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In geophysics, earthquakes (from Latin: terrae motus, which means “movement of the earth”), also called seisms or telluric tremors (from Latin Tellus, Roman goddess of the Earth), are vibrations or settlements of the Earth’s crust, caused by the sudden displacement of a rock mass in the subsoil.
This displacement is generated by the forces of tectonic nature that act constantly within the Earth’s crust causing a slow deformation until it reaches the breaking load with consequent release of elastic energy in an internal area of the Earth called hypocenter, typically located in correspondence of pre-existing fractures of the crust called faults. From the fracture created a series of elastic waves, called seismic waves, propagate in all directions from the hypocenter, giving rise to the phenomenon observed on the surface with the place of the Earth’s surface located on the vertical of the hypocenter, called epicenter, which is generally the most affected by the phenomenon. The branch of geophysics that studies these phenomena is seismology.
Almost all earthquakes that occur on the Earth’s surface are concentrated near the boundaries between two tectonic plates, where the contact is made by faults: these are in fact the areas tectonically active, that is, where the plates move more or less “rubbing” or “coercing” each other with respect to the other, thus generating interplate earthquakes. More rarely, earthquakes occur far from the border zones between plates, due to tectonic rearrangements. Localized earthquakes of minor intensity are recorded in volcanic areas due to the movement of magmatic masses in depth.
According to the model of plate tectonics, the movement of the plates is slow, constant and imperceptible (if not with special instruments), and shapes and distorts the rocks both on the surface and underground. However, in some moments and in some areas, due to internal forces (pressures, tensions and frictions) between the rock masses, such modeling stops and the surface involved accumulates tension and energy for tens or hundreds of years until, upon reaching the breaking load, the accumulated energy is sufficient to overcome the resistant forces causing the sudden and abrupt movement of the rock mass involved. This sudden movement, which in a few seconds releases energy accumulated for tens or hundreds of years, thus generates the seismic waves and the associated earthquake.
An earthquake (or seism) originates when the collision between two crustal plates causes a rapid vibration of the Earth’s crust capable of releasing very high amounts of energy, regardless of the effects it causes. Every day on Earth there are thousands of earthquakes: experimentally it is observed that the majority of earthquakes in the world, as well as volcanic eruptions, occur along the so-called pacific fire belt, oceanic ridges and subduction zones or boundaries between tectonic plates and therefore often affects the oceanic crust as a trigger zone or fracturing. Only a few dozen are felt by the population and most cause little or no damage. The average duration of a tremor is much less than 30 seconds; for the strongest earthquakes, however, it can be up to a few minutes.
The source of the earthquake is generally distributed in an internal zone of the earth’s crust. In the case of the most devastating earthquakes this can have an extension of the order of a thousand kilometers but it is ideally possible to identify a precise point from which the seismic waves originated: this is called the “hypocenter” and here the movement originated from the pre-existing fracture (fault) or its sudden generation. The vertical projection of the hypocenter on the earth’s surface is instead called the “epicenter”, and it is the point where the greatest damage usually occurs. The elastic waves that propagate during an earthquake are of different types, and in some cases may result in a predominantly horizontal (undulatory shaking) or vertical movement of the ground (subsulatory shaking).
Some earthquakes occur or are preceded by seismic swarms (foreshocks) more or less long and intense, characterized by more earthquakes repeated over time and particularly circumscribed in a particular area, others instead occur immediately and suddenly with one or more main shocks (main shock), another form are the seismic sequences, each characterized by more earthquakes released in close succession and not circumscribed in a particular area. Earthquakes of greater magnitude are usually accompanied by secondary events (not necessarily less destructive) that follow the main shock and are called aftershocks. When several events occur at the same time or almost at the same time, these may be induced earthquakes (the earthquake triggers the fracturing of other rock that was already close to the critical point of fracture).
