Update on Crandall Canyon Mine Seismic Activity
August 17, 2007, 04:00 p.m. (MDT)
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.
Press Release August 9, 2007, 05:00 p.m. (MDT)
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.
Press Release August 6, 2007, 03:40 p.m. (MDT)
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.