Observations of mine seismicity in the eastern Wasatch Plateau,Utah, U.S.A.: A possible case of implosional failure

In the summer of 1984, a three-dimensional, high-resolution microearthquake network was operated in the vicinity of two coal mines beneath Gentry Mountain in the eastern Wasatch Plateau, Utah. During a six-week period, approximately 3,000 seismic events were observed of which the majority were impulsive, higher frequency (>10 Hz), short duration (<2–3 sec) events probably associated with the caving of the roof from a longwall operation. In contrast, 234 of the largest located events appeared to occur predominantlybeneath the mines to a depth of 2 to 3 km consistent with previous studies. The magnitudes of these events ranged from less thanM _c 0 to 1.6. In addition to the unusual depths of these latter events, an anomalous aspect displayed by the events was an apparent dilatational focal mechanism suggesting a non-double-couple, possibly implosional source. Implosional events have been observed in other studies of mine seismicity; however, the generally inadequate instrumental coverage of the focal sphere has cast some doubt on the validity of such mechanisms. Previously suggested source mechanisms for such implosional events have included tensional failure through strata collapse, and a shear-implosional displacement mechanism. Shear failure must be involved in the failure process of the Gentry Mountain implosional events as evidenced by well-defined shear waves in the observed seismograms. Simultaneous monitoring in the East Mountain coal mining area to the south by the University of Utah revealed typical shear failure events mixed with implosional events. The observed double-couple, reverse focal mechanisms at East Mountain were similar to mechanisms determined in previous studies and a composite focal mechanism determined in this study for a sequence outside the mining areas. This suggested that the shear events within the mining areas are being influenced by the regional tectonic stress field. Thus in addition to the seismic events associated with caving of the roof from the longwall operation, there appear to be at least two other types of mining-induced seismic events occurring in the eastern Wasatch Plateau, both submine in origin: (1) events characterized by apparent non-double-couple possibly implosional focal mechanisms and well-defined shear waves; and (2) shear events, which are indistinguishable from tectonic earthquakes and may be considered mining triggered earthquakes. The small mining-induced stress changes that occur beyond a few hundred meters from the mine workings suggest both types of seismic events are occurring on critically stressed, pre-existing zones of weakness. Topography, overburden, method of mining, and mine configuration also appear to be significant factors influencing the occurrence of the implosional submine events.     

Analysis of mining-induced microseismic events at Strathcona Mine,Subdury, Canada

Rockbursts and mining-induced seismic events have serious socio-economic consequences for the Canadian mining industry, as their mines are extended to greater depths. Automatic multichannel monitoring systems (Electro-Lab MP250s) are routinely, used to detect the arrival times of seismic waves radiated by mining-induced events and sensed on an array of single component transducers installed throughout a mine. These arrival times are then used to locate the events and produce maps of areas of high activity for use in mine planning and design. This approach has limitations in that, it does not allow a detailed analysis of source mechanisms, which could be extracted if whole waveform signals are recorded and analyzed.A major research project, sponsored by the Natural Sciences and Engineering Research Council of Canada (NSERC) with the collaboration of the Canadian mining industry, is aimed at enhancing existing mine seismic monitoring technology in Canada, in order to carry out more advanced processing of data to obtain fundamental scientific information on mining-induced seismic events This paper describes preliminary results from seismic monitoring experiments carried out in a hard rock nickel mine in Sudbury, Canada. Existing seismic monitoring instrumentation was enhanced with a low cost microcomputer-based whole waveform seismic acquisition system. Some of the signals recorded during this experiment indicate anisotropic wave propagation through the mine rock masses, as observed by the splitting of shear waves and the relative arrival of two shear waves polarized in directions which may be related to the structural fabric and/or state of stress in the rock mass. Analysis of compressional wave first motion shows the predominance of shear events, as indicated by focal mechanism studies and is confirmed by spectral analysis of the waveforms. The source parameters were estimated fro typical low magnitude localized microseismic events during the initial monitoring experiments. The seismic moment of these events varied between 10⁶ N.m and 2.10⁸ N.m. with a circular source radius of between 1 m and 2 m with an estimated stress drop of the order of 1 MPa.     

