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The 1987 Whittier Narrows earthquake sequence in Los Angeles, southern California: Seismological and tectonic analysis

Hauksson, Egill and Jones, Lucile M. (1989) The 1987 Whittier Narrows earthquake sequence in Los Angeles, southern California: Seismological and tectonic analysis. Journal of Geophysical Research B, 94 (B7). pp. 9569-9589. ISSN 0148-0227. doi:10.1029/JB094iB07p09569.

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The October 1, 1987, Whittier Narrows earthquake (M_L = 5.9) was located at 34°2.96′N, 118°4.86′W, at a depth of 14.6±0.5 km in the northeastern Los Angeles basin. The focal mechanism of the mainshock derived from first motion polarities shows pure thrust motion on west striking nodal planes with dips of 25°±5° and 65°±5°, respectively. The aftershocks define an approximately circular surface that dips gently to the north, centered at the hypocenter of the mainshock with a diameter of 4–6 km. Hence the spatial distribution of the mainshock and aftershocks as well as the focal mechanisms of the mainshock indicate that the causative fault was a 25° north dipping thrust fault striking west and is confined to depths from 10 to 16 km. Although most of the 59 aftershock focal mechanisms presented here document a complex sequence of faulting, they are consistent with deformation of the hanging wall caused by the thrust faulting observed in the mainshock. A cluster of reverse faulting events on north striking planes occurred within hours after the mainshock, 2 km to the west of the mainshock. The largest aftershock (M_L = 5.3) occurred on October 4 and showed mostly right-lateral faulting on the same north-northwest striking plane within the hanging wall. Similarly, several left-lateral focal mechanisms are observed near the eastern edge of the mainshock rupture. The earthquake and calibration blast arrival time data were inverted to obtain two refined crustal velocity models and a set of station delays. When relocating the blast using the new models and delays, the absolute hypocentral location bias is less than 0.5 km. The mainshock was followed by nearly 500 locatable aftershocks, which is a small number of aftershocks for this magnitude mainshock. The decay rate of aftershock occurrences with time was fast, while the b value was low (0.67±0.05) for a Los Angeles basin sequence.

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Hauksson, Egill0000-0002-6834-5051
Jones, Lucile M.0000-0002-2690-3051
Additional Information:© 1989 by the American Geophysical Union. Received August 9, 1988; revised January 11, 1989; accepted January 14, 1989. We are grateful to all of the people who were involved in the collection of data from this earthquake. Paul Spudich, Andy Michael, and Chuck Bufe all provided data from portable recording systems deployed during the aftershock sequence. Kate Hutton supplied the magnitude data and, of course, supervised the network processing at Caltech. Greg Perkins kindly provided the Whittier Narrows blast data. We have benefited from stimulating discussions about this earthquake with Thorn Davis, Andy Michael, and Ross Stein. We thank Andy Michael, David Oppenheimer, Ross Stein, Chris Sanders, an anonymous reviewer, and John K. McRaney for critically reviewing the manuscript. This research was partially supported by USGS grant 14-08-0001-G1328 and USGS cooperative agreement 14-08-0001-A0264.
Group:Seismological Laboratory, Division of Geological and Planetary Sciences
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Issue or Number:B7
Record Number:CaltechAUTHORS:20130508-091114900
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Official Citation:Hauksson, E., and L. M. Jones (1989), The 1987 Whittier Narrows earthquake sequence in Los Angeles, southern California: Seismological and tectonic analysis, J. Geophys. Res., 94(B7), 9569–9589, doi:10.1029/JB094iB07p09569.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:38345
Deposited By: Ruth Sustaita
Deposited On:08 May 2013 16:24
Last Modified:09 Nov 2021 23:36

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