HR: 0800h
AN: S41B-0962 INVITED     [Abstracts]
TI: Modes of Rupture and Fault Maturation Processes in South African Gold Mines
AU: * Richardson, E
EM: eliza@geosc.psu.edu
AF: Penn State University, 406 Deike Bldg, University Park, PA 16802 United States
AU: Nyblade, A A
EM: andy@geosc.psu.edu
AF: Penn State University, 406 Deike Bldg, University Park, PA 16802 United States
AB: Deep mines are the best environment for studying small events (moment magnitude $-2 \le M \le 3.5$) whose dimensions are at or below the detection threshold of most surface-based seismic arrays. Mining-induced seismicity includes events directly triggered by blasting that are assumed to involve fresh fracturing of rock as well as those induced over longer timescales that have been hypothesized to be dominated by frictional slip. These distinctions have so far been based on spatio-temporal clustering statistics and spectral signatures of these two types of events. We have inverted for the moment tensors of 17 events from the Far West Rand gold-mining region of South Africa that range in size from $1.0 \le M \le 3.2$. These events occurred at 1-4 km depth and were recorded locally by four networks of 102 three-component geophones installed at depth throughout the active mining environment as well as regionally by a two-year Passcal deployment of 80 broadband seismometers. The moment tensors of these events are consistent with purely double-couple solutions. Therefore, we assert that these events are indeed proxies for natural tectonic earthquakes that nucleate via friction-dominated slip on planar surfaces. For each fault that produced one of these 17 events, we have constructed frequency-magnitude curves that span two to five years of seismicity on that particular fault. Since mine operations force seismic activity at a relatively fast rate, we can use this information to study the evolution of faults. Specifically, our study shows that many faults underground mature over time. They begin as individual patches that produce little seismicity or fresh-fracturing seismicity alone and gradually become large faults capable of producing large friction-dominated events. We hypothesize that this process is tied to the evolution of fault gouge and repeated slip events on these faults.
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting