RAPID: Monitoring Deep Crustal Earthquake Sequence, Sierraville, California

该项目将建立一个临时的遥测宽带阵列，以监测2001年8月在加利福尼亚州塞拉谷南部开始的一个充满活力和独特的莫霍面深度地震序列。该序列被限制在30-36公里深，与2003年在塞拉山谷东南约45公里处的北塔霍湖下发生的地震序列非常相似。到目前为止，在2011年的塞拉山谷序列中已经发生了大约1000次地震。这两个层序的高分辨率事件位置表明，它们都位于~N45W~50 NE向倾斜构造上；沿该构造已发现一个长周期(LP)事件，但都不在两个震群位置。现在在第二个序列中观察到的这一过程，被解释为通过注入较低密度的熔体而导致高强度上地幔莫霍面盖层的脉动和减弱，该熔体界定了内华达山脉微板块与盆地和山脉之间的基本边界。在这两个层序中，地震活动被限制在5-6公里的深度范围内，岩浆在两个层序中以非常相似的速度向上推进。活动的周期性增加很可能与整个堤防过程中的强化注水有关。新的数据提供了对锡尔兰地块内莫霍面深度震群的最全面的测量。10月26日发生的4.7级地震，整个地区都有震感，发生在15公里深的脆韧性转换深度，就在深部震群的正上方。根据与2003年深部震群有关的上地壳地震活动，我们可以预期会有额外的浅层地震活动；浅层触发地震活动要么与流体从深部传播有关，要么与莫霍面深度应变事件有关的形变。宽带仪器已经就位，以捕捉可能在额外的4.5级地震之前的浅层地壳活动。高密度宽带数据补充了区域监测和当地全球定位系统测量，以解决关于塞拉利昂北部地区内华达山脉微板块边界及其演化的基本问题。在2003年的序列中没有宽带站，2011年增加的站密度和传感器带宽提供了更好地观测与震群有关的莫霍面和下地壳火山过程的数据。该地区的几条断层有可能发生7级地震。对这些过程的详细观察为内华达大学雷诺分校、美国地质调查局和加州紧急事务管理署在为加州东部地区的公民提供准备和测量通知措施方面的协调努力提供了基础。

The project is to establish a temporary telemetered broadband array to monitor an energetic and unique Moho-depth earthquake sequence that began in southern Sierra Valley, California, in August 2001. The sequence is confined to 30-36 km depths and is remarkably similar to a sequence of earthquakes under North Lake Tahoe in 2003, about 45 km southeast of Sierra Valley. To date, ~1000 earthquakes have been located in the 2011 Sierra Valley sequence. High-resolution event locations from both sequences show that each lies on a ~N45W ~50 NE dipping structure; one long-period (LP) event has been identified along the structure, yet was not within either swarm location. The process, now observed in a second sequence, is interpreted to represent diking and weakening of a high strength upper mantle Moho cap by injection of a lower density melt defining a fundamental boundary between the Sierra Nevada microplate and the Basin and Range. In both sequences, seismicity is confined to a 5-6 km depth range and magma progresses upward at very similar rates in both sequences. Periodic increases in activity are most likely related to enhanced injection within the overall diking process. The new data provides the most comprehensive measurements of Moho-depth earthquake swarms within the Sierran block. An October 26th Mw 4.7 earthquake, felt throughout the region, occurred at 15 km depth, the brittle-ductile transition depth, directly above the deep swarm. Based on upper crustal seismicity associated with the 2003 deep swarm, we could expect additional shallow seismicity; the shallow triggered seismicity is related either to fluids propagating from depth or deformation associated a Moho depth strain event. Broadband instrumentation is in place to capture shallow crustal activity that may precede additional M 4.5 earthquakes. High-density broadband data complement regional monitoring and local GPS measurements to address fundamental questions about and evolution of the Sierra Nevada microplate boundary in the northern Sierra region. No broadband stations were in place during the 2003 sequence, and increased station density and sensor bandwidth in 2011 provides the data to better observe Moho and lower crustal volcanic processes associated with the swarm. Several faults in the region are capable of M ~7 earthquakes. Detailed observations of the processes provides the basis for coordinated efforts between the University of Nevada Reno, the U.S. Geological Survey, and California Emergency Management Administration in preparedness and gauged notification measures for citizens in the eastern California area.