RAPID: Monitoring Deep Crustal Earthquake Sequence, Sierraville, California

Board Of Regents, Nshe, Obo University Of Nevada, Reno, Reno NV

Investigators

Abstract

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.