Mining-induced Seismicity Research Articles

Analysis of Rock Mass Stability Based on Mining-Induced Seismicity: A Case Study at the Hongtoushan Copper Mine in China

In deep metal mines, seismic hazards (for example, rock bursts and roof collapses) occur easily due to the influence of complicated geological conditions, high-stress environments and strong blast disturbances. A microseismic (MS) monitoring technique was used to evaluate rock mass stability in the Hongtoushan copper mine in China. The changes in the multiple MS parameters, including the apparent volume, energy index, spatial correlation length, fractal dimension and b value, during the mining process were presented. The results showed that the MS sequences decayed following Omori’s power-law and that the rock mass returned to a relatively stable state after approximately 20 days of blasting. After each stoping, the proportion of large-scale fractures increased, the MS events become more concentrated, and the long-range correlations of the rock mass weakened. These changes caused b value, fractal characteristics and spatial correlation length to decrease significantly. Although the risk of a large-scale rock mass failure was reduced, the risk of a local failure increased. As the mining process continued, stress and deformation gradually increased, and the areas with concentrated stress and large deformation were not consistent. Control measures for hazards should be implemented according to the corresponding occurrence mechanisms as determined from the differences in the rock mass physics mechanics.

The b-value evolution of mining-induced seismicity and mainshock occurrences at hard-rock mines

• Estimate the b-value temporal evolution with mainshocks in mining-induced seismicity. • Evaluate the goodness of fit for determining the optimal magnitude of completeness. • Better utilize seismic monitoring system at underground hard-rock mines.

Mining-induced seismicity of a seam located in rock mass made of thick sandstone layers with very low strength and deformation parameters

Abstract The paper presents the results of calculations and analyses aimed at explaining the cause and mechanism of strong seismic events E ≥ 105 J (local magnitude, Ml ≥ 1.7). Their source are thick and at the same time weak layers of sandstone, theoretically unable to accumulate elastic energy during the exploitation of the seam 209 in hard coal mine Ziemowit in the edge area of seams 206/1, 206/1–2 and 207. In the developed method of forecasting the pressure/stresses in the areas of the rock mass, which are affected by mining exploitation, the formulas linking the results of geophysical measurements of the longitudinal wave anomaly to the pressure values in its impact zone are used. This method was used to locate areas where elevated pressure occurred compared to gravitational pressure, which had a decisive impact on confinement of layers affected by mining exploitation. In such areas the level of confinement/stiffening of the layers is very high, that is why their deformation in the direction of the gobs is limited. In the case of a formation of a large area of gobs, large-size rock blocks are formed in the rock mass, whose mining-induced separation causes their the impact on the ground. Seismic energy accompanying this event is a function of the impact energy reduced by damping resulting from the physiomechanical properties of the cracked substrate.

Seismicity induced by longwall coal mining at the Thoresby Colliery, Nottinghamshire, U.K.

The U.K. has a long history of deep coal mining, and numerous cases of mining-induced seismicity have been recorded over the past 50 years. In this study we examine seismicity induced by longwall mining at one of the U.K.’s last deep coal mines, the Thoresby Colliery, Nottinghamshire. After public reports of felt seismicity in late 2013 a local seismic monitoring network was installed at this site, which provided monitoring from February to October 2014. This array recorded 305 seismic events, which form the basis of our analysis. Event locations were found to closely track the position of the mining face within the Deep Soft Seam, with most events occurring up to 300 m ahead of the face position. This indicates that the seismicity is being directly induced by the mining, as opposed to being caused by activation of pre-existing tectonic features by stress transfer. However, we do not observe correlation between the rate of excavation and the rate of seismicity, and only a small portion of the overall deformation is being released as seismic energy. Event magnitudes do not follow the expected Gutenberg-Richter distribution. Instead, the observed magnitude distributions can be reproduced if a Truncated Power Law distribution is used to simulate the rupture areas. The best-fit maximum rupture areas correspond to the distances between the Deep Soft Seam and the seams that over- and underlie it, which have both previously been excavated. Our inference is that the presence of a rubble-filled void (or goaf) where these seams have been removed is preventing the growth of larger rupture areas. Source mechanism analysis reveals that most events consist of dip-slip motion along near-vertical planes that strike parallel to the orientation of the mining face. These mechanisms are consistent with the expected deformation that would occur as a longwall panel advances, with the under- and over-burdens moving upwards and downwards respectively to fill the void created by mining. This further reinforces our conclusion that the events are directly induced by the mining process. Similar mechanisms have been observed during longwall mining at other sites.

