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An Induced Earthquake Experiment in Deep Gold Mines—Natural Earthquake Laboratory in South African Mines: Margaret S. Boettcher


Project Title: An Induced Earthquake Experiment in Deep Gold Mines—Natural Earthquake Laboratory in South African Mines
Mendenhall Fellow: Margaret S. Boettcher, (650) 329-4878, mboettcher@usgs.gov
Duty Station: Menlo Park, CA
Start Date: January 9, 2006
Education: Ph.D., Geophysics, MIT/WHOI Joint Program, 2005
Research Advisors: Malcolm Johnston, 650) 329-4812, mal@usgs.gov; Art McGarr, (650) 329-5645, mcgarr@usgs.gov; Colin Williams, (650) 329-4881, colin@usgs.gov; David Lockner (650) 329-4826, dlockner@usgs.gov
  Margaret S. Boettcher


Project Summary: The Natural Earthquake Laboratory in South African Mines (NELSAM) provides a unique opportunity to access and monitor typically inaccessible seismogenic faults.  The most critical questions concerning earthquake predictability, such as whether the source processes of large and small earthquakes are the same and whether or not precursory signals exist,  can best be addressed with high-quality near-source recordings of seismic radiation, strain, and displacement.  While it is extremely difficult to obtain near-source records in most seismogenic regions, the South African mines allow for direct access to the likely nucleation zone of a near-future magnitude 2 to 3 earthquake.  In addition, mining-induced stresses guarantee earthquake occurrence on a daily basis, making hazard a prime concern in such a seismically active work environment.

By exploring two main research areas: earthquake nucleation and rupture propagation, we will target questions of how the earthquake source scales with the full event size. A primary goal of this project is to observe the presence or absence of a fault preparation process that may precede an earthquake. A second goal is to compute seismic radiated energy, which, obtained directly from the velocity power spectra, is a measure of an earthquakes’ potential for damage and offers a second measure of earthquake size independent of the seismic moment. The scaling of radiated energy with moment is integral to our understanding of how processes observed in laboratory friction experiments compare with those of destructive earthquakes. A related, third goal is to evaluate the scaling of apparent stress, which is proportional to the ratio of radiated energy to seismic moment. Apparent stress is a key indicator of how rupture propagation occurs.  If a constant value of apparent stress is observed over many orders of magnitude in earthquake moment, then rupture propagation in small and large earthquakes is inferred to be the same.  On the other hand, if apparent stress is found to increase with moment, then fault friction is thought to drop during rupture of large events due to mechanisms such as fault zone melting or fluid pressurization. 

Using this unique dataset to address the above topics, we aim to link observations of natural tectonic earthquakes and those from laboratory experiments and further our understanding of earthquake source processes.
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