The Kirovskiy Explosion of September 29, 1996: Example of a CTB Event Notification for a Routine Mining Blast

—?On September 29, 1996, a routine mining blast of about 390 metric tons was detonated underground at the Kirovskiy mine in the central Kola Peninsula. The United States was notified two weeks in advance that the blast was to take place and was given the date, approximate time, location and total charge. The explosion was detected and located by the prototype International Data Center (pIDC) and published in the Reviewed Event Bulletin (REB). Detailed information about the blast, including the type and depth of mining operation, the underground charge configuration, and the blasting delay pattern, is reviewed and combined with a seismological analysis of the event. The seismic analysis points to a possible associated tectonic component to the blast, consisting of a small rock burst or induced tremor, spall, or some combination of these mechanisms, that may have enhanced the shear waves, produced large Rg waves at low frequency, and small Pn/Sn and Pn/Lg amplitude ratios at high frequency. While these discriminants might identify the event as an earthquake, the spectral/cepstral analysis of the event clearly shows the ripple-fire delays. This event provides important confidence-building measures for both location calibration, in the form of travel-time corrections for location of mine events in this region, and for improved understanding of seismic discriminants expected for large mine blasts that may have an associated induced tectonic component (e.g., spall, mine tremor or rock burst).     

Seismic activation of tectonic stresses by mining

Hard coal mining in the area of the Bytom Syncline (Upper Silesia Coal Basin, Poland) has been associated with the occurrence of high-energy seismic events (up to 10⁹ J; local magnitude up to 4.0), which have been recorded by the local mining seismological network and regional seismological network. It has been noticed that the strongest seismic events occur when the mine longwall alignments coincide with the syncline axis. Data recorded by the improved local seismic network in the Bobrek Mine allow the estimation of the depths of the events’ hypocentres during excavation of longwall panel 3 as it approached the syncline axis. The recorded data were also used to estimate the location of the rupture surface and stress distribution in the seismic focus region. It was concluded that tectonic stresses, particularly horizontal stress components, are essential in the distribution of seismic tremors resulting from reverse faulting. The stresses induced by mining activity are only triggering tectonic deformations. The hypocentres of the strongest seismic events during mining of longwall panel 3/503 were located 300–800 m deeper than the level of coal seam 503.     

Studies of short and long memory in mining-induced seismic processes

Memory of a stochastic process implies its predictability, understood as a possibility to gain information on the future above the random guess level. Here we search for memory in the mining-induced seismic process (MIS), that is, a process induced or triggered by mining operations. Long memory is investigated by means of the Hurst rescaled range analysis, and the autocorrelation function estimate is used to test for short memory. Both methods are complemented with result uncertainty analyses based on different resampling techniques. The analyzed data comprise event series from Rudna copper mine in Poland. The studies show that the interevent time and interevent distance processes have both long and short memory. MIS occurrences and locations are internally interrelated. Internal relations among the sizes of MIS events are apparently weaker than those of other two studied parameterizations and are limited to long term interactions.     

应用震源机制方法研究鹤岗煤田开采区的构造应力环境

求解鹤岗强矿震震源机制解结果,表现出走滑伴随逆断层和正断层活动、非双力偶型的多样性。两组节面优势分布方向和节面的倾角优势分布不显著,两者分布无明显规律,反映出矿井下破裂面比较复杂。矿震震源主压应力释放优势方向北西310°左右,优势倾角为25°～60°;主张应力轴走向NE,主张应力场优势方向为北东60°左右,仰角在30～70°之间;中等应力轴(N)近于垂直,优势倾角为70～90°。矿震震源机制解显示的矿区最大主应力方向与区域构造应力场的最大主应力方向近似正交,矿震震源机制主应力轴优势倾角远大于区域构造地震,反映的是矿区采煤生产的次生构造应力环境重力应力场的贡献明显。     

