Correlation of Earthquake Locations with Volumetric Source Components in TauTona Gold Mine, South Africa
Abstract
We investigate the source characteristics of earthquakes in TauTona Gold Mine, South Africa, to test if the location of earthquakes relative to mining structures is correlated with significant isotropic source behavior. Earthquakes are well monitored in TauTona Mine, where underground near-source stations record smaller events and higher frequency energy than can generally be observed using surface stations. Our dataset includes -4 < Mw < 4 earthquakes recorded at hypocentral distances of tens of meters to a few kilometers. The locations of structures in the mine, including faults, dikes, tunnels, and stopes, are well known from detailed geologic mapping and surveyed mine plans. We use data collected between 2004 and 2009 from the in-mine array (1-6 kHz), the Natural Earthquake Laboratory in South African Mines (NELSAM) project stations (6-12 kHz), and a short-term PASSCAL experiment (200 Hz) to study source mechanism variability and correlation with mapped structures within the mine. Previous studies of earthquakes in mines suggest a relationship between earthquake size and isotropic moment tensor source characteristics. In TauTona Mine, earthquakes with significant implosive source characteristics tend to be infrequent, larger events (Mw > 1.5), whereas earthquakes with significant explosive source characteristics tend to be smaller (Mw < 0). A possible model for this variability in source behavior relates earthquake size to earthquake location relative to mining and natural structures. Larger events are more likely to produce closure of tunnels and stopes within the mine, whereas the smallest recorded explosive events can be interpreted as opening cracks that form at the edges of mining structures. Double-couple type sources occur throughout the full magnitude range, and are often located along mapped faults and dikes. We focus our analysis on earthquakes located near the NELSAM stations in the deepest part of the mine, and on earthquakes located at depths greater than the current extent of mining. High precision relative hypocenter relocation performed using the 3-D extensive seismic array provides excellent constraints on location of events. We compute full moment tensor solutions for events using amplitude measurements of individual arrival phases, including nearfield phases. By focusing on these two sets of events, within and below the mine, we are able to test whether earthquake location relative to structures has a direct effect on isotropic source behavior. This model predicts that events within the mine will exhibit stronger isotropic source components compared with events beneath the mine that are expected to predominantly exhibit double-couple source mechanisms.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.S43A2483K
- Keywords:
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- 7215 SEISMOLOGY Earthquake source observations;
- 7299 SEISMOLOGY General or miscellaneous