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San Fernando Earthquake Series, 1971: Focal Mechanisms and Tectonics

Whitcomb, James H. and Allen, Clarence R. and Garmany, Jan D. and Hileman, James A. (1973) San Fernando Earthquake Series, 1971: Focal Mechanisms and Tectonics. Reviews of Geophysics, 11 (3). pp. 693-730. ISSN 8755-1209. doi:10.1029/RG011i003p00693.

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The largest events in the San Fernando earthquake series, initiated by the main shock at 14h 00m 41.8s UT on February 9, 1971, were chosen for analysis from the first three months of activity, 87 events in all. C. R. Allen and his co-workers assigned the main shock parameters: 34°24.7′N, 118°24.0′W, focal depth h = 8.4 km, and local magnitude M_L = 6.4. The initial rupture location coincides with the lower, northernmost edge of the main north-dipping thrust fault and the aftershock distribution. The best focal mechanism fit to the main shock P wave first motions constrains the fault plane parameters to: strike, N67°(±6°)W; dip, 52°(±3°)NE; rake, 72° (67°−95°) left lateral. Focal mechanisms of the aftershocks clearly outline a down step of the western edge of the main thrust fault surface along a northeast-trending flexure. Faulting on this down step is left lateral strike slip and dominates the strain release of the aftershock series, which indicates that the down step limited the main event rupture on the west. The main thrust fault surface dips at about 35° to the northeast at shallow depths and probably steepens to 50° below a depth of 8 km. This steep dip at depth is a characteristic of other thrust faults in the Transverse ranges and indicates the presence at depth of laterally varying vertical forces that are probably due to buckling or overriding that causes some upward redirection of a dominant north-south horizontal compression. Two sets of events exhibit normal dip slip motion with shallow hypocenters and correlate with areas of ground subsidence deduced from gravity data. One set in the northeastern aftershock area is related to shallow extensional stresses caused by the steepening of the main fault plane. The other set is probably caused by a deviation of displacements along the down step of the main fault surface that resulted in localized ground subsidence near the western end of the main fault break. Several lines of evidence indicate that a horizontal compressional stress in a north or north-northwest direction was added to the stresses in the aftershock area 12 days after the main shock. After this change, events were contained in bursts along the down step, and sequencing within the bursts provides evidence for an earthquake-triggering phenomenon that propagates with speeds of 5–15 km/day. Seismicity before the San Fernando series and the mapped structure of the area suggest that the down step of the main fault surface is not a localized discontinuity but is part of a zone of weakness extending from Point Dume, near Malibu, to Palmdale on the San Andreas fault. This zone is interpreted as a decoupling boundary between crustal blocks that permits them to deform separately in the prevalent crustal shortening mode of the Transverse ranges region.

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Additional Information:© 1973 by the American Geophysical Union. Received February 13, 1973; accepted February 28, 1973. Article first published online: 14 Jun 2010. We are indebted to many organizations outside CIT for supplying their data from seismic stations for this study. They include the Earthquake Mechanisms Laboratory, National Ocean Survey (Don Tocher); the Las Vegas Branch of NOS (Stanley R, Brockman); Lamont-Doherty Geological Observatory (Christopher H. Scholz); University of Southern California (Ta-Liang Teng); the California Department of Water Resources; and the National Center for Earthquake Research, USGS (Robert L. Wesson, William H. K. Lee) for station SUS times. At CIT Charles F. Richter, Gladys R. Engen, and J. Leonard Blayney provided their personal seismic records, and John M. Nordquist and Mark Gapanoff read many of the records and ran many of the computer solutions. We give special thanks to the CIT technical personnel who installed and operated the portable stations during the aftershock series. We benefited greatly from discussions with Don L. Anderson, Pierre H. Jungels, Bernard J. Minster, Thomas C. Hanks, Thomas H. Jordan, Ralph W. Alewine III, and Donald V. Helmberger. This study was supported by the CIT Earthquake Research Affiliates and by the National Science Foundation (GA-29920). Contribution 2318, Division of Geological and Planetary Sciences California, Institute of Technology, Pasadena, California.
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Official Citation:Whitcomb, J. H., C. R. Allen, J. D. Garmany, and J. A. Hileman (1973), San Fernando Earthquake series, 1971: Focal mechanisms and tectonics, Rev. Geophys., 11(3), 693–730, doi:10.1029/RG011i003p00693.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:50001
Deposited By: Ruth Sustaita
Deposited On:24 Sep 2014 21:25
Last Modified:10 Nov 2021 18:50

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