Geotechnical Characterization of Bukov Underground Research Facility

Bukov Underground Research Facility (Bukov URF) is designed to operate as a test site to assess the properties and behavior of the rock mass analogous to selected seven candidate sites in the Czech Republic. It is situated at a depth of about 600 m beneath Earth's surface, which corresponds with the proposed storage depth of the final locality for the national deep repository of high-level radioactive waste. Bukov URF, the construction of which has started in 2013, is situated in the southern part of the Rožná uranium deposit, about 40km NNW from Brno. The rock mass is composed of relatively monotonous rock sequences mainly represented by migmatized biotite paragneisses up to stromatic migmatites, amphibole-biotite to biotite-amphibole gneisses, and amphibolites. Based on the analysis of physical and mechanical properties, intact rocks in the studied area exhibit a high to very high strength and may be only locally affected by metasomatic alterations. The research activities focus on a complex geological and geotechnical characterization of the rock mass of interest, which is necessary for further in situ research. This paper briefly describes the main results of the current stage of geotechnical exploration and research works made there by the Institute of Geonics of the CAS. These works especially include: determination and evaluation of physical and mechanical properties of the rocks taken from the drift walls and from the boreholes during driving, calculation of rock mass quality based on selected index rock mass classification systems, determination of stress and strain state of the rock mass using the methods of hydrofracturing, Goodman Jack, strain gauge probe measurements, long-term periodical tensometric and convergence measurements, and evaluation of the effects of technical and mining-induced seismicity on the rock mass of interest.

Application of mine-induced free-field and building foundation vibrations for evaluation of their harmfulness using mining scales

Free-field vibrations can be induced not only by earthquakes, but also by so-called paraseismic sources, for instance, mining tremors. Mining-induced seismicity has been recorded in many countries in mining areas with the underground exploitation. The mining rockbursts are a hazard to surface structures since ground motion can result in their damage. Legnica-Glogow Copperfield (LGC) is the most seismically active mining region in Poland. Records of mining-induced vibrations simultaneously registered on the free-field close to a building, and at the foundation level of the building, can differ significantly in this area. This occurs as a result of soil-structure interaction, which is visible in the transmission of ground vibrations to building foundations. Therefore, the responses of buildings determined on the basis of recorded free-field vibrations may be different from the responses obtained with an application of building foundation vibrations. Convenient in practice, simple, approximate way to assess the harmfulness of mine-induced vibrations to buildings, is the use of appropriately prepared scales. They are based on the parameters of ground vibrations. This paper deals with the evaluation of harmfulness of mine-induced vibrations in LGC region to an actual medium-rise apartment building. The Mining Intensity Scale GSI-2004/11 and SWD-II scale have been applied. Estimation of the intensity of surface vibrations according to these scales has been performed using recorded free-field and, for research purposes, also foundation vibrations. The influence of the place of surface vibrations measurements on the evaluated intensity of vibrations has been analysed. It was stated that significant differences can occur in the classification of the same mining tremor depending on whether the evaluation had been performed on the basis of the free-field vibrations or on the building foundation vibrations measured on the same time.

The forecast of mining-induced seismicity and the consequent risk of damage to the excavation in the area of seismic event

Abstract The Central Mining Institute has developed a method for forecasting the amount of seismic energy created by tremors induced by mining operations. The results of geophysical measurements of S wave velocity anomalies in a rock mass or the results of analytic calculations of the values of pressure on the horizon of the elastic layers are used in the process of calculating the energy. The calculation program which has been developed and adopted has been modified over recent years and it now enables not only the prediction of the energy of dynamic phenomena induced by mining but also the forecasting of the devastating range of seismic shock. The results obtained from this calculation, usually presented in a more readable graphic form, are useful for the macroscopic evaluation of locations that are potential sources of seismic energy. Forecasting of the maximum energy of seismic shock without prior knowledge of the location of the shock's source, does not allow shock attenuation that results from, for example, a distance of tremor source from the excavation which will be affected by seismic energy, to be taken into consideration. The phenomena of energy dissipation, which is taken into account in the forecasts, create a new quality of assessment of threat to the excavation. The paper presents the principle of a method of forecasting the seismic energy of a shock and the risk of damage to the excavation as a result of the impact of its energy wave. The solution assumes that the source of the energy shock is a resilient layer in which the sum of the gravitational stresses, resulting from natural disturbances and those induced by the conducted or planned mining exploitation, is estimated. The proposed solution assumes a spherical model for the tremor source, for which seismic energy is forecasted as a function of the longwall advance and the elementary value of seismic energy destroying the excavation. Subsequently, the following are calculated for the forecast of the seismic energy of a shock with the defined location of its source: value of the coefficient λ of dispersion/attenuation of seismic energy and the flux of seismic energy at predetermined distances r from the tremor source. The proposed solution for forecasting the seismic energy of tremors and the level of risk of damage to the excavation during the functioning of mining operations is helpful in the development of bump prevention. Changing the intensity of mining operations enables the level of the seismic energy induced by the operations both at the stage of its development and during the excavation of a seam using the longwall method to be “controlled”. The presented solution has been produced for an area disturbed by the mining of coal seam 510 in the hard coal mine, Jas-Mos. An original program developed by CMI was used for the calculations.

The influence of regional geological settings on the seismic hazard level in copper mines in the Legnica-Głogów Copper Belt Area (Poland)

The current level of rockburst hazard in copper mines of the (LGOM) Legnica- Glogow Copper Belt Area is mostly the consequence of mining-induced seismicity, whilst the majority of rockbursting events registered to date were caused by high-energy tremors. The analysis of seismic readings in recent years reveals that the highest seismic activity among the copper mines in the LGOM is registered in the mine Rudna. This study investigates the seismic activity in the rock strata in the Rudna mine fields over the years 2006-2015. Of particular interest are the key seismicity parameters: the number of registered seismic events, the total energy emissions, the energy index. It appears that varied seismic activity in the area may be the function of several variables: effective mining thickness, the thickness of burst-prone strata and tectonic intensity. The results support and corroborate the view that principal factors influencing the actual seismic hazard level are regional geological conditions in the copper mines within the Legnica-Glogow Copper Belt Area.

Assessment of Quantitative Aftershock Productivity Potential in Mining-Induced Seismicity

Strong mining-induced earthquakes exhibit various aftershock patterns. The aftershock productivity is governed by the geomechanical properties of rock in the seismogenic zone, mining-induced stress and coseismic stress changes related to the main shock’s magnitude, source geometry and focal mechanism. In order to assess the quantitative aftershock productivity potential in the mining environment we apply a forecast model based on natural seismicity properties, namely constant tectonic loading and the Gutenberg-Richter frequency-magnitude distribution. Although previous studies proved that mining-induced seismicity does not obey the simple power law, here we apply it as an approximation of seismicity distribution to resolve the number of aftershocks, not considering their magnitudes. The model used forecasts the aftershock productivity based on the background seismicity level estimated from an average seismic moment released per earthquake and static stress changes caused by a main shock. Thus it accounts only for aftershocks directly triggered by coseismic process. In this study we use data from three different mines, Mponeng (South Africa), Rudna and Bobrek (Poland), representing different geology, exploitation methods and aftershock patterns. Each studied case is treated with individual parameterization adjusted to the data specifics. We propose the modification of the original model, i.e. including the non-uniformity of M0, resulting from spatial correlation of mining-induced seismicity with exploitation. The results show that, even when simplified seismicity distribution parameters are applied, the modified model predicts the number of aftershocks for each analyzed case well and accounts for variations between these values. Such results are thus another example showing that coseismic processes of mining-induced seismicity reflect features of natural seismicity and that similar models can be applied to study the aftershock rate in both the natural and the mining environment.

