MECHANISM OF SEISMIC EVENTS AND MECHANICS OF ROCKBURST DAMAGE LABORATORY STUDIES, VISUAL AND SEISMIC OBSERVATIONS

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1 MECHANISM OF SEISMIC EVENTS AND MECHANICS OF ROCKBURST DAMAGE LABORATORY STUDIES, VISUAL AND SEISMIC OBSERVATIONS Page Keynote Lecture: Constraints on behavior of mining- induced earthquake inferred from laboratory rock mechanics experiments A. McGarr, M. Johnston, M. Boettcher, V. Heesakkers and Z. Reches Keynote Lecture: Forensic rock mechanics, Ortlepp shears and other mining induced structures G. van Aswegen New insight into the nature of size dependence and the lower limit of rock strength B.G. Tarasov and M.A. Guzev Fault formation in foliated rock insights gained from a laboratory study X. Lei, T. Funatsu and E. Villaescusa In-situ monitoring and modelling of the rock mass response to mining: Japanese-South African collaborative research H. Ogasawara, G. Hofmann, H. Kato, M. Nakatani, H. Moriya, M. Naoi, Y. Yabe, R. Durrheim, A. Cichowicz, T. Kgarume, A. Milev, O. Murakami, T. Satoh and H. Kawakata Quasi-static fault growth in a gabbro sample retrieved from a South African deep gold mine revealed by multi-channel AE monitoring T. Satoh, X. Lei, M. Nakatani, Y. Yabe, M. Naoi and G. Morema Relating tilt measurements recorded at Mponeng gold mine, South Africa, to the rupture of an M 2.2 event P. Share, A. Milev, R. Durrheim, J. Kuijpers and H. Ogasawara Determining the proneness of rock to strainburst P.M. Dight, B.G. Tarasov and A.W. O Hare Seismic dynamic influence of rock mass tremors on roadways according to the orientation of the rupture plane in the tremor source K. Stec Testing of the source processes of mine related seismic events D. Malovichko and G. van Aswegen

2 QUASI-STATIC FAULT GROWTH IN A GABBRO SAMPLE RETRIEVED FROM A SOUTH AFRICAN DEEP GOLD MINE REVEALED BY MULTI-CHANNEL AE MONITORING T. Satoh, X. Lei Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Japan M. Nakatani Earthquake Research Institute, University of Tokyo, Japan Y. Yabe Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Japan M. Naoi Earthquake Research Institute, University of Tokyo, Japan G. Morema SeismoGen CC., Republic of South Africa Using a gabbro sample retrieved from the Mponeng gold mine, South Africa, at about 3 km depth, a triaxial compression experiment was conducted and the space-time distribution of acoustic emission (AE) events was investigated in detail. The sample was cored from a dyke of about 30 m thick where a mining induced earthquake of magnitude M = 2 occurred on 27 December, Little changes in P-wave velocity and almost linear strain response during hydrostatic compression to the pre-determined confining pressure (75 MPa) suggest that the sample contains few pre-existing microcracks. Microscope image of thin section of this rock shows strong foliation structures. The sample was loaded to fracture under a constant confining pressure of 75 MPa, and fractured in a brittle manner with a steeply inclined fracture plane. We could locate hypocenters of sufficient number of AE events until the final dynamic rupture started. From the space-time distribution of AE events, we clearly found that the fault nucleated in an upper part of the sample and extended quasi-statically downward along the final fracture plane. Growth rate of the fault increased with time, and its area had reached to mm 2 when the dynamic fault growth started. The front of fault nucleation zone estimated from the AE distribution corresponds well to bends on the final fracture plane. Comparing the AE distribution, the fault geometry and the rock structure, it is indicated that the fault first had developed quasi-statically along the foliation, and changed its direction of growth to that more favourable for the shear fracture. These experimental results suggest that the faulting process from quasi-static to dynamic fault growth are strongly affected by the foliation structures, by which the final fault geometry becomes complicated. INTRODUCTION An earthquake of M = 2 occurred on 27 December, 2007 at about 3 km deep in Mponeng mine, South Africa. This event was located at about 30 m above a highfrequency ultramicroseismic network operated by JAGUARS (Japanese-German Underground Acoustic Emission Research in South Africa) (Yabe et al. 2009, Naoi