The University of Utah Seismograph Stations recorded a magnitude-1.6 seismic event that occurred near the Crandall Canyon Mine at about 6:39 PM MDT, close to the time of the reported accident in the mine that killed and injured rescuers searching for six miners trapped by an Aug. 6 collapse.

Thursday night's seismic event was very shallow--less than one mile from the surface. Depths this shallow are typical of mining- induced seismic events in the coal mining region of eastern Utah, but not of naturally-occurring earthquakes. The seismic waves recorded from this event all began with a downward motion of the earth--as was the case for the magnitude 3.9 event near the mine on Aug. 6 at 2:48 a.m. MDT. This downward motion 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.

As of 3:00 PM MDT on Friday, Aug. 17, the Aug. 6 collapse has been followed by 22 seismic events within a 2-mile radius that were large enough to be located. Twelve of these events occurred within two days of the original collapse. Seven of the 22 events have had magnitudes equal to or larger than the magnitude of Thursday's damaging event. However, the magnitudes of mining-induced seismic events are not necessarily indicative of the amount of damage that they cause to underground mines.

The University of Utah Seismograph Stations has installed five new seismometers near the mine in order to improve the detection threshold and location accuracy of seismic monitoring in the area of the mine. Thursday night's seismic event was detected both by the new seismometers and by part of the university's permanent seismic network.

Coal mining in eastern Utah takes place in an arc-shaped area within the Wasatch Plateau and along the Book Cliffs (see http://quake.utah.edu/MONRESEARCH/CM/overviewofminingseismicity.html). An analysis of decades of seismicity in that area by Walter Arabasz, director of the University of Utah Seismograph Stations, and his colleagues indicates that more than 98 percent of the seismicity in this coal-mining region is caused by mining activity.

A map and table showing all seismic events at the Crandall Canyon mine since Jan. 2, 2007, including those before and after the Aug. 6 collapse, is at: http://quake.utah.edu/MONRESEARCH/CM/cm_update.htm.

A shallow seismic event, magnitude 3.9, was recorded and located by the University of Utah Seismograph Stations at 02:48 AM MDT on August 6, 2007. The event was located in the vicinity of the Crandall Canyon Mine in the Wasatch Plateau coalfield of east-central Utah. Abundant mining-induced seismicity and less frequent natural earthquake activity have been instrumentally recorded in this area since 1962 by the University of Utah's regional seismic network. The largest of past seismic events related to mining activity in this region had magnitudes in the 3.5 to 4.2 range.

The M3.9 event does not have the characteristics of a typical, naturally occurring earthquake. Instead, preliminary observations suggest a shock induced by underground coal mining. Detailed seismological analyses by scientists at the University of California at Berkeley support this interpretation. The findings of the Berkeley team indicate that the source mechanism of the August 6 seismic event is most consistent with the collapse of an underground cavity (http://seismo.berkeley.edu/~peggy/Utah20070806.htm).

Twelve seismic events were recorded by the University's seismic network in the first 38 hours following, and in the vicinity of, the large event of August 6. These smaller events range in magnitude from less than 1.0 to 2.2. A shock of magnitude 2.1 occurred about 17 hours after the main event (at 8:05 PM MDT, August 6); another of magnitude 2.2 occurred about five hours later (at 01:13 AM, August 7). These shocks are interpreted to reflect settling of the rockmass following a cavity collapse.

Seismic monitoring in Utah is conducted by the University of Utah Seismograph Stations in partnership with the U.S. Geological Survey as part of the Advanced National Seismic System.

The preliminary location and magnitude of today's earthquake are consistent with the shock being a type of earthquake that is induced by underground coal mining. The general region of the earthquake's epicenter is an area that has experienced a high level of mining-induced earthquake activity for many decades. The largest of past mining-induced earthquakes had magnitudes in the 3.5 to 4.2 range, which encompasses the size of today's earthquake (3.9). On the basis of present evidence, however, the possibility that today's shock was a natural earthquake cannot be ruled out. The broad region of central Utah experiences normal tectonic earthquakes in addition to mining-induced earthquakes. For example, in 1988 a magnitude 5.2 earthquake occurred 40 km southeast of today's earthquake.

