Observation of numerous aftershocks of an Mw 1.9 earthquake with an AE network installed in a deep gold mine in South Africa

Yabe, Yasuo; Philipp, Joachim; Nakatani, Masao; Morema, Gilbert; Naoi, Makoto; Kawakata, Hironori; Igarashi, Toshihiro; Dresen, Georg; Ogasawara, Hiroshi; Jaguars,

doi:10.1186/bf03352963

Cited by 27 publications

(40 citation statements)

References 7 publications

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“…The mines provide a unique setting with in situ access to hypocenters of earthquakes of moment-magnitude -2 to *4, with the larger magnitudes often occurring along pre-existing fault-zones. Mining operations control the location, magnitude and timing of the earthquakes (COOK, 1963;SPOTTISWOODE and MCGARR, 1975;MCGARR, 1984;GIBOWICZ and KIJKO, 1994;MENDECKI, 1997;OGASAWARA et al, 2002a, b), and allow for monitoring at distances of 1-100 m from anticipated hypocenters (YABE et al, 2009). The NELSAM objectives that are addressed here include the characterization of a reactivated fault-zone, determination of the stress/ strain/strength distribution in the vicinity of this fault, and analysis of the rupture dynamic processes.…”

Section: Nelsam Projectmentioning

confidence: 99%

Earthquake Rupture at Focal Depth, Part I: Structure and Rupture of the Pretorius Fault, TauTona Mine, South Africa

Heesakkers

Murphy

Reches

2011

Pure Appl. Geophys.

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We analyze the structure of the Archaean Pretorius fault in TauTona mine, South Africa, as well as the rupture-zone that recently reactivated it. The analysis is part of the Natural Earthquake Laboratory in South African Mines (NELSAM) project that utilizes the access to 3.6 km depth provided by the mining operations. The Pretorius fault is a *10 km long, oblique-strikeslip fault with displacement of up to 200 m that crosscuts fine to very coarse grain quartzitic rocks in TauTona mine. We identify here three structural zones within the fault-zone: (1) an outer damage zone, *100 m wide, of brittle deformation manifested by multiple, widely spaced fractures and faults with slip up to 3 m; (2) an inner damage zone, 25-30 m wide, with high density of anastomosing conjugate sets of fault segments and fractures, many of which carry cataclasite zones; and (3) a dominant segment, with a cataclasite zone up to 50 cm thick that accommodated most of the Archaean slip of the Pretorius fault, and is regarded as the 'principal slip zone' (PSZ). This fault-zone structure indicates that during its Archaean activity, the Pretorius fault entered the mature fault stage in which many slip events were localized along a single, PSZ. The mining operations continuously induce earthquakes, including the 2004, M2.2 event that rejuvenated the Pretorius fault in the NELSAM project area. Our analysis of the M2.2 rupture-zone shows that (1) slip occurred exclusively along four, pre-existing large, quasi-planer segments of the ancient fault-zone; (2) the slipping segments contain brittle cataclasite zones up to 0.5 m thick; (3) these segments are not parallel to each other; (4) gouge zones, 1-5 mm thick, composed of white 'rock-flour' formed almost exclusively along the cataclasite-host rock contacts of the slipping segments; (5) locally, new, fresh fractures branched from the slipping segments and propagated in mixed shear-tensile mode; (6) the maximum observed shear displacement is 25 mm in oblique-normal slip. The mechanical analysis of this rupture-zone is presented in Part II (HEESAKKERS et al.,

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Section: Nelsam Projectmentioning

confidence: 99%

Earthquake Rupture at Focal Depth, Part I: Structure and Rupture of the Pretorius Fault, TauTona Mine, South Africa

Heesakkers

Murphy

Reches

2011

Pure Appl. Geophys.

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“…The reactivation of the Pretorius fault is an expected outcome of the mining around it as in the tectonically inactive region of central South Africa, gold mining is the main source of current seismic activity (COOK, 1963;SPOTTISWOODE and MCGARR, 1975;MCGARR, 1984;GIBOWICZ and KIJKO, 1994;MENDECKI, 1997;YABE et al, 2009). The mining here is conducted by removal of sub-horizontal layers (reef) of *1 m thick from a depth of 1.5 to 3.5 km for gold extraction.…”

Section: Introductionmentioning

confidence: 99%

“…Recognizing earthquakes of group (c) is more complicated (RICHARDSON and JORDAN, 2002;YABE et al, 2009). To identify them, BOETTCHER et al (2009) examined the seismic data of 12 years in TauTona mine (Part I, Fig.…”

Section: Introductionmentioning

confidence: 99%

“…Events recorded during relatively quiet hours in 2006 indicate that catalog detection limitations, not earthquake source physics, controlled the previously reported minimum magnitude in this mine. '' YABE et al (2009) used a local network of acoustic emission (AE) sensors to analyze the earthquake rupture of at depth of 3.3 km in Mponeng gold mine, close to TauTona mine, South Africa (Part I).…”

Section: Introductionmentioning

confidence: 99%

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Earthquake Rupture at Focal Depth, Part II: Mechanics of the 2004 M2.2 Earthquake Along the Pretorius Fault, TauTona Mine, South Africa

Heesakkers

Murphy

Lockner

et al. 2011

Pure Appl. Geophys.

