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Crustal geophysics and seismicity in southern California

Hauksson, Egill (2011) Crustal geophysics and seismicity in southern California. Geophysical Journal International, 186 (1). pp. 82-98. ISSN 0956-540X. doi:10.1111/j.1365-246X.2011.05042.x.

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The geographical distribution of the (1981–2005) seismicity in southern California forms a ±150 km broad zone adjacent to the Pacific–North America plate boundary, ranging from depths of ∼1–~30 km, with the bulk of the focal depths in the range of 2–12 km. The distribution of the seismicity that includes both mainshock–aftershock sequences and background events is affected by both static and kinematic geophysical parameters of the crust. The static parameters include heat flow, topography, crustal density, V_(p)/V_(s) ratio, hypocentral fault-distance and crustal thickness from receiver functions. The tectonic loading is represented by kinematic parameters such as the crustal shear strain rate field, and the dilatational strain rate field. In our analysis, we normalize the seismicity relative to the areal density of the range of values of each of the parameters. Most of the seismicity occurs in areas of average heat flow, low to intermediate topography, average Vp/Vs and high late Quaternary fault density, and forms seismogenic zones that extend through the brittle crust. The location of late Quaternary faults, often described as zones of weakness, influences the geographical distribution of seismicity more than any other parameter. Although above or below average crustal properties such as high heat flow, thin crust or very low V_(p)/V_(s) values exist, these properties only influence the spatial distribution of seismicity in a minor way. As an example, the Salton Trough area of low topography, high heat flow, high V_(p)/V_(s), high shear strain rate and thin crust has distributed seismicity within a thin seismogenic zone. Also, somewhat surprisingly, areas of high topography, low heat flow, low V_(p)/V_(s), low shear strain rate and thick crust have low seismicity rates but a thin seismogenic zone. We determine an empirical relationship between heat flow and crustal thickness to show how the ~400 °C temperature isotherm gradually deepens with crustal thickness and forms the base of the seismogenic zone for crustal thicknesses from 22 to 37 km. For crustal thickness ranging from 37 to 43 km, the ~250 °C isotherm forms the base of the seismogenic zone, suggesting that seismic faulting in these regions is confined to the top of the upper crust (12–14 km), and thus does not accommodate plate motion.

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Hauksson, Egill0000-0002-6834-5051
Additional Information:© 2011 The Author. Geophysical Journal International © 2011 RAS. Accepted 2011 April 9. Received 2011 March 10; in original form 2010 November 19. Article first published online: 31 May 2011. This research was supported by the U.S. Geological Survey Grant G10AP00017 and G11AP20032 to Caltech, and by the Southern California Earthquake Center. SCEC is funded by NSF Cooperative Agreement EAR-0529922 and USGS Cooperative Agreement 07HQAG0008. Most figures were done using GMT (Wessel & Smith 1998). H. Kanamori suggested the formulation in Appendix. L. Jones, M. Simons and W. Yang provided helpful comments. N. Fay provided the data used for the GPS dilatation and the modelled dilatation. C. Tape provided the data for the shear strain field. A. Plesch provided distance measurements between each hypocentre to the nearest PSZ. SCEC contribution number 1446. Contribution number 10052, Seismological Laboratory, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena.
Group:Seismological Laboratory
Funding AgencyGrant Number
Subject Keywords:Heat flow; Seismic cycle; Composition of the continental crust; Seismicity and tectonics; Continental tectonics: strike-slip and transform
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Other Numbering System NameOther Numbering System ID
Southern California Earthquake Center1446
Caltech Seismological Laboratory10052
Issue or Number:1
Record Number:CaltechAUTHORS:20110808-115026278
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Official Citation:Hauksson, E. (2011), Crustal geophysics and seismicity in southern California. Geophysical Journal International, 186: 82–98. doi: 10.1111/j.1365-246X.2011.05042.x
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:24739
Deposited On:08 Aug 2011 20:26
Last Modified:09 Nov 2021 16:26

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