Geophysical Journal International

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1 Geophysical Journal International Geophys. J. Int. (2011) 186, doi: /j X x Rock mechanical analysis of a M l = 4.0 seismic event induced by mining in the Saar District, Germany Michael Alber 1 and Ralf Fritschen 2 1 Ruhr-University Bochum, Bochum, Germany. michael.alber@rub.de 2 DMT GmbH & Co. KG, Essen, Germany Accepted 2011 April 19. Received 2011 April 19; in original form 2010 July 15 SUMMARY On 2008 February 23 a 4.0 magnitude seismic event occurred in close proximity to a deep underground longwall coal mine in the German Saar mining district. Preceding this event was a series of smaller seismic events, which can be traced back to geological structures. A seismic network was installed to observe and analyse the seismic events. Underground core drilling was performed to obtain rock samples for rock mechanical testing. Possible causes for the events, failure of rock mass or slip on unknown faults, are evaluated by numerical methods. Failure of rock mass is excluded as a cause since the stresses are well below the strength of the rock mass. Seismic events are of similar fault plane strike and are organized in several clusters, associated with local and regional tectonic elements. It was found from backcalculation that fault planes with friction angles as low as 8 must have been present. The role of pore pressure on the fault stability is discussed and found to play no role in generating the seismic events. Finally, the question of mining in a rock mass obviously close to failure is addressed and it was concluded that any mining activity would lead to a significant seismic response. Key words: Geomechanics; Rheology and friction of fault zones; Mechanics, theory and modelling. 1 INTRODUCTION Like in many other regions worldwide, coal mining in Germany is accompanied by mining induced seismic events. Today three areas hold active coal mines, one of them is located in the Saarland, Germany. Although industrial mining in the Saar region started in the middle of the 18th century, only a few cases of mining induced events are reported before the 1990s. In 1997, however, several seismic events were felt by the population of villages in the area of the Saar mine. As a result, the Saar mine decided to install a local seismic network to determine the peak particle velocities (PPVs) on site and to investigate possible relations between mining and seismicity. Quickly it became clear that the observed seismicity was directly related to mining activities in the seam Schwalbach. Almost all seismic events were located in the vicinity of active longwall faces. In 2006, mining of the seam Schwalbach started in the new field Primsmulde, planned as the mine s new main production field. The seam Schwalbach is termed after its location near a village close to Saarbrücken, Germany. The term Primsmulde refers to a syncline named after the river Prims. Mining in field Primsmulde was executed by the longwall method. This mining method means that a long wall of coal is mined in a single slice. The longwall panel (the block of coal that is being mined) is typically 3 4 km long and m wide. Here a double longwall was used with an overall span of 700 m and an expected length of 2500 m. Panel I was typically 100 m ahead of panel II. From the experience of mining induced events in other fields (Fritschen 2010) it was expected that seismic events would originate close to the seam. However, in the course of mining the double panel it turned out that one part of the mining induced seismic events originate in strata some 300 m above the seam while other events were located at seam level. The objective of this study was to provide rock mechanical reasons for the seismic events. Accordingly, a seismic network was installed and extended as the seismic events transpired, rock samples from the possible locations of seismic events were obtained by underground core drilling, laboratory tests were carried out to estimate strength and stiffness properties of the rock mass. Numerical modelling was employed to examine the possible causes for the seismic events by comparing strength characteristics of rock mass and faults with the applied stresses. 2 GEOLOGICAL SETTING OF THE MINE The mine is located on the western part of the Saar Nahe Basin (Fig. 1). The SW NE striking Saar Nahe Basin in SW Germany is one of the largest Permo-Carboniferous basins in the internal zone of the Variscides. Its north extension is denoted by the Hunsrück Boundary Fault (HBF). The structural style within the basin is characterized by normal faults parallel to the basin axis and orthogonal transfer fault zones. The carboniferous rocks are located in the northern part of the Saar Nahe Basin (Fig. 1) and show typically synclines and anticlines. The maximum sedimentary accumulation in the basin is some 7.5 km. Coal bearing measures start in the GJI Seismology C 2011 The Authors 359

