A Comparison of Ground Motions from the 2001 M 6.8 In-Slab Earthquakes in Cascadia and Japan

Are earthquake source and propagation properties for in-slab earthquakes in the Cascadia region (British Columbia and Washington State) similar to those in the subduction zone of southwest Japan? To answer this question, we compare ground motions for two recent earthquakes of similar magnitude and depth. The comparisons are made for horizontal-component 5% damped response spectral acceleration at frequencies from 0.5 to 10 Hz. The selected Cascadia event is the Nisqually earthquake of 28 February 2001, which occurred within the subducting Juan de Fuca plate with a moment magnitude of M 6.8 at a depth of 51 km. The Japan event is the Geiyo earthquake of 24 March 2001 with M 6.8, at a depth of 60 km within the subducting Philippine Sea plate. Generally, the response spectral amplitudes from the two events overlay each other, when amplitudes are plotted against distance. An interesting relationship is observed however. At low frequencies (0.5-1 Hz) the Nisqually data tend to have higher amplitudes, whereas at high frequency (5-10 Hz) the Nisqually data have lower amplitudes, in comparison to the Geiyo data. The observed "drop through" of the Nisqually data relative to the Geiyo data as frequency increases could be due to either source effects or site effects. (It is not a path effect because there is no indication of differences in the slope of the attenuation curve.) If the observed frequency-dependent amplitude differences are due to source effects, this could imply that in-slab earthquakes in Japan are fundamentally different from those that occur in the Cascadia subduction zone. This would have important implications, as it would cast doubt on the applicability of lessons learned in Japan to hazard assessment in Cascadia. On the other hand, if the observed differences are attributable to site effects, then Japanese observations and results are applicable to Cascadia once we have accounted for differences in site effects. To address the origin of the observed differences in ground motion between the Nisqually and Geiyo earthquakes, we evaluate the influence of site effects on the recorded motions. We correct all response spectra to an equivalent rock motion, using amplifications derived from quarter-wavelength site response calculations for representative regional soil profiles. The site-corrected rock motions from Nisqually and Geiyo are in reasonable agreement with each other at all frequencies, with the exception of some Nisqually observations from West Seattle. We conclude that the observed differences between the Nisqually and Geiyo ground motions are attributable to regional differences in site response.