HR: 11:20h
AN: S12B-05    [Abstracts]
TI: Micro- Nano- and Picoearthquakes at SAFOD: Implications for Earthquake Rupture and Fault Mechanics
AU: * Ellsworth, W L
EM: ellsworth@usgs.gov
AF: U. S. Geological Survey, MS 977, Menlo Park, CA 94025, United States
AU: Hickman, S H
EM: hickman@usgs.gov
AF: U. S. Geological Survey, MS 977, Menlo Park, CA 94025, United States
AU: Zoback, M D
EM: zoback@stanford.edu
AF: Stanford University, Department of Geophysics, Stanford, CA 94305, United States
AU: Imanishi, K
EM: imani@ni.aist.go.jp
AF: Geological Survey of Japan, AIST, Tsukuba, 305-8567, Japan
AU: Thurber, C H
EM: clifft@geology.wisc.edu
AF: University of Wisconsin, Department of Geology and Geophysics, Madison, WI 53706, United States
AU: Roecker, S W
EM: roecks@rpi.edu
AF: Rensselaer Polytechnic Institute, Department of Earth & Environmental Science, Troy, NY 12180, United States
AB: Seismometers in the main borehole of the San Andreas Fault Observatory at Depth (SAFOD) recorded a recurrence of the Mw 1.8 "Hawaii" repeating earthquake at 100 m range on August 11, 2006. We did not observe a Slow Initial Phase within the bandwidth of the seismometer (15 – 1600 Hz velocity response). The possible presence of a Seismic Nucleation Phase could not be determined due to the rapid clipping of the P wave on the recording system. Despite the clipping, a dynamic stress drop of 4 - 7 MPa during the first 0.5 - 1.0 ms of rupture could be measured using the Kostrov solution for a dynamically growing circular crack. This is comparable to the average stress drop for the entire event of 10 MPa determined with seismograms from nearby PASO surface and HRSN shallow borehole stations. An aftershock sequence of extremely small events was also observed in the SAFOD main hole, but went undetected in the SAFOD pilot hole, and at the HRSN and PASO stations. The first detected aftershock was noted as soon as the seismograms returned on scale at +2.5 s, and the activity rate declined following Omori's Law. Aftershock moment magnitudes range from M -1.9 (nanoearthquake) to below M -3.5 (picoearthquake). Because their corner frequencies lie above the pass band of the recording system, other source parameters could not be measured. Aftershock locations determined using the P-wave polarizations and S-P intervals (16 - 23 ms) define a northwest trending zone located 100 m below the borehole crossing at a depth of 2.7 km. Their very narrow depth range suggests that they define a "streak" as has been commonly observed along creeping faults in California. The aftershocks coincide with shallower of the two faults that have deformed the casing in the SAFOD main hole at 3190 m and 3300 m, measured depth. The 3190 m fault forms the southwest boundary of a 200-m- wide low velocity, low resistivity zone centered on the 3300 m fault. The "Hawaii" mainshock and its aftershocks behave like any normal crustal earthquake sequence occurring on a mature fault. They are just extremely small. The stress drop in the mainshock, beginning with the initiation of rupture, agrees well with measurements of neary earthquakes. By the time the rupture was no more than 1-2 m in length the dynamic stress release was nearly equal to the total average stress drop, suggesting crack-like behavior throughout the rupture process. Although source parameters of the picoearthquakes in the aftershock sequence could not be measured directly, their small moments and the 1600 Hz bound on corner frequency require that the smallest involve no more than a fraction of a mm of displacement. There appears to be nothing in these seismograms that contradicts laboratory-derived values for fault strength or slip-weakening distance.
DE: 7209 Earthquake dynamics (1242)
DE: 7215 Earthquake source observations (1240)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Seismology [S]
MN: 2007 Fall Meeting