2 360 M. Alber and R. Fritschen Figure 1. Simplified geological map of the Saar Nahe Basin showing major tectonic elements and sedimentary units. HBF denotes the Hunsrück Boundary Fault (simplified and modified after Stollhofen 1998). youngest Westfal D up to Stefan C and are extracted in deep mines at depths around m below surface. At a scale of approximately 4 km ( mine scale) the strata are dipping gently to N/NE with 15. The longwalls are located in a rock mass with no previous mining operations. Stresses at the site were measured by the hydraulic fracture method and reported to be (1) σ v = z (MPa) (2) σ h = σ v (MPa), direction 57 (3) σ H = σ v (MPa), direction 147. For the average depth z = 1411 m of the longwall under investigation the following in situ stresses are assumed: σ v = 37.4 MPa, σ h = 20.7 MPa and σ H = 43.2 MPa. Those stresses are aligned in accordance with the general stress orientation in central Europe (Reinecker et al. 2005). The ratio σ H /σ h is about 2, which is valid for many intraplate crustal regions for a depth of approximately 1 km (Rummel 2002). In the local situation, the major horizontal stress is oriented perpendicular to the long axis of the basin and in this case, perpendicular to the long axis of the longwall panels. The stress values obtained were not reduced by pore pressure estimates since all drill holes were reported to be dry and the mine personnel reported bone-dry conditions. The coal bearing strata in the Saar Nahe Basin belong to the Upper Carboniferous (Pennsylvanium, 318 to 299 Ma ago). Sediments in the basin consist of 7500 m thick molasse-type coal bearing strata with conglomerate, sandstone, siltstone and claystone as the dominant rock types. The thrusts are all oriented NE SW and are mainly steep dipping and are dissected by many steep dipping NW SE striking normal faults (Fig. 1). Those major features are shown in the left rosette plot in Fig. 2. Information about faults on the mine scale was obtained through deep boreholes, 3-D seismic campaign and surface as well as underground mapping. The orientation of the major tectonic features is shown in the centre rosette plot in Fig. 2. During development of the tunnels around the panels every 200 m drillholes were executed in the hangingwall and in the footwall, respectively. Here, small scale discontinuity orientations were observed from borehole inspections and summarized in the right rosette plot in Fig. 2. By comparison, the discontinuity patterns remain consistent on all scales (Fig. 2.). The setup of the field along with local faults as well as the in situ stresses at depth is given in Fig SEISMIC EVENTS AND THEIR ANALYSES 3.1 Local seismic network After several mining induced seismic events were felt in 1997 by the population living in the vicinity of the Saar Mine, a seismic

3 M l = 4.0 seismic event induced by mining 361 Figure 2. Orientation of tectonic elements. Major faults at 50 km scale (left), faults at 10 km scale (middle) and joints at 2 km scale (right). Figure 3. Plan view of the coal field with faults, panels and in situ stresses at depth. network of initially four stations was installed at surface. The network was not optimized for locating seismic events, but was primarily installed for vibration control. The recording was done according to the German vibration standards at building basements with threecomponent geophones having flat particle velocity responses in the range between 1 Hz and 80 Hz. Over the course of the measurements the network was continuously extended to 40 stations at surface and 11 stations in the mine. The locations of the stations are shown in Figs 4 and 5. The seismic stations recorded the seismicity in a triggered mode. All vibrations above a trigger level of 0.1 mm s 1 at surface and above 0.05 mm s 1 in the mine were stored as velocity seismograms and automatically transferred to a central processing station. The surface seismic stations were equipped with a DCF77 time receiver, which provided a time reference with an accuracy of approximately 2 ms. The underground stations were synchronized with a GPS clock at surface, with an accuracy of several microseconds. Event locations were carried out with a layered velocity model derived from sonic logs of nearby exploration wells. All events recorded by more than four stations were located. Since some of these events showed small impacts in the mine, the location accuracy could directly be estimated. When trying to locate all three spatial coordinates with a Geiger-type inversion algorithm, the epicentres could be determined with an accuracy of ±30 m. The error in focal depth was, however, often greater than 150 m. Also applying individual station corrections did not help to get a better depth estimate probably due to the limited accuracy of the surface station s time base (approximately 2 ms). After trying several approaches to improve the accuracy of the depth location, we decided to calculate only the epicentre coordinates using the seismic onset times from surface and underground stations together and estimated the focal depth directly by comparing the onset differences of the six nearest

