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Average Stress Drops of Southern California Earthquakes in the Context of Crustal Geophysics: Implications for Fault Zone Healing

Hauksson, Egill (2015) Average Stress Drops of Southern California Earthquakes in the Context of Crustal Geophysics: Implications for Fault Zone Healing. Pure and Applied Geophysics, 172 (5). pp. 1359-1370. ISSN 0033-4553. doi:10.1007/s00024-014-0934-4.

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To understand how fault healing processes affect earthquake stress drops, we search for a possible dependency of stress drops on crustal conditions and geophysical parameters. We reanalyze the stress drop values of ~60,000 earthquakes in southern California which were originally determined by Shearer et al. J Geophys Res 111:B06303, (2006) using a spectral method. We modify the dataset to include only stress drops that are derived from at least 10 spectra and with corner frequencies between 3 and 30 Hz, and correct the rupture velocity for increasing S-wave speed with depth. We see no dependence of stress drop on moment magnitude or depth, except for a small, poorly determined increase from 15 to 25 km. We use six crustal geophysics parameters to search for obvious correlations that may explain changes in the mean values of the stress drops: (1) crustal thickness, (2) isostatic gravity, (3) heat flow, (4) shear strain rate, (5) crustal stress regime, and (6) style of faulting. None of the variables reduce the scatter but most can explain up to 10–20 % variations in the mean stress drops. The geographical distribution of the grouped mean stress drops includes very high stress drops near Ridgecrest, eastern California, as well as near fault jogs within the San Andreas Fault system. Low stress drops dominate in trans-tensional regions. Heat flow and GPS-based shear strain rate estimates have the largest influence on stress drop variations. In the range of low to medium heat flow, the stress drops increase with increasing heat flow. In contrast, at high heat flow in thin crust, the stress drops decrease systematically with increasing heat flow. Increasing shear strain rate systematically correlates with decreasing stress drops. The crustal stress regime and style of faulting also influence the stress drops as demonstrated by lower stress drops for north-northeast trending principal horizontal stress and in areas of dip-slip faulting. The mean variations in stress drops with heat flow, stress regime, crustal thickness, and density can be explained in the context of fault healing (grain boundary growth) and corresponding increase in fault zone strength on time scales modulated by the tectonic shear strain rate.

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Hauksson, Egill0000-0002-6834-5051
Additional Information:© 2014 Springer Basel. Received March 12, 2014, revised September 12, 2014, accepted September 14, 2014, onlinedate October 15, 2014. We thank L. Jones, H. Kanamori, L. Rivera, S. Minson, T. Goebel, Y. Ben-Zion, and two anonymous reviewers for helpful discussions and reviews. E. Hauksson was supported by USGS/NEHRP grants G13AP00047 and G14AP00075. This research was also supported by the Southern California Earthquake Center, which is funded by NSF Cooperative Agreement EAR-0529922 and USGS Cooperative Agreement 07HQAG0008. This paper is SCEC contribution# 1923.
Group:Seismological Laboratory
Funding AgencyGrant Number
Southern California Earthquake Center (SCEC)UNSPECIFIED
Subject Keywords:Earthquakes; stress drop; Southern California; crustal geophysics; fault zone healing
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Other Numbering System NameOther Numbering System ID
Southern California Earthquake Center1923
Issue or Number:5
Record Number:CaltechAUTHORS:20150501-100705991
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:57142
Deposited By: Tony Diaz
Deposited On:01 May 2015 17:17
Last Modified:10 Nov 2021 21:08

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