Elastic Impacts of Fault Zone Structures as a Physical Alternative to the Brune-Haskell Model

Abstract:

The physics underlying the high-frequency ground motions produced by the Brune-Haskell model is that abrupt initiation and possibly ending of fault slip causes the radiation of high-frequency energy. While recent physics-based rupture models have emphasized the role of heterogeneous frictionally-governed slip in producing stronger high-frequency radiation than would otherwise be produced with smooth rupture, the fundamental reasons for the high-frequency motions remain the same, with earthquake stress drop being one of the most important factors in determining how strong these ground motions are. Here we propose an additional cause for high-frequency ground motions from elastic collisions of structures within a rupturing fault zone. The collision spectrum is set by an impact contact time that is proportional to the size of colliding structures so that spectra depend on fundamentally different physical parameters compared with slip models. When added to standard models, elastic collisions can reconcile various puzzling observations, including why faults that have had many earthquakes have less damaging ground motions, why earthquake damage is observed to occur more uniformly than previously predicted, why shallow earthquakes often have very strong ground motions, and why globally compiled stress drop measurements appear to be roughly magnitude independent while they are often quite variable regionally. If the model is correct, high-frequency earthquake ground motions and damage may be an outgrowth of fault-zone structure rather than sudden initiation of slip. Given the encouraging observational support for the elastic impact model and the challenges that traditional frictional slip models have in explaining the full range of observations, we suggest it is worthwhile to make further observations that would help confirm or reject the elastic impact model and the assumptions inherent in more traditional models.