4 362 M. Alber and R. Fritschen Figure 4. Location of seismic stations (triangles plane view). Figure 5. Location of subsurface seismic stations relative to Seam Schwalbach (cross-section). underground stations. The focal depth of the events could in this way be constrained to approximately ±30 m. Events with maximum PPVs at surface of more than approximately 1 mm s 1 were additionally recorded by the regional seismological network of Freiburg. Fig. 6 shows that the local magnitudes can be roughly related to the maximum PPV values that were recorded by the surface stations of the local network. The trigger levels of the surface stations of 0.1 mm s 1 correspond therefore to a local magnitude of approximately 1.3 and the perception threshold, which is in the range of 0.5 mm s 1, to a magnitude of approximately Seismicity of the Primsmulde field In 2004, the mine started to develop the coal seam Schwalbach in the new coalfield at a depth of approximately 1400 m below surface. The development of the field started 2 yrs before coal extraction with the driving of tunnels, which later encompassed the longwall panels. As with the older fields of Saar mine the extraction of seam Schwalbach was planned as a double panel system. It was not known whether this system in the new part of the mine would induce seismic events of intensities strong enough to cause damage to surface infrastructure (i.e. according to the German vibration standard events with PPVs of more than 5 mm s 1 ), because at that time both double panel systems with strong seismic events, that is, seismic events with PPVs of more than 12 mm s 1 and local magnitudes above 3 (old field I), and double panel systems without significant seismicity, PPVs of less than 1 mm s 1 and magnitudes below 2 (old field II), had been observed at this mine (cf. Fritschen 2010). A reliable explanation of the contrary seismicity found in these two fields was not available at that time. However, it was soon clear that the extraction of coal in the new field could lead to strong seismic events. In 2005 May, after driving the first tunnels, a seismic event of M l = 3.3 was induced. This was the first time that a seismic event, which was considerably felt at surface, was recorded in a German coal mine just by developing a new field, that is, even before mining started. Normally all events in the vicinity of German coal mines that are strong enough to be felt at surface can be related to coal extraction. This event was localized in the middle between two tunnels approximately at seam level (Fig. 7). No further seismicity was observed after that event in the field, until mining started in 2006 October. But even then seismicity was moderate, with only few events that were hardly detected and not felt at surface. The situation changed in 2007 June, approximately 9 months after start of longwall operations, when a magnitude 3.6 event with a PPV of 29mm s 1 was induced, more than 300 m in front of the double panel system. Both events, the event of 2005 May and the event of 2007 June, were located at seam level. The events marked the beginning of a series of strong seismic events, that is, of seismic events with a PPV of more than 5 mm s 1 (M l > 2.7). The epicentres of all strong events were located in an area of around 1 km 1 km. The focal depths of the seismic events that followed the 2007 June seismic events were, however, all located approximately 300 m above seam Schwalbach. The series of strong seismic events culminated in a M l = 4.0 event with a PPV of 94 mm s 1 on 2008 February 23. Mining was immediately stopped, notwithstanding, a last single event of PPV 1.2 mm s 1 occurred the next day. The M l 4.0 event caused damage to several buildings on the surface. 4 GEOMECHANICAL SETTING 4.1 Near seam The lithology around the longwall is known through numerous exploration boreholes and geological mapping while developing the tunnels. Additionally two high quality double barrel core drillings were executed: one borehole 50 m below the seam and one drilling 120 m upwards (Fig. 8). The typical coal measure rocks sandstone (fine to coarse grained) and siltstone in varying mixtures were encountered. The strata were all dry, typically flat lying and gently dipping with rather constant thickness. Of major concern were a few fluvial channel deposits known within the strata. Those sandstone channels were thought to be very strong compared to siltstone and claystone and it was believed that their failure might have been the cause for mining induced seismic events (Fritschen et al. 2004). Thus, the focus was first on the geomechanical characterization of

5 M l = 4.0 seismic event induced by mining 363 Figure 6. Correlation between PPV (v Max ) and local magnitude M l. rock mass. By measuring and rating certain rock mass features such as intact rock strength, frequency and condition of discontinuities a value is achieved that ranges from RMR = 100 (equivalent to intact rock) to RMR = 0 (equivalent to cohesionless soil). The lowest RMR was found in the coal seam (RMR = 40). The coal measure rocks are of good quality (50 < RMR < 75) with only a few exceptions. The Rock Mass Ratings were used in conjunction with the Hoek Brown failure criterion to estimate the uniaxial compressive rock mass strength σ CM as well as the rock mass deformation modulus E M (Hoek & Brown 1997). With this approach the RMR value is used to scale down the intact rock strength to in situ properties, or rock mass strength. If the RMR were 100 then the rock mass strength is close to the intact rock strength. With decreasing RMR values the rock mass strength is scaled with a negative exponential function. A similar procedure exists for the rock mass deformation modulus E M. This