Regional map view of instrumentally-determined ground motions, Nisqually earthquake of 28 February 2001

HAUGERUD, R.A., USGS, Seattle WA 98195, rhaugerud@usgs.gov; THOMAS, G., Dept. of Earth and Space Sciences, University of Washington, Seattle WA 98195, george@geophys.washington.edu; PALMER, S.P., Washington Dept. Natural Resources, Olympia WA 98501, steve.palmer@wadnr.gov.

Since the Nisqually earthquake we have maintained a map of instrumental strong motions (map). Data come from 43 instruments operated by the Pacific Northwest Seismograph Network (including 22 recently installed ANSS instruments) and 48 instruments maintained and (or) read by the USGS National Strong Motion Program (http://nsmp.wr.usgs.gov). Data from 6 additional instruments operated by other organizations are being collected and processed and will be included in the map. At present there is a broad suite of 3-component peak ground acceleration measurements and derived peak ground velocities. We look forward to the availability of spectral response parameters, Arias intensity, and other measures of shaking intensity. Peak ground accelerations (PGA) did not fall off monotonically with distance from the epicenter. Near-epicenter PGAs were as high as 0.25 g (Olympia). PGAs were markedly low (circa 0.06 g) in the Tacoma area, high in south Seattle and Bremerton (0.15 - 0.3 g), and high at Monroe (0.15 g). Our initial impression is that site soil conditions alone do not explain these effects. We are in the process of comparing PGAs and other measures of shaking intensity with at-instrument NEHRP soil classifications estimated from the 1:100,000-scale geologic maps of western Washington compiled by Washington Division of Geology and Earth Resources.