Slip Nucleation Process and Possibility of Aseismic Slip

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Along the northeastern Japan arc the Pacific plate is subducting beneath the continental plate on which the Japan islands lie. The subduction motion is not uniform either in space or in time. At shallow depths of the boundary between the two plates large earthquakes occasionally take place to produce almost instantaneously a significant amount of slip motion. At large depths, say deeper than 100 km, stable sliding seems to occur at the plate boundary. It may be reasonable to expect that aseismic but episodic slip occurs intermittently at intermediate depths. The present study is thus undertaken to examine the possibility of aseismic slip events propagating at very low speeds in relation to the slip nucleation process.Laboratory experiments were performed on a rock specimen with a pre-existing fault to generate stick-slip events with various propagation velocities. It is found that slip events with anomalously low propagation velocities are easy to occur on a composite fault made from g … More ranite and marble. This suggests the possibility of episodic slip events with very low rupture velocities on the inhomogeneous plate boundary. During the slip-weakening process acoustic emissions are found to be generated supposedly by microfractures of asperities on the fault. Their foci are located inside the slip nucleation zone. This implies that microscopically unstable phenomena (microfractures) are involved in a macroscopically stable phenomenon (stable sliding). In 1992 a large shallow earthquake of low-angle thrust fault type with the magunitude of 6.9 took place at the plate boundary in the region off the coast of Sanriku, Honshu, Japan. Following this shock noticeable post-seismic strain changes were recorded by extensometers at two stations, Miyako and Esashi, which are located near the coast of Sanriku. The observed strain changes are well explained by the model in which aseismic slip with a dacay constant of 12 hours is assumed to take place in the hypocentral region of the main shock. The seismic moment of the aseismic slip is found to be nearly the same as that of the main shock. Less