Assessment of failure mechanisms in deep longwall faces based on mining-induced seismicity

Failure mechanisms of the rock mass in the regions of maximum stress concentrations around a longwall face were assessed. In this respect, seismic events that result from changes in the stress field were analyzed to gain more knowledge about rock failure mechanisms in the proximity of the face area. A deep longwall mine developed at depths of about 3–3.5 km in South Africa was selected as a case study. Seismic moment tensor solutions were obtained for 32 seismic events with moment magnitudes in the range of 0.49 and 2.10. Through moment tensor decomposition, the dominant failure mechanisms were investigated by drawing focal mechanism plots. Further analysis was implemented by depicting the corresponding 3D radiation patterns of P-wave particle motions. Although the results cover various failure mechanisms, the dominant mechanisms are shear, implosional, and compressional failures. According to the results, most of the maximum principal stresses in the mine are compressive and oriented nearly vertical, which are in accordance with the gravitational collapses of the mined out areas. The results obtained from this research show that measuring and analyzing mining-induced seismicity can be a reliable measure to characterize the dominant failure mechanisms in a nondestructive manner and to provide a useful assessment of the stability of the longwall face in advance of extraction.

Magnitude‐based discrimination of man‐made seismic events from naturally occurring earthquakes in Utah, USA

AbstractWe investigate using the difference between local (ML) and coda/duration (MC) magnitude to discriminate man‐made seismic events from naturally occurring tectonic earthquakes in and around Utah. For 6846 well‐located earthquakes in the Utah region, we find that ML‐MC is on average 0.44 magnitude units smaller for mining‐induced seismicity (MIS) than for tectonic seismicity (TS). Our interpretation of this observation is that MIS occurs within near‐surface low‐velocity layers that act as a waveguide and preferentially increase coda duration relative to peak amplitude, while the vast majority of TS occurs beneath the near‐surface waveguide. A second data set of 3723 confirmed or probable explosions in the Utah region also has significantly lower ML‐MC values than TS, likely for the same reason as the MIS. These observations suggest that ML‐MC is useful as a depth indicator and could discriminate small explosions and mining‐induced earthquakes from deeper, naturally occurring earthquakes at local‐to‐regional distances.

Imaging of temporal stress redistribution due to triggered seismicity at a deep nickel mine

We conducted three-dimensional passive seismic tomography surveys by using mining-induced seismicity of a deep nickel mine. To investigate the stress evolution with the distribution of major events, we collected location information of monitored seismicity and arrival time picks of waves transmitted from seismicity at the Creighton Mine in Sudbury, Ontario, Canada to produce velocity tomograms. From June 22nd to July 24th, 2011, four major events (moment magnitude >1.0) were observed associated with 13630 microseismic events. We developed a large model and a small model, referring to scales of grid spacings and areas of territory, for double-difference tomographic inversion. The large model was used to examine the general trend of velocity distribution before and after the major events; the small model with finer grid spacing gave rise to a higher resolution to show results in detail. Seismic imaging results of velocity distribution showed good agreement with major events in different regimes, implying that the stress evolution was consistent with major events in the surrounding rock mass. By comparing the results obtained from tomographic inversion, we found that the stress in the region near major events started increasing before major events. High-stress areas, indicated by high-velocity anomalies, appeared and expanded adjacent to the region with major events.

Effect of deformation properties of discontinuities on sources of mining-induced seismicity in rocks. Part I: In situ observations

The authors analyze mine logs of seismic events in Poland, Finland, Canada, Russia and South Africa. For the analyzed events, induced seismic energy varies by 2–3 orders of magnitude at the same value of seismic moment. The upper and lower limits of the range correspond to “stiff” and “soft” sources, respectively. The most probable cause of the wide spread of the reduced energy values seems to be fluctuating properties of discontinuities due to the change in the composition of filler and water content of joints.

