Possible mechanisms of dynamic nucleation and arresting of shallow earthquake faulting

Abstract

The nucleation and arresting mechanisms of large shallow earthquakes are investigated from both observational and theoretical viewpoints. We show that two distinct phases are commonly observed at the initial part of seismograms of large shallow earthquakes. The first phase denotes the onset of the P wave, which shows a very gradual increase in amplitude with time. This gradual change is interrupted by the arrival of the second phase, which causes an abrupt change in the amplitude. Our seismological observation shows that the time interval between the onset of the two phases is strongly correlated with the magnitude of the earthquake. It is probable that the above two phases are related to some aspect of inhomogeneities in the earth's crust. One of the important sources of such inhomogeneities is known to be preexisting cracks and their interactions. We theoretically show in this paper that a rupture occurring in a zone of densely distributed cracks radiates elastic waves that can simulate the features of the above two phases; note that such a rupture is considerably affected by crack interactions. Theoretical calculation is also carried out to investigate the effect of crack interactions on the arresting of rupture propagation. A propagating crack can excite subsidiary cracks ahead of its crack tip, which gives rise to crack interactions. We show that these interactions sometimes facilitate the arresting of rupture propagation.