THREE-DIMENSIONAL S-WAVE VELOCITY MODEL OF NAPA VALLEY 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. The investigation area is approximately 12 km by 16 km at Napa, CA including the valley floor and surrounding hills, Mayacamas Mountains on the west and Vaca Mountains on the east, bounded by the West Napa Fault and the Soda Creek Fault. The 2014 South Napa earthquake occurred along the West Napa Fault. Another active fault, Soda Creek Fault is traced along the Napa river and east boundary of the valley. Data acquisition included MAM and H/V at approximately 100 sites. MAM was collected with seven 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. Joint inversion of a dispersion curve and H/V spectrum estimated Vs profiles to 30 m to 1000 m depth. We compiled all H/V and Vs profiles, and estimated a 3D Vs model to a depth of 1000 m. The average Vs to 30 m depth (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 (< 10 m) to greater than 300 m. The depth is greater than 300 m in the valley floor, whereas it is less than 100 m at the west (Mayacamas Mountains) and east (Vaca Mountains) hills. It is particularly shallow (< 50 m) at the eastern side of the Napa river where northeast of downtown Napa. Along the west-east Vs cross section across the downtown Napa, the shallow bedrock depth (Vs > 760 m/s) changed from 300 m to almost surface across the Napa river in a horizontal distance of about 1 km. The bedrock depth is relatively deep (about 100 m) at the west hills (Mayacamas Mountains) compared with the east hills (Vaca Mountains).