The magnitude 7.3 Landers earthquake of 28 June 1992 triggered a remarkably sudden and widespread increase in earthquake activity across much of the western United States. The triggered earthquakes, which occurred at distances up to 1250 kilometers (17 source dimensions) from the Landers mainshock, were confined to areas of persistent seismicity and strike-slip to normal faulting. Many of the triggered areas also are sites of geothermal and recent volcanic activity. Static stress changes calculated for elastic models of the earthquake appear to be too small to have caused the triggering. The most promising explanations involve nonlinear interactions between large dynamic strains accompanying seismic waves from the mainshock and crustal fluids (perhaps including crustal magma).

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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.