Earthquake

Global earthquake epicenters, 1963–1998

An earthquake is a sudden and sometimes catastrophic movement of a part of the Earth's surface. Earthquakes result from the dynamic release of elastic strain energy that radiates seismic waves. Earthquakes typically result from the movement of faults, planar zones of deformation within the Earth's upper crust. The word earthquake is also widely used to indicate the source region itself. The Earth's lithosphere is a patch work of plates in slow but constant motion (see plate tectonics). Earthquakes occur where the stress resulting from the differential motion of these plates exceeds the strength of the crust. The highest stress (and possible weakest zones) are most often found at the boundaries of the tectonic plates and hence these locations are where the majority of earthquakes occur. Events located at plate boundaries are called interplate earthquakes; the less frequent events that occur in the interior of the lithospheric plates are called intraplate earthquakes (see, for example, New Madrid Seismic Zone). Earthquakes related to plate tectonics are called tectonic earthquakes. Most earthquakes are tectonic, but they also occur in volcanic regions and as the result of a number of anthropogenic sources, such as reservoir induced seismicity, mining and the removal or injection of fluids into the crust. Seismic waves including some strong enough to be felt by humans can also be caused by explosions (chemical or nuclear), landslides, and collapse of old mine shafts, though these sources are not strictly earthquakes. These sources will also show a different seismogram than earthquakes

Characteristics

Large numbers of earthquakes occur on a daily basis on Earth, but the majority of them are detected only by seismometers and cause no damage .

Most earthquakes occur in narrow regions around plate boundaries down to depths of a few tens of kilometres where the crust is rigid enough to support the elastic strain. Where the crust is thicker and colder they will occur at greater depths and the opposite in areas that are hot. At subduction zones where plates descend into the mantle, earthquakes have been recorded to a depth of 600 km, although these deep earthquakes are caused by different mechanisms than the more common shallow events. Some deep earthquakes may be due to the transition of olivine to spinel, which is more stable in the deep mantle.

Large earthquakes can cause serious destruction and massive loss of life through a variety of agents of damage, including fault rupture, vibratory ground motion (i.e., shaking), inundation (e.g., tsunami, seiche, dam failure), various kinds of permanent ground failure (e.g. liquefaction, landslide), and fire or a release of hazardous materials. In a particular earthquake, any of these agents of damage can dominate, and historically each has caused major damage and great loss of life, but for most of the earthquakes shaking is the dominant and most widespread cause of damage. There are four types of seismic waves that are all generated simultaneously and can be felt on the ground. S-waves (secondary or shear waves) and the two types of surfaces waves ( Love waves and Rayleigh waves) are responsible for the shaking hazard.

Damage from the 1906 San Francisco earthquake.

Section of collapsed freeway after the 1989 Loma Prieta earthquake.

Most large earthquakes are accompanied by other, smaller ones, that can occur either before or after the principal quake — these are known as foreshocks or aftershocks, respectively. While almost all earthquakes have aftershocks, foreshocks are far less common occurring in only about 10% of events. The power of an earthquake is distributed over a significant area, but in the case of large earthquakes, it can spread over the entire planet. Ground motions caused by very distant earthquakes are called teleseisms. The Rayleigh waves from the Sumatra-Andaman Earthquake of 2004 caused ground motion of over 1 cm even at the seismometers that were located far from it, although this displacement was abnormally large. Using such ground motion records from around the world it is possible to identify a point from which the earthquake's seismic waves appear to originate. That point is called its "focus" or " hypocenter" and usually proves to be the point at which the fault slip was initiated. The location on the surface directly above the hypocenter is known as the " epicenter". The total size of the fault that slips, the rupture zone, can be as large as 1000 km, for the biggest earthquakes. Just as a large loudspeaker can produce a greater volume of sound than a smaller one, large faults are capable of higher magnitude earthquakes than smaller faults are.

Earthquakes that occur below sea level and have large vertical displacements can give rise to tsunamis, either as a direct result of the deformation of the sea bed due to the earthquake or as a result of submarine landslips or "slides" directly or indirectly triggered by it.

