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Velocity contrast across the San Andreas fault in central California: Small-scale variations from P-wave nodal plane distortion

McNally, Karen C. and McEvilly, Thomas V. (1977) Velocity contrast across the San Andreas fault in central California: Small-scale variations from P-wave nodal plane distortion. Bulletin of the Seismological Society of America, 67 (6). pp. 1565-1576. ISSN 0037-1106.

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Systematic variations in P-wave radiation patterns, evident in a data set of 400 central California earthquakes, have been analyzed for variations in velocity contrast across the San Andreas fault zone. Vertical strike-slip faulting characterizes the region, with radiation patterns well constrained by the dense local seismographic station network. A discontinuity in crustal velocity occurs across the San Andreas fault. The distribution of systematically inconsistent first motions indicates that first arrivals observed along the fault plane within the northeastern block have followed refracted paths through the higher velocity crustal rocks to the southwest, retaining P-wave polarities characteristic of the quadrant of origin, and thus appearing reversed. A simple geometrical interpretation, with P waves refracted at the fault plane near the focus, yields the velocity contrast across the fault zone; the distribution of hypocenters allows its mapping in time and space. The velocity contrast so determined ranges up to 15 per cent, for a depth range of 1 to 10 km. The observed pattern of contrast values is coherent, with the greatest contrast related apparently in space, and possibly in time, to the larger earthquakes occurring on the fault. We suggest the phenomenon reflects changes in stress state at the fault and, by virtue of its ease of measurement, offers a new and valuable technique in earthquake studies.

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Additional Information:© 1977 Seismological Society of America. Manuscript received June 28, 1977. This research was supported by the Advanced Research Projects Agency and was monitored by the Air Force Office of Scientific Research under Contract F44620-75-C-0049, as part of a cooperative study of the near-field characteristics of small earthquakes. Data from the USGS central California network stations were kindly made available by Menlo Park personnel. C.I.T. Contribution No. 2926.
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Advanced Research Projects Agency (ARPA)UNSPECIFIED
Air Force Office of Scientific Research (AFOSR)F44620-75-C-0049
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Caltech Division of Geological and Planetary Sciences2926
Issue or Number:6
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:49720
Deposited By: Tony Diaz
Deposited On:15 Sep 2014 20:58
Last Modified:03 Oct 2019 07:16

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