This review describes seismic activity caused by underground mine operations. Several links to sources of information and publications are given. Regulations, theory and arguments concerning mine-induced seismicity are also discussed.
Mine-induced seismicity is an ever present problem in most mining areas where there are deep mines and local populations. Rock bursts occur and the mining causes earthquake-like events that collapse mine workings, kill miners, and damage structures on the surface. In this technology review we touch on the subject and link to some of the many fine writings freely available on the web.
If you know more, or can add to this piece, please let us know.
Utah Mining Incident and USGS
For a comprehensive coverage of the seismic event in Utah, take a look at the information by USGS at this link
. Also see this site
for details about the University of California Berkeley findings regarding the event.
PERSONAL PERSPECTIVES & RECOLLECTIONS
My own interest in the subject of mine-induced seismicity was sparked by a tremor in Welkom, South Africa in 1976. It had a local Richter magnitude of 5.2. A mine rock burst event occurred and a large apartment block collapsed. Amazingly no-one was hurt thanks to the smarts of the caretaker who saw the cracks and evacuated the building before it collapsed.
A good client of the time, Fraser F. Alexander, was busy reworking a tailings impoundment above the local compound at the time. They had opened the side of the impoundment in order to liquefy the tailings and send them back to the process plant as a slurry for reworking to recover gold and uranium left behind in the initial processing. Sagely, they were concerned about full earthquake-induced liquefaction of the tailings of the inner part of the impoundment, which if they flowed would flood the compound and drown many. I was asked to consult and come up with a solution.
I came to Pasadena to a conference on earthquake engineering. I learnt that the topic was complex and that without sophisticated equipment, which at the time, we had no hope of acquiring in South Africa, it was difficult to establish the response of tailings to earthquakes. So we solved the problem in a farming manner: we simply plugged the opening in the tailings impoundment so that even if full liquefaction of the tailings occurred nothing would flow from the site.
Thus it was with great interest that I read a 2006 report and two of its appendices, 4.2 and 4.3 on the Welkom incident and a lot more about mine-induced seismicity in South Africa; these are large files, slow to download, but well worth the wait. In particular, Appendix 4.2 Origin, Intensity and Damage Potential of tectonic and Mining Seismic Event by Drs. Kijko and Cichowicz is, in my opinion, a masterpiece of clear writing and fine, factual content. I would love to meet them and/or read more of their writings.
As good a summary of the basics of mine-induced seismicity and related concepts is to be found on the SiM Consultants
As always there is much information to be found on the NIOSH sites; good solid information that is so constantly being added to I do not list it all here.
Here are some articles that talk about mine-induced seismicity, and in particular microseismic monitoring.
TechnoMine has run a series of articles on induced seismicity
written by Dan Oancea. They tell more cogently than more most why we should be bothered by induced seismicity, regardless of the cause: mining, reservoir construction, fluid injection, in situ leaching, heavy rainfall, or large excavations. Here is my favorite part of this four-part series:
A good online paper is the Induced Seismicity Associated with Enhanced Geothermal Systems study. I will only highlight a few of its findings beginning with the presentation of several well-documented cases:
- At Soultz, France fluid injection in two deep reservoirs (3,500 m and 5,000 m) generated micro earthquakes (up to 2.9) that determined the local population to intervene for curtailing the activity;
- At the Geysers Geothermal Field in Northern California, U.S.A., a strong correlation has been proved between injection and seismicity - a maximum 4.6 event occurred in 1982; numerous faults cut through the area;
- At the Cooper Basin, Australia, magnitude 3.0 earthquakes were detected since the beginning of injection / hydrofracturing; it is a sparsely populated area so no local resistance has been encountered by geothermal energy developers;
- Berlin, El Salvador, a seismically active region, it is marked by plate subduction and volcanic activity; some 2,500 m deep injection and production wells were located on the flanks of an inactive volcano; weeks after the shut-in of pumping operations a 4.4 magnitude event hit the area (it was the largest of a string of smaller magnitude earthquakes).
From South African regulations: At every mine where a significant risk of rockbursts or rock falls exist, the employer must engage the services of competent persons who...
- Proactively identify dangerous situations and take remedial action before persons are injured or workings are damaged
- Participate in any relevant planning activities that need to take account of, could influence or be influenced by, the risk of rockfalls or rockbursts.
- Evaluate all face layout positions to determine any significant risks relating to rockfalls or rockbursts created by or likely to be created by mining operations.
- Identify, review and make recommendations to the employer regarding systems, procedures, and techniques used or to be used by the employer to eliminate, control, or minimise rockfall or rockburst hazards.
- Establish monitoring, recording and reporting systems and procedures, that ensure that relevant information related to rockfalls and rockbursts is timeously provided to those persons involved in the planning and operating of mining activities.
- Approve plans for mining sequences to ensure that the probability of seismic events/rockbursts are minimised; the factors affecting the stability of off-reef excavations are taken into account; support systems accommodate current and anticipated rock conditions; and all precautions necessary for remnant mining are stipulated.
- In seismically active mines, advise the employer in developing a strategy for their mine's COP to reduce the incidence and the effects of rockbursts.
- On mines or sections of mines where the bulk of ore reserves are located in remnants or pillars, make a periodic detailed analysis of the whole mine with emphasis on sequencing or phasing the extrcation of pillars or remnants in such a way as to ensure that they are mined out as safely as possible.
- Monitor, interpret and analyse seismicity, where necessary assisted by seismic specialists using appropriate monitoring equipment.
- Regularly monitor pillar performance to ensure that they conform to design requirements.
- Inspect all major rockbursts and large or serious falls of grounds and submit a report making recommendations to the employer.
- Give input on risk assessment matters pertaining to rock related issues.
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