Earthquake Size

The first method of quantifying earthquakes was intensity scales. In the United States the Mercalli (or Modified Mercalli, MM) scale is commonly used, while Japan ( shindo) and the EU ( European Macroseismic Scale) each have their own scales. These assign a numeric value (different for each scale) to a location based on the size of the shaking experienced there. The value 6 (normally denoted "VI") in the MM scale for example is:

Everyone feels movement. People have trouble walking. Objects fall from shelves. Pictures fall off walls. Furniture moves. Plaster in walls might crack. Trees and bushes shake. Damage is slight in poorly built buildings. No structural damage.

A Shakemap recorded by the Pacific Northwest Seismograph Network that shows the instrument recorded intensity of the shaking of the Nisqually earthquake on February 28, 2001.

A Community Internet Intensity Map generated by the USGS that shows the intensity felt by humans by ZIP Code of the shaking of the Nisqually earthquake on February 28, 2001.

The problem with these scales is the measurement is subjective, often based on the worst damage in an area and influenced by local effects like site conditions that make it a poor measure for the relative size of different events in different places. For some tasks related to engineering and local planning it is still useful for the very same reasons and thus still collected. If you feel an earthquake in the US you can report the effects to the USGS.

The first attempt to qualitatively define one value to describe the size of earthquakes was the magnitude scale (the name being taking from similar formed scales used on the brightness of stars). In the 1930s, a California seismologist named Charles F. Richter devised a simple numerical scale (which he called the magnitude) to describe the relative sizes of earthquakes in Southern California. This is known as the “ Richter scale”, “Richter Magnitude” or “Local Magnitude” (M_L). It is obtained by measuring the maximum amplitude of a recording on a Wood-Anderson torsion seismometer (or one calibrated to it) at a distance of 600km from the earthquake. Other more recent Magnitude measurements include: body wave magnitude (m_b), surface wave magnitude (M_s) and duration magnitude (M_D). Each of these is scaled to gives values similar to the values given by the Richter scale. However as each is also based on the measurement of one part of the seismogram they do not measure the overall power of the source and can suffer from saturation at higher magnitude values (larger events fail to produce higher magnitude values).These scales are also empirical and as such there is no physical meaning to the values. They are still useful however as they can be rapidly calculated, there are catalogues of them dating back many years and are they are familiar to the public. Seismologists now favor a measure called the seismic moment, related to the concept of moment in physics, to measure the size of a seismic source. The seismic moment is calculated from seismograms but can also by obtained from geologic estimates of the size of the fault rupture and the displacement. The values of moments for different earthquakes ranges over several order of magnitude. As a result the moment magnitude (M_W) scale was introduced by Hiroo Kanamori, which is comparable to the other magnitude scales but will not saturate at higher values.

Larger earthquakes occur less frequently than smaller earthquakes, the relationship being exponential, ie roughly ten times as many earthquakes larger than 4 occur in a particular time period than earthquakes larger than magnitude 5. For example it has been calculated that the average recurrence for the United Kingdom can be described as follows:

an earthquake of 3.7 or larger every 1 year
an earthquake of 4.7 or larger every 10 years
an earthquake of 5.6 or larger every 100 years.

Causes

Most earthquakes are powered by the release of the elastic strain that accumulate over time, typically, at the boundaries of the plates that make up the Earth's lithosphere via a process called Elastic-rebound theory. The Earth is made up of tectonic plates driven by the heat in the Earth's mantle and core. Where these plates meet stress accumulates. Eventually when enough stress accumulates, the plates move, causing an earthquake. Deep focus earthquakes, at depths of 100's km, are possibly generated as subducted lithospheric material catastrophically undergoes a phase transition since at the pressures and temperatures present at such depth elastic strain cannot be supported. Some earthquakes are also caused by the movement of magma in volcanoes, and such quakes can be an early warning of volcanic eruptions. A rare few earthquakes have been associated with the build-up of large masses of water behind dams, such as the Kariba Dam in Zambia, Africa, and with the injection or extraction of fluids into the Earth's crust (e.g. at certain geothermal power plants and at the Rocky Mountain Arsenal). Such earthquakes occur because the strength of the Earth's crust can be modified by fluid pressure. Earthquakes have also been known to be caused by the removal of natural gas from subsurface deposits, for instance in the northern Netherlands. Finally, ground shaking can also result from the detonation of explosives. Thus scientists have been able to monitor, using the tools of seismology, nuclear weapons tests performed by governments that were not disclosing information about these tests along normal channels. Earthquakes such as these, that are caused by human activity, are referred to by the term induced seismicity.

