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WITHDRAWN Shear-wave Velocities in the Bellingham and Everett Basins, Washington State: Insights From Multimethod Characterization With krSPAC and Active-source Linear Arrays

Description: 

WITHDRAWN Characterizing earthquake ground motions through three-dimensional simulations has become an important focus for seismic hazard assessment in urbanized sedimentary basins. However, realistic ground motion prediction requires shear-wave velocity (Vs) at depths that capture the extent of the sedimentary column (i.e., usually >30 m), which is often poorly constrained. We acquired microtremor array data at 19 sites throughout the Everett and Bellingham basins in the Puget Lowland, Washington State, and applied the wavenumber-normalized spatial autocorrelation method (krSPAC) to characterize Vs at depths capable of delineating the Quaternary and upper Tertiary deposits, possibly >1000 m at some sites. In contrast to a traditional SPAC approach, where modeling high wavenumbers within the SPAC spectrum requires array symmetry, in the krSPAC approach we interpolate observed coherency-versus-frequency spectrum to coherency-versus-kr (where k and r are wavenumber and averaged interstation spacing, respectively), prior to Vs modeling in the kr domain. We deployed ten-sensor nested asymmetric triangular arrays with nominal interstation spacings that varied from ~100 m to 1000 m. We complement the krSPAC microtremor data with two colinear active-source, 72-sensor linear array seismic datasets (1.5 m sensor spacing) at the center of each site: one with single-component (1C) 4.5 Hz vertical and one with 1C 4.5 Hz horizontal sensors. We analyze the active-source horizontal-component data for both SH-wave refraction traveltimes and Love wave dispersion, and the vertical-component data for Rayleigh wave dispersion and P-wave refraction traveltimes. For modeling, we first use the active-source dispersion and traveltime datasets in an iterative least-squares joint inversion to constrain Vs in the upper ~50 m; these shallow profiles then constrain the krSPAC microtremor modeling, which resolves Vs to greater depths. Preliminary Vs profiles in both basins are generally consistent with those observed farther south in the Seattle and Tacoma basins, with Vs ranging from ~200 m/s to ~900 m/s in the upper 600 m at the deeper basin sites.

Session: Why Ignore the Structure? Soil-structure Interaction and Site Response at Local and Regional Scales [Poster]

Type: Poster

Date: 4/17/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: William

Student Presenter: No

Invited Presentation: 

Poster Number: 15

Authors