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THE MAGNITUDE 6.8 NISQUALLY
EARTHQUAKE OF FEBRUARY 28, 2001: SEISMOLOGICAL
ASPECTS
CROSSON, R.S.,
CREAGER, K.C., MALONE, S., THOMAS, G., LUDWIN, R.,
QAMAR, A., University of Washington, Seattle, WA,
98195, USA; WEAVER, C. S., PRATT, T., US Geological
Survey, Seattle, WA
The magnitude
(Mw) 6.8 Nisqually earthquake occurred at 10:54 PST (1854 UT) on
February 28, 2001. The epicenter of the earthquake was at 47.159 N,
123.733 W with an estimated hypocentral depth of 51 km. The source is
within the Wadati-Benioff zone of the Cascadia subduction zone, near
the same location as the 1949 magnitude 7.1 Olympia earthquake. The Mw
5.8 Satsop earthquake of July, 1999 occurred about 60 km to the west,
and the 1965 magnitude 6.5 Seattle-Tacoma earthquake originated about
40 km NNE of the Nisqually event. The double couple mechanism of the
main shock reflects apparent slab down-dip extension in either the E-W
direction (Harvard CMT solution) or ESE (PNSN short period polarity
mechanism). The difference between regional and teleseismic mechanisms
appears to be significant and may reflect complexities during
faulting. Resolution of the fault plane ambiguity awaits further work
on rupture kinematics and geodetic analysis. Aftershocks have been
extremely rare, consistent with worldwide observations of intraslab
earthquakes at this depth. Only 3 aftershocks were identified and
located in within the first 2 days of the mainshock. The largest of
these had a coda-duration magnitude of 3.4 and was located
approximately 6 km north and slightly deeper relative to the mainshock
hypocenter. Geodetic observations reflect surface deformation that is
consistent with CMT solutions derived from seismic data. The PNSN
network acquisition system performed well during the earthquake, and
the availability of 41 telemetered digital strong motion stations plus
a large number of autonomous strong motion stations provide a rich set
of on-scale observational data that will be useful for modeling source
kinematics. Observed peak accelerations are typically in the 10% g
range, with only two stations exceeding 25% g, consistent with the
generally moderate damage sustained from this earthquake.
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