Another type of movement of the Earth is observed by terrestrial spectroscopy. These oscillations of the earth are either due to the deformation of the Earth by tide caused by the Moon or the Sun, or other phenomena.

A recently proposed theory suggests that some earthquakes may occur in a sort of earthquake storm, where one earthquake will trigger a series of earthquakes each triggered by the previous shifts on the fault lines, similar to aftershocks, but occurring years later.

Preparation for earthquakes

Emergency preparedness
Household seismic safety
Seismic retrofit
Earthquake prediction

Specific fault articles

Alpine Fault
Calaveras Fault
Hayward Fault Zone
North Anatolian Fault Zone
New Madrid Fault Zone
San Andreas Fault
Great Sumatran fault

Specific earthquake articles

Shaanxi Earthquake (1556). Deadliest known earthquake in history, estimated to have killed 830,000 in China.
Cascadia Earthquake (1700).
Kamchatka earthquakes (1737 and 1952).
Lisbon earthquake (1755).
New Madrid Earthquake (1811).
Fort Tejon Earthquake (1857).
Charleston earthquake (1886). Largest earthquake in the Southeast and killed 100.
San Francisco Earthquake (1906).
Great Kanto earthquake (1923). On the Japanese island of Honshu, killing over 140,000 in Tokyo and environs.
Kamchatka earthquakes (1952 and 1737).
Great Chilean Earthquake (1960). Biggest earthquake ever recorded, 9.5 on Moment magnitude scale.
Good Friday Earthquake (1964) Alaskan earthquake.
Ancash earthquake (1970). Caused a landslide that buried the town of Yungay, Peru; killed over 40,000 people.
Sylmar earthquake (1971). Caused great and unexpected destruction of freeway bridges and flyways in the San Fernando Valley, leading to the first major seismic retrofits of these types of structures, but not at a sufficient pace to avoid the next California freeway collapse in 1989.
Tangshan earthquake (1976). The most destructive earthquake of modern times. The official death toll was 255,000, but many experts believe that two or three times that number died.
Great Mexican Earthquake (1985). 8.1 on the Richter Scale, killed over 6,500 people (though it is believed as many as 30,000 may have died, due to missing people never reappearing.)
Whittier Narrows earthquake (1987).
Armenian earthquake (1988). Killed over 25,000.
Loma Prieta earthquake (1989). Severely affecting Santa Cruz, San Francisco and Oakland in California. Revealed necessity of accelerated seismic retrofit of road and bridge structures.
Northridge, California earthquake (1994). Damage showed seismic resistance deficiencies in modern low-rise apartment construction.
Great Hanshin earthquake (1995). Killed over 6,400 people in and around Kobe, Japan.
İzmit earthquake (1999) Killed over 17,000 in northwestern Turkey.
Düzce earthquake (1999)
Chi-Chi earthquake (1999).
Nisqually Earthquake (2001).
Gujarat Earthquake (2001).
Dudley Earthquake (2002).
Bam Earthquake (2003).
Parkfield, California earthquake (2004). Not large (6.0), but the most anticipated and intensely instrumented earthquake ever recorded and likely to offer insights into predicting future earthquakes elsewhere on similar slip-strike fault structures.
Chuetsu Earthquake (2004).
Indian Ocean Earthquake (2004). One of the largest earthquakes ever recorded at 9.0. Epicenter off the coast of the Indonesian island Sumatra. Triggered a tsunami which caused nearly 300,000 deaths spanning several countries.
Sumatran Earthquake (2005).
Fukuoka earthquake (2005).
Kashmir earthquake (2005). Killed over 79,000 people. Many more at risk from the Kashmiri winter.
Lake Tanganyika earthquake (2005).