Earthquakes are caused by the sudden release of energy within the Earth's crust, leading to the shaking and vibration of the ground. This release of energy is primarily due to the movement of tectonic plates, which are large pieces of the Earth's lithosphere (the outermost layer of the Earth) that float on the semi-fluid asthenosphere beneath them. Tectonic plates are constantly in motion, albeit very slowly, and their interactions give rise to earthquakes.
The primary causes of earthquakes are as follows:
Tectonic Plate Movement: Most earthquakes occur at plate boundaries, where tectonic plates interact with each other. There are several types of plate boundaries:
a. Divergent Boundaries: Plates move away from each other, creating rift zones and mid-ocean ridges. As the plates separate, magma rises to fill the gap, leading to earthquakes.
b. Convergent Boundaries: Plates collide with each other, and one plate may be forced beneath the other in a process known as subduction. The immense pressure and friction at these boundaries can cause earthquakes.
c. Transform Boundaries: Plates slide past each other horizontally. The friction between the plates builds up stress until it is released suddenly, causing an earthquake.
Volcanic Activity: Earthquakes can also occur in volcanic regions as magma moves beneath the Earth's surface. These earthquakes are associated with volcanic activity and are often referred to as volcanic earthquakes.
Human Activities: Certain human activities, such as mining, reservoir-induced seismicity (caused by the filling of large reservoirs), and hydraulic fracturing (fracking) for oil and gas, can induce earthquakes. These human-induced earthquakes are typically smaller in magnitude but can still be significant.
When the accumulated stress along a fault (a fracture in the Earth's crust) exceeds the strength of the rocks holding it in place, the fault will suddenly slip, releasing the stored energy in the form of seismic waves. These waves travel through the Earth, causing the ground to shake and leading to the occurrence of an earthquake. The point within the Earth where the fault rupture begins is called the focus or hypocenter, and the point on the Earth's surface directly above the focus is called the epicenter. The severity of an earthquake is measured using various scales, such as the Richter scale or the moment magnitude scale (Mw).