Seismologists have not conclusively determined how the earthquake of August 6 might be related to the occurrence of a collapse at the nearby Crandall Canyon coal mine that, as of midday August 6, had left six miners unaccounted for. The epicenter of the seismic event is close to the mine. We do not have an authoritative report of the time at which the collapse occurred. If the collapse occurred nearly simultaneously with the earthquake, we would consider it likely that the earthquake is the seismic signature of the collapse. At this point, more information-- both from the mine and from more seismological analyses--will be needed to piece together cause and effect relations for today's M3.9 earthquake.

Data Explanation

The seismic events in this table are for the year 2007 only. This table and map will be updated periodically. Data are subject to revision as velocity models and location techniques are refined and as data are added from temporary seismic stations with onsite recorders. Locations on and after August 13 were computed using data from temporary seismic stations above and near the Crandall Canyon mine and with a local velocity model. These locations are likely to be more accurate in absolute location than those for earlier events.

Estimates of magnitude include the computed Richter local magnitude (M_L), when measurable, and/or coda magnitude (M_C), an empirical estimate of M_L calculated from signal durations. For mining-induced earthquakes, M_C tends to be systematically larger than M_L because of the effect of shallow surface waves. M_L is our preferred size estimate and is plotted as the size symbol on the map when available.

In the absence of a closest recording station comparable in distance to the depth of a seismic event, focal-depth resolution is poor, and the computed depths in this table for seismic events occurring before August 13 have large uncertainty. The closest stations at the time of the main seismic event on August 6 were situated 18 km or more away.

Information from UUSS August 15, 2007 (Last Updated Oct 10, 2007)
Date & Time (Local):	2007/08/06 2:48:40 AM (MDT)
Date & Time (UTC):	2007/08/06 08:48:40 AM (UTC)
Location:	21 km ESE of Mount Pleasant, Utah 39.465N, 111.237W
Depth (km):	1.6
Magnitude:	4.2 MW 3.9 ML 4.5 MC

Preliminary Seismological Report on the 6 August 2007 Crandall Canyon Mine Collapse
(poster, AGU Fall 2007 meeting)

"Mining-Induced Seismicity in the Crandall Canyon Coal Mine Area — Overview and Update" (presentation by W.J. Arabasz to Utah Mine Safety Commission, November 20, 2007)

Preliminary Seismological Report (abstract, AGU Fall 2007 meeting)

Overview of Mining-Induced Seismicity in Utah

Moment Tensor Inversion (Univ. of Calif., Berkeley)

Magnitude-Time Plots (2005-2007, May - August 2007)

Preliminary Relocation

Map and Listings of Seismic Events

Portable Seismic Network

Press Updates

FAQs About the Aug 6, 2007 Crandall Canyon Seismic Event

1. How common is seismic activity in central Utah's coalfields?
2. Why do seismologists say that the main seismic event on August 6, 2007, near the Crandall Canyon mine was not a naturally occurring earthquake?
3. Could sudden deformation in a mine cause a seismic event as large as magnitude 3.9?
4. University of Utah seismologists originally reported an "earthquake" at 02:48a.m. MDT on August 6. Did they change their minds?
5. How do the University of Utah and the U.S. Geological Survey fit into the picture of seismic monitoring in Utah?
6. How accurate is the location reported by seismologists for the main seismic event on August 6?
7. Is there any way to improve the accuracy of the location of the main seismic event on August 6?
8. Is anything being done to improve seismic monitoring in the Crandall Canyon mine area?
9. Did seismic activity occur near the Crandall Canyon mine before the August 6 shock?
10. What about the "aftershocks" that occurred following the main seismic event on August 6?

 

 

1. How common is seismic activity in central Utah's coalfields?

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. 

 

2. Why do seismologists say that the main seismic event on August 6, 2007, near the Crandall Canyon mine was not a naturally occurring earthquake?

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.  

 

3. Could sudden deformation in a mine cause a seismic event as large as magnitude 3.9?

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.

 

4. University of Utah seismologists originally reported an "earthquake" at 02:48a.m. MDT on August 6. Did they change their minds?

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.

 

5. How do the University of Utah and the U.S. Geological Survey fit into the picture of seismic monitoring in Utah?

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.”   

 

6. How accurate is the location reported by seismologists for the main seismic event on August 6?

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.

 

7. Is there any way to improve the accuracy of the location of the main seismic event on August 6?

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.     

 

8. Is anything being done to improve seismic monitoring in the Crandall Canyon mine area?

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.

 

9. Did seismic activity occur near the Crandall Canyon mine before the August 6 shock?

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.    

 

10. What about the "aftershocks" that occurred following the main seismic event on August 6?

 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.