A Caltech Library Service

State of Stress from Focal Mechanisms Before and After the 1992 Landers Earthquake Sequence

Hauksson, Egill (1994) State of Stress from Focal Mechanisms Before and After the 1992 Landers Earthquake Sequence. Bulletin of the Seismological Society of America, 84 (3). pp. 917-934. ISSN 0037-1106.

PDF - Published Version
See Usage Policy.


Use this Persistent URL to link to this item:


The state of stress in the Eastern California Shear Zone (ECSZ) changed significantly because of the occurrence of the 1992 M_w 6.1 Joshua Tree and the M_W 7.3 Landers earthquakes. To quantify this change, focal mechanisms from the 1975 Galway Lake sequence, the 1979 Homestead Valley sequence, background seismicity from 1981 to 1991, and the 1992 Landers sequence are inverted for the state of stress. In all cases, the intermediate principal stress axis (S2) remained vertical, and changes in the state of stress consisted of variations in the trend of maximum and minimum principal stress axes (S_1 and S_3) and small variations in the value of the relative stress magnitudes (ϕ). In general, the stress state in the ECSZ has S_1 trending east of north and ϕ = 0.43 to 0.65, suggesting that the ECSZ is a moderate stress refractor and the style of faulting is transtensional. South of the Pinto Mountain fault, in the region of the 1992 Joshua Tree earthquake, the stress state determined from the 1981 to 1991 background seismicity changed on 23 April and again on 28 June 1992. In the central zone, S_1 rotated from N14° ± 5°E to N28° ± 5°E on 23 April and back again to N16° ± 5°E on 28 June. Thus, the Landers mainshock in effect recharged some of the shear stress in the region of the M_w 6.1 Joshua Tree earthquake. Comparison of the state of stress before and after 28 June 1992, along the Landers mainshock rupture zone, showed that the mainshock changed the stress orientation. The S1 trend rotated 7° to 20° clockwise and became progressively more fault normal from south to north. Along the Emerson-Camp Rock faults, the variation was so prominent that the focal mechanisms of aftershocks could not be fit by a single deviatoric stress tensor. The complex distribution of P and T axes suggests that most of the uniform component of the applied shear stress along the northern part of the rupture zone was released in the mainshock. The San Bernardino Mountains region of the M_w 6.2 Big Bear earthquake has a distinctively different state of stress, as compared to the Landers region, with S_1 trending N3° ± 5°W. This region did not show any significant change in the state of stress following the 1992 M_w 6.2 Big Bear sequence.

Item Type:Article
Related URLs:
URLURL TypeDescription
Hauksson, Egill0000-0002-6834-5051
Additional Information:© 1994 Seismological Society of America. Manuscript received 18 August 1993. D. Wald, J. Stock, H. Kanamori, L. Jones, and two BSSA reviewers R. Weldon and R. Dokka provided helpful critical comments and reviews. I thank Kerry Sieh for stimulating discussions about the tectonics of the Landers earthquake and for providing a map of the surface rupture. I am grateful to the seismic analysts of Caltech and the USGS for quick and competent processing of the earthquake data. This research was partially supported by USGS Grant No. 14-08-0001-G1761 and USGS cooperative agreement 1434-92-A-0960 to Caltech and the Southern California Earthquake Center (SCEC) under NSF cooperative agreement No. EAR-8920136 and USGS cooperative agreement No. 14-08-0001-A0899. SCEC publication No. 74. Contribution No. 5318, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125.
Group:Seismological Laboratory, Division of Geological and Planetary Sciences
Funding AgencyGrant Number
Southern California Earthquake Center (SCEC)UNSPECIFIED
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Southern California Earthquake Center (SCEC)74
Caltech Division of Geological and Planetary Sciences5318
Issue or Number:3
Record Number:CaltechAUTHORS:20130227-133927074
Persistent URL:
Official Citation:Egill Hauksson State of stress from focal mechanisms before and after the 1992 landers earthquake sequence Bulletin of the Seismological Society of America, June 1994, v. 84, p. 917-934
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:37181
Deposited By: Ruth Sustaita
Deposited On:28 Feb 2013 16:01
Last Modified:03 Oct 2019 04:45

Repository Staff Only: item control page