Call for Models—A Test Case for the Source Inversion Validation: The 2014<i>M</i><sub>L</sub> 5.5 Orkney, South Africa, Earthquake

Moyer, P. A.; Boettcher, M. S.; Ellsworth, William L.; Ogasawara, Hiroshi; Cichowicz, Artur; Birch, Denver; Aswegen, G. van

doi:10.1785/0220160218

Cited by 3 publications

(6 citation statements)

References 10 publications

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“…They were seperated horizontally by around 2 km in which the initial origin is south of the main origin. An assimiable observation supporting our findings was reported by Moyer et al (2017). They identified an immediate foreshock precursory to the main shock which was located in about 1.4 km horizontal distance south of the main shock.…”

supporting

confidence: 93%

“…The residuals are used to shift the P-wave amplitude traces to account for velocity model uncertainties. Our results of the inversion for hypocenter location and origin time differ from results previously known from other sources (CGS, Manzunzu et al (2017), andMoyer et al (2017)). However, the computed traveltimes for the relocated earthquake hypocenter are now more consistent with the observation and a consistent traveltime-grid is needed for the application of MRPI.…”

contrasting

confidence: 92%

“…Since we consider here the surface network, we apply a more advanced but still rather simple velocity model which is listed in Table 2. It consists of two layers with homogeneous velocities and it was build based on input from AGA and Moyer et al (2017).…”

mentioning

confidence: 99%

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Statistics of the Seismic Sequence and Rupture Directivity of the M5.5 Earthquake in Orkney, South Africa

Dinske

Folesky

Kummerow

et al. 2020

Preprint

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Abstract. Despite our general knowledge of earthquake processes, it is still not fully understood how earthquake ruptures nucleate and propagate and why they stop. Also, the controlling factors of the frequency and of the size of earthquakes are subject of ongoing research. We aim to address these questions with a comprehensive study of seismicity in deep South African gold mines. We find here the unique situation that the seismicity consists of both induced earthquakes and aftershocks triggered by the M5.5 Orkney earthquake which occurred in August 2014. We separate the cataloged seismicity and group the events into three classes: the aftershock sequence, seismicity induced by fluids, and seismicity induced by mining activities. We examine statistical properties of earthquakes in each of the three classes. We conclude that the magnitude statistics of both aftershocks and induced earthquakes are influenced by the finite size and geometry of the rock volume of stress perturbation resulting in an absence of larger magnitude events. The magnitude frequency distributions obey the Lower Bound model of magnitude probability. The statistics of dynamic stress drop of aftershocks and induced earthquakes satisfy log-normal distributions but the value range is different, it means that aftershocks are generally characterized by higher stress drops. Another key aspect in our study is the imaging of the propagating rupture of the M5.5 earthquake. We apply the back projection imaging approach using seismological data from two different local networks and retrieve similar results. We conclude that the rupture of the M5.5 earthquake propagated predominantly unilaterally, nearly from North to South over a distance of about 6km. The hypocenters of the aftershock sequence are situated unilaterally in respect to the hypocenter of the main shock and are aligned to the South which confirms the obtained rupture propagation image and the directivity of the main shock.

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supporting

confidence: 93%

contrasting

confidence: 92%

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Statistics of the Seismic Sequence and Rupture Directivity of the M5.5 Earthquake in Orkney, South Africa

Dinske

Folesky

Kummerow

et al. 2020

Preprint

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“…The dense in-mine geophone network, deployed to the south of the hypocentre of the Orkney M5.5 earthquake, delineated a nearly vertically-dipping planar distribution of aftershocks (See Figure 2). The CGS surface strong motion network covers the areas both to the south and north of the epicentre, allowing the locating of aftershocks to the north of the epicentre and investigation the rupture process of the M5.5 mainshock, Ellsworth et al (2017), Moyer et al (2017) and Mori et al (2018). The deepest mining horizons are 3 km below surface, and the upper fringe of the aftershock zone several hundreds of metres below them, which is within the range of the hydraulic drilling rigs that are used for routine mine exploration and cover drilling (pilot drilling in advance of tunnel development).…”

Section: The 2014 Orkney M55 Earthquakementioning

confidence: 99%

“…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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What Governs the Spatial and Temporal Distribution of Aftershocks in Mining-Induced Seismicity: Insight into the Influence of Coseismic Static Stress Changes on Seismicity in Kiruna Mine, Sweden

Kozłowska

Orlecka‐Sikora

Dineva

et al. 2020

Bulletin of the Seismological Society of America

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Strong mining-induced earthquakes are often followed by aftershocks, similar to natural earthquakes. Although the magnitudes of such in-mine aftershocks are not high, they may pose a threat to mining infrastructure, production, and primarily, people working underground. The existing post-earthquake mining procedures usually do not consider any aspects of the physics of the mainshock. This work aims to estimate the rate and distribution of aftershocks following mining-induced seismic events by applying the rate-and-state model of fault friction, which is commonly used in natural earthquake studies. It was found that both the pre-mainshock level of seismicity and the coseismic stress change following the mainshock rupture have strong effects on the aftershock sequence. For mining-induced seismicity, however, we need to additionally account for the constantly changing stress state caused by the ongoing exploitation. Here, we attempt to model the aftershock sequence, its rate, and distribution of two M≈2 events in iron ore Kiruna mine, Sweden. We could appropriately estimate the aftershock sequence for one of the events because both the modeled rate and distribution of aftershocks matched the observed activity; however, the model underestimated the rate of aftershocks for the other event. The results of modeling showed that aftershocks following mining events occur in the areas of pre-mainshock activity influenced by the positive coulomb stress changes, according to the model’s assumptions. However, we also noted that some additional process not incorporated in the rate-and-state model may influence the aftershock sequence. Nevertheless, this type of modeling is a good tool for evaluating the risk areas in mines following a strong seismic event.

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Call for Models—A Test Case for the Source Inversion Validation: The 2014M_L 5.5 Orkney, South Africa, Earthquake

Cited by 3 publications

References 10 publications

Statistics of the Seismic Sequence and Rupture Directivity of the M5.5 Earthquake in Orkney, South Africa

Statistics of the Seismic Sequence and Rupture Directivity of the M5.5 Earthquake in Orkney, South Africa

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

What Governs the Spatial and Temporal Distribution of Aftershocks in Mining-Induced Seismicity: Insight into the Influence of Coseismic Static Stress Changes on Seismicity in Kiruna Mine, Sweden

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Call for Models—A Test Case for the Source Inversion Validation: The 2014ML 5.5 Orkney, South Africa, Earthquake

Cited by 3 publications

References 10 publications

Statistics of the Seismic Sequence and Rupture Directivity of the M5.5 Earthquake in Orkney, South Africa

Statistics of the Seismic Sequence and Rupture Directivity of the M5.5 Earthquake in Orkney, South Africa

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

What Governs the Spatial and Temporal Distribution of Aftershocks in Mining-Induced Seismicity: Insight into the Influence of Coseismic Static Stress Changes on Seismicity in Kiruna Mine, Sweden

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Call for Models—A Test Case for the Source Inversion Validation: The 2014M_L 5.5 Orkney, South Africa, Earthquake