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Preliminary Site Response Results across the San Gabriel and San Bernardino Basins Utilizing the Ambient Noise Spectral Ratio Method

Anisha D. Tyagi, Margaret Grenier, Rachel Kreuziger, Jacob S. Kays, & Jascha Polet

Published August 15, 2018, SCEC Contribution #8792, 2018 SCEC Annual Meeting Poster #111

Sedimentary basins, such as the San Gabriel and San Bernardino Basins, are known to amplify earthquake ground motions; therefore, it is important to characterize these Basins and determine site response parameters across these structures to mitigate seismic hazard. One of the effects causing ground motion amplification in basins is resonance. Basins have a specific resonance period that depends on their geometry and material properties. If this resonance period is the same as that of a building, the resulting double resonance effect will further increase the potential for damage. The San Gabriel and San Bernardino Basins are densely populated regions that are surrounded by a network of faults, which have the capacity to produce significant earthquakes. Additionally, the energy from a potential major rupture on the southern San Andreas Fault could be funneled through the San Gabriel and San Bernardino Basins, acting as a waveguide, into the Los Angeles Basin (Denolle et al., 2014).

Linear seismic arrays consisting of hundreds of nodes were deployed across the San Gabriel and San Bernardino Basins in 2017 and nearly one month of three-component waveform data was collected. The ambient noise waveform data was processed using the Geopsy software to determine Horizontal-to-Vertical Spectral Ratio (HVSR) curves, which inform us about the resonance frequency and ground motion amplification at each site. Site Effects Assessment Using Ambient Excitations (SESAME) guidelines are applied to the HVSR curves to determine reliability and clarity for all processed data. The analysis of the spectral ratio curves shows a significant variation in both resonance periods and amplification factors across both Basins. Results for the Western San Gabriel Basin line indicate peak frequencies above 1 Hz for nodes situated to the north of the Raymond Fault and peak frequencies that range from 0.2-0.3 Hz for nodes south of the Raymond Fault. Higher peak frequencies along the Central Basin line suggest a shallower basin. Nodes in the southern portion of the San Bernardino Basin, specifically in the southern half of the Chino Basin, indicate peak frequencies around 0.4 Hz. The peak HVSR amplitudes range from 2.0-4.5, indicating potential for significant ground motion amplification. Future work includes comparison of our resonance periods with those predicted by structural models of the Basins to help improve our understanding of their deep subsurface structure.

Tyagi, A. D., Grenier, M., Kreuziger, R., Kays, J. S., & Polet, J. (2018, 08). Preliminary Site Response Results across the San Gabriel and San Bernardino Basins Utilizing the Ambient Noise Spectral Ratio Method. Poster Presentation at 2018 SCEC Annual Meeting.

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