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Shallow crustal imaging in Southern California using ambient noise and fault zone trapped waves

Fan-Chi Lin, Elizabeth Berg, Amir A. Allam, Hongrui Qiu, Yadong Wang, & Yehuda Ben-Zion

Published August 15, 2017, SCEC Contribution #7764, 2017 SCEC Annual Meeting Poster #041

Better determination of shallow crustal structure is critical for earthquake hazard assessment. Here we present two efforts using detailed wavefield variation to investigate high-resolution crustal structure. In the first application, we extract Rayleigh waves from multi-component ambient noise cross-correlations using data recorded by 315 Southern California seismic stations in 2015. We measure phase dispersion and Rayleigh wave ellipticity (or H/V ratio) from ambient noise cross-correlation between 6 and 18 second periods, preserving relative amplitude of the three components of motion by performing spectral and temporal normalization on all three simultaneously. The complimentary sensitivity of phase velocity and H/V ratio enables simple and accurate resolution of geological features from the surface to 20km depth. We compare the observed H/V ratios and phase velocities to predictions generated from the current regional models (SCEC UCVM), finding strong correspondence where the near-surface structure is well-resolved by the models. This includes high H/V ratios in the LA Basin, Santa Barbara Basin and Salton Trough; and low ratios in the San Gabriel, San Jacinto and southern Sierra Nevada mountains. Disagreements in regions such as the Western Transverse Ranges, Salton Trough, San Jacinto and Elsinore fault zones motivate further work to improve the community models. A new updated 3D isotropic model of the area is derived via a joint inversion of Rayleigh phase dispersions and H/V ratios.

In the second application, we use near-fault 1D arrays of hundreds of three-component geophones deployed with ~10m spacing to observe the full spatial eigenfunction of fault zone trapped wave normal modes from local earthquakes. We show that the spatial wavefield variation of the trapped waves are consistent from event to event, and are controlled by damage zone structure rather than source properties. By applying a series of spatial filters, we can determine the characteristic length scale of the trapped waves as a function of fault-normal distance. Using mode theory, we translate this length scale variation into measurement of lateral velocity variation. Combined with ambient noise cross-correlation analysis across the dense arrays, we will present new 2D velocity models of the San Jacinto fault that can be used to evaluate many important fault zone properties including damage zone width, velocity reduction, asymmetry, and across-fault velocity contrast.

Key Words
ambient noise; trapper waves; fault zone; dense array

Lin, F., Berg, E., Allam, A. A., Qiu, H., Wang, Y., & Ben-Zion, Y. (2017, 08). Shallow crustal imaging in Southern California using ambient noise and fault zone trapped waves. Poster Presentation at 2017 SCEC Annual Meeting.

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