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Structure and Qp-Qs Relations in the Seattle and Tualatin Basins From Converted Seismic Phases

Session: Advances in Upper Crustal Geophysical Characterization I

Type: Oral

Date: 4/21/2021

Presentation Time: 09:45 AM Pacific

Description: 

We use converted body wave phases from local earthquakes to constrain depth-to-basement and average attenuation relations for the Seattle Basin in Washington State and the Tualatin Basin in Oregon. P-, P-to-S- (Ps), S-to-P- (Sp), and S-wave arrivals from M2.5-4.0 earthquakes are present in three-component seismic records from these basins. Based on their relative travel times, these phases are attributed to body wave conversions at the basement-to-basin contact, or to high impedance contrast interfaces within the basins. Depth-to-basement values are calculated using the differential travel times between direct and converted phases, as well as average P- and S-wave velocity values. We also identify a high impedance contrast layer in the Tualatin Basin that likely represents a laterally extensive deposit of volcanic materials embedded between the basement contact and the Columbia River Basalt Group. In addition, we calculated the average Qp-Qs attenuation relation for each station by taking the spectral ratio of converted phases to their parent body wave arrivals. For the Seattle Basin, our analysis yields an average Qp value of 73 and an average Qs value of 60 for waves with frequencies between 2-25 Hz. In the Tualatin Basin, a much reduced Qp-Qs relation suggests that average body wave attenuation is likely higher than in the Seattle Basin. The converted phase techniques presented here provide a reliable way to develop estimates of basin depth and attenuation structure for under-characterized regions using simple passive source seismic records.

Presenting Author: Ian Stone

Student Presenter: No

Authors