Slip Nucleation Process and Possibility of Aseismic Slip

沿着日本东北部的弧线，太平洋板块正俯冲到日本岛屿所在的大陆板块之下。俯冲运动在空间和时间上都不是均匀的。在两个板块交界的浅层深处，偶尔会发生大地震，几乎在瞬间产生大量的滑动运动。在较大的深度，比如深度超过100公里，稳定的滑动似乎发生在板块边界。可以合理地预期，在中等深度处间歇性地发生地震性但偶发性的滑动。因此，目前的研究是为了检查地震滑动事件以非常低的速度传播的可能性，与滑动成核过程有关。在室内实验中，对存在断层的岩石试样进行了不同传播速度的粘滑事件模拟。研究发现，在由花岗岩和大理岩构成的复合断层上容易发生传播速度异常低的滑动事件。这表明，在非均匀板块边界上，有可能发生极低破裂速度的偶发性滑动事件。在滑移减弱过程中，发现声发射可能是由断层上的微裂缝产生的。它们的焦点位于滑移成核带内。这意味着微观不稳定现象（微断裂）涉及宏观稳定现象（稳定滑动）。1992年，在日本本州三陆海岸附近的板块边界发生了一次震级为6.9级的低角逆冲断层型浅层大地震。在这次地震之后，位于三陆海岸附近的宫古和Esashi两个站点的延伸计记录了明显的震后应变变化。假定在主震的震源中心区域发生日常数为12小时的地震滑动，这种模型很好地解释了观测到的应变变化。发现地震滑动的地震矩与主震的地震矩几乎相同。少

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