method is common in the rock engineering practice to estimate strength and deformation characteristics of a rock mass. With the exemption of a strong layer of fine-grained sandstone, 10 m above the hangingwall, strata have more or less similar rock mass strength and rock mass stiffness (Fig. 10). Figure 7. Development tunnels into the Schwalbach seam at a depth of approximately 1400 m below surface (approximately 1200 m below sea level). In the middle of the field a magnitude 3.3 event was induced at seam depth (circle) in 2005 May, before any mining in the field started. the immediate 100 m hangingwall. Towards this, an extensive laboratory program with the focus on uniaxial and triaxial strength tests was carried out according to the suggested methods of the ISRM (Ulusay and Hudson 2007). Some results are summarized in Fig. 9 and it may be seen that the coarse-grained channel sandstones do not exhibit significantly higher strength compared to the surrounding rock types. In contrast to the common belief, the fine-grained sandstones and siltstones form strong beds and exhibit higher stiffness than the channel rocks. The cores were evaluated using the Geomechanics Classification after Bieniawski (1989) without using the rating adjustment for discontinuity orientation. The Geomechanics Classification or Rock Mass Rating (RMR) system is a method to evaluate the quality of a m above seam Numerous mining induced seismic events originate in strata some 300 m above the seam. Accordingly, a double barrel drill was vertically executed up to 320 m height above seam to obtain high quality rock cores for further investigation (Fig. 8). No fault zone was detected from inspection of the cores. Special focus was placed on the mechanical characterization of the last 30 m of cores as the possible origin of seismicity. The laboratory program included tests on Brazilian Tensile Strength (17), Uniaxial Compressive Strength (23), Triaxial Compressive Strength (95) and residual Triaxial Compressive Strength (87). In addition, dry densities, compressional wave velocities and shear wave velocities were determined (66). For triaxial testing the confining pressures were up to 25 MPa to place the rocks in primary as well secondary stress conditions that

6 364 M. Alber and R. Fritschen Figure 8. Fault plane solutions (lower hemisphere equal area projection) of seismic events having peak particle velocities > 5 mm s 1 or M l >2.7.The lines show the roadways surrounding both panels. The triangles mark the positions of underground stations. BH1 indicates the location of the boreholes for recovering cores 50 m below seam and 120 m above seam. BH2 indicates the location of the boreholes for recovering cores up to 320 m above seam level. The numbered lines within the panels indicate the position of the face by months. The event with M l = 4.0 is denoted by the fault plane solution in dark red. may be encountered at a depth of 1200 m below surface, that is, 300 m above the double panel. The rock types recovered from drilling at 300 m above the panels are interbedded siltstones, fine- to coarse-grained sandstones and conglomerates. It was first attempted to characterize each rock type separately but the interbedding was so close that only few specimen with only one individual rock type could be prepared. The Triaxial Compressive Strength Tests left the specimen in a fractured state. The inclination of the fractures is typically 20 to 30 from direction of axial loading (cf. Fig. 11). The specimens were again triaxially stressed up to their residual strength. With this test the existing fracture is loaded with high shear stresses and low normal stresses so that slip (or failure) of the fracture is enforced. This strength of a specimen fractured in the most unfavourable orientation with respect to the applied stresses is assumed to be the lower boundary of the rock mass strength. Mohr-circles of peak and residual strengths of all rock types (Fig. 12) were used to estimate peak and residual strengths values. Strength and elastic parameters of sandstone and siltstone are summarized in Tables 1 and 2. Figure 9. Schematic view of rock types and some petrophysical parameters (σ C, uniaxial compressive rock strength; E, static Young s modulus; v P, compressional wave velocity) from double core drilling into the hangingwall strata. 4.3 Geomechanical modelling To evaluate the possible failure mechanism of both events that took place at the level of the seam (±25 m) the 3-D boundary element package EXAMINE 3-D (Rocscience 2008) was employed to

