Slip Nucleation Process and Possibility of Aseismic Slip

沿着日本东北部的弧线，太平洋板块正在俯冲到日本群岛所在的大陆板块之下。俯冲运动在空间和时间上都是不均匀的。在两个板块之间边界的浅部，偶尔会发生大地震，几乎同时产生大量的滑动运动。在大深度，比如说100公里以下，板块边界似乎发生了稳定的滑动。可以合理地预计，在中等深度，无地震但间歇性的滑动发生。因此，本研究旨在探讨与滑移成核过程有关的无地震滑动事件以极低速度传播的可能性。在具有预先存在的断层的岩样上进行了实验室实验，以产生具有不同传播速度的粘滑事件。研究发现，在由g-…组成的复合断层上，极易发生传播速度异常低的滑动事件更多的是雷诺石和大理石。这表明在不均匀的板块边界上有可能发生破裂速度很低的幕式滑动事件。在滑动减弱过程中，发现声发射可能是由断层上凹凸体的微裂隙产生的。它们的焦点位于滑移形核区内。这意味着宏观上的稳定现象(稳定滑动)涉及到微观上的不稳定现象(微破裂)。1992年，在日本本州三陆外海板块边界发生了一次规模为6.9级的小角度逆冲断裂型浅源大地震。这次地震后，位于三陆海岸附近的宫古和江西两个台站的伸缩仪记录到了明显的震后应变变化。该模型很好地解释了观测到的应变变化，该模型假定在主震的震中区发生了12小时的无震常数滑动。无震滑动的地震矩与主震的地震矩基本相同。较少

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