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)
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ring
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Study Area
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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?
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Velocity (m/s)
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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
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Frequency (Hz)
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f c 1= 212 Hz
∆σ1 = 13 MPa
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Time (s)
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Three component spectral ratios
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Relative Amplitude
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Relative Amplitude
Velocity (m/s)
Stacked spectral ratio
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Frequency (Hz)
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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
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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
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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.
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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
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Global comparison of earthquake parameters with lines of
constant stress drop (From Allmann and Shearer, 2009)
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MP Pa
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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
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This study, initial results
aG
old
Allmann and Shearer (2009)
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Hough (1996)
Mi
ne
Boatwright (1994)
Mori and Frankel (1990)
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Tajima and Tajima (2007)
Humphrey and Anderson (1994)
Archuleta et al. (1982)
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Venkataraman and Kanamori (2004)
Abercrombie (1995)
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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
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∆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
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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
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Global median
for intraplate
earthquakes
5.95 +/- 1.01 MPa
2
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/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.