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Willamette Valley Site Characterization With HVSR

Description: 

Oregon’s population density is mostly concentrated within the Willamette Valley, a foreland basin with sedimentary cover. Sedimentary fill in basins or valleys will amplify ground motion at frequencies related to regional structure, a key component of accurately assessing hazard and understanding the impacts on the built environment. One methodology to constrain spectral site response and amplification is from horizontal-to-vertical spectral ratios (HVSR). This relies on sufficient seismic activity and instrumentation, which have both been low historically in the Willamette Valley. Recently, instrumentation has increased due to the buildout of the Pacific Northwest Seismic Network, providing additional constraints to use in the region.

We present our analysis of earthquake HVSR (eHVSR) data on permanent broadband (BB) and strong motion (SM) instrument recordings within the Willamette Valley, following the SESAME criteria for identifying fundamental site frequencies. We found that, of the 17 fundamental peak frequencies (f0) identified throughout the Willamette Valley, 82% were < 3 Hz, 76% were < 2 Hz, and 47% were < 1 Hz, and SM instruments were less reliable than BB in detecting f0 < 1 Hz due to their corner frequency (fc) limits. Some sites had just a few reliable and clear peaks, however others were affected by basin effects, which produced a significant number of qualifying peaks across a large range of frequencies due to trapped waves within the sediments. Our ability to fully characterize site response is still affected by limited permanent instrumentation in the center of the Willamette Valley. Future HVSR studies of this region should consider a higher density placement of BB seismic instruments, microtremor inclusion, and azimuthal processing to observe true fundamental frequencies on a microzonation-scale in order to gain a better understanding of expected ground motion across Oregon’s most densely populated areas.

Session: Why Ignore the Structure? Soil-structure Interaction and Site Response at Local and Regional Scales [Poster]

Type: Poster

Date: 4/17/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Charity

Student Presenter: Yes

Invited Presentation: 

Poster Number: 13

Authors