Microscopic processes on a fault plane and their implications for earthquake dynamics

Abstract

Seismic radiation exhibits complex wave forms. The complexity can be due to both static and dynamic mechanisms. Static mechanisms responsible for complexity are roughness and heterogeneities of rock properties on a fault plane. Many dynamic mechanisms are possible, but thermal processes caused by frictional heating are most likely to cause complex and chaotic behavior of seismic rupture through nonlinear feedback mechanisms. Similarity of the beginning of small and large earthquakes suggests that a cascade process that can be simulated by percolation models provides a realistic model of complex earthquake rupture. A percolation process results in a relation similar to the magnitude-frequency relationship for earthquakes. As the state of the crust changes from a sub-critical state to a critical state, the magnitude-frequency diagram changes from an upward convex curve to a more straight line. Thus, the magnitude-frequency relationship could be used to diagnose the state of the crust regarding its proximity to a critical state. For purposes of forecasting seismic activity in a particular area, it would be useful to know how critical (i.e. how close to failure) the crust is in that area. The concept of criticality is important because it suggests the use of other methods for diagnostic purposes; e.g., electric and magnetic methods to monitor migration of fluid flow in the crust. When fluid migrates in the crust and weakens some parts of fault zones, the crust would approach a critical state and is likely to produce electric or magnetic noises and other premonitory processes such as foreshocks and slow deformation. A better understanding of the physics of microscopic processes would be important for interpreting premonitory phenomena and understanding earthquake dynamics.

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