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Open-File Report 96-532

National Seismic Hazard Maps: Documentation June 1996

By Arthur Frankel, Charles Mueller, Theodore Barnhard, David Perkins, E.V. Leyendecker, Nancy Dickman, Stanley Hanson, and Margaret Hopper

WUS Attenuation Relations

Crustal Events. For peak ground acceleration (PGA) we used three, equally-weighted attenuation relations: 1) Boore, Joyner, and Fumal (BJF; 1993, 1994a) with later modifications to differentiate thrust and strike-slip faulting (Boore et al., 1994b), 2) Sadigh et al. (1993) and 3) Campbell and Bozorgnia (1994). When calculating spectral response values, we used BJF (1993) and Sadigh et al. (1993). For 1) we calculated ground motions for a site with average shear-wave velocity of 760 m/sec in the top 30m, using the relations between shear-wave velocity and site amplification in Boore et al. (1994a). For 2) and 3) we used their "rock" values. Joyner (1995) reported velocity profiles compiled by W. Silva and by D. Boore showing that WUS rock sites basically spanned the NEHRP B/C boundary. When calculating ground motions for each fault, we used the relations appropriate for that fault type (e.g, thrust). All of the relations found higher ground motions for thrust faults compared with strike slip faults.

For all calculations we included the variability of ground motions. For 1) we used the sigma values reported in BJF (1994b). For 2) and 3) we used the magnitude-dependent sigmas found in those studies. Our hazard code incorporates magnitude-dependent variability.

Of course, the distance measure from fault to site varies with the attenuation relation and this was accounted for in the hazard codes.

Deep events (> 35 km). Most of these events occurred beneath the Puget Sound region. although some were in northwestern California. For these deep events, we used only one attenuation relation by Geomatrix (1993; with recent modification for depth dependence provided by R. Youngs, written comm., 1996) which is based on empirical data of deep events recorded on rock sites. We did not use the relations of Crouse (1991), because they were for soil sites. We have found that the ground motions from Geomatrix (1993) are somewhat smaller than those from Crouse (1991), by an amount consistent with soil amplification. These events were placed at a depth of 40 km for calculation of ground motions.

Cascadia subduction zone. For M8.3 events on the subduction zone we used two attenuation relations (with equal weights) following the lead of Geomatrix (1993): 1) Sadigh et al. (1993) for crustal thrust earthquakes and 2) Geomatrix (1993) for interface earthquakes. For the M9.0 scenario we could not use Sadigh et al. (1993) formulas, since they are invalid over M8.5. Therefore, we used only Geomatrix (1993). Again the values from Geomatrix (1993) were somewhat smaller than the soil values in Crouse (1991).

 

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