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1 Department of Geology
University
of California, Davis
1 Shields Avenue
Davis, California
95616
manaker{at}geology.ucdavis.edu
(D.M.M.)
2 Western Earthquake Hazards
Team
U.S. Geological Survey
345 Middlefield Road, M/S 977
Menlo Park,
California
94025-3591
michael{at}usgs.gov
(A.J.M.)
3 Department of Earth and Planetary
Science
University of California, Berkeley
385 McCone Hall
Berkeley,
California
94720
burgmann{at}seismo.berkeley.edu
(R.B.)
The Calaveras and Hayward faults are major components of the San Andreas fault system in the San Francisco Bay region. Dextral slip is presumed to transfer from the Calaveras fault to the Hayward fault in the Mission Hills region, an area of uplift in the contractional stepover between the two faults. Here the estimated deep slip rates drop from 15 to 6 mm/yr on the Calaveras fault, and slip begins on the Hayward fault at an estimated 9 mm/yr. A lineament of microseismicity near the Mission fault links the seismicity on the Calaveras and Hayward faults and is presumed to be related directly to this slip transfer. However, geologic and seismologic evidence suggest that the Mission fault may not be the source of the seismicity and that the Mission fault is not playing a major role in the slip transfer.
We perform a joint inversion for hypocenters and the 3D P-wave velocity structure of the stepover region using 477 earthquakes. We find strong velocity contrasts across the Calaveras and Hayward faults, corroborated by geologic, gravity, and aeromagnetic data. Detailed examination of two seismic lineaments in conjunction with the velocity model and independent geologic and geophysical evidence suggests that they represent the southern extension of a northeasterly dipping Hayward fault that splays off the Calaveras fault, directly accounting for the deep slip transfer. The Mission fault appears to be accommodating deformation within the block between the Hayward and Calaveras faults. Thus, the Calaveras and Hayward faults need to be considered as a single system for developing rupture scenarios for seismic hazard assessments.
Online material: 3D interactive visualizations of the Mission and Alum Rock hypocenters.
This article has been cited by other articles:
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  J. L. Hardebeck, A. J. Michael, and T. M. Brocher Seismic Velocity Structure and Seismotectonics of the Eastern San Francisco Bay Region, California Bulletin of the Seismological Society of America, June 1, 2007; 97(3): 826 - 842. [Abstract] [Full Text] [PDF]
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 P. Mann Global catalogue, classification and tectonic origins of restraining- and releasing bends on active and ancient strike-slip fault systems Geological Society, London, Special Publications, January 1, 2007; 290(1): 13 - 142. [Abstract] [Full Text] [PDF]
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R. W. Graymer, V. E. Langenheim, R. W. Simpson, R. C. Jachens, and D. A. Ponce Relatively simple through-going fault planes at large-earthquake depth may be concealed by the surface complexity of strike-slip faults Geological Society, London, Special Publications, January 1, 2007; 290(1): 189 - 201. [Abstract] [Full Text] [PDF]
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S. Hartzell, S. Harmsen, R. A. Williams, D. Carver, A. Frankel, G. Choy, P.-C. Liu, R. C. Jachens, T. M. Brocher, and C. M. Wentworth Modeling and Validation of a 3D Velocity Structure for the Santa Clara Valley, California, for Seismic-Wave Simulations Bulletin of the Seismological Society of America, October 1, 2006; 96(5): 1851 - 1881. [Abstract] [Full Text] [PDF]
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 R. Burgmann, G. Hilley, A. Ferretti, and F. Novali Resolving vertical tectonics in the San Francisco Bay Area from permanent scatterer InSAR and GPS analysis Geology, March 1, 2006; 34(3): 221 - 224. [Abstract] [Full Text] [PDF]
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