An earthquake is the result of a sudden release of energy in the
Earth's
crust that creates
seismic waves. Earthquakes are recorded with a
seismometer, also known as a seismograph. The
moment magnitude of an earthquake is conventionally reported, or the related and mostly obsolete
Richter magnitude, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale.
At the Earth's surface, earthquakes manifest themselves by a shaking and sometimes displacement of the ground. When a large earthquake
epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a
tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.
In its most generic sense, the word earthquake is used to describe any seismic event—whether a natural
phenomenon or an event caused by humans—that generates
seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear experiments.
An earthquake's point of initial rupture is called its
focus or
hypocenter. The term
epicenter means the point at ground level directly above this.
Most naturally occurring earthquakes are related to the tectonic nature of the
Earth. Such earthquakes are called tectonic earthquakes. The Earth's
lithosphere is a patchwork of plates in slow but constant motion caused by the release to space of the heat in the Earth's mantle and core. The heat causes the rock in the Earth to flow on geological timescales, so that the plates move slowly but surely.
Plate boundaries lock as the plates move past each other, creating frictional
stress. When the frictional stress exceeds a critical value, called local strength, a sudden failure occurs. The boundary of tectonic plates along which failure occurs is called the
fault plane. When the failure at the fault plane results in a violent displacement of the Earth's
crust, energy is released as a combination of radiated elastic
strain seismic waves, frictional heating of the fault surface, and cracking of the rock, thus causing an earthquake. This process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the
Elastic-rebound theory. It is estimated that only 10 percent or less of an earthquake's total energy is radiated as seismic energy. Most of the earthquake's energy is used to power the earthquake
fracture growth or is converted into heat generated by friction. Therefore, earthquakes lower the Earth's available elastic potential energy and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat out from the Earth's deep interior.
[1]The majority of tectonic earthquakes originate at depths not exceeding tens of kilometers. In
subduction zones, where older and colder
oceanic crust descends beneath another tectonic plate,
Deep focus earthquakes may occur at much greater depths (up to seven hundred kilometers). These seismically active areas of subduction are known as
Wadati-Benioff zones. These are earthquakes that occur at a depth at which the subducted
lithosphere should no longer be brittle, due to the high temperature and pressure. A possible mechanism for the generation of deep focus earthquakes is faulting caused by
olivine undergoing a
phase transition into a
spinel structure.
[2]Earthquakes also often occur in volcanic regions and are caused there, both by tectonic faults and by the movement of
magma in
volcanoes. Such earthquakes can serve as an early warning of volcanic eruptions.
Sometimes a series of earthquakes occur in a sort of
earthquake storm, where the earthquakes strike a fault in clusters, each triggered by the shaking or stress redistribution of the previous earthquakes. Similar to
aftershocks but on adjacent segments of fault, these storms occur over the course of years, and with some of the later earthquakes as damaging as the early ones. Such a pattern was observed in the sequence of about a dozen earthquakes that struck the
North Anatolian Fault in Turkey in the 20th century, the half dozen large earthquakes in
New Madrid in 1811-1812, and has been inferred for older anomalous clusters of large earthquakes in the Middle East and in the Mojave Desert.
There are many effects of earthquakes including, but not limited to the following:
Shaking and ground rupture
Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings or other rigid structures. The severity of the local effects depends on the complex combination of the earthquake
magnitude, the distance from
epicenter, and the local geological and geomorphological conditions, which may amplify or reduce
wave propagation. The ground-shaking is measured by ground
acceleration.
Specific local geological, geomorphological, and geostructural features can induce high levels of shaking on the ground surface even from low-intensity earthquakes. This effect is called site or local amplification. It is principally due to the transfer of the
seismic motion from hard deep soils to soft superficial soils and to effects of seismic energy focalization owing to typical geometrical setting of the deposits.
Ground rupture is a visible breaking and displacement of the earth's surface along the trace of the fault, which may be of the order of few metres in the case of major earthquakes. Ground rupture is a major risk for large engineering structures such as
dams,
bridges and
nuclear power stations and requires careful mapping of existing faults to identify any likely to break the ground surface within the life of the structure.
Landslides and
avalanchesEarthquakes can cause landslides and avalanches, which may cause damage in hilly and mountainous areas.
FiresFollowing an earthquake, fires can be generated by break of the
electrical power or gas lines. In the event of water mains rupturing and a loss of pressure, it may also become difficult to stop the spread of a fire once it has started.
Soil liquefaction
Soil
liquefaction occurs when, because of the shaking, water-saturated
granular material temporarily loses its strength and transforms from a
solid to a
liquid. Soil liquefaction may cause rigid structures, as buildings or bridges, to tilt or sink into the liquefied deposits.
TsunamisUndersea earthquakes and earthquake-triggered landslides into the sea, can cause Tsunamis. See, for example, the
2004 Indian Ocean earthquake.
Human impacts
Earthquakes may result in
disease, lack of basic necessities, loss of life, higher insurance premiums, general property damage, road and bridge damage, and collapse of buildings or destabilization of the base of buildings which may lead to collapse in future earthquakes.
This can cause total devastation for those affected as the country may not have the funds for the regeneration of people lives and possessions. An earthquake can ruin someones life forever, only 3% of buildings in kobe, for instance, have earthquake insurance; therefore un-enabling them to get back ont their feet again.
Preparation for earthquakes
Earthquake preparednessHousehold seismic safetyHurriQuake nail (for resisting hurricanes and earthquakes)
Seismic retrofitSeismic hazardMitigation of seismic motionEarthquake prediction