Earthquakes are the natural phenomena that have captured the human imagination throughout history. They are sudden and deadly, and their victims range from a few to hundreds of thousands. No other natural force can cause so much damage in such a short time. How often do earthquakes occur on our planet?
Earthquakes occur constantly all over the world. There are millions of small earthquakes every year that humans may not even feel. Seismographs annually record more than 300,000 cases of seismic activity in all regions of the Earth. People do not feel most of them or feel them weakly. On average, earthquakes of magnitude 2.0 or less occur several hundred times a day around the world. Strong earthquakes that are dangerous to the population occur with the following frequency:
- tremors of magnitude greater than 7.0 – about 20 times a year;
- earthquakes of magnitude 0 occur once a year;
- devastating disasters with magnitude of 9.0 and higher occur once every 20 years.
Although the annual number of earthquakes detected and localized by modern seismic equipment is increasing, but this doesn’t mean that the actual number of earthquakes is growing. However, population growth and industrial development in tectonic fault zones lead to increased potential seismic risks.
Seismic cycle
Most earthquakes are tectonic in nature, which means they are formed along faults. A tectonic fault is a two-dimensional structure in rock, the movement of tectonic plates along which has already been in the past. Therefore, the tectonic fault is a weak zone and is a potential site for a new shift. Thus, the frequency of earthquakes is an important factor for determining the seismic risks of the region.
Active faults follow the basic principles of strain energy accumulation and release, which is called the seismic cycle. The most active faults follow three main models that determine how plates move at the fault zone:
- compression, when the upper plate rises above the lower one along the fault plane. This shows both the horizontal convergence and the relative vertical movement of the two plates. This type of fault is characteristic of both tectonic subduction zones (one oceanic plate dives beneath another) and continental collisions, where two lithospheric blocks collide.
- extension, where the upper plate moves down along the sloping fault plane. Such faults occur in large tectonic zones, such as along ocean ridges, where plates are pulled apart by pressure of rocks from the mantle;
- strike-slip fault, occur when two plates slide horizontally along each other. This type of fault can be observed at the boundary of two plates that move parallel to one another without converging or diverging. This movement is characteristic of the San Andreas Fault in California.
Research shows that the size of the fault, the strength of earthquakes and their frequency depend on the nature of plate movement.
Frequency of earthquakes
A fault that regularly causes earthquakes is cyclical. This recurrence is reflected in the seismic cycle, which includes a period of accumulation of strain. During the interseismic period, the fault is locked, and the rocks deform under the tectonic forces.
However, rocks rupture when they can no longer withstand additional elastic deformation. The plates suddenly begin to slide along the fault, causing earthquakes. This period is called coseismic. The amount of energy released during the movement of tectonic plates determines the magnitude of an earthquake.
After a series of tremors, the fault again enters the phase of energy accumulation. The longer the period between earthquakes along a fault, the greater the risk of another powerful earthquake. Research shows that the most active faults are located in zones of compression and shear of lithospheric plates. If tectonic plate movement involves both compression and shear, or more than two lithospheric plates meet in the fault zone, interseismic periods can be short, and the power of earthquakes can be devastating.