Preliminary report on site characterization using noninvasive single- and multi-station methods at southern California seismic stations

Alan Yong, Antony Martin, Jennifer Pfau, Devin McPhillips, Marcos Alvarez, Scott S. Lydeen, Fiona Clerc, & Nolan Leue

Published August 15, 2016, SCEC Contribution #6801, 2016 SCEC Annual Meeting Poster #226

In-situ measurements of shear-wave velocity (Vs) are used commonly to evaluate seismic response at earthquake monitoring station and project sites. Vs30, the time-averaged Vs in the upper 30 m, is a common parameter used to capture seismic site response and is used in almost all modern ground motion prediction equations. Traditional invasive downhole methods directly measure Vs; however, these methods are often cost- and/or environmentally-prohibitive and their results do not always reflect the lateral variability of seismic conditions beyond the immediate vicinity of the test site. In comparison, noninvasive methods record active- or passive-source data consisting of surface or body waves and are less prohibitive to use. Moreover, these methods use multiple horizontally-spaced surface receivers (multi-station array), thus, lateral variability beneath the array is accounted for in their results. Most noninvasive methods, however, indirectly measure Vs, and thus have inherent uncertainties. We have used a suite of noninvasive methods at six stations in southern California. We record microseisms using standalone single-stations, located at the end- and mid-points of the measurement array, and over the same period, we also collect records from the seismic station. Using both single- and seismic-station records, we calculate the horizontal-to-vertical-spectra-ratios (HVSR), resonance frequency, and power spectral density to study site characteristics, including noise levels. For soil sites, we generally find insignificant lateral variability in subsurface conditions beneath our multi-station arrays by matching similar spectral peaks and frequencies in the three HVSR records; for rock sites, the magnitudes of the HVSR values are not as discernible. While we find general agreement in Vs30 computed using a variety of methods at each site, preliminary results for low-noise sites using standalone passive methods have large uncertainty in their computed Vs30 values.

Key Words
seismic site characterization, noninvasive, surface waves, body waves, Vs, Vs30

Yong, A., Martin, A., Pfau, J., McPhillips, D., Alvarez, M., Lydeen, S. S., Clerc, F., & Leue, N. (2016, 08). Preliminary report on site characterization using noninvasive single- and multi-station methods at southern California seismic stations. Poster Presentation at 2016 SCEC Annual Meeting.

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