Kinematics of postseismic relaxation from aftershock focal mechanisms of the 1994 Northridge, California, earthquake

Abstract

Geodetic observations of surface deformation associated with the 1994 Northridge, southern California, earthquake generally are reproduced by simple models of a large-scale elastic dislocation on a blind or buried thrust fault. The smaller-scale aftershocks of the Northridge earthquake are distributed throughout much of the volume of crust that appears to have deformed elastically during the mainshock. These aftershocks, averaged over volumes that are large relative to their rupture radii, reflect a distributed, permanent deformation that is accommodated by local brittle fracture. We use a micropolar continuum model to invert the aftershocks in such volumes for the average incremental strain, and we compare that deformation both with the elastic strain from the dislocation model of the mainshock and with geodetically measured strain. Aftershock deformation that occurred at depths below about 6 km, and which is associated with the primary rupture zone, is consistent with slow continuation of the southwest-side-up reverse slip on the blind Northridge thrust fault. In contrast, aftershock deformation from the upper 5-7 km of the hanging wall block directly above the thrust fault can be characterized by horizontal NE-SW shortening and horizontal NW-SE (i.e., fault-parallel) extension. This pattern of deformation is similar to that associated with the mainshock, as observed geodetically and as calculated from the elastic dislocation model. We interpret that the aftershock activity in the hanging wall represents the quasi-ductile accommodation by brittle deformation mechanisms of a permanent strain distributed through the hanging wall block. The aftershocks along the mainshock rupture zone are interpreted as resulting from either (1) the time-dependent release along a weakened fault zone of part of the remaining accumulated elastic strain in the upper crust or (2) the continued slip in the weakened fault zone driven by the deformation of a ductile-elastic lower crustal layer that relaxes under the stress transferred by the coseismic loss of cohesion in the upper crust. In either case, the aftershock activity suggests that the crust undergoes quasi-ductile flow as a brittle-elastic material, and is not a strictly elastic material.

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