Routine United States Mining Seismicity : Goal and Scope of the Catalog

"Routine Mining Seismicity in the United States" is an effort to catalog the larger of the many seismic events in the U.S. that are caused by mineral-resource industries in the course of their normal operations. Most of these seismic events are planned explosions; some are planned roof collapses associated with longwall mining.

The USGS/National Earthquake Information Center (USGS/NEIC) is responsible for monitoring the occurrence of earthquakes within the United States. The USGS/NEIC uses seismographic data from the U.S. National Seismographic Network (USNSN) and many other cooperating networks in the United States and throughout the world. USGS/NEIC personnel attempt to obtain estimates of the locations and magnitudes of earthquakes that are large enough to be felt in the U.S. For much of the U.S., earthquakes of magnitude 2.5 or larger are located and cataloged by the USGS/NEIC. In other parts of the U.S., the threshold of earthquakes that can be located and cataloged is higher, due either to the remoteness of those regions from seismographs or due to earthquake levels that are so high that a higher threshold of cataloging still reveals the important features of local seismicity. Within the conterminous U.S., the level of completeness for earthquake location by the USGS/NEIC is probably in the magnitude 3.0 - 3.5 range.

Intentionally detonated mine or quarry explosions and planned roof collapses produce seismic signals that are broadly similar to the seismic signals produced by earthquakes, and the process of monitoring natural seismicity has therefore involved identifying these routinely occurring mining seismic events and ensuring that they are not misidentified as natural earthquakes. The USGS/NEIC has historically dealt with routine mining seismic events by identifying them at an early stage of analysis and removing them from further analysis procedures, so that full effort could be devoted to earthquakes that might reflect seismically hazardous situations. Many of the U.S. regional networks that cooperate with the USGS/NEIC have had the same policy. Now, however, with the U.S. being a signatoree of the Comprehensive Nuclear-Test-Ban Treaty (CTBT), there is an interest in maintaining publically-accessible catalogs of mine seismicity as a CTBT confidence-building measure. The possible impact of mine seismicity on the CTBT is reviewed in a document published by the Department of Energy (Mine Seismicity and the Comprehensive Nuclear-Test_Ban Treaty, Report of a Working Group from Government, Industry, and National Laboratories, Department of Energy Report LA-UR-99-384).

The epicenters listed in the "Routine Mining Seismicity in the United States" catalogs are calculated from the arrival times of seismic phases recorded on seismographs that record at the USGS/NEIC. The epicenters are therefore approximate, with accuracies that depend on the location of the mines with respect to the recording seismographs (see Explanation of Catalog Listings and Mining Seismicity Source Regions for discussions of epicentral accuracies).

The magnitudes assigned to the events in the "Routine Mining Seismicity in the United States" catalogs are calculated from the amplitudes of local and regional seismic phases. The magnitudes assigned to events that occur east of the Rocky Mountains are mbLg magnitudes and should be approximately equal to the mb magnitudes that would be assigned to those events if P-wave readings were available from teleseismic distances. The magnitudes assigned to events that occur in the western U.S. are ML magnitudes; a preliminary analysis suggests that they tend to be somewhat smaller than the mb magnitudes that would be assigned if P-wave readings were available from teleseismic distances. Other studies (e.g., Anandakrishnan and others, 1997; Carr and Garbin, 1998; Khalturin and others, 1998) indicate that neither the mbLg nor ML magnitude provides an accurate measure of the total charge size of a given explosion: the explosions are ripple fired, and the amplitudes of short-period seismic waves generated by a multiple-charge explosion are greatly influenced by the sizes, spacing and timing of individual charges.

In any given region, the magnitude level for essentially complete detection and location of mining associated seismicity will occur at, or slightly above, the magnitude level at which earthquakes are essentially completely detected and located. There are several factors that may make it more difficult to detect/locate mine seismicity than to detect/locate earthquakes.

A. P-waves from mine-explosions tend to be more emersive than P-wave from earthquakes, making automatic P-wave detection more difficult for explosion signals than for earthquake signals.

