Evidence Used in Identifying Routine Mining Seismicity
USGS/NEIC personnel identify mine explosions and roof collapses induced by longwall mining on the basis of the following evidence:
- Computed locations -- Locations of many of the provisionally identified explosions occur within or near well-known mining districts that have large surface mines, and provisionally identified explosions occur where other similar size events have regularly occurred at the same time of day. Roof collapses that are large enough to be detected and located by our procedure are much less numerous in our catalogs than explosions: they are identified by virtue of occurring in groups in the neighborhoods of some underground coal-mines that use longwall technology.
- Time of day -- Mine explosions tend to be set off during local daytime hours, even if the mines are operating 24 hours a day.
- Seismic waveforms -- Seismograms at a given station for explosions at the same mine tend to be similar from event-to-event, both in the relative times and amplitudes of different seismic phases within each seismogram and in the absolute amplitudes of the seismic phases. Seismograms may have the general characteristics expected for mine explosions -- emergent beginning of phases due to ripple-firing, no S, presence of Rg phase.
- Events not reported as felt -- Calculated magnitudes of seismic events in some mining districts are large enough that, if the events were earthquakes, they would probably have been felt at nearby towns and reported to the USGS/NEIC or to regional seismographic network operators.
- Independent knowledge of operators of regional seismographic networks.
The evidence described above may be regarded as circumstantial , because the USGS/NEIC does not obtain direct confirmation of each explosion or roof collapse from the mine where it occurred. Furthermore, for many events not all types of evidence are available. Finally, several of the above-cited characteristics of mine-explosions and planned roof collapses are also characteristics of unplanned rockbursts, which are earthquakes near mine openings that are caused by unbalancing of rock stresses due to the mining process. For these reasons, it is likely that a small percentage of events that we have labeled as routine mining seismic events are in fact natural earthquakes that are unrelated to mining activities, and another small percentage are probably unplanned rockbursts. In our discussion of "generalized source regions" we note those regions that are known to have significant natural seismicity or unplanned rockburst activity.
Longwall roof-collapses and non-planned mining-associated rockbursts and mine collapses have usually have been reported in regular USGS/NEIC earthquake catalogs if their magnitudes were of a size that they might affect estimates of seismic hazard in the regions in which they occur. Some large and otherwise atypical mining explosions might also be occasionally listed in the regular USGS/NEIC earthquake catalogs. Information on mining related seismic events that have been listed in the regular earthquake catalogs of the USGS/NEIC may be obtained from the ComCat Earthquake Catalog (see also Mining-Induced Events).