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Site response, basin amplification, and earthquake stress drops in the Portland, Oregon area

December 15, 2020

Site response, sedimentary basin amplification, and earthquake stress drops for the Portland, Oregon area were determined using accelerometer recordings at 16 sites of 10 local earthquakes with MD">MDMD 2.6–4.0. A nonlinear inversion was applied to calculate site response (0.5–10 Hz), corner frequencies, and seismic moments from the Fourier spectra of the earthquakes. Site amplifications at lower frequencies of 0.1–2.0 Hz were determined from Fourier spectra of four regional earthquakes with Mw">MwMw 5.8–6.4. Amplifications were calculated relative to a stiff‐soil site outside the Portland and Tualatin basins. Sites on artificial fill and Holocene alluvium show strong amplification peaks (factor of 5) around 1–2 Hz. Sites on the Portland Hills, consisting of thin soil over basalt, display spectral peaks at 4–5 Hz (factor of 4). Spectral peaks at both sites are similar to those predicted for vertically propagating S waves from VS">VSVS profiles determined at these sites using a borehole and refraction microtremor analysis. The largest amplifications at 0.1–1 Hz were found at stiff‐soil sites in the Tualatin basin, based on recordings of regional earthquakes. Amplifications of a factor of 10, at about 0.3 Hz, were observed for a site in the deeper portion of the Tualatin basin and a factor of 7 at 0.5–0.6 Hz for two adjacent sites closer to the border of that basin. Stiff‐soil sites in the Portland basin exhibit amplifications of 2–3 at frequencies of about 0.3–0.8 Hz. The frequencies of the amplification peaks for the deep Tualatin basin site can be explained by S‐wave resonance in the shallow sediments, but the observed amplification is underestimated. Earthquake stress drops determined from the inversion range from 3 to 11 MPa, with no overall dependence on seismic moment.

Publication Year 2021
Title Site response, basin amplification, and earthquake stress drops in the Portland, Oregon area
DOI 10.1785/0120200269
Authors Arthur Frankel, Alex R. R. Grant
Publication Type Article
Publication Subtype Journal Article
Series Title Bulletin of the Seismological Society of America
Index ID 70236083
Record Source USGS Publications Warehouse
USGS Organization Earthquake Hazards Program; Earthquake Science Center