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Seismotectonics of the Newport-Inglewood fault zone in the Los Angeles basin, southern California

Hauksson, Egill (1987) Seismotectonics of the Newport-Inglewood fault zone in the Los Angeles basin, southern California. Bulletin of the Seismological Society of America, 77 (2). pp. 539-561. ISSN 0037-1106.

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The Newport-Inglewood fault zone (NIF) strikes northwest along the western margin of the Los Angeles basin in southern California. The seismicity (1973 to 1985) of M_L ≧ 2.5 that occurred within a 20-km-wide rectangle centered on the NIF extending from the Santa Monica fault in the north to Newport Beach in the south is analyzed. A simultaneous full inversion scheme (VELEST) is used to invert for hypocentral parameters, two velocity models, and a set of station delays. Arrival time data from three quarry blasts are included to stabilize the inversion. The first velocity model applies to stations located along the rim and outside the Los Angeles basin and is well resolved. It is almost identical to the starting model, which is the model routinely used by the CIT/USGS southern California seismic network for locating local earthquakes. The second velocity model applies to stations located within the Los Angeles basin. It shows significantly lower velocities down to depths of 12 to 16 km, which is consistent with basement of Catalina Schist below the sediments in the western Los Angeles basin. The distribution of relocated hypocenters shows an improved correspondence to mapped surface traces of late Quaternary fault segments of the NIF. A diffuse trend of seismicity is observed along the Inglewood fault from the Dominguez Hills, across the Baldwin Hills to the Santa Monica fault in the north. The seismicity adjacent to Long Beach, however, is offset 4 to 5 km to the east, near the trace of the subsurface Los Alamitos fault. The depth distribution of earthquakes along the NIF shows clustering from 6 to 11 km depth, which is similar to average seismogenic depths in southern California. Thirty-nine single-event focal mechanisms of small earthquakes (1977 to 1985) show mostly strike-slip faulting with some reverse faulting along the north segment (north of Dominguez Hills) and some normal faulting along the south segment (south of Dominguez Hills to Newport Beach). The results of an inversion of the focal mechanism data for orientations of the principal stress axes and their relative magnitudes indicate that the minimum principal stress is vertical along the north segment while the intermediate stress is vertical along the south segment. The maximum principal stress axis is oriented 10° to 25° east of north. Reverse faulting along the north segment indicates that a transition zone of mostly compressive deformation exists between the Los Angeles block and the Central Transverse Ranges.

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Hauksson, Egill0000-0002-6834-5051
Additional Information:© 1987 Seismological Society of America. Manuscript received 20 June 1986. I thank L. M. Jones, J. I. Ziony, and G. V. Saldivar for critical reviews. A. J. Michael made copies and documentation of his stress inversion programs available. W. L. Ellsworth and R. S. Cockerham provided advice and copies of the VELEST programs. L. M. Jones and L. K. Hutton made copies of the CIT/USGS southern California network data available. G. V. Saldivar assisted with rechecking the USC arrival time data. This research was supported by U.S. Geological Survey Grants 14-08-0001-G983 and Gl158 and U.S. Geological Survey Contract 14-08-0001-A0264. The USC seismic network is partially supported by the THUMS Long Beach Company. The figures were drafted by J. Dodds and the manuscript typed by S. Turnbow.
Group:Seismological Laboratory, Division of Geological and Planetary Sciences
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Issue or Number:2
Record Number:CaltechAUTHORS:20130222-071943411
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Official Citation:Egill Hauksson Seismotectonics of the Newport-Inglewood fault zone in the Los Angeles basin, southern California Bulletin of the Seismological Society of America, April 1987, v. 77, p. 539-561
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:37061
Deposited By: Ruth Sustaita
Deposited On:22 Feb 2013 17:57
Last Modified:03 Oct 2019 04:44

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