基于密集台阵研究2019辽宁抚顺M2.4矿震震源参数

分析矿震破裂机制及微震的时空分布能够为矿区灾害评估提供更多的有效信息.本研究基于密集台阵观测对2019年11月12日辽宁抚顺2.4级矿震开展震源参数研究,震源机制解显示地震破裂包含明显的非双力偶分量,表现为体积压缩的塌陷机制,且震源深度较浅,最佳拟合矩心深度为0.6 km.同时,对11月3日—25日记录的连续地震波形开展微震扫描,新检测出324个微震事件(-0.5~2.0级),定位结果显示在M 2.4矿震发生前M>1.0级事件显著增多,且在矿震位置存在近南北向的微震条带分布,微震序列随时间向深部迁移(约1.5 km),暗示存在断层活化迹象.结合震源破裂机制,我们认为此次事件与矿区塌陷破裂密切相关,同时伴随先存断裂的剪切滑动.本研究表明,基于密集台阵观测的地震矩张量反演和微震检测研究,对判定矿震类型及防范矿区灾害具有重要的研究意义.     

A Temporary Network For Seismological Monitoring In West-Saxony: First Results

Since December 1997 a network of 10 autonomously recording digital seismic stations operates in W-Saxony to monitor microseismic activity generated by tectonic movements in the Gera-Jáchymov fault zone and induced by the flooding of the uranium mining area near Aue. Further goals to be followed with these data are the in situ verification of engineering seismological parameters and the study of the transmission properties of seismic waves across the Gera-Jáchymov fault zone to improve seismic risk evaluation. So far 19 tectonic and more than 50 induced events have been located and source parameters have been determined. Event clusters near Zwickau and within the Aue mining area could be spatially resolved with a master event technique. The cluster near Zwickau occured on a common fault plane striking parallel to the Gera-Jáchymov fault zone and dipping at an angle of about 60° towards ENE. The induced events near Aue occured within a volume of approximately 5 km ³ . The majority of these events were located in the granite body underneath the mining area. Fault plane solutions show a variety of focal mechanisms with no clear relation to the regional stress field.     

Passive Seismic Monitoring of Mine-scale Geothermal Activity: A Trial at Lihir Open Pit Mine

The Lihir open pit mine in Papua New Guinea is located inside an old volcano where geothermal activity is strongly present. Outbursts of hot water and steam into the mining areas were a major safety concern. Passive seismic monitoring was carried out at the mine to investigate whether the geothermal activities could be detected and located using microseismic techniques in a mining environment. In this trial, sixteen triaxial geophones which can withstand temperature up to 200°C were used and installed in four deep boreholes inside the pit. The microseismic events were discriminated using the STA/LTA triggering criterion. During 6 weeks of monitoring, more than 17,000 events were recorded. Approximately 12% of the events showed harmonic vibration characteristics similar to those observed in other geothermal and volcanic areas, suggesting that the geothermal activity inside the pit was captured by the microseismic monitoring system. More than 75% of the events present both P and S waves and they were interpreted to be associated with rock fracturing due to stress release near the bottom of the pit. Many geothermal-type events were located in areas where shear events occurred, implying that the detected geothermal events were not far from the mining area below the pit and they may also be associated with mining. The borehole installation of the geophones significantly reduced the interference of mining noise and achieved good observation of the seismic events. However, equipment installation requires great attention as the geophones may be destroyed due to unexpected rising temperature within the boreholes.     

Mining-related and tectonic seismicity in the East Mountain area Wasatch Plateau,Utah, U.S.A.

