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An earthquake occurs when two blocks of the Earth slide past each other along a surface known as a fault or fault plane. The point beneath the Earth’s surface where the earthquake originates is called the hypocenter, while the corresponding point directly above it on the Earth’s surface is referred to as the epicentre.
Occasionally, an earthquake may be preceded by smaller earthquakes known as foreshocks, which occur in the same location as the larger earthquake that follows. It is often difficult for scientists to identify these foreshocks until a larger earthquake occurs. The largest and most significant earthquake in a sequence is called the mainshock, which is invariably accompanied by aftershocks. Aftershocks are smaller earthquakes that occur in the same area as the mainshock and can persist for weeks, months, or even years depending on the magnitude of the mainshock.
Plate boundaries consist of numerous faults, and a majority of earthquakes worldwide happen along these faults. The rough edges of the plates become locked together while the rest of the plate continues its movement. Eventually, when the plate has shifted sufficiently, the locked edges suddenly release along one of the faults, resulting in an earthquake. Let us analyse the various causes of an earthquake.
Natural Causes of Earthquakes
Earthquakes result from the sudden release of energy within the Earth’s rocks, which can be triggered by various natural processes. Among these processes, the release of elastic strain is considered the most significant cause. Elastic strain energy accumulates in rock masses over time, and when it exceeds the strength of the rocks, it can lead to major disturbances. These disturbances are often associated with tectonic earthquakes, which occur along fault lines.
Tectonic Earthquakes
Tectonic earthquakes are explained by the elastic rebound theory, which was formulated by the American geologist Harry Fielding Reid after the 1906 San Francisco earthquake. According to this theory, strains in rock masses accumulate until the resulting stresses exceed the strength of the rocks, causing sudden fracturing. The fractures propagate rapidly through the rock along fault lines, sometimes extending over long distances.
Volcanism and Earthquakes
Volcanic earthquakes are a separate type of earthquake associated with volcanic activity. While the exact mechanisms behind volcanic earthquakes are not fully understood, it is believed that they may result from the sudden slip of rock masses adjacent to volcanoes, releasing stored elastic strain energy. Additionally, the heat provided by magma moving in reservoirs beneath the volcano or the release of gas under pressure may contribute to these seismic events.
Artificial Induction of Earthquakes
Human activities can also induce earthquakes. These activities include the injection of fluids into deep wells, large underground nuclear explosions, mining operations, and the filling of large reservoirs. Deep mining, for example, can produce changes in the strain around tunnels, leading to slip on adjacent faults or outward shattering of rock. In fluid injection, the lubrication of fault surfaces by the injected liquid can induce slip, similar to tectonic earthquakes.
Reservoir Induction
Reservoirs, particularly those exceeding certain depths and volumes, can induce earthquakes. The weight of water in the reservoir can trigger fault rupture, especially if nearby rocks are already strained by tectonic forces. Examples of reservoirs where such connections have likely occurred include the Hoover Dam, the Aswan High Dam, and the Kariba Dam. While most large reservoirs do not produce hazardous earthquakes, some have induced seismic events.
Seismology and Nuclear Explosions
Seismology plays a crucial role in detecting and studying earthquakes, including those induced by human activities such as underground nuclear explosions. Recent advancements in seismological techniques have improved our ability to distinguish between natural and induced earthquakes. For instance, underground nuclear explosions typically generate seismic waves with much larger amplitudes than surface waves. International monitoring networks of seismographic stations help detect and locate seismic events of significant magnitude, aiding in earthquake monitoring and hazard assessment. The explanation for earthquakes associated with reservoirs suggests that the rocks near the reservoir are already under strain from regional tectonic forces, nearing the point where nearby faults are close to slipping. The presence of water in the reservoir creates a pressure disturbance that can trigger the fault rupture. Additionally, the pressure effect may be amplified by the reduced strength of rocks along the fault due to increased water-pore pressure. Despite these factors, the majority of large reservoirs being filled have not caused earthquakes significant enough to pose a hazard.
In summary, earthquakes can be caused by various natural and human-induced processes, including the release of elastic strain, volcanic activity, human activities such as fluid injection and nuclear explosions, and the filling of large reservoirs. Each of these mechanisms contributes to the complex and dynamic nature of seismic events, highlighting the importance of understanding their causes and effects for earthquake preparedness and mitigation efforts.