7 M l = 4.0 seismic event induced by mining 365 Figure 10. Geotechnical profile depicting uniaxial compressive rock strength σ C, Rock Mass Rating, rock mass deformation Modus E M and the uniaxial compressive rock mass strength σ CM of the 16 geotechnical units. Figure 11. Setup for obtaining the compressive triaxial strength of intact rock (left-hand side). The residual triaxial strength was obtained by loading the fractured specimen again to enforce slip of the existing, e.g. unfavourably oriented, fracture. estimate stresses between the development tunnels as well as ahead of the double panel. This software is an established numerical tool in the mining industry and is scientifically used to estimate 3-D stresses around underground excavations (Vijayakumar et al. 2000; Diederichs et al. 2004). The tunnels modelled have dimensions of 700 m 5m 5m(length width height) subjected to an in situ stress field as shown in Fig. 3. Stresses were evaluated at grid points between the tunnels with spacing of 1 m. The tunnels of dimension 5 5 m did not greatly influence the state of stress, a slight reduction (<<1 MPa) of the major principal stress was numerically evaluated. By the same token, the numerical model shows stresses 300 m ahead of the face similar to the in situ stresses with a slight reduction (<<1 MPa) of the minor principal stress oriented towards the double panel. A comparison of induced stresses versus rock mass strength (Fig. 13) suggests that even for the most adverse rock mass condition, that is the low quality shale immediately above seam Schwalbach, the rock mass is stronger than the applied stresses. Even if the failure may be initiated, there will be not enough energy released from these poor quality strata with a probably low shear modulus to produce events of magnitudes 3.3 and 3.6, respectively. It was concluded from those numerical models that there must exist some unknown zone(s) of weakness in the rock mass. Application of the Mohr Coulomb-criterion suggests a mobilized angle of friction φ mob 20 which would resist shearing of conjugate planes as shown in the insert of Fig. 14. Particularly the SE NW striking plane coincides with discontinuity/fault orientation as shown in Figs 2 and 3 and slickensided faults in shale with very low friction angles are well known in coal measure rocks (Barton 2007). Fig. 8 depicts the locations of the stronger seismic (PPV > 5 mm s 1 or M l > 2.7) events with respect to the panel faces as well as the strike of the typically very steep dipping fault planes. The auxiliary flat lying fault planes were not considered to represent probable slip planes due to their low shear stresses. The fault plane solutions, that is, the determination of focal plane orientation and direction of relative movement on that plane, were calculated using

8 366 M. Alber and R. Fritschen Figure 12. Strength envelopes in red of intact rock (left-hand side) and residual strength (right-hand side) of fractured rock. The blue Mohr circles depict the state of stress (σ 1 and σ 3 ) at failure. The strength envelopes were determined by a least square fit. Table 1. Average geomechanical data from laboratory tests of two rock types from 300 m above seam (σ C, uniaxial compressive strength; v P, compressional wave velocity; E, Young s Modulus; m i, constant in the Hoek Brown failure criterion). Rock type (no. of tests) σ C (MPa) v P (km s 1 ) E(GPa) m i Sandstone, coarse (12) Siltstone/claystone (39) Table 2. Peak (intact) and residual strength in terms of Mohr Coulomb strength parameters (cohesion c, friction angle φ) of the rocks described in Table 1. Rock type Intact Intact Residual Residual c (MPa) φ ( ) c (MPa) φ ( ) Sandstone coarse Siltstone/claystone the polarities of the recorded P waves and the polarisation angles of the S waves. We used a grid search algorithm with a resolution of 5. All stronger seismic events (M l > 2.7) could be explained by shear failures. Subsequent events are of similar strike of the fault plane and organized in clusters. The event locations are typically high above the panels and few are at panel level. In the following focus will be placed on the stronger events high above the seam. In a first step the stresses at the respective depths were estimated and compared with the intact and residual rock strength. The numerical model covers an excavation 750 m long 700 m width and 2 m height with panel m ahead of panel 2. The stresses are computed on planes at 250 m, 300m and 350 m above the seam an grid points with spacing 1 m (Fig. 15). The principal stresses are plotted in Fig. 16 versus the lowest boundaries of intact rock strength and residual rock strength, respectively. The stresses are well below the strength and failure of rock or rock mass is not expected. It was concluded from these numerical models that not rock or rock mass failure but slip on existing planes/zones of weakness might be the source of seismic events. The planes/zones of weakness are presumably in similar directions as shown in Fig. 2. These tectonic elements originated during formation of the basin in the Variscan orogenesis and are still stressed in the same direction by the Alpine orogenesis. In a next step three planes of weakness with orientation (dipdirection/dip) 45 /90, 135 /90 and 170 /90, respectively, were se- Figure 13. States of stress (in situ and induced stresses) for the seismic event caused by developing the tunnels. The strength of most unfavourable rock mass (interbedded coal and shale) is close but always higher than the applied stresses. lected to represent tectonic elements on all scales. The assumed fault planes were subjected to numerical analyses for their behaviour around the longwall panels. Towards this end the ubiquitous joint approach (Kazakidis & Diederichs 1993) was used. This approach allows interpreting relevant stress data, such as normal and shear stresses, with respect to an omnipresent joint of given orientation. The planes of weakness were assumed to be cohesion less, as it is typically the case with joints or faults (Barton 1976; Byerlee 1978). The mobilized friction angle φ mob at various positions around the longwall panels was computed for the different planes of weakness. The friction angle φ is derived from the linear Mohr Coulombs criterion with cohesion set to zero as explained above so that τ = σ N tanφ. τ is the shear strength of a plane subjected to a normal stress σ N and an internal angle of friction φ. For any point on a given plane the shear and normal stresses can be estimated from numerical modelling. The mobilized friction angle for limit

