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Geophysical characterization of twelve CSMIP station sites in Riverside County, California

Antony Martin, Lauren Demine, William Dalrymple, Nolan Leue, & David Carpenter

Published August 15, 2016, SCEC Contribution #6997, 2016 SCEC Annual Meeting Poster #252

The California Geological Survey recently funded geophysical characterization of 12 California Strong Motion Instrument Program (CSMIP) stations in western Riverside County, California. The purpose of the investigation was to develop a shear wave velocity (Vs) profile to a depth of 40 m and an estimate the time-averaged shear-wave velocity of the upper 30 meters (Vs30) at each site. The CSMIP stations are located in rural, suburban, and urban environments. Geologic conditions ranged from shallow to relatively deep sedimentary basins.

Surface wave techniques, including the active-source multi-channel analysis of surface waves (MASW), and passive-source array microtremor and horizontal-to-vertical spectral ratio (HVSR) techniques, were used for site characterization. Because many of the sites had significant noise from traffic on adjacent roads, the MASW technique was only expected to be able to image Vs to depths of 20 to 30 m. Array microtremor measurements, primarily made using nested triangle or L-shaped arrays, ensured that depth of investigation exceeded 40 m. The vertical component of Rayleigh waves were recorded during all active and passive surface wave acquisition. However, complex Rayleigh wave propagation at two sites necessitated the acquisition of the horizontal (radial) component of the Rayleigh wave and Love wave MASW data. This demonstrates the importance of a flexible field approach, where multiple surface wave methods are available to be deployed as necessary.

Depth of investigation of the combined active and passive surface wave soundings was in the 40 to 100 m range. The fundamental mode assumption was generally valid for modeling Rayleigh and Love dispersion data acquired during this investigation. However, at 3 sites it was necessary to model Rayleigh wave data using an effective mode assumption to simulate a jump from the fundamental to first higher mode at low frequencies. Crystalline rock, which ranged in depth from about 10 to 65 m, was encountered within the depth of investigation at 7 of the 12 sites. The predicted HVSR peak from the Vs models, based on the diffuse field assumption, was relatively close to the observed HVSR peak at most sites where the depth of investigation was sufficient to estimate bedrock depth. Vs30 at the 12 sites ranged from about 270 to 550 m/s; half of the sites classified as NEHRP Site Class D and the other half as Site Class C.

Key Words
CSMIP station, surface wave, Vs30

Martin, A., Demine, L., Dalrymple, W., Leue, N., & Carpenter, D. (2016, 08). Geophysical characterization of twelve CSMIP station sites in Riverside County, California. Poster Presentation at 2016 SCEC Annual Meeting.

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