et al ), whose sensitivity was at least as small as moment magnitude M W = -4.4 (Kwiatek et al ). More than aftershocks of this event were precisely located by JAGUARS s network (Naoi et al ), and it was found that the M = 2 event occurred in a gabbroic dyke, called Pink and Green (PG) dyke, of about 30 m thick. The geometry of fault plane delineated by the aftershock hypocenter distribution is complicated having some bends and branchings. When the M = 2 earthquake occurred, the subvertical PG dyke had been left unmined as a pillar. Near vertical compressive stress had increased by 18 MPa in the dyke, and 14 MPa in the host rock during 6 months before the main shock (Katsura 2009 ). This situation is very similar to laboratory triaxial compression experiment. We conducted a triaxial compression experiment using a gabbro sample which was retrieved from PG dyke near the M = 2 event. A detailed fracture process was discussed based on spacetime distribution of acoustic emissions (AEs). EXPERIMENT A cylindrical sample with 40 mm in diameter and 100 mm in length was prepared from a BX core drilled from the dyke. Modal components of the sample are listed in Table I. Microscope image of thin section of this rock shows strong foliation structures. P- and S-wave velocities and apparent density measured at atmospheric pressure before the experiment are 6.67 km/sec, 3.86 km/sec and 2.96 g/cm 3, respectively. Young s modulus, rigidity and Poisson s ratio are thus GPa, 44.1 GPa and 0.25, respectively. P-wave velocity of the gabbro sample is higher than that of the quartzite host rock by 15%. QUASI-STATIC FAULT GROWTH IN A GABBRO SAMPLE

3 Thirty longitudinal-type PZT transducers with 2 MHz resonant frequency were mounted on the sample s side surface to monitor AE waveforms and to measure ultrasonic wave velocity. Six cross strain gauges with 5 mm gauge length were also glued onto the center of sample side surface with sixty degree intervals to measure the axial and circumferential strains. Table I Mineral mode composition of the sample determined by point counting method Mode % Amphibolite (Colorless) Amphibolite (Green) 4.25 Clay Mineral Chlorite Epidote 7.00 Quartz 5.25 Calcite 2.50 Leucoxene 2.00 Magnetite 1.00 P-wave velocities were measured when confining pressure was increased hydrostatically to a predetermined value (75 MPa). P-wave velocity at 75 MPa was 6.78 km/sec, which is only 1.6 % higher than that at atmospheric pressure (Figure 1). Strain response to pressure was almost linear. These facts indicate that the sample contains few pre-existing microcracks. The sample was loaded to fracture under a constant confining pressure of 75 MPa, which corresponds to overburden pressure at the depth where the sample was collected. The sample failed at 597 MPa in differential stress. After that, the differential stress decreased gradually and the sample fractured in a brittle manner at 20 seconds after the beginning of failure forming a steeply inclined fault plane with a dip angle of about 60 degree (Figure 2). We can find a bend of the fault in an upper part of the sample. Above the bend, the fault is parallel to the foliation. On the other hand, it is steeper and more favourable for shear fracture below the bend. When the dynamic rupture occurred, the differential stress had dropped by about 20 MPa from the peak stress. The AE signals were digitized with a sampling interval of 50 nanoseconds and a resolution of 12 bits, and were recorded with a 1024 words (51.2 microseconds) record length for each event. The AE measurement system can record about events a second, and hence we could record AE waveforms with few missed events until the final rupture. The AE hypocenter locations were calculated using P-wave first arrival times. When the dynamic rupture occurred, the AE activity became very high and the AE waveforms overlapped to each other. Thus it was difficult to locate AE hypocenter after the dynamic rupture started. Figure 1. Waveforms of P-wave velocity measurement RaSiM8

