Using Microtremor-Based Horizontal-to-Vertical Spectral Ratios to Improve Linear Site Response Predictions in the Sacramento-San Joaquin Delta Region of California
Description:
Sites located in the Sacramento-San Joaquin Delta region of California typically have peaty organic soils near the ground surface which are characteristically soft, with shear wave velocities as low as 30 m/s. These unusually soft geotechnical conditions will produce site effects that are not represented in datasets used to derive existing global ergodic site amplification models. We therefore perform nonergodic site response analyses using weak ground motion data recorded at 36 seismic stations in the Delta region. First-order site effects are modeled using a period-dependent multilinear VS30-scaling model, which provides unbiased predictions for Delta sites as a whole by smoothing over site-specific effects. Microtremor-based horizontal-to-vertical spectral ratios (mHVSR) are developed for 34 sites, from which additional site parameters such as peak frequency (fp) and average H/V amplitude over some frequency bandwidth (μmHVSR) are derived. Sites with prominent mHVSR peaks are interpreted as exhibiting site resonance effects, while sites without prominent peak features do not. A hybrid Ricker wavelet – Gaussian pulse function conditioned on fp is used to model peak resonance effects, while a short-period constant computed from μmHVSR which smoothly transitions to zero at long periods is used to model general levels of amplification. These mHVSR-informed models are implemented as additive components to the VS30-scaling model. The regionally-calibrated VS30-scaling model and mHVSR-informed variant significantly reduce bias when compared to predictions provided by an ergodic model. The VS30-scaling model does not appreciably change the aleatory variability (ϕS2S) for periods shorter than about 1.5 s, however significant reductions around the order of 0.1 (natural log units) are observed at long periods. When the mHVSR-informed model components are used, ϕS2S is reduced by about 0.05 to 0.1 (natural log units) for short-to-intermediate periods. Ongoing work is investigating nonlinear effects for the purpose of developing a comprehensive regional site response model for forward application.
Session: Site-specific Modeling of Seismic Ground Response: Are We Quantitative Enough to Predict?
Type: Oral
Date: 4/19/2023
Presentation Time: 11:15 AM (local time)
Presenting Author: Tristan E. Buckreis
Student Presenter: No
Invited Presentation:
Authors
Tristan Buckreis Presenting Author Corresponding Author tristanbuckreis@ucla.edu University of California, Los Angeles |
Pengfei Wang p1wang@odu.edu Old Dominion University |
Scott Brandenberg sjbrandenberg@g.ucla.edu University of California, Los Angeles |
Jonathan Stewart jstewart@seas.ucla.edu University of California, Los Angeles |
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Using Microtremor-Based Horizontal-to-Vertical Spectral Ratios to Improve Linear Site Response Predictions in the Sacramento-San Joaquin Delta Region of California
Category
Site-specific Modeling of Seismic Ground Response: Are We Quantitative Enough to Predict?