Earthquake site response across tectonically complex regions of the continental United States

Haiyang Kehoe, Oliver S. Boyd, James W. Atterholt, Morgan P. Moschetti, Ebru Bozdag, & Emilia A. Caylor

Submitted September 7, 2025, SCEC Contribution #14917, 2025 SCEC Annual Meeting Poster #TBD

Across the continental United States (US), diverse tectonic environments necessitate the characterization of laterally varying site effects to improve seismic hazard estimates and guide resilient infrastructure design. Horizontal-to-Vertical Spectral Ratio (HVSR) measurements provide inexpensive, noninvasive estimates of site-specific resonant frequencies that constrain the depths of layered geologic units. Additionally, the polarization of maximum horizontal amplification reveals lateral variations in subsurface geology. Here, we complete microtremor-based HVSR measurements at 2,194 Transportable Array sites across the continental US and characterize resonant site effects using an automated peak-identification procedure. In the central and eastern US, we find spatially coherent site frequencies that correspond with the thicknesses of mapped sedimentary deposits. In the western US and Alaska, we observe considerable variation in site frequencies across short distances. In tectonically active, high-hazard regions, greater spatial variability in site conditions underscores the need to resolve fine-scale, location-dependent site effects. We use published sediment thickness maps and resonant site frequencies to invert for region-specific shear-wave velocity profiles to understand the prevailing subsurface controls on earthquake site response. We develop a conceptual model to explain the combined roles of subsurface geology and overlying topography on observed site response. Our results provide a uniform, continental-scale, US-wide HVSR dataset that can be used to refine community velocity models and seismic hazard models. We also identify regions where targeted local and regional HVSR datasets may constrain basin-edge effects, refine ground motion simulations, and reduce uncertainty in site response modeling. These advances require the integration of geological and geophysical datasets to capture crustal complexity and produce more accurate, spatially dependent seismic hazard assessments.

Key Words
horizontal vertical spectral ratio HVSR site effects

Citation
Kehoe, H., Boyd, O. S., Atterholt, J. W., Moschetti, M. P., Bozdag, E., & Caylor, E. A. (2025, 09). Earthquake site response across tectonically complex regions of the continental United States. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology