Constraining Shear-Wave Velocity Profiles in Anchorage, Alaska, Through Inversion of Microtremor Horizontal-to-Vertical Spectral Ratios
Description:
We modify shear-wave velocity (VS) profiles, obtained from modeling of active-source multimethod site characterization data at 19 strong motion stations in the Anchorage, Alaska, region using microtremor horizontal-to-vertical-spectral-ratio (mHVSR) data. The acquisition sites were located throughout the City of Anchorage and northward through the Eagle River and Palmer-Wasilla areas. We deployed one 20-s-to-100-Hz three-component broadband seismometer for mHVSR data acquisition as part of our multimethod site characterization approach. We first calculate the mHVSR curves through standard processing techniques (time-series detrending, mean amplitude removal, bandpass filtering from 0.5 to 50 Hz, calculation of time-window averaged Fourier amplitude spectra, spectral smoothing). Then we model the mHVSR fundamental frequency (F0) through inversion by an elitist genetic algorithm (EGA) technique. The initial VS models at each site were developed from iterative least-squares joint inversion of active-source P- and S-wave refraction data complemented with Rayleigh and Love dispersion data, as previously described by Stephenson et al. (2022). However, these models were often limited to less than 50 m depth because of acquisition logistics, particularly at sites in urban areas, and these models commonly were not sufficiently deep to predict the observed mHVSR F0. By adding a layer beneath the initial model depths and allowing up to 10% variability in the shallow layers defined by the active-source model, we extend the VS profiles to greater depth through the EGA inversion of the mHVSR data. In addition, the mHVSR modeling generally corroborate the depths of shallow high VS impedance boundaries in the initial active-source models. The new VS profiles incorporating mHVSR modeling will be used in support of ground motion modeling and related earthquake hazards studies in the region.
Session: How Well Can We Predict Broadband Site-Specific Ground Motion and Its Spatial Variability So Far? [Poster Session]
Type: Poster
Date: 5/1/2024
Presentation Time: 08:00 AM (local time)
Presenting Author: William
Student Presenter: No
Invited Presentation:
Authors
William Stephenson Presenting Author Corresponding Author wstephens@usgs.gov U.S. Geological Survey |
Utpal Dutta udutta2@alaska.edu University of Alaska Anchorage |
Noah Lindberg nlindberg@usgs.gov U.S. Geological Survey |
Alena Leeds aleeds@usgs.gov U.S. Geological Survey |
Annika Goozen annikagoozen@yahoo.com University of Alaska Anchorage |
John Thornley john.thornley@wsp.com WSP Global |
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Constraining Shear-Wave Velocity Profiles in Anchorage, Alaska, Through Inversion of Microtremor Horizontal-to-Vertical Spectral Ratios
Session
How Well Can We Predict Broadband Site-Specific Ground Motion and Its Spatial Variability So Far?