Beneath the Earth’s crust, which forms the surface of our planet, are deeper and hotter layers, such as the mantle and core. The Earth’s crust reaches several tens of kilometers in thickness and forms a lithosphere – a mosaic of several rigid tectonic plates, including 7 major and 46 minor. These plates reach the plastic layers of the mantle (asthenosphere) and are in constant motion due to convection processes.
The plates can gradually diverge, converge, subduct beneath one another, and slide horizontally in different directions. The movement of tectonic plates causes the formation of new continents, mountain ranges and underwater volcanoes, and also contributes to the accumulation of stresses, leading to powerful earthquakes along fault zones.
How can lithospheric plates move
Geophysicists distinguish three types of tectonic (lithospheric) plates movement:
- two plates diverge – they once formed a single structure that eventually fractured, and its parts are removed in two opposite directions. The boundary of divergent tectonic plates is formed mainly on the ocean floor. In a fault zone, hot magma can rise, cool and form new oceanic crust on the surface. Divergent tectonic plates form long chains of underwater mountains and ocean ridges;
- two plates slide parallel to one another – they rub against each other, forming transform fault zones. The energy released by sliding plates can cause the strongest earthquakes, as occurs in Southern California (USA), Anatolia (Turkey) or in certain parts of New Zealand;
- two tectonic plates collide – the event can develop in two ways. In one way, the plates converge and the heavier one slides beneath the other, forming a convergent or destructive boundary. The second drags the main one, and two continental plates meet on the surface, forming mountain ranges and causing powerful earthquakes (Himalayan or Alpine). In the other way, one plate subducts beneath another, which leads to the accumulation of deformation energy and results in destructive earthquakes (the Pacific Belt in Chile and Alaska).
The movement of tectonic plates is accompanied by friction and accumulation of huge amounts of energy. Resisting stress, the plates behave like a tightly compressed spring. And upon reaching the limit, they release internal energy in a few seconds in the form of heat and vibration. Seismic waves soon reach the surface, causing an earthquake. After the deformation is eliminated, the contact zone of the tectonic plates is blocked, starting the accumulation of new energy until the next earthquake.
It should be noted that such a deformation mechanism operates only in the upper part of tectonic plates. At a deeper level, due to the high temperature, rocks have a different mechanical behavior. They are more plastic, regularly deforming through continuous sliding, without causing the accumulation of energy that causes earthquakes.
Zones of increased seismicity
The largest number of earthquakes occur on faults of tectonic plates, in areas with low mechanical resistance, susceptible to deformations caused by the movement of tectonic plates. Areas of increased seismicity are mainly concentrated along the boundaries of large tectonic plates, where the relative movements of massive layers are greatest. These include the boundaries of the Pacific plate. Strong earthquakes regularly occur in South America, Alaska, Japan, Kamchatka, as well as on the periphery of the Indian Ocean (seismic activity in Indonesia and the Himalayas). In these regions, plate movement reaches several centimeters of relative displacement per year, which leads to regular strong earthquakes.
Sometimes rigid plates are able to transmit the forces generated in the contact zones remotely over considerable distances. Earthquakes far from plate boundaries occur in this way, for example in the central-eastern part of the United States, Australia or mainland France. Earthquakes can also be caused by other natural phenomena on the Earth’s surface. Melting glaciers and erosion lead to an increase in deformations acting on segments of the Earth’s crust, which can trigger earthquakes. In addition, there are induced earthquakes caused by human activity. The development of mineral resources in fault zones, deep storage of gas and water can provoke increased seismicity and increase the risk of earthquakes.