The 1991 Sierra Madre Earthquake Sequence in Southern California: Seismological and Tectonic Analysis
- Creators
- Hauksson, Egill
Abstract
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.
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.Attached Files
Published - 1058.full.pdf
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Additional details
- Eprint ID
- 37184
- Resolver ID
- CaltechAUTHORS:20130227-143843340
- 14-08-0001-G1761
- USGS
- Created
-
2013-02-28Created from EPrint's datestamp field
- Updated
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2019-10-03Created from EPrint's last_modified field
- Caltech groups
- Seismological Laboratory, Division of Geological and Planetary Sciences
- Other Numbering System Name
- Southern California Earthquake Center (SCEC)
- Other Numbering System Identifier
- 55