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Rayleigh Phase Velocities in the Upper Mantle of the Pacific-North American Plate Boundary in Southern California

Lennin Escobar1, Dayanthie Weeraratne1, and Monica Kohler2
1California State University, Northridge, Department of Geological Sciences, Northridge, CA 91330
2California Institute of Technology, Department of Mechanical and Civil Engineering, Pasadena, CA 91125

The Pacific-North America plate boundary, located in Southern California, presents an opportunity to study a unique tectonic process that has been shaping the plate tectonic setting of the area since the Miocene. The rapid subduction of the Farallon plate outpaced the spreading rate of the East Pacific Rise rift system causing it to be subducted beneath southern California 30 Ma years ago. The details of microplate capture, reorganization, and lithospheric deformation on the area is not well understood, but may have important implications for fault activity, stresses, and earthquake hazard analysis. We use Rayleigh waves recorded by an array of 34 ocean bottom seismometers deployed offshore southern California for a 12 month duration from August 2010 to 2011. Our array recorded teleseismic earthquakes at distances ranging from 30° to 120° for magnitudes Mw ≥ 5.9. We consider seismic periods between 15 - 78 seconds. The inversion technique considers non-great circle path propagation by representing the arriving wave field as two interfering plane waves. This takes advantage of statistical averaging of a large number of paths that travel offshore southern California allowing for improved resolution and parameterization of lateral seismic velocity variations at lithospheric and sublithospheric depths. We present phase velocity results for periods sampling mantle structure down to 100 km depth along the west coast margin. With this study, we seek to understand the strength and deformation of the Pacific oceanic lithosphere resulting from plate convergence and subduction beneath Southern California 30 Ma as well as translational stresses present today.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013