9 M l = 4.0 seismic event induced by mining 367 Figure 14. Numerically evaluated state of stress for the first seismic event caused by developing the tunnels. The dotted lines in the insert show the most probable orientations of planes of failure. Figure 15. Numerical setup and the planes in which the stresses were evaluated. Panel 1 is 100 m ahead of panel 2 and the planes cover an area of 750 m 750 m at various heights above the seam. The orientations of the principal stresses are shown and the magnitudes are σ H = σ 1 = 43.2 MPa, σ V = σ 2 = 37.4 MPa and σ h = σ 3 = 20.7 MPa, respectively. equilibrium is computed by φ = arctan (τ/σ N ). The mobilized friction angle indicates the demand on the strength of the discontinuity in terms of friction; if it exceeds the actual friction angle, the plane slips. The first cluster of events (Fig. 17) on NW SE striking planes occurred approximately 200 m ahead of the face of panel 2. The mobilized friction angle at those locations was calculated to range from 20 to 24, that is, any plane with a lower friction angle would fail. The next clusters (Fig. 18) of events were localized 100 m ahead of panel 2 and involved ENE WSW striking planes. Here the mobilized friction angles were estimated to be 8 10.Thefinal events, which took place when panel 1 was 200 m ahead of panel 2, were mainly located m behind the face of panel 1 and high above the seam (Fig. 19). Here, the mobilized angle of friction on these NE SW striking planes was calculated to be as low as DISCUSSION Due to an extensive seismic network with both surface and subsurface stations, mining induced seismic events could be located

10 368 M. Alber and R. Fritschen Figure 16. Principal stresses evaluated at planes m above seam level. The strengths of both intact and fractured rock are higher than the stresses, that is, there is no stress state in the rock mass between 250 m and 350 m that may lead to a fracturing of intact or even to a failure of already fractured rock. and analysed for their fault plane solution. Laboratory tests and underground mapping led to rock strength estimates at depth. Rock stress measurements yielded results consistent with stress data from central Europe and enabled numerical modelling with the goal of analysing the failure mechanisms of the seismic events. Mining of such a huge underground excavation with dimension 700 m wide and more than 1000 m long lead to large scale stress redistribution up to 350 m above the seam. This means that stresses at a distance of up to 350 m above or below the longwall face are different from the in situ stresses. In contrast, stresses at the seam level diminish soon to the in situ levels. It has been shown that it is unlikely that the events occurred as a result of stress driven failure of the rock mass as the triaxial rock mass strength of even the most unfavourable rock mass is higher than the induced stresses. Consequently, all events may be contributed to the category activation of faults as discussed by Hasegawa et al. (1989), Boler et al. (1997), Gale et al. (2001) or may be addressed as triggered events as distinct from induced events in the narrow sense of this term (Fritschen 2010). The analysis of a seismic event induced by driving solely the tunnels suggests that an SE NW striking plane may slip when the actual friction angle of the plane falls below 20. This low frictional strength is common in slickensided faults in shale and the plane in consideration is in accordance with the existing ones (cf. Fig. 2). The analyses of the events which occurred during the longwall excavations revealed insights in the rock mass around the two panels: (1) the events appeared in clusters either high above the seam or at seam level, (2) aside from the final events, hypocentres were located well ahead of the faces; (3) the events did not occur at locations where the highest friction angle would be mobilized. The existence of several clusters of events suggests that the rock mass features distinct zones with planes of weakness, which are activated by the advancing longwalls. The mobilized friction angles range from only 8 up to 32. Those friction angles are well below Byerlee s law (Scholz 2002) in the low stress range where φ are the critical friction angles when faults start to slip. However, slickensides in fine-grained sediments such as shale and siltstone may indeed show very low friction angles. The notion that there exist distinct zones with planes of weakness is further supported by the fact that slip was not induced in locations where the highest friction angle was required. Moreover, the orientation of the planes of weakness coincides with the orientation of tectonic elements (Fig. 20).

