HR: 1330h
AN: S42A-0152    [PDF]
TI: Using the Local Magnitude Scale to Determine Seismic Site Response in the Pacific Northwest
AU: * Qamar, A
EM: tony@ess.washington.edu
AF: University of Washington, Dept. of Earth and Space Sciences, PO Box 351310, Seattle, WA 98195 United States
AU: Wright, A
EM: amyw@ess.washington.edu
AF: University of Washington, Dept. of Earth and Space Sciences, PO Box 351310, Seattle, WA 98195 United States
AU: Thomas, G
EM: george@ess.washington.edu
AF: University of Washington, Dept. of Earth and Space Sciences, PO Box 351310, Seattle, WA 98195 United States
AB: Using new wide dynamic-range digital seismographs and accelerometers of the Pacific Northwest Seismograph Network in Washington and Oregon we have reevaluated the magnitudes of earthquakes in the Pacific Northwest using the Local (Richter) Magnitude scale, ML. We have done this by synthesizing Wood-Anderson Seismograms from broadband instruments. This procedure is a departure from our traditional use of the coda-magnitude scale. We have computed the ML of 76 earthquakes that occurred from 2000-2003 with 2.5$<$M$<$6.8 using observations from a total of 2812 seismograms. We utilized a method of Joint Magnitude Determination in which earthquake magnitudes and station corrections are determined simultaneously for all earthquakes. The computed magnitude corrections at the stations range between -0.4 to +0.4 magnitude-units. On average, the magnitude corrections are correlated with, and can be reasonably predicted from, the NEHRP soil classification assigned to each site. Computed magnitudes tend to be higher at sites where the estimated surface shear wave velocity is low. The variations in computed magnitude do not seem to depend on the presence of deep-basin structures in the subsurface. On the other hand, such structures have been shown to affect the amplitudes of very low frequency seismic waves. We suspect that this discrepancy is caused by the fact that Richter magnitudes (ML) are intrinsically sensitive to higher frequency ground motions. We conclude that variations in Richter magnitude can be used to detect shallow subsurface site conditions likely to affect earthquake shaking.
DE: 7223 Seismic hazard assessment and prediction
SC: Seismology [S]
MN: 2003 Fall Meeting