Exploring the Relationship Between Early Rupture History and Final Earthquake Size

Gilead Wurman, David D. Oglesby, and Richard M. Allen


AGU Fall Meeting, San Francisco, December 2007.

A question of considerable contention in the seismological community is whether earthquakes follow the cascade model or the preslip model of earthquake rupture. In the preslip model, earthquake ruptures are generated by an initial slip distribution in a nucleation zone, which loads the fault in proportion to the magnitude of this early slip. In this model, earthquake size is a deterministic result of the early rupture process, and can be determined through observations while the rupture is ongoing. In the cascade model, the rupture is initiated on a particular asperity on the fault, and that failure applies increased stress to adjacent portions of the fault. Each adjacent patch either propagates the rupture forward or stops its propagation, depending on whether local conditions on each individual patch are favorable to rupture or not. The local geometry of the fault and the strength of the host rock, as well as the local distribution of stresses, all affect how favorable a given fault patch is to rupture. Earthquake size in this model is inherently non-deterministic, as no portion of the rupture front feels the effects of barriers (unfavorable patches) until it has propagated into them and stopped. As a result, this model does not allow for the final size of the earthquake to be known until the rupture has completely stopped propagating.

We use a Support Operators (Ely et al., 2007) model to simulate dynamic rupture of a rectangular planar fault with stochastic heterogeneous initial shear stress. By varying the initial shear stress near the point of nucleation we can control the intensity of the early rupture in terms of stress drop, moment release and other parameters. We investigate the effect of different characteristics of early rupture on the final distribution of slip on the fault. Results suggest that under realistic initial stress distributions, earthquake rupture exhibits elements of both cascade and preslip behavior, and that it is possible to alter significantly the final size of the earthquake with comparatively small changes in the character of the early rupture.

© Richard M Allen