HR: 13:55h
AN: S33D-02 INVITED    [Abstracts]
TI: Constraints on Seismogenesis of Small Earthquakes (-2.8 ≤ M_\rm{W} ≤ 3) Defined by the Dense Instrument Network of the Natural Earthquake Laboratory in South African Mines (NELSAM)
AU: * Boettcher, M S
EM: mboettcher@usgs.gov
AF: U.S. Geological Survey, USGS MS #977, 345 Middlefield Rd, Menlo Park, CA 94025, United States
AU: McGarr, A
EM: mcgarr@usgs.gov
AF: U.S. Geological Survey, USGS MS #977, 345 Middlefield Rd, Menlo Park, CA 94025, United States
AU: Johnston, M
EM: mal@usgs.gov
AF: U.S. Geological Survey, USGS MS #977, 345 Middlefield Rd, Menlo Park, CA 94025, United States
AU: Jordan, T H
EM: tjordan@usc.edu
AF: Southern California Earthquake Center, Department of Earth Sciences University of Southern California, Los Angeles, CA 90089, United States
AU: Heesakkers, V
EM: heesakkers@ou.edu
AF: University of Oklahoma, College of Earth and Energy School of Geology and Geophysics 100 East Boyd Street Suite 810, Norman, OK 73019, United States
AU: Reches, Z
EM: reches@ou.edu
AF: University of Oklahoma, College of Earth and Energy School of Geology and Geophysics 100 East Boyd Street Suite 810, Norman, OK 73019, United States
AB: The Natural Earthquake Laboratory in South African Mines (NELSAM) is designed to investigate the nucleation and rupture propagation of small (-2.8 ≤ M_\rm{W} ≤ 3) earthquakes in quartz host rock at seismogenic depths. Acceleration, velocity, and strain meters are installed at a depth of 3.6 km within and surrounding the ancient Pretorius Fault zone in TauTona Mine, South Africa, which is a site of enhanced seismicity. Data from the 9 NELSAM instruments, recorded at 12 kHz, are supplemented by recordings from the mine-operated geophone arrays, which include ~25 stations in TauTona Mine and ~30 stations in nearby Mponeng Mine. This dense seismic network allows us to resolve detailed waveforms from earthquakes located meters to hundreds of meters from the seismometers and investigate their source properties. Additionally, we have obtained an extensive catalog from the mine-operated network that includes 733,000 earthquakes M_\rm{W} ≥ -2 that occurred between 1995 and 2007. Using recent recordings from the NELSAM dataset in conjunction with catalog data from the mine-operated network, we address questions of how to distinguish between the multiple populations of mining-induced seismicity and whether a minimum magnitude, M_\rm{min}, exists for the population thought to be most similar to tectonic earthquakes. During quiet hours in 2006-2007 with no ore production, we find tectonic-like earthquakes with magnitudes as low as -2.8 and a catalog completeness threshold of -1.2. Thus, we find that catalog detection limitations, not earthquake source physics, control the apparent M_\rm{min} of "tectonic" earthquakes. Furthermore, assuming that laboratory results (Lockner and Okubo, 1983) from similar sized events in faults of similar composition can be directly applied to the mining environment, and that earthquake nucleation processes are dependent on the fault zone parameters, we estimate that earthquake nucleation in the mines entails a moment release equivalent to M_\rm{W} ≤ -3. The nucleation patch radius and critical slip distance are estimated to be ~0.2 m and ~20 microns respectively, consistent with data from a magnitude -2.8 earthquake recently recorded in TauTona Mine. This study was supported by NSF Continental Dynamic grant 0409605.
DE: 7203 Body waves
DE: 7215 Earthquake source observations (1240)
DE: 7294 Seismic instruments and networks (0935, 3025)
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Seismology [S]
MN: 2007 Fall Meeting