
About 100,000 earthquakes are recorded on our planet every year. Most of them are not felt by humans, but the most powerful seismic tremors are catastrophic and lead to tragic consequences. But how do they occur, and why does an earthquake begin?
Seismic activity, such as earthquakes, is characterized by ground vibrations, which can be very brief (a few seconds) and hardly noticeable or, conversely, last longer and be more intense. The tremors and vibrations are caused by the sudden release of energy accumulated in rocks due to stress. The energy is released when a rupture occurs along an existing fault.
Types of earthquakes
Earthquakes are divided into 4 types, depending on the causes of origin:
- Tectonic earthquakes, which occur due to rupture of a fault along the contact zone of moving lithospheric plates;
- Volcanic earthquakes, triggered by powerful eruptions with deep magma movements;
- Glacial earthquakes, caused by fracturing of large ice masses and extending into the Earth’s crust;
- Explosion earthquakes, both natural and induced, which arise due to the collapse of cavities within rock masses.
Tectonic earthquakes are the most frequent and destructive. A significant number of these earthquakes occur at the boundaries of tectonic plates, where huge blocks of rock slide past each other.
What causes earthquakes
The Earth’s surface is made up of tectonic plates, like pieces of a puzzle forming the continents and ocean floor. These plates are in constant motion due to convection processes occurring in the Earth’s interior. They slide, collide and subduct beneath each other along major fault lines.
Movement along the fault is blocked during interseismic periods (between earthquakes), and energy is accumulated due to elastic deformation of rocks. When the deformation reaches its limit, this stored energy is released suddenly, causing the tectonic plates at the fault to move and triggering an earthquake. The longer the interseismic period, the more energy accumulates in the deformation zone. The amount of accumulated energy, the type of fault, and its size determine the strength and destructive effects of an earthquake.
Earthquake precursors
There are often precursors that indicate an impending earthquake:
- abnormal animal behavior;
- changes in the level and chemical composition of groundwater;
- underground sounds;
- unusual phenomena in the atmosphere.
These signals serve as a reminder that it is important to be prepared for an earthquake by developing an emergency plan and gathering a survival kit.
Precursors may vary or be absent due to the characteristics of seismic activity in the fault zone. For example, the catastrophic earthquake in Turkey and Syria 2023 began completely suddenly. There was no sign of its approach, and municipalities were unable to provide warnings to the population. This fact once again underlines the unpredictability of earthquakes and the impossibility of controlling their onset.
How an earthquake begins
Earthquakes occur along tectonic plate boundaries and fault lines, where elastic deformations accumulate from rock friction. Seismic activity does not occur along the entire length of the fault. It begins from the hypocenter, then the rupture propagates along the fault. The accumulated energy is released in two stages:
- Primary body waves (P-waves), which are the fastest and earliest, cause an initial seismic shock.
- Secondary waves (S-waves) cause vertical vibrations on the Earth’s surface that people feel as tremors during an earthquake.
If a layer of solid rock near the surface can propagate vibrations at high speed, then surface acoustic waves (Love and Rayleigh waves) are formed during an earthquake. These waves amplify vibrations at the surface, causing extensive destruction hundreds of kilometers from the epicenter.
Foreshocks and aftershocks
Depending on the type of fault (horizontal, vertical shear, displacement of plates in both planes), an earthquake occurs according to different scenarios. In normal faults, where one tectonic plate shifts down relative to another, the approach of the main earthquake may be indicated by precursor such as foreshocks. In some cases, they can be very strong and signal the imminent onset of an even more powerful earthquake.
In subduction zones, where one plate dives under another, earthquakes are often followed by aftershocks, which can number in the hundreds. Despite the fact that they are weaker than the main earthquake, these aftershocks are dangerous for people, as they can destroy buildings that have already been weakened by previous tremors.