Site Response Characterization of the San Gabriel, Chino, and San Bernardino Basins from Ambient Noise Horizontal-to-Vertical Spectral Ratio Measurements
Abstract
The San Gabriel, Chino and San Bernardino Basins are densely populated sedimentary basins located between the Los Angeles Basin to the west and the San Andreas Fault to the east. Sedimentary basins are known to amplify earthquake ground motions and increase their duration. The subsurface structure of these Basins is poorly constrained, but is critical for understanding their expected ground motion, and how seismic energy will be transferred from potential earthquakes on the San Andreas Fault to the greater Los Angeles area.
In the past three years, a series of dense 2D arrays of nodal seismometers were deployed across these Basins, collecting three component waveforms over a period of one month. Short-term deployments of broadband seismometers are also in progress to verify results obtained for a number of these nodes. We are processing ambient noise data from the nodal transects, and permanent and temporary broadband seismometers, using the Geopsy software, with the Site Effects Assessment Using Ambient Excitations protocol for assessment of the quality of the results. The resulting Horizontal-to-Vertical Spectral Ratio (HVSR) curves provide information about the site resonance frequency and ground motion amplification. Our HVSR results show strong variation in site response, even at scales of less than 1 km, emphasizing the importance of micro-zonation. Long period HVSR peak amplitudes range from 2 to 5.5, indicating significant amplification due to basin resonance. In the Western San Gabriel Basin (SGB), the Raymond fault appears to separate, relatively high peak frequencies (>1.0 Hz) to the North, from peak frequencies between 0.15-0.30 Hz to the South, suggesting a basin depth of 1-1.5 km. In the Northern section of the Central SGB, the HVSR curves show two sets of marginal peaks, with peaks at ~0.45 Hz suggesting the existence of a relatively shallow basin. In the Chino Basin, lower peak frequencies in the Southern section suggest it is deepest there, although shallower than the Western SGB. For all three basins, secondary intermittent shorter period peaks may be explained using edge effects, the presence of small-scale basins, and/or topographic effects. Calculations for the San Bernardino Basin are in progress, as are comparisons to the Southern California Earthquake Center Community Velocity Models.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.S33E0639G
- Keywords:
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- 7212 Earthquake ground motions and engineering seismology;
- SEISMOLOGY;
- 7299 General or miscellaneous;
- SEISMOLOGY