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Subsurface Geologic Controls on Seismic Site Response Across the Continental United States

Description: 

Earthquake ground motions are modified by local geologic site conditions, and the evaluation of site response is a crucial component of seismic hazard analyses. Horizontal-to-Vertical Spectral Ratio (HVSR) measurements provide inexpensive and noninvasive estimates of site resonant frequencies and amplifications. Here, we complete microtremor HVSR measurements at over 2,000 Transportable Array sites across the continental United States (US) and characterize the fundamental site resonance using an automated peak-identification procedure. In the central and eastern US (CEUS), we find smoothly varying fundamental site resonances that correspond to previously mapped subsurface geologic structures. Low-frequency fundamental site resonances (0.1–1 Hz) in this region are controlled by deep (>100 m) impedance contrasts, principally by the Great Unconformity separating overlying Phanerozoic sedimentary strata from Precambrian basement across the North American midcontinent as well as thick sedimentary deposits beneath the Atlantic and Gulf Coastal Plains. Higher-frequency site resonances (1–15 Hz) that vary smoothly across the CEUS are largely governed by shallower (<100 m) glacial sediments in the north-central and northeastern US. Given the presence of both shallow glacial sediments and the deeper Great Unconformity in this region, we investigate the prevailing controls on site resonance and amplification from layered subsurface structures. In the western US (WUS) and Alaska, where geologic features are shaped by complex tectonic histories, we find predominantly flat spectral responses and high variance in the fundamental site resonances among neighboring sites. Across the continental US, our results demonstrate the role of regional geology on seismic site response and hazard. We highlight the continued importance of detailed subsurface geologic models, dense seismic observations, and standardized site characterization techniques to improve existing seismic hazard estimates.

Session: New Directions in Environmental, Seismic Hazard and Mineral Resource Exploration Studies - I

Type: Oral

Date: 4/17/2025

Presentation Time: 02:45 PM (local time)

Presenting Author: Haiyang

Student Presenter: No

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