As part of a larger multi-institutional seismic monitoring experiment during June–August 1984 in the eastern Wasatch Plateau, Utah, data from a subarray of 20 portable seismographs were used to investigate seismicity in the East Mountain area, an area of active underground coal mining and intense microseismicity. Eight stations of the subarray were concentrated on top of East Mountain, about 600 m above mine level, at an average spacing of 2 to 3 km. The primary objective was the accurate resolution of hypocenters and focal mechanisms for seismic events originating at submine levels. Data from high-resolution seismic reflection profiles and drill-hole sonic logs yielded a detailed velocity model. This model features a strong velocity gradient in the uppermost 1 km, which has a significant effect on takeoff angles for first-arrivingP-waves from shallow seismic events. Two hundred epicenters located with a precision of ±500 m cluster within an area about 5 km in diameter and show an evident spatial association with four sites of longwall mining during the study period. A special set of foci rigorously tested for focal-depth reliability indicates submine seismicity predominating within 500 m of mine level and extending at least to 1 km, and perhaps to 2 km, below mine level. Continuous monitoring for a 61-day period (June 15–August 15) bracketed a 16-day mining shutdown (July 7–22) during which significant seismicity, comparable to that observed before the shutdown, was observed. Ten focal mechanisms for seismic events originating at or down to 2 km below mine level nearly all imply reverse faulting, consistent with previous results and the inferred tectonic stress field. Enigmatic events recorded with all dilatational first motions can be fit with double-couple normal-faulting solutions if they in fact occurabove mine level, perhaps reflecting overburden subsidence. If these events are constrained to occur at mine level, their first-motion distributions are incompatible with a double-couple source mechanism.     

Extending the double difference location technique—improving hypocenter depth determination

Locating the seismic event hypocenter is the very first issue undertaken when studying any seismological problem. Thus, the accuracy of the obtained solution can significantly influence consecutive stages of an analysis, so there is a continuous demand for new, more efficient, and accurate location algorithms. It is well recognized that there exists no single universal location algorithm which performs equally well in all situations. Seismic activity and its spatial variability over time, seismic network geometry, and the controlled area’s geological complexity are factors influencing the performance of location algorithms. For example, in the case of mining applications, the planarity of the seismic network usually operated at the exploitation level becomes an important issue limiting the accuracy of location of the hypocenter depths. In this paper, we push forward the discussion on the performance of the newly proposed location algorithm called the extended double difference (EDD), concentrating on the reliability of source depth estimation for mining-induced seismic events. We demonstrate that the EDD algorithm very efficiently uses information originating from the nonplanarity of the seismic network, improving the hypocenter depth estimates with respect to the classical double difference technique. Methodological considerations are illustrated by real data analysis of selected events from the Rudna copper mine (Poland).     

Investigation of mining-induced earthquakes in Iran within a time window of 2006–2013

Iran is one of the most seismically active areas of the world and frequently suffers from destructive earthquakes. Rare studies on anthropogenic-induced seismicity in Iran may be related to less attention to triggered events and more concern to natural origin of earthquakes. Hence, the present study as a frontier research aims to investigate the mining-induced earthquakes in Iran. For this purpose, distribution of ~?76,000 seismic events was investigated between the years 2006 and 2013. This study considered a correlation test to investigate the possible mining triggering of the seismic events based on a network of 194 geographical pixels (1°?×?1°) in ArcGIS. Results conveniently confirmed a positive meaningful relation between all earthquake events with magnitudes M?>?0.5 and mining activities in Iran (R?=?0.42). Detailed results confirmed that the most of earthquake swarms (at least ~?60%) had mining-induced origin, which were spatially located in same pixels of metallic mineral mining sites. The correlation test between earthquake swarms and mining activities indicated positive and meaningful relationships in four regions of Alborz, Kopet Dag, Kerman, and Zagros, respectively (R?=?0.61, 0.54, 0.51, and 0.50). Hence, aforementioned seismic regions exposed sensitive seismic responses toward mining triggering effects in Iran.     

