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

摘要建议EAR0105508(PH#25x)标题：1992年加利福尼亚州兰德斯地震后断层修复的继续研究，地震：南加州大学的李永刚和加州大学洛杉矶分校的约翰·E·维代尔我们建议继续研究加州兰德斯破裂带的地震后愈合过程，使用在破裂带内爆炸的断裂带地震波。1994年、1996年、1998年和2000年的多次调查表明，断裂带岩石的剪切速度在1994-1996年间增加了约1.2%，在1996-1998年间进一步增加了约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所致。1994-1998年间观测到的速度增加与断裂带内视裂纹密度降低约0.03相一致。我们还发现，断层附近的裂缝部分充满了水，并随着时间的推移变得更加潮湿。然而，最近一次实验记录的数据表明，在1998年至2000年期间，浅层破裂带的波速并没有进一步增加，而是略有下降，推测在1992年地震后的6-8年内，断层的愈合可能是看不见的。然而，我们推测1999年10月25日发生在兰德斯以东仅25公里的赫克托矿7.1级地震可能影响了兰德斯破裂带的愈合过程。InSAR数据显示，赫克托矿主震在邻近断层上触发了滑动，并可能由于该地区强烈的震动和平均应力的变化而导致地壳浅层裂隙张开或合并。我们将在2001-2002年进行一次重复的实验。新的数据将帮助我们确定兰德斯破裂带的愈合过程是否受到赫克托矿地震的影响，或者已经变得不可见。我们将把兰德斯破裂带的愈合过程与我们从2000年11月开始监测的赫克托地雷破裂带的愈合过程进行比较。我们将建立一个基于裂纹扩容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.