Experimental Data Set of Mining-Induced Seismicity for Studies of Full-Scale Topographic Effects

This paper describes two large, high-quality experimental data sets of ground motions collected with locally dense arrays of seismometers deployed on steep mountainous terrain with varying slope angles and topographic features. These data sets were collected in an area of central-eastern Utah that experiences frequent and predictable mining-induced seismicity as a means to study the effects of topography on small-strain seismic ground motions. The data sets are freely available through the George E. Brown, Jr. Network for Earthquake Engineering Simulation data repository ( NEEShub.org ) under the DOI numbers 10.4231/D34M9199S and 10.4231/D3Z31NN4J. This paper documents the data collection efforts and metadata necessary for utilizing the data sets, as well as the availability of supporting data (e.g., high-resolution digital elevation models). The paper offers a brief summary of analyses conducted on the data sets thus far, in addition to ideas about how these data sets may be used in future studies related to topographic effects and mining seismicity.

Application of a moment tensor inversion code developed for mining-induced seismicity to fracture monitoring of civil engineering materials

A moment tensor inversion (MTI) code originally developed to compute source mechanisms from mining-induced seismicity data is now being used in the laboratory in a civil engineering research environment. Quantitative seismology methods designed for geological environments are being tested with the aim of developing techniques to assess and monitor fracture processes in structural concrete members such as bridge girders. In this paper, we highlight aspects of the MTI_Toolbox programme that make it applicable to performing inversions on acoustic emission (AE) data recorded by networks of uniaxial sensors. The influence of the configuration of a seismic network on the conditioning of the least-squares system and subsequent moment tensor results for a real, 3-D network are compared to a hypothetical 2-D version of the same network. This comparative analysis is undertaken for different cases: for networks consisting entirely of triaxial or uniaxial sensors; for both P and S-waves, and for P-waves only. The aim is to guide the optimal design of sensor configurations where only uniaxial sensors can be installed. Finally, the findings of recent laboratory experiments where the MTI_Toolbox has been applied to a concrete beam test are presented and discussed.

Predictive Regression Models of Monthly Seismic Energy Emissions Induced by Longwall Mining

Abstract This article presents the development and validation of predictive regression models of longwall mining-induced seismicity, based on observations in 63 longwalls, in 12 seams, in the Bielszowice colliery in the Upper Silesian Coal Basin, which took place between 1992 and 2012. A predicted variable is the logarithm of the monthly sum of seismic energy induced in a longwall area. The set of predictors include seven quantitative and qualitative variables describing some mining and geological conditions and earlier seismicity in longwalls. Two machine learning methods have been used to develop the models: boosted regression trees and neural networks. Two types of model validation have been applied: on a random validation sample and on a time-based validation sample. The set of a few selected variables enabled nonlinear regression models to be built which gave relatively small prediction errors, taking the complex and strongly stochastic nature of the phenomenon into account. The article presents both the models of periodic forecasting for the following month as well as long-term forecasting.

Soil mechanics and the observational method: challenges at the Zelazny Most copper tailings disposal facility

This paper illustrates the geotechnical aspects of the development of one of the world's largest copper tailings disposal facilities, located at Zelazny Most in south-west Poland. Its operation commenced in 1977 and, by the end of 2013, 527 × 106m3of tailings had already been stored within a confining embankment dam (‘ring dam') of about 14·3 km in total length. The foundations of the ring dam lie on Pleistocene deposits, underlain by a thick sequence of Pliocene sediments. The period of operation of the facility will continue until exhaustion of the ore body, which is estimated to occur in 2042, when the total volume of tailings stored will reach 1000 × 106m3. The development and the use of the Zelazny Most facility is affected by a number of significant geotechnical hazards, including the phenomenon of flow liquefaction, as is often the case for such structures. However, the most important geotechnical hazard at Zelazny Most is a consequence of its location in an area that, during the Pleistocene, experienced at least three major glaciations. The ice sheets, over 1000 m thick, which overrode the area, induced substantial glacio-tectonic phenomena, the most geotechnically important of which is the formation of extensive sub-planar shear planes in the Pliocene clays that extend to depths of around 100 m. The shear strength in these shear zones, which occur mostly in high-plasticity clays, is close to residual, thus controlling the stability of the confining dams. Moreover, the area of the disposal is also subject to a moderate degree of mining-induced seismicity. Discussion of this is omitted from this paper because of its relatively minor influence on the design of the depository. Given the large scale of the disposal facility, the complexity of the geological and geotechnical settings, and its anticipated life span, the owners KGHM (the Polish acronym for Copper Mine and Mill Company), on the advice of the World Bank, appointed an international board of experts (IBE). The IBE, supported by a Polish geotechnical expert as local liaison, have the responsibility to oversee the safe development of the tailings dams by applying Peck's observational method. This paper describes the monitored behaviour of the dams, with particular reference to the presence of the glacio-tectonic phenomena and the remedial measures adopted for their mitigation. The geotechnical characterisation of the foundation soils and of the tailings is described, and the stability analyses of the tailings dams are discussed in the context of the safe development of the whole facility.

