Continuing Study of the Post-Seismic Fault Healing of the 1992 M7.5 Landers, California, Earthquake

EAR 0105508（PH #25 x）提案摘要标题：1992年加州兰德斯M7.5地震后断层愈合的持续研究主要研究者：李永刚，南加州大学，和John E. Vidale，加州大学，洛杉矶我们建议继续我们的研究兰德斯，加州，断裂带的地震后愈合过程中使用断裂带地震波激发的爆炸内引爆破裂带。1994年、1996年、1998年和2000年的重复测量表明，断裂带岩石的剪切速度在1994年至1996年期间增加了约1.2%，在1996年至1998年期间进一步增加了约0.7%。这一趋势表明兰德斯破裂带在主震后通过加强而愈合，很可能是由于1992年地震期间打开的裂缝闭合。在1994年和1998年之间观察到的速度增加是一致的？0.03在破裂区内的表观裂纹密度的减少。我们还发现，断层带附近的裂缝部分充满了水，随着时间的推移变得更加潮湿。然而，在最近的实验中记录的数据表明，在浅破裂带的波速没有进一步增加，但与1998年和2000年之间的剪切速度略有下降，推断断层愈合可能是在1992年地震后6至8年变得不可见。然而，我们推测兰德斯破裂带的愈合过程可能受到1999年10月25日发生在兰德斯以东约25公里处的7.1级赫克托矿地震的影响。如干涉合成孔径雷达数据所示，赫克托矿主震触发了邻近断层的滑动，并可能由于该地区的强烈震动和平均应力变化而导致地壳浅层裂缝张开或合并。我们将在2001-2002年进行重复试验。新的数据将帮助我们确定兰德斯断裂带的愈合过程是否受到赫克托煤矿地震的影响，或者已经变得不可见。我们将比较兰德斯断裂带和赫克托矿断裂带的愈合过程，我们从2000年11月开始监测赫克托矿断裂带。我们将发展一个基于裂纹双稳理论的力学模型来定量解释断层愈合。

Abstract for proposal EAR0105508 (PH # 25x)Title: Continuing Study of the Post-Seismic Fault Healing of the 1992 M7.5 Landers, California, EarthquakePI's: Yong-Gang Li, University of Southern California, and John E. Vidale, University of California, Los Angeles We propose to continue our study of the post-seismic healing process on Landers, California, rupture zone using fault-zone seismic waves excited by explosions detonated within the rupture zone. Repeated surveys in 1994, 1996, 1998, and 2000 have revealed that the shear velocity of the fault zone rock increased by ~1.2% between 1994 and 1996, and increased further by ~0.7% between 1996 and 1998. This trend indicates the Landers rupture zone has been healing by strengthening after the mainshock, most likely due to the closure of cracks that opened during the 1992 earthquake. The observed velocity increase between 1994 and 1998 is consistent with a decrease of ~0.03 in apparent crack density within the rupture zone. We also found that cracks near the fault zone was partially water-filled and became more wet with time. However, the data recorded in the most recent experiment showed that wave velocities in the shallow rupture zone did not increase further more but with a slight decrease for shear velocity between 1998 and 2000, inferring that the fault healing probably became invisible since 6 to 8 years after the 1992 earthquake. However, we speculate that the healing process on Landers rupture zone might have been affected by the M7.1 Hector Mine earthquake on October 25, 1999, occurring only ~25 km east of Landers. The Hector Mine mainshock triggered slip on the neighboring faults as shown in the InSAR data and could cause crack opening or coalescence in the shallow crust due to strong shaking and change in the mean stress in the region. We shall conduct a repeated experiment in 2001-2002. The new data will help us to determine if the healing process on the Landers rupture zone was affected by the Hector Mine earthquake or has become invisible. We shall compare the healing process on the Landers rupture zone with that on the Hector Mine rupture zone for which we are monitoring from November of 2000. We shall develop a mechanical model based on crack dilatancy theory to interpret the fault healing quantitatively.