SITE RESPONSE IN WESTERN WASHINGTON FROM SPECTRAL RATIOS

We analyze 3-component broadband ground motion data from the 2012 M 6.1 Vancouver Island earthquake and a M4.5 aftershockto study site response and basin effects in 18 stations in western Washington. Seismic stations were chosen to cover different geologic units close to the Seattle and Tacoma basins and other areas in western Washington.We compare results from three popular methods: (1) simple spectral ratios (SSRs) of soft soil sites to a reference site, horizontal-to-vertical (H/V) ratios of shear wave arrivals, and (3) horizontal-to-vertical spectral ratios (HVSRs) of ambient noise. Preliminary results show the largest amplification in 0.1 to 2Hz for both the main shock and aftershock, which likely corresponds to depth to basement in the vicinity of the stations. Although previous studies noted a shift in lower frequency amplification of larger events (main shocks) relative to smaller events (aftershocks), this phenomenon is only observed at some sites. Further investigation of the relationship of site to frequency content, amplification, and geologic setting is warranted. However, there is a generally good agreement between the preliminary results from this study and previous studies in the area, a result that suggests the stability of resonant peaks over time.