Mining-Induced Seismicity during Development Works in Coalbeds in the Context of Forecasts of Geomechanical Conditions

Mining-induced seismicity in the area of development works and proper mining operations is one of the major determinants of the rockburst hazard level in underground mines. Rockburst hazard assessment in Polish collieries is performed by a variety of mining and geophysical methods, including seismic and seismoacoustic techniques, borehole surveys, small diameter drilling, rock strata profiling and analyses of geomechanical properties of rocks, geological structure and geological mining conditions. In the case of zones particularly exposed to potential hazards, it is recommended that analytical or numerical forecasts of the state of stress in the vicinity of workings should be used already at the stage of planning of mining operations. This study summarises the comparative analysis of seismic test data and analytical forecasts of the state of stress in five selected headings in one of the burst-prone collieries within the Upper Silesia Coal Basin in Poland (USCB). As regards the seismic data, duly defined quantitative indicators and energy criteria of the registered seismic activity are recalled in the assessment of rockburst hazard level during the roadheading operations. Analytical simulations utilise a developed geomechanical model and stress–strain relationships stemming from the principles of elastic media mechanics. From the standpoint of mining engineering practice, interpretation of results obtained by the two methods reveals how effective analytical models will be in prognosticating or verification of rockburst hazard conditions.

Spatial Distribution of Triggered Earthquakes in the Conditions of Mining-Induced Seismicity

Abstract—The spatial distribution of the triggered seismic events in mining conditions in the tectonically loaded rock masses is studied using the example of seismicity in the Khibiny Mountains. It is shown that the distribution of distances from the triggering to triggered events, on average, obeys the power-law with a parameter independent of the magnitude of the triggering event. The model of the maximum distances from a triggering event’s hypocenter to the triggered shocks expected with a given probability is derived. It is shown that the model is consistent with the real data. Based on the error diagram analysis, the guidelines are proposed for the practical use of the model.

An approach to assessment of post mining-induced seismic hazard in Kolar Gold Fields mines – a review

A common challenge faced in underground hardrock mines worldwide is post mining-induced seismicity, as the events have been quite disastrous, causing risk to the structures and lives. In the recent years, many of the worked out mining areas are slowly getting populated and in due course of time shall be posing environmental threat to the people residing above and to the surface structures like sudden void formations or sudden ground collapse becoming visible on the surface. Worked out or closed mines have most of the time shown existence of post mining-induced seismicity signatures. Some of the closed mines showing post mining induced seismicity in Korea, South Africa, Sweden and India are being discussed. Post mining induced seismicity observed in Kolar Gold Fields worked out mine still being felt since closure of deeper levels is discussed. As mining depth increases especially in hard rock mines, magnitude of stress increases, hence, the occurrence and severity of postmining induced seismicity also increases. The problem becomes more serious if proper fund allocation is not done to investigate these areas, may be due to the absence of economic interest once the mine site has been abandoned and in many cases, direct investigations inside the mines may not be possible due to stability problems or due to the ingress of water into the void spaces of the mining area. Several approaches and techniques adopted by researcher’s world over are being discussed in this paper, with a view to gaining insight into the techniques of evaluation of seismic hazard. Seismic vulnerability assessment should integrate the effects of all the seismic events occurring at different locations of mining area during mining and post mining, along with their uncertainties also being considered. Based on the recorded data and some of the derived parameters from previous years, an attempt should be made to evaluate the existing risk prone areas. The past records of induced seismicity due to mining should be used as a precursor for identification of impending future events and their expected probable locations of occurrence. The methods discussed here for assessment of seismic hazard are based on direct waveform and seismic source parameters, parameters from indirect waveform methods, frequency-magnitude relationship based, and frequency content analysis based. From the assessment it is found that the choice of method that can be used depends on the period of monitoring (short-term monitoring, intermediate-term or long-term monitoring) and the objective of the study required to be achieved, this varies on site-to-site basis. The main focus is to show the importance and need to install a micro seismic monitoring system for long term assessment of seismic risk especially in abandoned/worked out mines showing post mining-induced seismicity.

What Governs the Spatial and Temporal Distribution of Aftershocks in Mining-Induced Seismicity: Insight into the Influence of Coseismic Static Stress Changes on Seismicity in Kiruna Mine, Sweden

ABSTRACT Strong mining-induced earthquakes are often followed by aftershocks, similar to natural earthquakes. Although the magnitudes of such in-mine aftershocks are not high, they may pose a threat to mining infrastructure, production, and primarily, people working underground. The existing post-earthquake mining procedures usually do not consider any aspects of the physics of the mainshock. This work aims to estimate the rate and distribution of aftershocks following mining-induced seismic events by applying the rate-and-state model of fault friction, which is commonly used in natural earthquake studies. It was found that both the pre-mainshock level of seismicity and the coseismic stress change following the mainshock rupture have strong effects on the aftershock sequence. For mining-induced seismicity, however, we need to additionally account for the constantly changing stress state caused by the ongoing exploitation. Here, we attempt to model the aftershock sequence, its rate, and distribution of two M≈2 events in iron ore Kiruna mine, Sweden. We could appropriately estimate the aftershock sequence for one of the events because both the modeled rate and distribution of aftershocks matched the observed activity; however, the model underestimated the rate of aftershocks for the other event. The results of modeling showed that aftershocks following mining events occur in the areas of pre-mainshock activity influenced by the positive coulomb stress changes, according to the model’s assumptions. However, we also noted that some additional process not incorporated in the rate-and-state model may influence the aftershock sequence. Nevertheless, this type of modeling is a good tool for evaluating the risk areas in mines following a strong seismic event.