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OVERVIEW OF STRONG-MOTION RECORDINGS OF THE M6.8 NISQUALLY, WASHINGTON, EARTHQUAKE
FRANKEL, A. and CARVER, D., U.S. Geological
Survey, MS 966, Box 25046, DFC, Denver, CO 80225,
afrankel@usgs.gov, carver@usgs.gov;
MALONE, S. and
THOMAS, G., University of Washington, Geophysics Dept.,
Box 351650, Seattle, WA 98195,
steve@geophys.washington.edu,
george@lobo.geophys.washington.edu;
WEAVER, C., USGS,
Univ. of Washington, Box 351650, Seattle, WA 98195,
craig@usgs.gov;
STEPHENS, C., USGS, 345 Middlefield Rd.,
Menlo Park, CA 94025, cdstephens@usgs.gov;
PORCELLA, R.,
USGS, 2665 N. Air-Fresno Dr., Fresno, CA 93727,
porcella@usgs.gov;
BENZ, H., USGS, Denver, CO 80225,
benz@usgs.gov;
FILSON, J., USGS, 12201 Sunrise Valley
Dr., Reston, VA 20192, jfilson@usgs.gov;
WONG, I., URS
Greiner-Woodward Clyde, 500 12th St., Suite 200,
Oakland, CA 94607, ivan_wong@urscorp.com;
BICE, T.,
USGS, Golden, CO 80225, bice@usgs.gov;
NORRIS, R., USGS,
Univ. of WA, Box 351650, Seattle, WA 98195,
norris@usgs.gov;
PETERSEN, M. and HARMSEN, S., USGS,
Golden, CO 80225, mpetersen@usgs.gov and
harmsen@usgs.gov.
The Nisqually earthquake of Feb 28, 2001 was
recorded by more than 90 digital and analog
accelerographs at epicentral distances between 12 and
100 km. Three major sets of strong-motion data are
available. The first is from the University of
Washington strong-motion network of about 40 stations,
part of the Advanced National Seismic System. Data from
most of these sensors were telemetered in realtime and
used to produce shakemaps released rapidly on the
Internet showing intensity, peak acceleration, and other
values (www.geophys.washington.edu). Peak accelerations
and velocities from the digital instruments of the
National Strong Motion Program (NSMP) of the USGS were
released shortly after the event and records from about
40 NSMP sites are available (nsmp.wr.usgs.gov). The
third data set is from an array of 25 digital recorders
in Seattle operated by the USGS office in Golden, CO,
consisting of sites on soft soils south of downtown and
closely-spaced sites in downtown, Queen Anne Hill, and
other locations (groundmotion.cr.usgs.gov). Peak ground
accelerations ranged from about 5%g to 30%g within a
hypocentral distance of 100 km. These values are
generally lower than those predicted from current
attenuation relations for intraslab events. Stations
located on soft soils show substantial amplification at
frequencies below 3 Hz, compared to firm-rock
sites. Stiff-soil sites also exhibit some amplification
below 2 Hz. Comparison of soil site to rock site
spectral ratios for the mainshock and a ML 3.4
aftershock show the loss of high-frequency amplification
(3-8 Hz) for soft-soil sites during the mainshock and
shifts of spectral peaks to lower frequency. This
indicates nonlinear response at the soft-soil sites,
some of which had nearby liquefaction. Many of the
mainshock records from soft-soil sites display spiked
arrivals after the S-wave, another symptom of
nonlinearity.
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