assessments for the east than in the west because:
- the historic seismicity record (300 years) is too short compared to the recurrence periods of major damaging events (of hundreds or thousands of years in the east, at any given location). This leads to great uncertainties in estimates of the seismicity levels, of the spatio-temporal stationarity of the seismicity, and in the assignment of regional maximum magnitudes;
- only few sets of eastern strong-motion data exist. They marginally constrain the attenuation of shaking with distance, i.e. the shaking’s dependence on magnitude, distance and depth of the earthquake.
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Last modified: 5/7/2001 3:38:57 PM
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