4 QUASI-STATIC FAULT GROWTH Figure 3 shows differential stress and z coordinate of AE hypocenters as functions of time for 1000 seconds before the final fracture. We can find a clustered AE activity in an upper part of the sample. This activity initiated at about 900 seconds before the final fracture, expanded gradually until the peak stress, and then rapidly accelerated to the final fracture. The AE events in this cluster are distributed on the final fracture plane (Figure 2, Figure 5). This activity reflects transition from quasi-static fault growth to dynamic one. Figure 4 is the same plot as Figure 3, but for about 20 seconds from the peak stress to the final rupture. In Figure 4a), drop rate of the differential stress is also plotted as well as the differential stress. Based on the stress drop rate, the period from the peak stress to the final rupture can be divided into three stages as indicated by vertical dashed lines in Figure 4. We call these three stages Stage II, Stage III and Stage IV as shown in Figure 4. Stage I is assigned to the period from the beginning of loading to the peak stress (Figure 3). Figure 5 shows orthographic projections of AE hypocenters for these four stages. Strike of the fault plane is almost parallel to the x axis. Dashed lines in the x-z projection are major bends on the fault plane (Figure 6). Figure 2. Photo of the fractured rock sample Figure 3. Differential stress (a), b-value (b) and z coordinate (c) of AE hypocenters as functions of time for final 1000 seconds before the final fracture Figure 4. Same as Figure 3, but for 20 seconds before the final fracture. In a), drop rate of the differential stress ( d diff / dt ) is also plotted by closed dots. Based on the stress drop rate, the period after the peak stress can be divided into three stages as indicated by vertical dashed lines QUASI-STATIC FAULT GROWTH IN A GABBRO SAMPLE

5 Figure 5. Orthographic projections of AE hypocenter distributions for a) Stage I shown in Figure 3, and b) d) Stages II, III and IV shown in Figure 4. Strike of the fault plane is almost parallel to the x axis. Dashed lines in the x-z projection are major bends on the fault surface (Figure 6) The fault growth rates for the four stages estimated from the y-z projection of AE hypocenters are 0.03 mm/s, 0.5 mm/s, 4 mm/s and 20 mm/s. Acceleration of quasistatic fault growth was also observed by Lei et al These behaviour is also similar to pre-slip preceding an unstable sliding (Okubo and Dieterich 1984, Ohnaka and Kuwahara 1990 ). In Figures 3 and 4, b -value in Gutenberg-Richter magnitude frequency relation, log 10 N a bm, is also plotted. Where, N is number of events whose magnitude is greater than M, a and b are constants. Relative magnitude (M ) is defined as logarithm of the peak amplitude of each event. We estimated b -value by the maximum likelihood method (Aki 1965 and Utsu 1965 ). We can find that the b -value decreased from 1.2 at the peak stress to less than 1 at the beginning of the final rupture. This result is consistent with Scholz 1968 and Lei and Satoh From the x-z projection of AE hypocenters (Figure 5), we can find that the fault size just before the final dynamic rupture (the end of Stage IV) had reached to about mm 2. We can also find that the front of fault nucleation zone estimated from the AE distribution corresponds well to bends on the final fracture plane (Figure 6). The AE events had been distributed above the bends until the differential stress reached to its peak (Stage I, Figure 5a). After that, the AE activity extended beyond the bends (Stage II-IV, Figure 5b-d). This indicates that the fault first grew quasi-statically along the foliation in Stage I, and the direction of fault growth became steeper in Stage II-IV, resulting in the fault bends. Figure 6. Photo of the fault surface. White dashed lines indicate approximate locations of major bends on the surface SUMMARY AND DISCUSSIONS We conducted a triaxial compression experiment of gabbro under a confining pressure of 75 MPa. From the space-time distribution of AE events, we clearly found that the fault nucleated in an upper part of the sample and extended downward along the final fracture plane. Growth rate of the fault increased with time, and its area had reached to mm 2 when the dynamic fault growth RaSiM8