Application of Kalman Filtering Techniques for Microseismic Event Detection

—?Microseismic monitoring systems are generally installed in areas of induced seismicity caused by human activity. Induced seismicity results from changes in the state of stress which may occur as a result of excavation within the rock mass in mining (i.e., rockbursts), and changes in hydrostatic pressures and rock temperatures (e.g., during fluid injection or extraction) in oil exploitation, dam construction or fluid disposal. Microseismic monitoring systems determine event locations and important source parameters such as attenuation, seismic moment, source radius, static stress drop, peak particle velocity and seismic energy. An essential part of the operation of a microseismic monitoring system is the reliable detection of microseismic events. In the absence of reliable, automated picking techniques, operators rely upon manual picking. This is time-consuming, costly and, in the presence of background noise, very prone to error. The techniques described in this paper not only permit the reliable identification of events in cluttered signal environments they have also enabled the authors to develop reliable automated event picking procedures. This opens the way to use microseismic monitoring as a cost-effective production/operations procedure. It has been the experience of the authors that in certain noisy environments, the seismic monitoring system may trigger on and subsequently acquire substantial quantities of erroneous data, due to the high energy content of the ambient noise. Digital filtering techniques need to be applied on the microseismic data so that the ambient noise is removed and event detection simplified. The monitoring of seismic acoustic emissions is a continuous, real-time process and it is desirable to implement digital filters which can also be designed in the time domain and in real-time such as the Kalman Filter. This paper presents a real-time Kalman Filter which removes the statistically describable background noise from the recorded seismic traces.     

长江三峡工程周边地区的采矿诱发地震

地震观测和实地调查表明,长江三峡工程周缘广泛存在采矿诱发地震。采矿诱发地震主要是由于矿山采空区的出现使浅部地壳差应力增大岩体失稳而产生的,它们不同于天然地震。水库蓄水前应对这些地震的背景进行详细研究。在进行区域地震震情分析和预报、区域应力场及活动断裂研究时应严格加以区别     

Fractal analysis of earthquake swarms of Vogtland/NW-Bohemia intraplate seismicity

The special type of intraplate microseismicity with swarm-like occurrence of earthquakes within the Vogtland/NW-Bohemian Region is analysed to reveal the nature and the origin of the seismogenic regime. The long-term data set of continuous seismic monitoring since 1962, including more than 26000 events within a range of about 5 units of local magnitude, provides an unique database for statistical investigations. Most earthquakes occur in narrow hypocentral volumes (clusters) within the lower part of the upper crust, but also single event occurrence outside of spatial clusters is observed. Temporal distribution of events is concentrated in clusters (swarms), which last some days until few month in dependence of intensity. Since 1962 three strong swarms occurred (1962, 1985/86, 2000), including two seismic cycles. Spatial clusters are distributed along a fault system of regional extension (Leipzig-Regensburger Störung), which is supposed to act as the joint tectonic fracture zone for the whole seismogenic region. Seismicity is analysed by fractal analysis, suggesting a unifractal behaviour of seismicity and uniform character of seismotectonic regime for the whole region. A tendency of decreasing fractal dimension values is observed for temporal distribution of earthquakes, indicating an increasing degree of temporal clustering from swarm to swarm. Following the idea of earthquake triggering by magma intrusions and related fluid and gas release into the tectonically pre-stressed parts of the crust, a steady increased intensity of intrusion and/or fluid and gas release might account for that observation. Additionally, seismic parameters for Vogtland/NW-Bohemia intraplate seismicity are compared with an adequate data set of mining-induced seismicity in a nearby mine of Lubin/Poland and with synthetic data sets to evaluate parameter estimation. Due to different seismogenic regime of tectonic and induced seismicity, significant differences between b-values and temporal dimension values are observed. Most significant for intraplate seismicity are relatively low fractal dimension values for temporal distribution. That observation reflects the strong degree of temporal earthquake clustering, which might explain the episodic character of earthquake swarms and support the idea of push-like triggering of earthquake avalanches by intruding magma.     

