FAQs about the Aug 6, 2007 Crandall Canyon Seismic EventAugust 15, 2007
- How common is seismic activity in central Utah's coalfields?
- Why do seismologists say that the main seismic event on August 6, 2007, near the Crandall Canyon mine was not a naturally occurring earthquake?
- Could sudden deformation in a mine cause a seismic event as large as magnitude 3.9?
- University of Utah seismologists originally reported an "earthquake" at 02:48a.m. MDT on August 6. Did they change their minds?
- How do the University of Utah and the U.S. Geological Survey fit into the picture of seismic monitoring in Utah?
- How accurate is the location reported by seismologists for the main seismic event on August 6?
- Is there any way to improve the accuracy of the location of the main seismic event on August 6?
- Is anything being done to improve seismic monitoring in the Crandall Canyon mine area?
- Did seismic activity occur near the Crandall Canyon mine before the August 6 shock?
- What about the "aftershocks" that occurred following the main seismic event on August 6?
Hundreds to thousands of seismic events induced by underground coal mining in the Wasatch Plateau and Book Cliffs coalfields of east-central Utah are large enough to be located each year by the University of Utah’s regional seismic network (click here for a seismicity map and overview). Most are smaller than magnitude 3; the majority, smaller than magnitude 2.5. During 2006, more than 2200 such events were recorded and located.
An early clue was that seismic recordings made by vertically-oriented sensors throughout the University of Utah’s network all began with a downward (dilatational) motion. Recordings of naturally occurring earthquakes resulting from slip on a fault should include a mix of both upward (compressional) and downward first motions in a predictable pattern. Click here for a more detailed explanation. Observing all downward first motions is consistent with a type of deformation in a mine in which the roof and floor of part of the mine suddenly move toward each other as the space between them closes, either partially or totally. Depending on the distribution of recording stations, first-motion information alone may not conclusively determine what kind of source produced the observations.
A good diagnostic tool for determining the source mechanism of seismic events involves the analysis of long-period seismic waves recorded on high-quality digital seismographs. A team of seismologists at the University of California at Berkeley, consulting with seismologists at the University of Utah and at the U.S. Geological Survey, completed such a study for the August 6, 2007, seismic event. The findings of the Berkeley team (click here for report) show that most of the recorded seismic wave energy is consistent with a source mechanism involving an underground collapse, but not with a tectonic earthquake source.
Seismic events as large as magnitude 4.2 have been related to underground mining in Utah’s coalfields. In 1981, a collapse-type failure in a room-and-pillar mine located 12 km (7 miles) east-northeast of the Crandall Canyon mine produced a magnitude 3.8 seismic event. In 1995, a collapse in a trona (sodium evaporite) mine in southwestern Wyoming produced a magnitude 5.2 seismic event. Click here for a University of Utah report that includes a review of larger mine tremors in Utah.
When energy is suddenly released in the Earth, producing seismic waves that shake the ground or make vibrations recorded by seismographs, seismologists generally report the result as an “earthquake.” (This term is not used for seismic recordings of an explosion or a sonic boom.) Shocks caused by underground mining and related to energy release at mine openings or in the surrounding rockmass are commonly referred to as “mining-induced earthquakes.” Using the generic term “seismic event” when referring to energy release caused by underground mining may help avoid confusion with naturally occurring earthquakes.
Seismic monitoring in Utah is conducted by the University of Utah Seismograph Stations (UUSS) in partnership with the U.S. Geological Survey (USGS) as part of the Advanced National Seismic System (ANSS). The UUSS maintains and operates more than 160 regional and urban seismic stations in the Utah region (click here for map) with funding from the state of Utah and the USGS/ANSS. There is continuous data exchange between the UUSS network operation center on the University of Utah campus in Salt Lake City and the USGS National Earthquake Information Center in Golden, Colorado. The latter provides 24/7 backup to UUSS as part of the ANSS. UUSS and USGS seismologists coordinate closely in responding to significant earthquakes in the Utah region. For virtually all earthquake locations in Utah that are posted on the USGS earthquake Web site, the source of the earthquake details is identified as the “University of Utah Seismograph Stations.”
Due to the relatively large spacing of seismographs in the vicinity of the Crandall Canyon mine at the time of the August 6 seismic event, the estimated uncertainty (95% confidence bounds) in the computed epicenter or map location is ±0.8 km (±0.5 mile). There may be added uncertainty due to unknown variations in the seismic velocities of the Earth’s crust. In computing the depth of the event, there is even greater numerical uncertainty, which depends on the distance to the nearest seismic instruments. The initially reported depth was 1.6 km (1.0 mile). Any depth shallower than 6.0 km (3.7 miles) fits the data equally well.
Seismologists at the University of Utah are using various tools to try to refine the location of the main seismic event and other seismic events in the vicinity of the Crandall Canyon mine. Some techniques help improve the relative location of seismic events with respect to each other. Other techniques aim to find correction terms for the travel times of seismic waves to recording stations so that the absolute location of individual seismic events can be improved (click here for map). In the case of the Crandall Canyon seismic event, one aim will be to evaluate the significance of any differences between “ground truth," namely the location and time of the mine collapse (as eventually verified), and corresponding seismological information for the source of the largest seismic event on August 6.
Between August 7 and 9, 2007, UUSS field personnel installed five portable seismographs above and near the Crandall Canyon mine (click here for more information). While they are operating, these instruments will improve the detection threshold and location accuracy of seismic monitoring in the area of the mine. Data from these instruments will also help calibrate the regional network to refine retroactively the location of the main seismic event and its early after-events. Seismic data from three UUSS instrument packages are being continuously telemetered to the University of Utah campus and integrated into our UUSS network recording. Data from two other instruments, provided by the USGS, are only being recorded onsite, and the recordings have to be physically retrieved.
The earthquake catalog generated by the University of Utah’s regional seismic network provides a continuous record of mining-related seismicity, above a certain size, in the Wasatch Plateau–Book Cliffs coalfields. Figure A is a plot of magnitude versus time for 152 seismic events in the catalog located within 3.0 km (1.9 miles) of the inferred location of the damaged part of the Crandall Canyon mine and occurring during a two-year period preceding the August 6 shock. Such a plot cannot be properly evaluated without independent information regarding details of mining operations within the mine. The number of seismic events is not in itself unusual for an active mine in the Wasatch Plateau–Book Cliffs area.
Figure B is an expanded plot for seismic events in Figure A occurring after May 1, 2007. Events that occurred after the main event on August 6 and through August 14 have been added.
During the first 38 hours after the main seismic event at 02:48 a.m. MDT on August 6, twelve subsequent seismic events as large as magnitude 2.2 were recorded and located by the University of Utah’s regional seismic network in the vicinity of the Crandall Canyon mine. Relatively small magnitude seismic events continue to occur more than a week after the main event. Individually and collectively, these seismic events show no indication of being tectonic earthquakes. Just as for the main seismic event, discernible first motions are consistently down. Focal depths, computed with the benefit of seismic instruments temporarily installed near the mine after the main event, are shallow. Available evidence suggests that these events are occurring at or very close to mine level and reflect settling of the rockmass following the August 6 seismic event.