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Published October 1981 | Published
Journal Article Open

P_n velocity anisotropy in Southern California

Abstract

We analyze P_n propagation as a function of azimuth across a 28-station, 150-km aperture subarray of the SCARLET network centered near the central Transverse Ranges, California. We selected signals from 81 earthquakes and explosions with epicentral distances ranging from 150 to 400 km, covering all azimuths except a 40° gap from the southwest and a lesser gap from the northeast direction. For each source the apparent velocity of P_n was determined using a one-norm measure of misfit. The apparent P_n velocity does not show any systematic variation with epicentral distance but exhibits a strong azimuthal dependence. Our preferred interpretation calls for a slightly dipping (2° to N40W) planar moho, with 3 to 4 per cent anisotropy of subcrustal material. Transverse isotropy with a nearly horizontal symmetry axis is sufficient to explain the data; the direction of sagittal symmetry is N50W. The isotropic velocity of P_n is 7.8 km/sec. In contrast, a higher (8.1 km/sec) P_n velocity is found in the Mojave block, with no indication of anisotropy. These observations are consistent with a subcrustal model of the Pacific-North America plate boundary where ductile flow is characterized by simple shear in a vertical plane with strike parallel to the direction of relative plate motion.

Additional Information

© 1981, by the Seismological Society of America. Manuscript received February 27, 1981. We are grateful to D. L. Anderson and H. Kanamori for useful discussions. C. Johnson and P. German were very helpful in retrieving CEDAR data and provided the computer program to compute apparent velocities using the L 1 norm. W. L. Rodi provided helpful comments which led to improvements in the manuscript. Further comments from S. Crampin, B. Raleigh, and D. Hadley are gratefully acknowledged. This research was supported by the U.S. Geological Survey under Contract 14-08-0001-18321.

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August 19, 2023
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