Differential Waveform Analysis to Test for a Mid-Crustal Low-Velocity Zone Beneath the Western Mojave

William K. Eymold, Thomas H. Jordan, & David A. Okaya

Submitted August 15, 2016, SCEC Contribution #6847, 2016 SCEC Annual Meeting Poster #232

Lee et al. (JGR, 2014a) have applied full-3D tomography to a large set of three-component earthquake waveforms and vertical-component ambient-field correlagrams to obtain a revised community velocity model for Southern California, CVM-S4.26. An interesting feature of this model is a distinct mid-crustal low-velocity zone (LVZ) beneath the western Mojave region with a minimum P velocity of 6.0 km/s and a minimum S velocity of 3.4 km/s at 14-km depth, which is overlain by a high-velocity zone (HVZ) with maxima of 6.7 km/s and 3.9 km/s, respectively, at 6 km. Although CVM-S4.26 provides a good fit to three-component seismograms for paths crossing this region (Lee et al., SRL, 2014b), the model is constrained to be isotropic. Geologic reconstructions of Mojave-block development suggest that the region could be underlain by highly anisotropic schists emplaced during the transition from subduction to transform faulting. One issue, therefore, is whether the HVZ-LVZ reversal is an artifact resulting from the inversion of three-component waveform data under an isotropic assumption. To test this hypothesis, we have analyzed the frequency dispersion of single-component waveforms for Mojave-crossing paths with sources at LVZ depths. Using a 1D approximation, we show that a diagnostic feature of the HVZ-LVZ structure is a reduction of the differential phase delays between the S wave and surface wave at frequencies tuned to the near-surface and mid-crustal waveguides. We describe our preliminary attempts to verify the existence of the mid-crustal LVZ in this region using the available earthquake data.

Eymold, W. K., Jordan, T. H., & Okaya, D. A. (2016, 08). Differential Waveform Analysis to Test for a Mid-Crustal Low-Velocity Zone Beneath the Western Mojave. Poster Presentation at 2016 SCEC Annual Meeting.

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