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On the depth extent of co-seismic rupture

N. M. Beeler, Terry E. Tullis, & Greg H. Hirth

In Preparation December 24, 2015, SCEC Contribution #6163

We investigate the implications of laboratory deformation experiments for the coseismic down-dip extent of rupture in quasi-static, whole cycle, earthquake models of a fault in which the depth of the brittle-ductile transition (BDT) depends on strain rate. The calculations use a dislocation fault model from Tse and Rice [1986] with a vertical strike-slip orientation, mode III rupture, and variable along strike length. Two representations of the BDT were used, a transition between rate-weakening friction and dislocation creep and a transition between rate-weakening and rate-strengthening friction. In either case, slip extends >1 to ~ 2 km below the fixed temperature (depth) that is traditionally used to define the rheological transition at the plate motion rate. Thus slip extends below the depth assumed in some geodetic models, kinematic slip inversions and seismic hazard models where the seismogenic depth is constrained by microseismicity or a chosen fixed temperature contour from a thermal model. Though significant slip occurs below the plate rate BDT, since the areal extension is relatively small, so is the associated moment, < 10% of the total. Furthermore this is a region that is rheologically distinct, and, for our favored model where the deep rheology is dislocation creep, negative stress drop is associated with this deep coseismic slip. Average and surface slip increase with the along strike rupture length in a manner consistent with the limited natural observations. This property results from the stiffness of the fault decreasing with fault length, an intrinsic property of the particular lithospheric scale fault model used.

Beeler, N. M., Tullis, T. E., & Hirth, G. H. (2015). On the depth extent of co-seismic rupture. Bulletin of the seismological society of america, (in preparation).