The Alabama,U.S.A., seismic event and strata collapse of May 7, 1986

On May 7, 1986, the residents of Tuscaloosa, Alabama, felt a seismic event of local magnitude 3.6 that occurred at the same time as a rock burst and roof collapse in an active longwall coal mine. Visual inspection of the seismograms reveals a deficiency in energy at frequencies above 20 Hz compared to tectonic earthquakes or surface blasts. The predominance of energy below 5 Hz may explain reports of body wave magnitudes (m _b ) greater than 4.2. Also, 1.0 Hz surface waves were more strongly excited than body waves and may explain local felt effects more typically associated with greater epicentral distances. All recorded first motions were dilatational. The concentration of stations in the northern hemisphere allows reverse motion on an east-trending near-vertical plane or strike-slip motion on northwest or southeast trending planes. The reverse focal mechanism is preferred, because the area of roof collapse and the area of active longwall mining are located between two east-striking loose vertical fracture zones. The characteristics of the seismic event suggest that it might have been sudden shear failure resulting from accumulated strain energy in overlying strata behind an active longwall. Although an alternate interpretation of the focal mechanism as an implosion or shear failure in the strata above previously mined out areas is also allowed by the first motion data, this alternate intepretation is not supported by geological data.     

A GIS approach to seismic risk assessment with an application to mining-related seismicity in Johannesburg,South Africa

The majority of seismic activity in South Africa is related to extensive mining operations, usually in close proximity to densely populated areas where a relatively weak seismic event could cause damage. Despite a significant decrease in mining operations in the Witwatersrand area, the number of seismic events appears to be increasing and is attributed to the acid mine drainage problem. The increased seismicity is raising concern amongst disaster management centres and in the insurance industry. A better understanding is required of the vulnerability and the size of the potential loss of people and infrastructure in densely populated Johannesburg and its surrounding areas. Results of a deterministic seismic risk, vulnerability, and loss assessment are presented by making use of a geographic information system (GIS). The results illustrate the benefits of using GIS and contribute to a better understanding of the risk, which can assist in improving disaster preparedness.     

矿山地震与瓦斯突出的相关性及其在震源物理研究中的意义

介绍了矿山地震与瓦斯突出等煤矿灾害及成因, 并通过若干煤矿瓦斯突出和矿山地震的同震现象,论述了这些灾害在动力过程中的内在关系.这些震例表明,在高瓦斯煤矿, 矿山地震与瓦斯突出存在密切的相关.认为较大矿震伴随瓦斯的低值延时响应可能是瓦斯突出的预警信号.从矿震定位、震源机制、矿震成因、瓦斯突出条件等分析了矿震与瓦斯突出相关的机理. 进一步介绍了瓦斯流体对矿震的触发作用,尤其超临界流体的特殊性质在矿震发生中的重要作用.依据矿震与构造地震在机制方面的相似性, 讨论了上述结果在震源物理研究中的意义.      

Changes in the Frequency-Energy Distribution of Seismic Events During Mining in the Ostrava-Karviná Coal Field

The relation between the number of seismic events N and their magnitude and/or energetic magnitude is one of the basic relations in the statistics of earthquakes and mining-induced events. A detailed analysis of the b-values in the relation log N =a–b logE for the induced seismic events occurring in the Ostrava-Karviná Coal Basin is given in this paper. The least-squares method, as well as the maximum likelihood method were applied in solving this problem mathematically. Based on the results of frequency-energy distribution analysis, it was proved that the variations of b-values in space and time do exist, and that these values are inversely proportional to the level of induced seismicity influenced by the various mining activities being carried out in-situ. This statement could, in general, be supported by the fact that lower b-values correspond to higher seismic activity, while higher b-values correspond to low and moderate seismic activity.