The aftershock sequence of the 5 August 2014 Orkney earthquake (ML 5.5), South Africa

Manzunzu, B.; Midzi, V.; Mangongolo, A.; Essrich, F.

doi:10.1007/s10950-017-9667-z

Cited by 15 publications

(9 citation statements)

References 28 publications

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“…Regardless of the specific transient stress change that brings a fault to failure, the resulting rupture mechanism will however be consistent with the regional tectonic stress field as was observed in many other studies (e.g. Manzunzu et al 2017;Midzi et al 2018).…”

Section: Introductionsupporting

confidence: 81%

“…The current seismic swarm may be either a result of stress transfer within a set of faults that are rupturing in response to heterogeneous stress fields or the presence of pre-existing crustal weaknesses within which deformation is localised and possibly triggered by loading, unloading or non-tectonic sources (e.g. Calais et al 2016;Manzunzu et al 2017). This can accommodate localised seismic swarms as seen to occur in the region.…”

Section: Seismotectonic Analysismentioning

confidence: 99%

“…The occurrence of intraplate earthquakes has been a major concern for seismic hazard analysis studies (e.g. Gupta and Johnston 1998;Calais et al 2016;Manzunzu et al 2017;Allen, 2020;Labidi Abstract Recently, a swarm of seismic events occurred within the Kunene region (NW of Namibia) with magnitude values going up to M L 4.6. Some of the events alarmed the Anker community and have become of national interest.…”

Section: Introductionmentioning

confidence: 99%

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Seismotectonic analysis of the Anker area, Kunene region north-western Namibia

et al. 2022

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Recently, a swarm of seismic events occurred within the Kunene region (NW of Namibia) with magnitude values going up to ML 4.6. Some of the events alarmed the Anker community and have become of national interest. In order to understand the source of this seismic activity, the Geological Survey of Namibia (GSN) and the Council for Geoscience (CGS) of South Africa installed a temporary network of 10 seismic stations in the Anker area to monitor seismicity for a period of 3 months from 20 June to 24 September 2018. More than 1600 microseismic events were recorded and located within the boundaries of the array, which was roughly 50 km × 50 km. The stations were installed in a 50 km × 50 km radius to have an accurate overlap of the network coverage and also by the spatial distribution of the known seismicity in the study area. The magnitude of the events ranged from ML − 1.1 to 3.6 and occurred at depths of less than 20 km. To improve the location of earthquakes, a new velocity model was developed using high-quality seismic dataset recorded by the temporary network. The seismic data was used to determine a minimum 1-D velocity model, which was used for the relocation of the earthquakes using the double difference method. The relocated solutions provided information to explain the seismotectonics of the region and establish the source of the current seismic swarm. Lineaments were mapped using magnetic data to identify the sources of the seismicity. Focal mechanism analysis of some of the events indicates the area is highly faulted with predominantly normal faulting.

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Section: Introductionsupporting

confidence: 81%

Section: Seismotectonic Analysismentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Seismotectonic analysis of the Anker area, Kunene region north-western Namibia

et al. 2022

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“…The 2014 Orkney earthquake (M5.5) occurred on 5 August 2014 below the Moab Khotsong gold mine near Orkney, southwest of Johannesburg, South Africa (Figure 1a; Midzi et al., 2015). The mainshock and aftershocks were clearly recorded at the surface by strong‐motion meters operated by the South African Council for Geoscience (Manzunzu et al., 2017; Midzi et al., 2015) and by in‐mine geophones and strainmeters (Ogasawara et al., 2017, 2019). Analysis of the seismograms revealed that the hypocenter was 4.7 km below the surface and identified a distinct NNW–SSE striking vertical planar aftershock cloud 3.5–7.0 km below the ground surface (Figure 1b).…”

Section: Introductionmentioning

confidence: 99%

Characteristics of Fault Rocks Within the Aftershock Cloud of the 2014 Orkney Earthquake (M5.5) Beneath the Moab Khotsong Gold Mine, South Africa

Miyamoto

Hirono

Yokoyama

et al. 2022

Geophysical Research Letters

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Cores recovered during the International Continental Scientific Drilling Program project “Drilling into Seismogenic zones of M2.0 to M5.5 earthquakes in deep South African Gold Mines” include fault breccia from within the aftershock cloud of the 2014 Orkney earthquake (M5.5). The breccia and surrounding intrusive rocks, probably lamprophyres rich in talc, biotite, calcite, and amphibole, had high magnetic susceptibilities owing to the presence of magnetite. All of these characteristics can be attributed to fluid‐related alteration. Both the breccia and the lamprophyres had low friction coefficients and showed evidence of velocity strengthening, which is inconsistent with the occurrence of earthquakes. Variable amounts of talc, biotite, calcite, and amphibole within the lamprophyres might have produced complex frictional properties and spatial heterogeneity of fault stability. The altered lamprophyres may be the host rocks of the 2014 Orkney earthquake, but frictional complexity may have governed the magnitudes of the main‐ and aftershocks and their distributions.

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“…This national network was enhanced with 15 strong motion meters within a radius of 25 km in the Klerksdorp district, which could record the Orkney M5.5 earthquake on 5 August 2014, and the aftershocks, Midzi et al (2015). Together with the strong motion data, 46 in-mine seismic sensors, mainly geophones, installed at depths of 2 km -3 km at distances of 3 km -8 km from the M5.5 hypocentre, allowed detailed investigation of the event (Moyer et al (2017) , Imanishi et al (2017), Mori et al (2018) and Manzunzu et al (2017), Figure 2). This M5.5 earthquake was atypical because the mechanism was strike-slip faulting instead of typical normal-faulting, and it took place at a depth significantly greater than the mining horizons in a normal-faulting stress regime.…”

Section: Introductionmentioning

confidence: 99%

2019 status report: Drilling into seismogenic zones of M2.0–M5.5 earthquakes in South African gold mines (DSeis project)

Ogasawara¹,

Liebenberg²,

Rickenbacher³

et al. 2019

Proceedings of the Ninth International Conference on Deep and High Stress Mining

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In 2014, a M5.5 earthquake ruptured the range of depths between 3.5 km and 7 km near Orkney, South Africa. The main and aftershocks were very well monitored in the nearfield by dense, surface, strong motion meters and a dense underground seismic network in the deep gold mines. The mechanism of this M5.5 earthquake was left-lateral strike-slip faulting, differing from typical mining-induced earthquakes with normalfaulting mechanisms on the mining horizons shallower than 3.5 km depth. To understand why such an unusual event took place, the aftershock zone was probed by full-core NQ drilling during 2017-2018, with a total length of about 1.6 km, followed by in-hole geophysical logging, core logging, core testing, and monitoring in the drilled holes. These holes also presented a rare opportunity to investigate deep life. In addition, seismogenic zones of M2-M3 earthquakes were probed on mine horizons that were also very well monitored by acoustic emission networks. This paper reviews the early results of the project.

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The aftershock sequence of the 5 August 2014 Orkney earthquake (ML 5.5), South Africa

Cited by 15 publications

References 28 publications

Seismotectonic analysis of the Anker area, Kunene region north-western Namibia

Seismotectonic analysis of the Anker area, Kunene region north-western Namibia

Characteristics of Fault Rocks Within the Aftershock Cloud of the 2014 Orkney Earthquake (M5.5) Beneath the Moab Khotsong Gold Mine, South Africa

2019 status report: Drilling into seismogenic zones of M2.0–M5.5 earthquakes in South African gold mines (DSeis project)

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