Using HVSR to Assess Seismic Site Effects in Oregon's Tualatin Basin

The Tualatin Basin is a deep (>5 km), fault-bound sedimentary basin situated just west of Portland, OR. With the basin's proximity to the Cascadia Subduction Zone and local crustal faults, understanding how it may amplify seismic waves is critical to mitigating seismic hazard. In June 2021, the USGS deployed seven broadband seismometers across the region, with five instruments overlying the deepest portions of the Tualatin Basin, one on the edge of the basin, and one in the Portland Basin. We calculate horizontal-to-vertical spectral ratios (HVSR) using these data to find and evaluate the resonant frequency of the basin at each location. All of the seismometers centered in the basin exhibit a low frequency peak between 0.12-0.15 Hz, likely due to the large depth to basement at these locations. Larger peak frequencies between 1.0-20.0 Hz are also observed at many sites, likely due to shallow layers or discontinuities in the basin, including the buried Columbia River Basalt Group. We compare the observed resonant frequencies to frequencies calculated using estimated velocity profiles and subsurface geologic structures. Of these comparisons, many observed and calculated frequencies are close to one another, implying the velocity and depth profiles are reasonably accurate for most sites. Finally, we analyze a subset of teleseismic arrivals to better understand how shaking is amplified inside the basin relative to sites outside the basin. These results suggest that there are discrete, geologically-controlled frequencies (particularly, 0.12-0.15 Hz) at which earthquake shaking is significantly amplified in the Tualatin Basin, which should be considered during seismic hazard analysis.