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Three-dimensional S-wave Velocity Model of Napa, California Obtained from Microtremor Array Measurements and Horizontal to Vertical Spectral Ratio

We estimated a three-dimensional (3D) S-wave velocity (Vs) model of the Napa, California, U.S. using microtremor array measurements (MAM) and horizontal to vertical spectral ratio (H/V) at approximately 100 sites. MAM was collected with eight to twenty 2 Hz geophones, and the maximum receiver spacing ranged from 30 to 1500 m. Ambient noise for MAM and H/V were collected for 20-120 minutes. A spatial auto-correlation (SPAC) method calculated phase velocities from the vertical component of ambient noise. Minimum frequency of dispersion curves ranged from 1 to 10 Hz. H/V was calculated from three-component (3C) seismic ambient noise using a single 3C 2Hz geophone. The peak frequency of H/V ranged from 0.25 Hz to 10Hz. There were clear differences between valley floor and surrounding hills both dispersion curves and H/V. In the H/V spectra, there is a clear peak of H/V at a frequency of 0.3 Hz in the valley floor sites whereas there is no clear H/V peak below 1 Hz in the hill sites. Joint inversion of a dispersion curve and H/V spectrum estimated Vs profiles to 30 m to 1000 m depth. There is a large difference in resultant Vs profiles. Depth to a shallow engineering bedrock with Vs of 760 m/s is 300 m and 30 m at typical valley floor sites and hill sites respectively. It indicates that the velocity model changed considerably along the Soda Creek Fault. The result of inversion and geological model implies that the low frequency peak of 0.3 Hz at the valley floor site is mainly due to a deep bedrock with Vs more than 2500 m/s at approximately 1000 m depth. We compiled all Vs profiles together with the 3D Vs model based on geological information and estimated a preliminary 3D Vs model to a depth of 1000 meters. The VS30 obtained from the MAM ranged between 200 m/s and 970 m/s. Clear H/V peak frequencies of 0.25 to 0.4 Hz were consistent in the valley floor. The depth to the bedrock with Vs of 760 m/s ranged between almost surface to greater than 300 m.

Session: Site Response Characterization in Seismic Hazard Analysis I

Type: Oral

Room: Regency E-G

Date: 4/22/2022

Presentation Time: 09:00 AM Pacific

Presenting Author: Koichi Hayashi

Student Presenter: No

Additional Authors