Regional Moment Tensor Solutions for Source-Type Identification: The Crandall Canyon Mine Collapse

Regional Moment Tensor Solutions for Source-Type Identification: The Crandall Canyon Mine Collapse Fig 1.
A) Source type plot showing separation of earthquake, explosion and collapse (yellow star shows the 08/06/2007 mine-collapse event). B) Location of the 08/06/2007 mine-collapse event and the 6 closest stations. C) Observed (black) and predicted (red) displacement waveforms (0.02 to 0.1 Hz).
Seismic moment tensor methods are now routinely applied at many scales from the study of micro-earthquakes to the characterization of damaging great earthquakes, and applied at regional distances they are proving to be a reliable tool for distinguishing between earthquakes and events associated with volcanic processes [e.g. Dreger et al., 2000], and other man-made sources of seismic radiation such as from explosions, or mining activity [e.g. Ford et al., 2009]. On August 6, 2007 a magnitude 3.9 seismic event was associated with the tragic collapse of a Utah coal mine. The event was well recorded by UUSS, USGS, and Earthscope USAarray stations (Fig. 1) The moment tensor inversion of complete, three-component, low-frequency (0.02 to 0.10 Hz) displacement records recovers a mechanism that is most consistent with the gravity driven vertical collapse of a horizontally oriented underground cavity (Fig. 1a). The seismic moment tensor of the event, is comprised 78% from a closing horizontal crack and a secondary 22% non-crack component, which results from the fitting of the large amplitude Love waves that were observed on the tangential component (Fig. 1c). Plausible interpretations of the secondary source include sympathetic vertical dip-slip faulting, non-uniform crack closure, and elastic relaxation in response to the mine collapse [Ford et al., 2008]. The moment tensor solution produces a pure dilatational P-wave first-motion mechanism consistent with the P-wave polarity observations [Pechman et al., 2008]. The source-type diagram [Hudson et al., 1989] in Fig. 1a illustrates the deviation from a pure earthquake double-couple (DC) source at the center in terms of a volumetric component (explosion or implosion) on the ordinate, and deviatoric component in terms of a volume compensated linear vector dipole (CLVD) on the abscissa. The mine- collapse event plots in the region of a negative or closing crack similar to solutions obtained for other mine and Nevada Test Site (NTS) cavity collapses [Ford et al., 2009]. The application of seismic moment tensor analysis to non-tectonic seismic events such as buried explosions or underground collapses as illustrated here, demonstrates the feasibility of continuous monitoring of regional distance seismic wavefields for source-type identification useful for nuclear explosion monitoring and possible emergency response.
</p><p>Dreger, D. S., H. Tkalcic, and M. Johnston (2000). Dilational processes accompanying earthquakes in the Long Valley Caldera, Science, 288, 122-125.
</p><p>Ford, S., D. Dreger and W. Walter (2008). Source Characterization of the August 6, 2007 Crandall Canyon Mine Seismic Event in Central Utah, Seismol. Res. Lett., 79, 637-644.
</p><p>Ford, S. R., D. S. Dreger, and W. R. Walter (2009), Identifying isotropic events using a regional moment tensor inversion, J. Geophys. Res., 114, B01306.
</p><p>Acknowledgements: We acknowledge DOE BAA contract DE-FC52-06NA27324 (BSL) and Contract W-7405-Eng-48 (LLNL) for support of this work.</p>


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