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

Jeffrey Unruh, Robert J. Twiss, & Egill Hauksson

Published April 10, 1996, SCEC Contribution #489

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 d3 . 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.

Citation
Unruh, J., Twiss, R. J., & Hauksson, E. (1996). Seismogenic deformation field in the Mojave block and implications for tectonics of the eastern California shear zone. Journal of Geophysical Research, 101(B4), 8335-8361. doi: 10.1029/95JB03040.