An earthquake is a sudden, sharp shaking of the Earth’s crust caused by the movement of rock layers, volcanic or magmatic activity of our planet. Earthquakes occur unexpectedly, without warning, tremors can last from a few seconds to minutes, leading to the destruction of buildings and critical infrastructure, loss of life, and material damage. Secondary disasters, resulting from movements of the Earth’s crust such as tsunamis, floods, landslides, and the release of toxic materials, pose a great danger.
During an earthquake, there is a rupture of the Earth’s layers, their sudden sliding along a seismic fault, which arises as a result of the accumulation of stresses from the constant movement of lithospheric plates. At this moment the accumulated energy is suddenly released and disperses in all directions as waves. Their penetration to the surface is felt as ground vibrations. Compressions or shifts of the Earth’s crust will spread in rocks, forming new seismic waves and triggering further tremors.
What is the connection between plate tectonics and earthquakes?
Continental or oceanic tectonic plates form the rigid, sturdy outer layer of the Earth (the lithosphere). The lithospheric plates are in constant motion due of strong convective currents acting on the outer shell from the underlying hot mantle. Their movement leads to the convergence, divergence, or sliding of adjacent plates, creating deformations in the contact zone, also known as a fault. In such areas, the movement of plates slows down, leading to significant deformational stresses. When the permissible stress threshold is exceeded, the layers of the Earth’s crust in the contact zone rupture, causing an earthquake.
Cities located near tectonic faults, as well as their population, are in a zone of constant seismic danger. The following areas have the greatest risk of being affected by strong earthquakes due to tectonic processes:
- the West coast of the United States with the famous San Andreas Fault;
- the south and east of Japan, Kamchatka, the Far Eastern coast of Russia;
- Chile and Peru;
- Indonesia and the Philippines;
- Turkey and the European Mediterranean coast.
If the epicenter of the fault occurs in the oceanic part of the Earth’s crust, seismic waves trigger tsunamis destroying everything in their path.
Are artificial earthquakes possible on the Earth?
In addition to natural earthquakes caused by tectonic processes, the Earth’s crust can experience tremors of artificial origin. These include:
- earthquakes associated with volcanic activity. Thus, the famous eruption of the Krakatau volcano, which occurred on August 26, 1889, was accompanied by explosions and tremors that almost destroyed the island of the same name;
- underground testing of nuclear weapons. If the atomic bombs dropped on Hiroshima and Nagasaki were detonated at a depth of 2-3 km, tremors with a magnitude of 5.5 could be felt on the surface;
- oil and gas extraction, exploitation of geothermal sources;
- filling large dams with water;
- industrial disasters – rock collapses in old mines.
Anthropogenic earthquakes (excluding nuclear explosions) are typically accompanied by tremors not exceeding magnitude 4.0 and are rarely felt by the population of the region.
What is the duration of an earthquake?
Usually, the epicenter of earthquakes is located within 20 km below the Earth’s surface. More massive catastrophes occur in fault zones and can reach depths of up to 700 km, reaching the mantle layer. The duration of tremors felt by people depends on several factors:
- the time during which a sudden movement of plates occurs in the fault – from 1-2 seconds for an earthquake of magnitude 4.0 to approximately 1.5 minutes if the event is caused by tremors of magnitude over 8.0;
- the type of seismic vibrations – their duration is related to the movement of fast (P-waves) and slow (S-waves) surface waves;
- the features of rocks and soil in the earthquake zone – for example, in Alluvial Basins, vibrations can cause resonance and trigger tremors with a long-term spreading effect.
Usually, the first strong earthquakes are followed by subsequent tremors, which scientists call aftershocks. Their number and magnitude decrease over time. Sometimes aftershocks can have a higher magnitude than the main earthquake. For instance, after the February 2023 earthquake in Turkey with a magnitude of 7.5, there were aftershocks with a magnitude of 7.8.