11 M l = 4.0 seismic event induced by mining 369 Figure 17. Mobilized friction on ubiquitous NW SE (45 /90 ) striking planes of weakness. The straight lines indicate the panels and the circle denotes the location of the events approximately 150 m ahead of the longwall and 300 m above the seam. Figure 19. Mobilized friction on ubiquitous NW SE (135 /90 ) striking planes of weakness evaluated at a plane 300 m above seam level. The straight lines indicate the panels. The black circles denote the locations of the events ahead of the longwalls. The grey circle shows the location of the final events within the area of panel 1. Figure 18. Mobilized friction on ubiquitous ENE WSW (170 /90 ) striking planes of weakness evaluated at a plane 300 m above seam level. The straight lines indicate the panels and the circle denotes the location of the events approx. 100 m ahead of panel 2. It has been shown (Alber et al. 2009) that multiple seam mining with remnant pillars and gob-solid boundaries may lead to stress concentrations which exceed the strength of the rock mass leading to mining induced seismic events of magnitudes up to M l = 3.0. Here, where no old mine workings are involved, it is evident that not the rock mass strength but the strength of existing planes of weakness within the rock mass is exceeded, leading to the activation of faults which were probably generated during the Variscan orogenesis. The observed seismic events were attributed to slip on existing fault planes. The possibility exists that water pressure may have played a role in the failure of the rock mass. When examining the stress and strength characteristics as shown in Fig. 16 it may be deduced that water pressure in the order of P = 8.5 MPa is necessary to initiate failure of the rock mass (Fig. 21). This translates to a hydraulic head of approximately 850 m, which was not observed during mining activities. By the same token, the shear strength of a joint or fault is affected by joint water pressure. The necessary discontinuity water pressure to initiate slip of either fault orientation was computed (Fig. 22). If the NW SE and NE SW striking faults had a friction angle φ 35 as often assumed with faults then at least 5 MPa of water pressure were necessary to sufficiently reduce the normal stresses on the discontinuity for failure to occur. For the ENE WSW striking faults pore pressure in the order of 22 MPa is necessary for failure. Both values are high and could not be substantiated by observations.

