Inter-Method HVSR Bias and the Resultant Vs30-fd Relationship for Measured-Vs30 Stations in the Western United States
Session: How Well Can We Assess Site Effects So Far? III
Type: Oral
Date: 4/21/2021
Presentation Time: 02:30 PM Pacific
Description:
Two important ground motion modeling parameters are currently used to account for seismic site effects at network strong-motion monitoring stations: 1) the time-averaged shear wave velocity (VS) of the upper 30 m from the surface (VS30), derived through VS profiles determined from in situ multi-station array-based measurements; and 2) the dominant site frequency (fd), derived from the horizontal-to-vertical spectral ratio (HVSR) of ground motions recorded by the single-station method. Although the effectiveness of using one or more (or the combination) of these indices as site terms is an ongoing research topic, fd—particularly, based on microtremor sources—is substantially less costly to determine than VS30. As both indexes are known to fundamentally represent seismic site conditions, we follow previous studies and regress measured VS30 against fd but use newly available data consisting of 60 network stations in the Western U.S. However, there exist many different methods for computing HVSR, which may result in varying fd estimates, whereas VS30 methods are relatively better established. We examine possible fd -related biases by using a common set of microtremor recordings from each network station and by computing their Fourier amplitude spectrum (FAS), power spectral density (PSD), and response spectrum (RS) based HVSRs. The RS approach is shown to be inappropriate for use on microtremor data because the damping in the single-degree-of-freedom (SDOF) system does not allow for a significant accumulation of energy during the long-duration, low-amplitude microtremor recordings. HVSR computed from FAS and PSD produce similar shapes; however, from 0.2–50 Hz the amplitudes generally differ by a factor of up to two, and fd varies between the two methods on average by ± 0.5 Hz. We discuss the pros and cons of the FAS, PSD, and RS approaches to computing fd, then present a relationship between VS30 and fd, and compare the relationship to those proposed in recent studies.
Presenting Author: Kenneth S. Hudson
Student Presenter: Yes
Authors
Kenneth Hudson Presenting Author Corresponding Author khud27@g.ucla.edu University of California, Los Angeles |
Samantha Palmer spalme22@uwo.ca Western University |
Sean Ahdi sahdi@usgs.gov U.S. Geological Survey |
Behzad Hassani behzad.hassani@bchydro.com BC Hydro |
Gabriel Toro toro@lettisci.com Lettis Consultants International, Inc. |
Alan Yong yong@usgs.gov U.S. Geological Survey |
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Inter-Method HVSR Bias and the Resultant Vs30-fd Relationship for Measured-Vs30 Stations in the Western United States
Category
How Well Can We Assess Site Effects So Far?