Mining-induced seismicity at open pit mines in Kuzbass (Bachatsky earthquake on June 18, 2013)

The data on the world-strongest earthquake induced by hard mineral mining—Bachatsky earthquake (local magnitude 6.1) in Kuzbass on June 18, 2013—are reported. Seismic activation of the territory at the Bachatsky Open Pit Mine in 2012–2013 awoke three large earthquakes, sensibly affecting the Kuzbass towns. The authors have analyzed the results of experiments on temporal seismology nets in 2012 and the records of the after-shocks of the Bachatsky earthquake in 2013. The low energy seismic activity is revealed in rocks under the Bachatsky and Shestaki OPMs. It is found that small earthquakes cluster under the center of the open pit mine bottoms, and the larger earthquakes crowd under the pitwalls, and the depth of the earthquake sources is 3–4 km.

Fine Details of Mining-Induced Seismicity at the Trail Mountain Coal Mine Using Modified Hypocentral Relocation Techniques

Abstract Master event and double‐difference techniques were used to relocate mining‐induced seismicity (MIS) at the Trail Mountain Mine, a longwall coal mine in central Utah. Travel‐time data were collected by Arabasz et al. (2002) using a surface seismic network with stations at elevations both above and below mine level (because of the topography) and a single in‐mine station. Arabasz et al. (2002) only used surface stations above mine level to determine locations. Using this network geometry, they were only able to constrain focal depths for 321 of 1829 events. In contrast, we use all stations, creating a 3D network. Hypocentral locations are improved by implementing a master event methodology to reduce the effects of uncertainties in the velocity structure, though the resulting locations do not correspond with known structures or stratigraphy. The mismatch between the locations and geology is likely due to fracturing of the rock mass by the mining process, thereby decreasing the seismic velocity near mined‐out regions. Assuming a 10% velocity decrease places the MIS in the roof of the mine. A double‐difference procedure is used to mimic a time‐varying velocity structure. The time‐varying velocity structure results in locations that approximate the dip of the coal seam. By using all available stations and allowing for a time‐varying velocity structure, we find the MIS is located immediately above the coal seam and closely follows the position of the coalface. The epicenters align with the roads along the longwall panel, where stress concentrations are expected during mining. Online Material: Animations of the progression of seismicity along the longwall panel.

Natural and induced seismic activity in Kuzbass

Based on the plotted times of occurrence of seismic events, their timing within a work week and location of epicenters, it is illustrated that since the 1960s the seismicity in the Kuzbass territory has been a complicated natural and mining-induced phenomenon. Since the late 1980s, the natural and mining-induced seismicity has entered the second stage of development, with clustering of low-energy seismic events and strong shallow earthquakes in the areas under heavy mining and, in the first instance, at deep open pit mines. The representative event of this kind is the magnitude 5.2 earthquake at the Bachatsky Coal Open Pit Mine on June 19, 2013. It has been found that the natural and induced seismic activity is associated with deep faults that weakly show themselves in the upper layer of the earth crust but appear in the local relief, which is an evidence of their live emergence onto the daylight surface due to the mining impact. The article proposes the comprehensive research program for the natural and mining-induced seismic activity in the Kuzbass area and the development of early identification of seismically active zones.