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Seismogenic deformation field in the Mojave block and implications for tectonics of the eastern California shear zone

Unruh, Jeffrey R. and Twiss, Robert J. and Hauksson, Egill (1996) Seismogenic deformation field in the Mojave block and implications for tectonics of the eastern California shear zone. Journal of Geophysical Research B, 101 (B4). pp. 8335-8361. ISSN 0148-0227. doi:10.1029/95JB03040.

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From the aftershocks of the 1992 Landers earthquake, we infer the orientation of the principal strain rate axes (d_1 > d_2 > d_3; d_1 lengthening), their relative magnitude, and the relative spin of fault blocks by using a micropolar continuum model to invert the seismic P and T axes. The seismogenic deformation is consistent with the geodetic measurements of the coseismic displacement and with the secular deformation of the central Mojave block. Regionally, the aftershock data define two major domains within the central Mojave block: (1) the western Mojave block, including the San Bernardino Mountains and the epicentral area of the Big Bear earthquake, which is characterized by E-W d_1 (lengthening) and N-S d_3 (shortening); and (2) the central Mojave block, including the Landers surface rupture zone, which is characterized by NW-SE d_1 and NE-SW d_3. Inversion for the principal strain axes of geodetically measured coseismic displacements across the Big Bear and Landers seismogenic zones gives results similar to the aftershock inversions for those areas, indicating that the aftershocks accommodate a deformation similar to the main rupture and do not reflect elastic rebound or residual stresses. The background seismicity for 1981 to 1991 shows the same characteristic d_1 and d_3 orientations for the two domains, indicating that the secular seismogenic strain has the same regional geometry as the 1992 coseismic deformation. The micropolar inversion also provides values of the relative vorticity parameter W, which reflects a difference between the vorticity of a shearing continuum and the vorticity of fault-bounded blocks rotating within tabular seismogenic shear zones. The observed fault geometry along the Kickapoo fault suggests a pinned-block model for the local block rotation that is consistent with the values of W obtained from our inversions. We interpret the regional NW-SE orientation of d_1 in the central Mojave block to be characteristic of the dextral eastern California shear zone, which transfers approximately 22% of the Pacific-North American plate motion from the San Andreas system to the Walker Lane Belt in eastern California. Our results and geodetic determinations of the secular shear strain in the central Mojave block indicate that the locus of NW dextral shear generally lies between the San Bernardino Mountains and the Pisgah fault.

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Hauksson, Egill0000-0002-6834-5051
Additional Information:© 1996 American Geophysical Union. Received March 27, 1995; revised September 25, 1995; accepted September 29, 1995. We thank the following people for their assistance in carrying out this study: Rick Allmendinger for the use of his program Stereonet 4.7-IIa with which we plotted our stereonet diagrams; Andrea Donnellan for her inversion program on which we based our program GPSTRN for calculating strains and rotations from geodetic data; and Jeff Freymueller for providing us the subroutines CONVCO and POLY44, which we used to calculate distances between geodetic stations from latitude/longitude data. Support for this research was provided to J.R.U. by a grant from the Department of Interior, U.S. Geological Survey (National Earthquake Hazards Reduction Program, contract 1434-94-G-2463); and to R.J.T. by a grant from the National Science Foundation (project EAR-9219633). The contents of this paper do not neccessarily represent the policy of the U.S. Geological Survey, however, and the endorsement of the federal government should not be assumed.
Group:Seismological Laboratory, Division of Geological and Planetary Sciences
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Issue or Number:B4
Record Number:CaltechAUTHORS:20131127-092629526
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Official Citation:Unruh, J. R., R. J. Twiss, and E. Hauksson (1996), Seismogenic deformation field in the Mojave block and implications for tectonics of the eastern California shear zone, J. Geophys. Res., 101(B4), 8335–8361, doi:10.1029/95JB03040.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42753
Deposited By: Ruth Sustaita
Deposited On:02 Dec 2013 16:21
Last Modified:10 Nov 2021 16:27

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