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Shallow Shear-Wave Velocity and Crustal Structure in the Seattle and Tacoma Basins From Microtremor Array Analysis

Session: Advances in Upper Crustal Geophysical Characterization [Poster]

Type: Poster

Date: 4/21/2021

Presentation Time: 11:30 AM Pacific

Description: 

We characterize shear-wave velocity (Vs) structure in the Seattle and Tacoma basins, Washington State, by analyzing three microtremor array datasets with either the 1) multi-mode spatial autocorrelation (MMSPAC) method coupled with a Bayesian Monte Carlo inversion (BMCI) or 2) the wavenumber-normalized SPAC (krSPAC) method. Dataset 1 included seven sites in the western Seattle and Tacoma basins acquired to image Vs in the upper 1 km that are suitable for krSPAC analysis; each array consisted of 10 sensors deployed simultaneously in an asymmetric nested triangular pattern, with the largest interstation spacings of roughly 1 km and minimal spacings of about 57 m. When compared to the regional Cascadia velocity model (CVM) the unconsolidated sedimentary thickness interpreted in the krSPAC Vs profiles generally agrees with the CVM; however, the krSPAC profiles suggest lower Vs in the upper 100 m and a typically higher velocity gradient to the interpreted rock interface. Dataset 2 consisted of microtremor data at 17 sites located within the city of Seattle. The microtremor data were acquired with four sensors deployed in three sequentially recorded equilateral triangles with interstation spacings of 33.3 m, 100 m and 300 m, respectively. Dataset 3 consisted of microtremor data at 20 sites in the Tacoma basin simultaneously acquired with 10 sensors in nested equilateral triangular arrays similar in size to dataset 2. Both datasets 2 and 3 were analyzed using the MMSPAC/BMCI approach and imaged to no greater than 300 m depth. Comparison of Vs30 (time averaged Vs to 30 m depth) for each of the datasets suggests the shallowest sediments within the southern Puget Lowland are broadly comparable between the three data regions (nominally ~420 m/s), with mean Vs30 in the Seattle-area data being ~13% less than the other areas. Interpreted Vs within the basin sediments generally ranges from 250 m/s to 700 m/s in the upper 150 m, with shallow rock Vs ranging from 900 m/s to >1200 m/s.

Presenting Author: William J. Stephenson

Student Presenter: No

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