Geographic deaggregation of seismic hazard in the United States

The seismic hazard calculations for the 1996 national seismic hazard maps h ave been geographically deaggregated to assist in the understanding of the relative contributions of sources. These deaggregations are exhibited as ma ps with vertical bars whose heights are proportional to the contribution th at each geographical cell makes to the ground-motion exceedance hazard. Bar colors correspond to average source magnitudes. We also extend the deaggre gation analysis reported in Harmsen et al. (1999) to the western contermino us United States. In contrast to the central and eastern United States (CEU S); the influence of specific faults or characteristic events can be clearl y identified. Geographic deaggregation for 0.2-sec and 1.0-sec pseudo spectral accelerati on (SA) is performed for 10% probability of exceedance (PE) in 50 yr (475-y r mean return period) and 2% PE in 50 yr (2475-yr mean return period) for f our western U.S. cities, Los Angeles, Salt Lake City, San Francisco, and Se attle, and for three central and eastern U.S. cities, Atlanta, Boston, and Saint Louis. In general, as the PE is lowered, the sources of hazard closer to the site dominate. Larger, more distant earthquakes contribute more sig nificantly to hazard for 1.0-sec SA than for 0.2-sec SA. Additional maps of geographically deaggregated seismic hazard are available on the Internet f or 120 cities in the conterminous United States (http://geohazards.cr.usgs. gov/eq/) for 1-sec SA and for 0.2-sec SA with a 2% PE in 50 yr. Examination of these maps of hazard contributions enables the investigator to determine the distance and azimuth to predominant sources, and their mag nitudes, This information can be used to generate scenario earthquakes and corresponding time histories for seismic design and retrofit. Where fault d ensity is lower than deaggregation cell dimensions, we can identify specifi c faults that contribute significantly to the seismic hazard at a given sit e. Detailed fault information enables investigators to include rupture info rmation such as source directivity, radiation pattern, and basin-edge effec ts into their scenario earthquakes used in engineering analyses.