Investigating Source Properties of Mining
Investigating Source Properties of Mining
Investigating Source Properties of Mining-Induced Earthquakes and the Resulting Seismic Hazard in TauTona Gold Mine, South Africa Pamela A. 1 Moyer ([email protected]) and Margaret S. 1 Boettcher ([email protected]) Why study mining-induced microseismicity at TauTona Gold Mine, South Africa Fencing erected to stabilize a fault in TauTona mine in a region where the tunnel collapsed during a M1.8 earthquake on 12 December, 2004. (Photo provided by V. Heesakkers and Z. Reches) Seismic stations placed at depth create a natural laboratory to study earthquake sources and relate laboratory seismic experiments to complex tectonic fault systems Left: A miner and a scientist inspect a fault in a tunnel in TauTona Gold Mine. ~3.6 km Pr et Fa ori ul us t Surface TauTona mine seismic stations The Natural Earthquake Laboratory South African Mines (NELSAM) seismic network is installed within the Pretorius fault zone in the deepest part of the mine to provide high-quality and high-sample rate records of very small earthquakes (Z. Reches, 2006) Bea ring rp sdo r e t en eef V a on tact R T u Ta Con dor s r te n e V ef e g R en act n o t Mp Con Ree f Study Area f Natural Earthquake Laboratory NELSAM South African Mines (NELSAM) Cross-section of TauTona Gold Mine and the location of seismic stations at depth including the high-sample rate NELSAM network. The TauTona and nearby Mponeng mines are well instrumented to monitor the abundant seismic activity within the mines. How is spectral analysis used to obtain stress drop for microseismicity at TauTona Gold Mine? 0 í 0 P S 0.05 M 0.1 0.1 Time (s) Velocity (m/s) x 10 P E 0 0.1 x 10 P S 0.05 0.1 x 10 í 0.15 20070718040341258166 Z í Time (s) 1. Earthquakes less than 100 m apart, with at least one order of magnitude difference, and have similar waveforms are paired and used for analysis N 2 10 Z 1 10 10 100 10 2 10 3 Frequency (Hz) M -0.9 0.1 0.15 10 1 1 10 0 M -0.9 P S í 0.1 0.15 0.05 Time (s) E 2 f c 1= 212 Hz ∆σ1 = 13 MPa 101 10 0 M -0.9 P S í 0.1 0.15 0.05 Time (s) S M 0.1 Time (s) N í 10 P Three component spectral ratios 2 í 0.15 0.05 í 0 í í S M 0.1 Time (s) Z 0 í 0.15 0.05 −5 N Relative Amplitude E 102 20070726020747529338 Relative Amplitude Velocity (m/s) Stacked spectral ratio 0 10 10 2 10 3 Frequency (Hz) 0 10 10 2 10 3 Frequency (Hz) 2. Earthquake records for each pair are converted to the frequency domain and divided to obtain a spectral ratio Stress drop calculated from S-wave corner frequency 7 fc ∆σ = M 0 16 0.21β 3 where f c 2= 534 Hz ∆σ2 = 7 MPa 102 103 Frequency (Hz) ratio data stack of ratio data model to stacked ratio Combined geophysical evidence suggests that large normal stresses due to nearby mining result in earthquakes with high stress drops (Z. Reches, 2006; Heesakkers et al., 2011a; Lucier et al., 2009; Boettcher et al., in prep.; Heesakkers et al., 2011b) Continuing work and analysis Seismic data can be used to obtain stress drop, an earthquake source parameter related to the energy of an earthquake, and may be linked to crustal strength. (e.g. Fletcher and McGarr, 2006; Baltay et al. 2011) The mean stress drop for earthquakes at TauTona Gold Mine is consistent with other studies of both large and small magnitude earthquakes (e.g. Allmann and Shearer, 2009; Baltay et al., 2011) Mining-induced microearthquakes have a stress drop similar to that of natural faults and show no evidence for a weak fault system ee R r e d ea L n o arb aC n o T Tau Block diagram of the TauTona study area and the Pretorius fault off-setting the gold-bearing reef. The mine environment allows for seismic stations to be placed in the seismogenic zone next to hundreds of earthquakes that occur everyday. 10 32 -1 < M < 2 0.2 to 119 MPa 12 MPa 5 MPa Stress drop distribution for earthquakes at TauTona Gold Mine show a median value comparable to the global median for intraplate earthquakes ~3.6 km Mponeng mine seismic stations p Gol d Ma Global comparison of earthquake parameters with lines of constant stress drop (From Allmann and Shearer, 2009) gn itu 10 de 0 M ra ng 10 P eo a M fm 0.1 1 M Pa icr MP Pa os a e ism To compare results for P-waves from Allmann and Shearer (2009), we multiplied our S-wave corner frequency results by the predicted factor of 1.5 ici ty 2 10 at Ta u To n This study, initial results aG old Allmann and Shearer (2009) 1 10 Hough (1996) Mi ne Boatwright (1994) Mori and Frankel (1990) 0 10 Tajima and Tajima (2007) Humphrey and Anderson (1994) Archuleta et al. (1982) í 10 