Group , Poster #211, Seismology

NEAR-SURFACE SHEAR WAVE VELOCITY AND ATTENUATION AT GARNER VALLEY, CALIFORNIA

Lei Qin, Jamison H. Steidl, Kenneth S. Hudson, Timothy Lamere, Hongrui Qiu, & Yehuda Ben-Zion
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Poster Presentation

2021 SCEC Annual Meeting, Poster #211, SCEC Contribution #11432 VIEW PDF
We analyze data from seismic and pore pressure stations at 0-50 m depths, and a cross-hole array experiment, from the Garner Valley Downhole Array, to estimate shear-wave velocities and attenuation in the near-surface structure. Impulse response functions (IRFs) between the 6 m and 22 m borehole stations resolve clear free surface reflected S-waves recorded by the 6m-deep borehole station. Analysis of the IRF based on 15-year earthquake data indicates S-wave velocity of ~200 m/s with ~10% seasonal variations averaged in the top ~6 m. The seasonal variations are correlated with the precipitation (i.e. water table) and temperature with a phase delay of ~64 days. The cross-hole array experiment... includes two geophones with a sampling rate of 2000 Hz and a solenoid-activated dual-direction borehole hammer source, all located at 5 meters depth. Data from weekly hammer tests are analyzed between the two 5-m geophones to estimate near-surface shear wave velocity and attenuation. Compared with the IRF results that represent average values in the top 6 m, the cross-hole array experiment shows similar S-wave velocity (~200 m/s) at 5 m depth, but with less (~5%) velocity changes over the course of the seasonal water table variations. This suggests a decrease of S-wave velocity variations with depth, perhaps related to variations induced by thermoelastic strains in the top ~1 m. Updated results will be presented in the meeting.
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