12 370 M. Alber and R. Fritschen Figure 20. Clusters of seismic events with similar ranges of fault plane strike and associated mobilized friction angles. The inserts show the orientation of tectonic elements at 10 km (left-hand side) and 2 km scale (right-hand side). The event with M l = 4.0 is denoted by the fault plane solution in dark red. factor of safety ) of the three identified fault orientations 300 m above the seam. A double panel with one panel 100 m ahead of the second was oriented from strike 0 to 360 in steps of 45.Itmay be seen in Table 3 that irrespective of the orientation of the panels the ENE SWS striking faults will always fail while the other two fault orientations would suffer failure only for specific directions of mining. Figure 21. Necessary pore pressure p for initiating rock mass failure 300 m above the panels. Having examined and discussed the geomechanical situation at hand, now the question arises whether it would have been possible to avoid the seismic events by changing the direction of mining. Thus, numerical modelling was executed to estimate the ratio of shear strength to shear stress (equivalent to the engineering term 6 CONCLUSION This research has shown that barely elevated stresses would potentially trigger mining induced seismic events when extracting coal in a rock mass never mined before. Existing faults planes at distances of several 100 m above and in front of the excavation face may be activated. The apparently weak strength of the fault planes may be expressed by low friction angles. This study suggests that even without mining activities the earth s crust is close to failure, a conclusion which is supported by the observations of Cornet et al. (2007) made at the deep geothermal reservoir at Soultz-sous-forêt. It has been shown that it is unlikely that pore fluid pressure played a role in the mining induced seismicity as unreasonably high pressures should have been present for initiating failure. Even if the direction of mining was different the dominating discontinuities in either orientation would have been activated. It appears that in this location any mining activity could induce seismic events no matter which counter measures were pursued. ACKNOWLEDGMENT This research was partly supported by the collaborative research centre Rheology of the Earth, SFB 526 funded by the German

13 M l = 4.0 seismic event induced by mining 371 Figure 22. Necessary fault water pressure P to initiate slip on the assumed faults with different friction angles. The hatched area denotes the range of the Byerlee fault friction angle. Table 3. Factor of safety (shear strength/shear stress) of the three assumed faults as a function of the mining direction and friction angle φ. Fault Direction of mining Orientation φ / /90 26 >2 >2 1 2 >2 >2 >2 >2 >2 170 / Research Society (DFG). The mine provided the underground core drilling and the mine personnel were always helpful and open to discussions. The reviewers helped to significantly improve the manuscript. We appreciate their efforts. REFERENCES Alber, M., Fritschen, R., Bischoff, M. & Meier, T., Rock mechanical investigations of seismic events in a deep longwall coal mine, Int. J. Rock Mech. Min. Sci., 46, Barton, N., The shear strength of rock and rock joints, Int. J. Rock Mech. Min. Sci., 13, Barton, N., Rock Quality, Seismic Velocity, Attenuation, and Anisotropy. Taylor & Francis/Balkema, Leiden. Bieniawski, Z.T., Engineering Rock Mass Classifications. John Wiley and Sons, New York, NY. Boler, F.M., Billington, S. & Zipf, R.K., Seismological and energy balance constraints on the mechanism of a catasrophic bump in the book cliffs coal mining District, Utah, U.S.A. Int. J. Rock Mech. Min. Sci., 34(1), Byerlee, J., Friction of rocks, Pure appl. Geophys., 116, Cornet F., Bérard, Th., & Bourouis, S., How close to failure is a granite rock mass at a 5 km depth. Int. J. Rock Mech. Min. Sci., 44, Fritschen R., Loske, B., Uhl, O., & Polysos N., Seismological and geological investigations of seismic events at the Ensdorf mine, Das Markscheidewesen, 21, (in German). Fritschen, R., Mining induced seismicity in the Saarland, German, Pure appl. Geophys., 167, 77 89, doi: /s Gale, W. J., Heasley, K. A., Iannacchione, A. T., Swanson, P.L., Hatherly, P. & King, A., Rock damage characterization from microseismic monitoring, in Proceedings of the 38th U.S. Symposium of Rock Mechanics, Washington DC, pp Hasegawa, H.S., Wetmiller, R. J. & Gendzwell, D. J, Induced seismicity in mines in Canada An overview, Pure appl. Geophys., 129(3 4), Hoek, E. & Brown E.T Practical estimates of rock mass strength, Int. J. Rock Mech. Min. Sci. Geomech. Abstr., 34(8), Kazakidis, V.N. & Diederichs, M.S., Understanding jointed rock mass behaviour using a ubiquitous joint approach, Int. J. Rock Mech. Min. Sci. Geomech. Abstr., 30(2), Reinecker, J., Heidbach, O., Tingay, M., Sperner, B. & Müller, B., The release 2005 of the World Stress Map. Available at: (last accessed 2011 April 12). Rummel, F., Crustal stress derived from fluid injection tests in boreholes, in: In-Situ Characterisation of Rocks, pp eds. Sharma, V.M. & Saxena, K.R., Balkema, Lisse. Rocscience EXAMINE 3D, 3D Stress Analysis for Underground Excavations, Version , Toronto, CA. Scholz, C.H., The Mechanics of Earthquakes and Faulting, 2nd edn. Cambridge University Press, Cambridge.

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