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Modeling of the surface static displacements and fault plane slip for the 1979 Imperial Valley earthquake

Slade, Martin A. and Lyzenga, Gregory A. and Raefsky, Arthur (1984) Modeling of the surface static displacements and fault plane slip for the 1979 Imperial Valley earthquake. Bulletin of the Seismological Society of America, 74 (6). pp. 2413-2433. ISSN 0037-1106.

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Synthesis of geodetic and seismological results for the 1979 Imperial Valley earthquake is approached using three-dimensional finite element modeling techniques. The displacements and stresses are calculated elastically throughout the modeled region. The vertical elastic structure in the model is derived from compressional and shear wave velocities as used in the seismic data analysis (Fuis et al., 1981) combined with a sediment density profile. Two strategies for applying initial conditions are followed in this modeling. In the first strategy, a sample seismological estimate for fault plane slip is used to predict the resultant surface motions. We show that the geodetic strain results over distances of tens of kilometer from the fault (Snay et al., 1982) are basically consistent with the model seismic fault displacements. Geodetic results from within a few kilometers of the fault trace (Mason et al., 1981) seem to require more slip at shallow depths than appears at seismic time scales. This is consistent with the occurrence of aftercreep at shallow depths in less well-consolidated material, which would bring surface displacements into line with maximum slip at depth, but not greatly affect the net moment. In the second strategy, we consider stresses on the fault plane, rather than displacements, as model variables. To constrain this part of our numerical modeling, we assume that the fault driving stress is governed by ambient tectonic stress and an opposing Coulomb friction derived from experiment. The coseismic stress drop from point to point on the failed fault is given by the difference between the tectonic shear stress and the frictional stress. After arriving at such a uniform model which adequately represents the Snay et al. results, we further modify a small region near the seismic “asperity” to make the fault plane motions qualitatively and quantitatively resemble the model of coseismic motions used in the first strategy. The observed offset on the fault trace (Sharp et al., 1982) is approximated in this final stress-driven model by removing the driving stress on the southern third of the fault. Thus, the principal features of the coseismic slip pattern are explained by a stress-driven fault model in which: (a) a spatially unresolved asperity is found equivalent to a stress drop of 18 MPa averaged over an area of 15 km^2, and (b) driving stress is essentially absent on the fault segment overlapping the 1940 earthquake rupture zone.

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Additional Information:© 1984 Seismological Society of America. Manuscript received 22 July 1983. We are very indebted to Professor R. F. Scott of Caltech Earthquake Engineering for his calculations relating to the fault friction model and for helpful comments on the manuscript. We thank S. H. Hartzell and T. H. Heaton for many valuable discussions. The comments of an anonymous reviewer have resulted in substantial improvements to the paper. The work described in this paper was carried out by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
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Issue or Number:6
Record Number:CaltechAUTHORS:20140903-081436089
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:49170
Deposited By: Tony Diaz
Deposited On:03 Sep 2014 16:20
Last Modified:03 Oct 2019 07:11

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