A Caltech Library Service

The 1991 Sierra Madre Earthquake Sequence in Southern California: Seismological and Tectonic Analysis

Hauksson, Egill (1994) The 1991 Sierra Madre Earthquake Sequence in Southern California: Seismological and Tectonic Analysis. Bulletin of the Seismological Society of America, 84 (4). pp. 1058-1074. ISSN 0037-1106.

PDF - Published Version
See Usage Policy.


Use this Persistent URL to link to this item:


The (M_L 5.8) Sierra Madre earthquake of 28 June 1991 occurred at a depth of 12 km under the San Gabriel Mountains of the central Transverse Ranges. Since at least 1932 this region had been quiescent for M ≧ 3. The mainshock focal mechanism derived from first-motion polarities exhibited almost pure thrust faulting, with a rake of 82° on a plane striking N62°E and dipping 50° to the north. The event appears to have occurred on the Clamshell-Sawpit fault, a splay of the Sierra Madre fault zone. The aftershock sequence following the mainshock occurred at a depth of 9 to 14 km and was deficient in small earthquakes, having a b value of 0.6. Twenty nine single-event focal mechanisms were determined for aftershocks of M > 1.5. The 4-km-long segment of the Clamshell-Sawpit fault that may have ruptured in the mainshock is outlined by several thrust focal mechanisms with an east-northeast-striking fault plane dipping to the north. To the west, several thrust aftershocks with east-striking nodal planes suggest some complexity in the aftershock faulting, such as a curved rupture surface. In addition, several strike-slip and normal faulting events occurred along the edges of the mainshock fault plane, indicating secondary tear faulting. The tectonic stress field driving the coexisting left-lateral strike-slip and thrust faults in the northern Los Angeles basin is north-south horizontal compression with vertical intermediate or minimum principal stress axis.

Item Type:Article
Related URLs:
URLURL TypeDescription
Hauksson, Egill0000-0002-6834-5051
Additional Information:© 1994 Seismological Society of America. Manuscript received 28 May 1993. I thank L. Jones and D. Oppenheimer for critical review of the manuscript. I thank D. Morton, L. Silver, and K. Sieh for interesting discussions about the tectonics of the central Transverse Ranges. I am grateful to the seismic analysts of Caltech and the USGS for quick and competent processing of the data. This work was supported by USGS Grant No. 14-08-0001-G1761. Division of Geological and Planetary Sciences, California Institute of Technology, Contribution No. 5132. Southern California Earthquake Center Contribution No. 55.
Group:Seismological Laboratory, Division of Geological and Planetary Sciences
Funding AgencyGrant Number
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Caltech Division of Geological and Planetary Sciences5132
Southern California Earthquake Center (SCEC)55
Issue or Number:4
Record Number:CaltechAUTHORS:20130227-143843340
Persistent URL:
Official Citation:Egill Hauksson The 1991 Sierra Madre earthquake sequence in southern California: Seismological and tectonic analysis Bulletin of the Seismological Society of America August 1994 84:1058-1074
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:37184
Deposited By: Ruth Sustaita
Deposited On:28 Feb 2013 16:06
Last Modified:03 Oct 2019 04:45

Repository Staff Only: item control page