Venkataraman and Kanamori (2004) Abercrombie (1995) í 10 í í í í 0 1 2 Mw 4 5 6 7 8 Are ancient intraplate faults in a mining environment strong or weak? overburden Seismic stations have been placed throughout the mine to monitor the abundant seismic activity x 10 10 ∆σ = stress drop M 0 = seismic moment f c = corner frequency β = S-wave velocity 3. The spectral ratios for each 4. Stress drop for each earthquake is calculated using the corner component and station are frequency from spectral analysis stacked and modeled to find and a catalog seismic moment the corner frequency TauTona median Mining has reactivated the Pretorius fault, the largest fault system in the mine, which was last active over 2 billion years ago x 10 4 A constant stress drop for earthquakes over a wide magnitude range suggests that intraplate mining-induced microearthquakes have the same rupture processes as large earthquakes in tectonic fault systems Seismicity throughout TauTona Gold Mine along the mined portion of the Carbon Leader Reef. Each colored dot represents an earthquake. Approximately ~300,000 earthquakes are shown. Recording microseismicity in the vicinity of an ancient intraplate fault No. of earthquakes: Magnitude range: Stress drop range: Mean stress drop: Median stress drop: 4 1XPEHURI(DUWKTXDNHV TauTona Gold Mine ~80 km west of Johannesburg, South Africa (from Heesakkers et al., 2011a) x 10 10 fc (Hz) Seismicity contributes to hazardous conditions within the mine where even small earthquakes are very dangerous í 5 Stress drop results based on the Madariaga (1976) source model for an initial set of earthquakes recorded at TauTona Gold Mine: Hundreds of earthquakes are recorded each day (-4 < M < 4) triggered by active mining í The University of New Hampshire, Durham, NH USA Is the rupture process of mining-induced microearthquakes the same as that of large earthquakes in tectonic fault systems? TauTona is the deepest gold mine in the world with mining to depths of ~4 km í 1 3 Global median for intraplate earthquakes 5.95 +/- 1.01 MPa 2 1 í 1 /RJVWUHVVGURS03D 2 Stress drop distribution of 32 microearthquakes recorded at TauTona Gold Mine (global median from Allmann and Shearer, 2009) Geologic observations of a subparallel segment of the Pretorius fault that slipped after a M2.2 earthquake on 12 December, 2004. Heesakkers et al. (2011a) Acknowledgements and References Obtain stress drop and additional source parameters for hundreds more earthquakes in the vicinity of the Pretorius Fault and throughout TauTona Gold Mine using spectral analysis techniques We gratefully acknowledge NSF funding for this project to MSB Compare source parameters of earthquakes occurring in the vicinity of the Pretorius Fault to earthquakes occurring near other natural and manmade structures throughout the mine (such as dikes and stopes) Fletcher, J. B., and A. McGarr (2006), Distribution of stress drop, stiffness, and fracture energy over earthquake rupture zones, J. Geophys. Res., 111, B03312, doi:10.1029/2004JB003396. Investigate in detail source parameters of large (M > 2) earthquakes recorded in TauTona Gold Mine and thought to have anomalously high stress drop Compare source parameter results throughout the mine with faulting type Allmann, B. P., and P. M. Shearer (2009), Global variations of stress drop for moderate to large earthquakes, J. Geophys. Res., 114, B01310, doi:10.1029/2008JB005821. Baltay, A., S. Ide, G. Prieto, and G. Beroza (2011), Variability in earthquake stress drop and apparent stress, Geophys. Res. Lett., 38, L06303, doi:10.1029/2011GL046698. Heesakkers, V., S. Murphy, and Z. Reches (2011a), Earthquake rupture at focal depth, Part I: Structure and rupture of the Pretorius fault, TauTona mine, South Africa, Pure Appl. Geophys., doi 10.1007/s00024-011-0354-7. Heesakkers V., S. Murphy, D. A. Locker, and Z. Reches (2011b) Earthquake rupture at focal depth, Part II: Mechanics of the 2004 M2.2 earthquake along the Pretorius fault, TauTona mine, South Africa, Pure Appl. Geophys., doi 10.1007/s00024-011-0355-6. Lucier, A. M., M. D. Zobeck, V. Heesakkers, Z. Reches, and S. K. Murphy (2009), Constraining the far-field in situ stress state near a deep South African gold mine, Int. J. Rock Mech. & Mining Sci., vol. 46, pp. 555-567. Reches, Z., and the Drilling Active Faults in South African Mines and the Natural Earthquake Laboratory in South African Mines Teams (2006), Building a natural earthquake laboratory at focal depth (DAFSAM-NELSAM Project, South Africa), Scientific Drilling doi 10.2204/iodp.sd.3.06.2006, 30–33.