KINEMATICS OF POSTSEISMIC RELAXATION FROM AFTERSHOCK FOCAL MECHANISMSOF THE 1994 NORTHRIDGE, CALIFORNIA, EARTHQUAKE

Geodetic observations of surface deformation associated with the 1994 Northridge, southern California, earthquake generally are reproduced b y simple models of a large-scale elastic dislocation on a blind or bur ied thrust fault. The smaller-scale aftershocks of the Northridge eart hquake are distributed throughout much of the volume of crust that app ears to have deformed elastically during the mainshock. These aftersho cks, averaged over volumes that are large relative to their rupture ra dii, reflect a distributed, permanent deformation that is accommodated by local brittle fracture. We use a micropolar continuum model to inv ert the aftershocks in such volumes for the average incremental strain , and we compare that deformation both with the elastic strain from th e dislocation model of the mainshock and with geodetically measured st rain. 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 bl ind Northridge thrust fault. In contrast, aftershock deformation from the upper 5-7 km of the hanging wall block directly above the thrust f ault can be characterized by horizontal NE-SW shortening and horizonta l NW-SE (i.e., fault-parallel) extension. This pattern of deformation is similar to that associated with the mainshock, as observed geodetic ally and as calculated from the elastic dislocation model. We interpre t that the aftershock activity in the hanging wall represents the quas i-ductile accommodation by brittle deformation mechanisms of a permane nt strain distributed through the hanging wall block. The aftershocks along the mainshock rupture zone are interpreted as resulting from eit her (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 deformati on of a ductile-elastic lower crustal layer that relaxes under the str ess transferred by the coseismic loss of cohesion in the upper crust. In either case, the aftershock activity suggests that the crust underg oes quasi-ductile flow as a brittle-elastic material, and is not a str ictly elastic material.