6 started. The front of fault nucleation zone estimated from the AE distribution corresponds well to bends on the final fracture plane. Comparing the AE distribution, the fault geometry and the rock structure, it is indicated that the fault first had developed quasi-statically along the foliation, and changed its direction of growth to that more favourable for the shear fracture. It is suggested that the faulting process from quasi-static to dynamic fault growth is strongly affected by the foliation structures, by which the final fault geometry becomes complicated. This result is in agreement with Lei et al When the M = 2 earthquake occurred, the subvertical PG dyke had been left unmined as a pillar. Increases in near vertical compressive stress by 18 MPa and 14 MPa during 6 months before the main shock were observed by strainmeters (Ishii et al ) installed in the PG dyke and the quartzite host rock, respectively (Katsura 2009 ). This situation is very similar to the triaxial compression experiment that we showed in this paper. Dip angle of the fault plane estimated from the aftershock distribution is about 60 degree (Yabe et al and Naoi et al ), which is a typical angle for brittle fracture of rock under confining pressure. CMT focal mechanism solution of the main shock is normal fault type one (Naoi et al ). Yabe et al and Naoi et al thus concluded that this event was a Mohr- Coulomb failure that occurred in the PG dyke. The precisely located aftershock hypocenters (Naoi et al ) indicates that the fault of this event has a complicated structure having some bends and branchings. The main shock hypocenter was located near a bend (Naoi et al ). Although we have to take into account the differences of size, boundary conditions etc. between laboratory rock fracture experiments and mining induced earthquakes, these results might suggest that the fault geometry and faulting process are strongly affected by the geological structure as shown in our experiment. Yabe et al found foreshock activities on the fault plane. Although it is not significant because of the insufficient number of events before the main shock, the b -value for the foreshocks tends to be smaller than that for the aftershocks. Naoi et al observed quasi-static growth of planar AE clusters up to about 20 m, at 1000 m depth in a gold mine in South Africa. They found that the b -value decreased with the growth of the cluster activity. The laboratory AE experiments should give some important information on source process including seismicity leading to larger mining induced events. ACKNOWLEDGEMENTS This work was supported by Grants-in-Aid for Scientific Research (A ), the Observation and Research Program for the Prediction of Earthquakes and Volcanic Eruptions of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, and the Science and Technology Research Partnership for Sustainable Development (SATREPS). REFERENCES AKI, K. Maximum likelihood estimate of b in the formula log N a bm and its confidence limits, Bull. 10 Eartq. Res. Inst., Univ. Tokyo, vol. 43, pp ISHII, H., YAMAUCHI, T. and KUSUMOTO, F. Development of high sensitivity bore hole strain meters and application for rock mechanics and earthquake prediction study, in Rock Stress (ed. Balkema, A.A.), Brookfield, Vermont, pp KATSURA, T. Rock mass deformation before, at and after an M = 2.1 earthquake within only 20 m from two Ishii strainmeters, B.S. Thesis, Ritsumeikan University, Kusatsu, Japan, pp (in Japanese). 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7 during stick-slip shear failure, Tectonophys., vol. 175, pp OKUBO, P.G. and DIETERICH, J.H. Effects of physical fault properties on frictional instabilities produced on simulated faults, J. Geophys. Res., vol. 89, pp SCHOLZ, C.H. The frequency-magnitude relation of microfracturing in rock and its relation to earthquakes, Bull. Seism. Soc. Am., vol. 58, pp UTSU, T.A method for determining the value of b in a formula log10 n a bm showing the magnitudefrequency relation for earthquakes, Geophys. Bull. Hokkaido Univ., vol. 13, pp (in Japanese). YABE, Y., NAKATANI, M., PHILIPP, J., NAOI, M., MOREMA, G., PLENKERS, K., KWAITEK, G., OGASAWARA, H., STANCHTS, S., KAWAKATA, H. and DRESEN, G. AE activity prior to an M = 2.1 earthquake in a South African deep gold mine, in Prog. Abs. 7th General Assembly ASC and Fall Meeting SSJ, Tsukuba, Japan, p. 59. YABE, Y., PHILIPP, J., NAKATANI, M., MOREMA, G., NAOI, M., KAWAKATA, H., IGARASHI, T., DRESEN, G., OGASAWARA, H. and JAGUARS GROUP. Observation of numerous aftershocks of an M W = 1.9 earthquake with an AE network installed in a deep gold mine in South Africa, Earth Planets Space, vol. 61, pp. e49 e RaSiM8