Seismological Report on the 6 August 2007 Crandall Canyon Mine Collapse in Utah

Abstract

A large and tragic underground collapse occurred in the Crandall Canyon coal mine in eastcentral Utah on 6 Aug 2007, causing the loss of six miners and attracting national attention. This collapse was accompanied by a local magnitude (M L) 3.9 seismic event having a location and origin time coincident with the collapse, within current uncertainty limits. Two lines of evidence indicate that most of the seismic wave energy of this event was generated by the mine collapse rather than a naturally-occurring earthquake: (1) the observation that all of the observed P-wave first motion directions are down and (2) the results of a moment tensor inversion by Ford et al. (2008). We propose one possible model for the collapse that has dimensions of 920 m E-W by 220 m N-S and an average roof-floor closure of 0.3 m. This model is consistent with the seismic moment, volumetric constraints on the amount of closure, available underground observations, and our best location for the M L 3.9 epicenter. This epicenter is near the western end of our proposed collapse area, suggesting that the collapse propagated mostly eastward from its initiation point. Our locations for the M L 3.9 event and for other seismic events that occurred in the area before and after it were greatly improved by the use of a double difference method and data from a 5-station temporary network that the University of Utah deployed near the mine beginning on 8 Aug. The Crandall Canyon Mine is in an area of Utah where there is abundant mining-induced seismicity, including events with both collapse and shear-slip sources. Prior to the 6 Aug 2 collapse, and within a 3 km radius of it, there were 28 seismic events during 2007 that were large enough to be detected and located as part of the routine data processing for the University of Utah regional seismic network: 8 in the 2.5-week period prior to the collapse (M L ≤ 1.9) and 15 during an earlier period of activity in late February and early March (M L ≤ 1.8). These events occurred primarily in areas where there was concurrent or recent mining activity. By the end of August, the 6 Aug collapse had been followed by 37 locatable seismic events of M L ≤ 2.2, which clustered near the eastern and inferred western ends of the collapse area. One of these "aftershocks" (M L 1.6) occurred in conjunction with the violent burst of coal from the mine walls on 17 Aug (UTC) that killed three rescuers and injured six others. The aftershocks have an exponential frequency-magnitude distribution with a lower ratio between the frequencies of smaller-and larger-magnitude events (lower b-value) than for the prior events in the area. Aftershock rates generally decreased with time through August. However, there was a noteworthy 5.8-day hiatus in activity, above a completeness threshold of coda magnitude (M C) 1.6, that began 37 hours after the collapse.

A Catalog of Regional Moment Tensors in Utah from 1998 to 2011

“…One solution is for an event in southwestern Wyoming on 30 January 2000 (number 10, Ⓔ Table S1 in the supplement, M w 4.3, F -test 100%), which is associated with a roof fall (collapse) involving three room and pillar sections in an underground trona mine (McCarter, 2001). The other solution (number 32, Ⓔ Table S1 in the supplement, M w 4.2, F -test 100%) is for the well-documented August 2007 Crandall Canyon coal-mine collapse (see Pechmann et al, 2008). Our solution for the Crandall Canyon event is similar to that of Ford et al (2008).…”

“…Before completing the F -test, we suspected that a third event (number 11, Ⓔ Table S1 in the supplement) might also have a significant isotropic component because it occurred within the Willow Creek coal mine, closely in time with seven reported roof falls (Ellenberger et al, 2001;McCarter, 2001). This is one of the largest seismic events associated with underground coal mining in Utah (see Pechmann et al, 2008). The full moment tensor solution has a relatively large isotropic component of 21% (not shown in Ⓔ Table S1 in the supplement as the deviatoric solution is the only one reported).…”

Recommendation for the discrimination of human-related and natural seismicity

“…single or few significant earthquakes (e.g. Ottemöller et al, 2005;Pechmann et al, 2008, Cesca et al, 2011, swarms of weak, shallow earthquakes concentrated in space and time (e.g. Häring et al, 2008;Cuenot et al, 2008), slow slip events (slow earthquakes) with emergent ground motions (e.g.…”

Fine Details of Mining-Induced Seismicity at the Trail Mountain Coal Mine Using Modified Hypocentral Relocation Techniques

“…MIS that is relocated using the double-difference method can be positioned in absolute space if there is ground truth (Pechmann et al, 2008) or by using the mine layout and sequencing Sonley et al, 2009). For this analysis, the hypoDD locations are shifted relative to the position of the solution from the modified master event locations that was best located using the criteria from the master event technique.…”

“…Recent analyses of both tectonic earthquakes (e.g., Waldhauser and Ellsworth, 2000) and MIS (e.g., Pankow et al, 2008;Pechmann et al, 2008;Rudziński and Dębski, 2012) have demonstrated the ability of enhanced location techniques, such as master event and double-difference methodologies, to improve hypocentral resolution even with velocity model uncertainties and less-than-perfect station distributions. In this study, a variation of the master event methodology and double-difference relocation (Waldhauser and Ellsworth, 2000) with temporal clustering were applied to improve hypocentral resolution of MIS at the TMM.…”