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USGS seismicity map of South Africa

Desription

Mining-induced seismicity can damage buildings and surface infrastructure and cause fatalities. Seismic event means the trembling or shaking movement of the earth's surface that produces ground motions.

Abstract:

Flaws in building design have been highlighted by recent seismic activity in South Africa, mainly caused by mining. These activities are different from tectonic events. They affect structures in unique ways. A case study of the March 9, 2005 Stilfontein DRDGOLD's Hartebeesfontein mine [[earthquake] revealed that the orientation of buildings, particularly those facing North, contributed to structural failure during moderate seismic events.

Introduction

In Southern Africa, seismic resistance is often overlooked in architectural and engineering design. This is largely due to the lack of significant destructive earthquakes in recent years. The 1969 Ceres earthquake, which measured 6.3 on the Richter scale and caused nine deaths and extensive property damage, is the most notable recent event. Significant damage has also been caused by smaller earthquakes, particularly in the mining sector. An earthquake in Mozambique in 2006, which was felt as far away as Durban and caused damage to buildings, was the most recent major seismic event. Although the boundaries of Africa's tectonic plates lie outside the continent, there is intra-plate seismic activity, particularly in the East African Rift, and mine-induced tremors in South Africa. In view of these risks, the consideration of seismic aspects in the design of infrastructure is of paramount importance. In South Africa, it's crucial to map and understand geohazards like seismic risks and dolomite-related ground movement to support sustainable infrastructure development. Seismic hazard refers to the likelihood of an earthquake exceeding a specific ground motion threshold within a certain timeframe. This information helps assess risk for building codes and land use planning. Although South Africa is an intraplate region with low to moderate seismicity, mapping hazards is complicated due to limited data. Continuous seismic monitoring is essential to reduce uncertainties. Additionally, understanding geological structures, depth to bedrock, and human activities is important for a comprehensive assessment of seismic hazards and risks.

Seismicity of southern africa

Seismic studies in southern Africa began around 1910. In recent decades, significant research has been carried out on earthquake hazard mapping in South Africa, Mozambique, Zimbabwe, Botswana, Namibia, Lesotho and Swaziland. Researchers such as have made notable contributions to the study of seismicity in southern africa. The region has a low level of seismicity due to its location within major tectonic plates, but it does have complex seismic characteristics with over 90% of the world's natural earthquakes occurring at plate boundaries, most significant earthquakes are of tectonic origin. The seismicity of Southern Africa includes sporadic shallow earthquakes, with notable events such as the 1976 Welkom earthquake (magnitude 5.5) and the 2005 Klerksdorp earthquake (magnitude 5.3).

While certain areas in the region can experience earthquakes with a magnitude of more than 7, the majority of events are below magnitude 5 and are often mine-induced, typically occurring at depths of less than 2 km. Natural earthquakes pose a greater threat to the region than mine-induced tremors, which mainly affect areas close to mines, despite their lower frequency.

USGS ShakeMap illustrating the intensity of the 2014 Orkney earthquake near Klerksdorp

Earth tremors relating to mining activities

According to Professor Andrzej Kijko from the University of Pretoria's Natural Hazard Centre, mining can activate natural faults. He believes that 95% of South Africa's earthquakes are caused by mining, especially around the areas of Klerksdorp, Welkom and Carletonville. According to earth science consultant Dr Chris Hartnady, "This part of Africa is in the vicinity of the African Rift system, which is being pulled apart by a few millimetres annually." He says "earthquakes are caused by a slip on a fault line and the release of stored elastic energy" and mining activity can trigger earthquakes.Free State. A large number of events are also recorded each year around the Carleton and Klerksdorp areas. Mining-induced tremors have distinct characteristics when compared to natural (tectonic) earthquakes. It has been reported by that the region's tremors have characteristically high peak horizontal accelerations and velocities up to 0.45 g and 67 mm/s respectively. Carletonville (1986) measured a peak acceleration of 0.45g. This is even higher than the peak acceleration of 0.36 g recorded in the well-known El Centro Mexico earthquake of 1940. The difference between the two seismic events is that the El Centro earthquake caused massive structural damage, whereas the Carleton damage to structures, the Carletonville earthquake caused only minor damage and cracking to structures close to the epicentre. Looking at other characteristics of the mine tremors, one can see the reason for the differences in the destructive capacity of the two types of seismic activity: The frequencies of mine-induced are usually high, ranging from 10 Hz to 50 Hz. This means that mine tremors are unlikely to cause structural response in structures with low natural frequencies. In general, structures with natural frequencies of less than 2 Hz will not generate structural responses to this type of event. Tall buildings will not respond to this type of seismic excitation because they have natural frequencies well below 2 Hz. Mine tremors occur at relatively shallow epicentres, typically at depths of 2 - 4 km. This means that the spread is over a small area. In addition, the decay of mine vibrations is rapid. For example, an acceleration of 0.39 g measured at 2.5 km is reduced to 0.13 g at 10 km.

Earthquake data

For earthquakes prior to the modern era, the magnitude and epicentre location are only approximate, and were calculated based on available reports from the time. The magnitude where given is measured using the Richter scale ($M_{L}$) unless stated otherwise.

See also

• Geology of South Africa
• List of earthquakes in 2014
• List of earthquakes in South Africa

External links

• Council for Geoscience
• https://www.volcanodiscovery.com/earthquakes/south-africa/largest.html#:~:text=Quakes%20since%201900%3A,between%20magnitude%205%20and%206

This page will be placed in the following categories if it is moved to the article namespace.

Categories:

• Lists of earthquakes by country
• Lists of events in South Africa
• Earthquakes in South Africa
• South Africa environment-related lists