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Fault systems of the 1971 San Fernando and 1994 Northridge earthquakes, southern California: Relocated aftershocks and seismic images from LARSE II

Fuis, Gary S. and Clayton, Robert W. and Davis, Paul M. and Ryberg, Trond and Lutter, William J. and Okaya, David A. and Hauksson, Egill and Prodehl, Claus and Murphy, Janice M. and Benthien, Mark L. and Baher, Shirley A. and Kohler, Monica D. and Thygesen, Kristina and Simila, Gerry and Keller, G. Randy (2003) Fault systems of the 1971 San Fernando and 1994 Northridge earthquakes, southern California: Relocated aftershocks and seismic images from LARSE II. Geology, 31 (2). pp. 171-174. ISSN 0091-7613. http://resolver.caltech.edu/CaltechAUTHORS:20121001-133438812

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Abstract

We have constructed a composite image of the fault systems of the M 6.7 San Fernando (1971) and Northridge (1994), California, earthquakes, using industry reflection and oil test well data in the upper few kilometers of the crust, relocated aftershocks in the seismogenic crust, and LARSE II (Los Angeles Region Seismic Experiment, Phase II) reflection data in the middle and lower crust. In this image, the San Fernando fault system appears to consist of a decollement that extends 50 km northward at a dip of ∼25° from near the surface at the Northridge Hills fault, in the northern San Fernando Valley, to the San Andreas fault in the middle to lower crust. It follows a prominent aseismic reflective zone below and northward of the main-shock hypocenter. Interpreted upward splays off this decollement include the Mission Hills and San Gabriel faults and the two main rupture planes of the San Fernando earthquake, which appear to divide the hanging wall into shingle- or wedge-like blocks. In contrast, the fault system for the Northridge earthquake appears simple, at least east of the LARSE II transect, consisting of a fault that extends 20 km southward at a dip of ∼33° from ∼7 km depth beneath the Santa Susana Mountains, where it abuts the interpreted San Fernando decollement, to ∼20 km depth beneath the Santa Monica Mountains. It follows a weak aseismic reflective zone below and southward of the main-shock hypocenter. The middle crustal reflective zone along the interpreted San Fernando decollement appears similar to a reflective zone imaged beneath the San Gabriel Mountains along the LARSE I transect, to the east, in that it appears to connect major reverse or thrust faults in the Los Angeles region to the San Andreas fault. However, it differs in having a moderate versus a gentle dip and in containing no mid-crustal bright reflections.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1130/0091-7613(2003)031<0171:FSOTSF>2.0.CO;2DOIUNSPECIFIED
http://geology.gsapubs.org/content/31/2/171PublisherUNSPECIFIED
http://geology.geoscienceworld.org/content/31/2/171PublisherUNSPECIFIED
Additional Information:© 2003 Geological Society of America. Manuscript received 25 March 2002; Revised manuscript received 2 October 2002; Manuscript accepted 3 October 2002. We are indebted to many government agencies, organizations, companies, and private individuals who granted permission and, in many cases, vital assistance to LARSE II (see Table 3 in Fuis et al., 2001a). This research was supported by the U.S. Geological Survey (USGS Cooperative Agreement 00HQGR0076 and internal funds), the National Science Foundation (NSF Cooperative Agreement EAR-97-25413), the Southern California Earthquake Center (SCEC, which is funded by NSF Cooperative Agreements EAR-8920136 and USGS Cooperative Agreements 14-08-0001-A0899 and 1434-HQ-97AG01718), the Deutsche Forschungsgemeinschaft, and the GeoForschungsZentrum, Potsdam, Germany. Instruments were supplied by IRIS/ PASSCAL, University of Texas, El Paso, Geophysical Instrument Pool, Potsdam, Canadian Geological Survey, Copenhagen University, SCEC, and USGS. This is SCEC contribution 665. Reviews by Bill Ellsworth, Rufus Catchings, and Dave Wald substantially improved this paper, as did discussions with Jim Mechie, Tom Hanks, and Keith Richards-Dinger.
Funders:
Funding AgencyGrant Number
USGS Cooperative Agreement00HQGR0076
USGS Internal FundsUNSPECIFIED
NSF Cooperative AgreementEAR-97-25413
Southern California Earthquake Center (SCEC)UNSPECIFIED
NSF Cooperative AgreementEAR-8920136
USGS Cooperative Agreement14-08-0001-A0899
USGS Cooperative Agreement1434-HQ-97AG01718
Deutsche Forschungsgemeinschaft (DFG)UNSPECIFIED
GeoForschungsZentrumUNSPECIFIED
Subject Keywords:crustal structure; tectonics; earthquakes; seismic imaging; southern California
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Southern California Earthquake Center (SCEC)665
Record Number:CaltechAUTHORS:20121001-133438812
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20121001-133438812
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:34597
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:01 Oct 2012 21:09
Last Modified:01 Oct 2012 21:09

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