INSIGHT TO THE MECHANICS OF FAULTING FROM 3-D SLIP DISTRIBUTIONS DURING STRIKE-SLIP EARTHQUAKES

Observations from earthquakes provide unique insight to the mechanics of rupture nucleation, propagation, and the accumulation of slip. We use 3-D distributions of slip and stress drop, determined from inversions of geophysical data, to investigate the relative importance of a cohesive end-zone (CEZ) in controlling stress changes, slip distributions, and rupture extent of four well-studied earthquakes (1984 Morgan Hill, Ca; 1992 Landers, CA; 1999 Izmit, Turkey; 1999 Hector Mine, CA). Observations indicate areas of (1) low slip and slip gradients and (2) negative stress drop, which we interpret as a CEZ. We model co-seismic slip using a new 3-D, elastic-plastic fracture mechanics solution for slip along an elliptical fault that incorporates a CEZ. This model for fault slip reproduces the general characteristics of the source parameter scaling relations for these and other strike slip earthquakes (as well as the scatter), by invoking strength heterogeneities along the rupture surface, or specifically, velocity strengthening frictional behavior in the CEZ and velocity weakening along the main portion of the rupture. We observe that among similar sized ruptures, variations in stress drop result from variations in the area of the CEZ, and is therefore the primary reason for the failure of a universal (constant stress drop) scaling law. Our results imply average resolved shear stresses and CEZ shear strengths on the order of 10² MPa, and CEZ lengths of ~10-30 km (~20-40% of rupture length), an order of magnitude larger than that inferred from characteristics of high-frequency seismic radiation (i.e., f_max). In our new model of fault slip, stress drop is predicted to be two orders of magnitude less than fault strength, and thus supports a partial stress drop model of earthquake rupture. Importantly, we also observe a correspondence between the location of earthquake nucleation and the tail of the CEZ, which is consistent with the relative displacement fracture criterion in CEZ models of faulting, and suggests that the difference in mechanical properties between the velocity strengthening and velocity weakening part of the fault surface provides an important first-order control on initiating frictional slip.