An earthquake may also be accompanied by loud noises that may resemble roars, rumbles, thunder, gunfire sequences, etc.: these sounds are due to the passage of seismic waves to the atmosphere and are more intense in the vicinity of the epicenter. The most seismic countries and areas in the world are Afghanistan, Alaska, Albania, Armenia, Azerbaijan, California, Chile, Colombia, Croatia, Ecuador, Philippines, Georgia, Japan, Greece, Indonesia, Iran, Iceland, Italy, Mexico, Montenegro, Nepal, New Guinea, Peru, Polynesia, Taiwan and Turkey.
In general, earthquakes are caused by sudden movements of rock masses (more or less large) within the earth’s crust. The earth’s surface is in fact in slow, but constant movement (see plate tectonics) and earthquakes occur when the resulting tension accumulated by mechanical stress exceeds the capacity or resistance of the rocky material to withstand it, that is, exceeds the so-called breaking load. This condition most often occurs at tectonic plate boundaries. Seismic events that occur at plate boundaries are called tectonic earthquakes, those less frequent that occur within the plates of the lithosphere are instead called intraplate earthquakes.
Almost all the earthquakes that occur on the Earth’s surface are therefore concentrated in very precise areas, that is, near the borders between one tectonic plate and another: these are in fact the tectonically active areas, that is, where the plates move more or less slowly and suddenly with respect to each other. According to plate tectonics, the surface of the Earth is in fact modeled as if it were composed of about a dozen large tectonic plates that move very slowly, due to convection currents of the Earth’s mantle placed under the Earth’s crust. Because they do not all move in the same direction, the plates often collide directly by sliding sideways along the edge of another (transform fault). In general, the movement of the plates is slow, imperceptible (if not with special instruments) and constant, but at certain times and in some areas, the movement stops and the area involved accumulates energy for decades or centuries until it reaches the so-called breaking load, when due to internal forces, or the balance between pressures, tensions and friction between the rock masses, these movements occur suddenly and abruptly releasing the accumulated energy and thus developing an earthquake.
The disposition of the seismic zones is localized for the most part along the margins between the tectonic plates (ex. The disposition of the seismic zones is localized mostly along the margins between the tectonic plates (e.g. fire belt) and in particular along the abyssal pits (subduction zones), where the sinking of the oceanic crust below other portions of the Earth’s crust leads to the friction fusion of part of the rocky zone of contact, or along the oceanic ridges where the magma of the Earth’s mantle rises to the surface through the fractures of the oceanic crust and once solidified goes to “weld” the plates themselves; the earthquakes along the ridges are therefore the effect of the sudden rupture of these welds upon reaching a certain level of mechanical stress. In these areas, seismic phenomena are often associated with volcanism due to the concomitance of the tectonic forces at play and for this reason volcanic eruptions are often preceded by earthquakes.
It is therefore assumed that the dislocation of the plates is the triggering mechanism of earthquakes. A secondary cause is magmatic movement within a volcano, which can be an indication of an impending eruption along with the characteristic tremor. In very rare cases, earthquakes have been associated with the accumulation of large masses of water in dam reservoirs, as with the Kariba Dam in Zambia, Africa, and with the injection or extraction of fluids from the earth’s crust (Rocky Mountain Arsenal). Such earthquakes occur because the strength of the Earth’s crust can be changed by fluid pressure.
Detection and measurement
Seismic waves are detectable and measurable through particular instruments called seismographs, commonly used by seismologists, and displayed on seismograms; the cross processing of data from several seismographs scattered over a territory at a certain distance from the earthquake allows to estimate quite accurately the epicenter, the hypocenter and the intensity of the earthquake; The latter can be evaluated through the so-called seismic scales, mainly the Richter scale, the Mercalli scale and the seismic moment magnitude scale.
Recognizing the orientation of arrival of the shocks along the three reference planes, and understanding whether the first arrival of the shock was compressive or expansive, allows one to determine the focal mechanism of the shock and thus to understand what type of fault originated the earthquake.
The tectonic displacement of the Earth’s crust in the three spatial coordinates following a strong earthquake can be accurately measured by remote sensing techniques such as geodetic surveys and radar-satellite interferometry using SAR (Synthetic Aperture Radar) over the entire affected area from the epicenter.