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We analyze here the rupture mechanics of the 2004, M2.2 earthquake based on our observations and measurements at focal depth (Part I). This event ruptured the Archean Pretorius fault that has been inactive for at least 2 Ga, and was reactivated due to mining operations down to a depth of 3.6 km depth. Thus, it was expected that the Pretorius fault zone will fail similarly to an intact rock body independently of its ancient healed structure. Our analysis reveals a few puzzling features of the M2.2 rupture-zone:(1) the earthquake ruptured four, non-parallel, cataclasite bearing segments of the ancient Pretorius fault-zone; (2) slip occurred almost exclusively along the cataclasite-host rock contacts of the slipping segments; (3) the local in-situ stress field is not favorable to slip along any of these four segments; and (4) the Archean cataclasite is pervasively sintered and cemented to become brittle and strong. To resolve these observations, we conducted rock mechanics experiments on the fault-rocks and host-rocks and found a strong mechanical contrast between the quartzitic cataclasite zones, with elastic-brittle rheology, and the host quartzites, with damage, elastic-plastic rheology. The finite-element modeling of a heterogeneous fault-zone with the measured mechanical contrast indicates that the slip is likely to reactivate the ancient cataclasitebearing segments, as observed, due to the strong mechanical contrast between the cataclasite and the host quartzitic rock.

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“…Two almost vertically dipping diorite-gabbro dykes are in close proximity to the seismic event's hypocentre . From Figure 1 it is evident that the hypocentre of the M w = 1.9 seismic event was located within one of the two dykes, the Pink-Green dyke (Yabe et al, 2009).…”

Section: Introductionmentioning

confidence: 99%

Mining induced static stress transfer and its relation to a high-precision located Mw = 1.9 seismic event in a South African gold mine

Ziegler¹,

Reiter²,

Heidbach³

et al. 2014

Rock Engineering and Rock Mechanics: Structures in and on Rock Masses

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On 27 December 2007 a M w 1.9 seismic event occurred in the Mponeng gold mine, South Africa. The seismic event's hypocentre, as well as the fore-and aftershocks were located with the JAGUARS acoustic emission array, placed at mining level 116. Thus, hypocentre depth (3509 m) and focal mechanism are very well constrained. Since no mining activity took place more than two days before the event, dynamic triggering due to blasting is ruled out. We investigate the hypothesis that static stress transfer due to excavation of the gold reef induced the event. To estimate these mining-induced stress changes, we set up a small scale (450 × 300 × 310 m 3 ) high resolution 3D geomechanical-numerical model, containing most of the seismic events detected. The model is made of the four different rock units present in the mine: quartzite (footwall), hard lava (hanging wall), conglomerate (gold reef), and diorite (dykes). The virgin in-situ stress, determined in nearby TauTona mine, is used to implement initial conditions. Elastic rock properties are taken from laboratory measurements. For the numerical solution, we are using the finite element method, with a discretised mesh of ∼1 million elements. From the computed 3D stress tensor we estimate the Coulomb failure stress change ( CFS) on the fault plane of the event in two ways: The first with shear stresses resolved in rake direction ( CFS slip ) and the second using the maximum shear stress ( CFS max ) on the plane. The CFS slip values are negative, indicating that, according to the model, mining activity brought slip in the resolved direction on the resolved fault plane further away from failure. Considering model uncertainties with respect to fault plane geometry, rake vector, and elastic parameters, this result remains stable. In contrary, the CFS max value is positive, indicating that, given the modelled stress is correct, a high potential for triggering a rupture is available.

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Observation of numerous aftershocks of an Mw 1.9 earthquake with an AE network installed in a deep gold mine in South Africa

Cited by 27 publications

References 7 publications

Earthquake Rupture at Focal Depth, Part I: Structure and Rupture of the Pretorius Fault, TauTona Mine, South Africa

Earthquake Rupture at Focal Depth, Part I: Structure and Rupture of the Pretorius Fault, TauTona Mine, South Africa

Earthquake Rupture at Focal Depth, Part II: Mechanics of the 2004 M2.2 Earthquake Along the Pretorius Fault, TauTona Mine, South Africa

Mining induced static stress transfer and its relation to a high-precision located Mw = 1.9 seismic event in a South African gold mine

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