B. In many mining districts, numerous explosions occur during working hours, making more difficult the automatic association of specific arrival-times with specific epicenters/origin times than is the case with earthquakes.

C. Felt reports are important for detection/location of small earthquakes in regions that are not well-instrumented with seismographs, but mine explosions and planned longwall collapses tend not to be reported as felt.

The above considerations suggest that mine explosions and roof collapses in the magnitude (ML or mbLg) 3.0 - 3.5 range, or slightly larger, could occur in some parts of the conterminous U.S. and escape detection/location. In fact, in Mining Seismicity Source Regions we note a case of a magnitude 3.4 explosion in Minnesota that could not have been located solely with data recorded at the USGS/NEIC, and we note a case of a well-recorded mine explosion in West Virginia, magnitude 3.4, that was not detected by the USGS/NEIC automatic event-detection algorithm, apparently because the algorithm attempted to recognize an event in the signals of more than one explosion. We also note that the number of mining-associated seismic events located in West Virginia and vicinity dropped dramatically in 1999 -- 2000 compared with 1997; this reduction is probably a consequence of a temporary reduction in the number of stations from this region that are recording at the USGS/NEIC. Stations are still being added to the network that records at the USGS/NEIC; for some regions, the new stations will dramatically lower the magnitude level at which explosions are completely detected and located. It must be emphasized, however, that the completeness and accuracy of the catalog in many regions is strongly dependent on the health of local seismograph networks with which the USGS/NEIC is cooperating. The future quality of the catalog will degrade for regions in which local networks are cut back due to lack of funding.

At present, our bulletins, catalogs, and maps cover only the 48 conterminous states of the United States. Mining Seismicity Source Regions lists one mining region in Alaska that has been recognized to detonate explosions that are recorded on regional seismographic stations.

For the period May - September, 1997, the catalog lists epicenters for all suspected mining seismic events for which we had sufficient data to obtain a stable location. These epicenters represent only a small fraction of the total number of mining seismic events detected by seismographs reporting to the USGS/NEIC, because many seismic events are only recorded by one or two seismographs.

For October-November, 1997, we focused cataloging efforts on events for which computed magnitudes were 2.0 or higher and for which stable calculations of the epicenters could be achieved in several computation attempts.

For December, 1997 - April, 1998, we suspended the cataloging of mine seismicity except for a few events of special interest.

May-July, 1998, was a training period for a new analyst; cataloged events were selected for their usefulness in training, rather than according to a magnitude criterion.

From August, 1998, to June, 1999, and from October, 1999 to the present, mining-associated seismic events having magnitude (ML, USGS) 2.5 or greater in the western U.S., or having magnitude (mbLg, USGS) 3.0 or greater in the eastern U.S., are listed if they are automatically detected and located by the USGS/NEIC near-real time system and if, after examination of the data by the analyst, a stable epicenter can be computed. In addition, smaller events are catalogued if, (1), they provide information on the characteristics of a region's mine-associated seismicity that cannot be obtained with the larger events or, (2), their epicenters have been reliably determined in the course of searching for larger events.

For the period July -- September, 1999, we listed only suspected mining-associated events that were reported in the Reviewed Event Bulletin (REB) of the Prototype International Data Center (PIDC).

Beginning October, 2002, we list only suspected mining-associated events that were reported in the Reviewed Event Bulletin (REB) of the International Data Center (IDC).

REFERENCES

Anandakrishnan, S., Taylor, S.R., and Stump, B.W., 1997, Quantification and characterization of reigonal seismic signals from cast blasting in mines: a linear elastic model: Geophysical Journal International, v. 131, p. 45-60.

Carr, D.B., and Garbin, H.D., 1998, Discriminating ripple-fired explosions with high-frequency (> 16Hz) data: Bulletin of the Seismological Society of America, v. 88, p. 963-972.

Khalturin, V.I., Rautian, T.G., and Richards, P.G., 1998, The seismic signal strength of chemical explosions: Bulletin of the Seismological Society of America, v. 88, p. 1511-1524.

 

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