Earthquake

What should you do during an EQ?

2007-09-09T06:23:00.000-07:00

If you are INDOORS--STAY THERE! (Get under a desk or table and hang on to it, or move into a hallway or get against an inside wall. STAY CLEAR of windows, fireplaces, and heavy furniture or appliances. GET OUT of the kitchen, which is a dangerous place (things can fall on you). DON'T run downstairs or rush outside while the bldg is shaking or while there is danger of falling and hurting yourself or being hit by falling glass or debris.

If you are OUTSIDE-- get into the OPEN, away from bldgs, power lines, chimneys, and anything else that might fall on you.

If you are DRIVING--stop, but carefully. Move your car as far out of traffic as possible. DO NOT stop on or under a bridge or overpass or under trees, light posts, power lines, or signs. STAY INSIDE your car until the shaking stops. When you RESUME driving watch for breaks in the pavement, fallen rocks, and bumps in the road at bridge approaches.

If you are in a MOUNTAINOUS AREA--watch out for falling rock, landslides, trees, and other debris that could be loosened by quakes.

What emergency supplies do you need?

2007-09-07T22:34:00.001-07:00

Fire extinguisher

Adequate supplies of medications that you or family members are taking

Crescent and pipe wrenches to turn off gas and water supplies

First-aid kit and handbook

Flashlights with extra bulbs and batteries

Portable radio with extra batteries

Water for each family member for at least two weeks (allow at least 1 gallon per person per day) and purification tablets or chlorine bleach to purify drinking water from other sources

Canned and package foods, enough for several days and MECHANICAL can opener. Extra food for pets if necessary

Camp stove or barbecue to cook on outdoors (store fuel out of the reach of children)

Waterproof, heavy-duty plastic bags for waste disposal.

During an EQ should you head for the doorway?

2007-09-07T22:30:00.000-07:00

Only if you live in an old, unreinforced adobe house. In modern homes doorways are no stronger than any other parts of the house and usually have doors that will swing and can injure you. YOU ARE SAFER PRACTICING THE DUCK, COVER, AND HOLD under a sturdy piece of furniture.

How are earthquakes recorded? How are earthquakes measured? How is the magnitude of an earthquake determined?

2007-09-06T09:37:00.000-07:00

Earthquakes are recorded by a seismographic network. Each seismic station in the network measures the movement of the ground at the site. The slip of one block of rock over another in an earthquake releases energy that makes the ground vibrate. That vibration pushes the adjoining piece of ground and causes it to vibrate, and thus the energy travels out from the earthquake in a wave.

There are many different ways to measure different aspects of an earthquake.Magnitude is the most common measure of an earthquake's size. It is a measure of the size of the earthquake source and is the same number no matter where you are or what the shaking feels like. The Richter scale measures the largest wiggle on the recording, but other magnitude scales measure different parts of the earthquake.

Intensity is a measure of the shaking and damage caused by the earthquake, and this value changes from location to location.

What are the differences between explosions and earthquakes?

2007-09-05T10:32:00.000-07:00

Both earthquakes and nuclear tests can rapidly release a large amount of energy. The energy source for small yield (typically less than 50 kilotons) thermonuclear devices is the splitting of heavy radioactive isotopes. This process produces about 20 million times the energy of each reacting atom in a chemical explosive. The energy source for an earthquake is tectonic strain accumulated by the relative motion of Earth's tectonic plates which is driven by mantle heat flow in the presence of the earth's gravitational field. In a nuclear test, all of the energy is suddenly (within milliseconds) released in the form of heat from a relatively small volume surrounding the thermo- nuclear device. The tremendous heat causes rapid expansion of a spherical cavity, which in turn generates seismic waves. The heat gradually conducts away from the cavity into the surrounding rock. However, rock is a poor conductor of heat so it can take many years for the thermal signature of the thermonuclear explosion to subside and the increase in the surface temperature above the explosion is insignificant.

Nuclear tests are also very shallow sources with the depth of burial generally less than a few hundred meters (the depth of burial is typically proportional to the cube root of the expected yield). The estimated yields of the larger Indian and Pakistani tests are approximately 2-40 kilotons. In a large earthquake, the elastic strain energy stored in the Earth's crust is released, within a few seconds to a few tens of seconds, by rupture along a fault and the strain energy is released from a relatively large volume of rock surrounding the fault rupture. For example, the magnitude 6.5 earthquake in Afghanistan (5/30/98 at 06:22:28 UTC 37.4 N, 70.0 E) had a source duration of about 5 seconds and an estimated source volume of order 4000 cubic kilometers. This earthquake also had a focal depth of 18 km. The energy release is equivalent to a 2000 kiloton nuclear explosion.

What is a seismometer? What are seismographs? How do they work?

2007-09-04T09:36:00.000-07:00

A seismometer is the internal part of the seismograph, which may be a pendulum or a mass mounted on a spring; however, it is often used synonymously with "seismograph". Seismographs are instruments used to record the motion of the ground during an EQ--installed in the ground throughout the world and operate as seismographic network. The first one was developed in 1890. The earliest "seismoscope" was invented by the Chinese philosopher Chang Heng in A.D. 132. This did not record earthquakes, however. It only indicated that there was one occurring.

A seismograph is securely mounted onto the surface of the earth so that when the earth shakes, the entire unit shakes with it, EXCEPT for the mass on the spring which has inertia, and remains in the same place. As the seismograph shakes under (in the example below) the mass, the recording device on the mass records the realtive motion between itself and the rest of the instrument, thus recording the ground motion. In reality, these mechanisms are no longer manual, but instead work by measuring electronic changes produced by the motion of the ground with respect to the mass.

How much energy is released in an earthquake?

2007-09-03T08:16:00.000-07:00

The total energy from an earthquake includes energy required to create new cracks in rock, energy dissipated as heat through friction, and energy elastically radiated through the earth. Of these, the only quantity that can be measured is that which is radiated through the earth. It is the radiated energy that shakes buildings and is recorded by seismograph.

The radiated energy can be obtained in various ways. Historically, the radiated energy was estimated empirically (from observations) from magnitude Ms through the Richter formula, log Es = 4.8 + 1.5Ms, where Es is seismic energy in Joules. In this formula, magnitude is measured first, after which the formula is used to obtain Es. With modern instrumentation, energy can be measured directly from velocity seismograms and converted to a magnitude. If Es is energy in joules, the energy magnitude Me is obtained by Me = (2/3) log Es -2.9. If Me is not available, the seismic moment Mo of an earthquake can provide an empirical estimate of radiated energy. After Mo is measured, it is converted to a moment magnitude Mw by Mw = (2/3) log Mo – 6.0 where Mo is in Newton-meters (Joules). Mw is then used as the magnitude in the Richter formula to obtain an estimate of radiated energy.

[Note that Me and Mw do not necessarily have the same numerical value because they measure different physical quantities. Mw is a magnitude that is derived from low-frequency displacement spectra whereas Me is measured from higher frequency velocity spectra. Mw is a measure of the area of rupture and the average slip across the fault, whereas is Me is a measure of the shaking from an earthquake.

What is the Modified Mercalli Intensity Scale?

2007-09-03T08:13:00.000-07:00

The Mercalli Scale is based on observable earthquake damage. From a scientific standpoint, the magnitude scale is based on seismic records while the Mercalli is based on observable data which can be subjective. Thus, the magnitude scale is considered scientifically more objective and therefore more accurate.

For example a level I-V on the Mercalli scale would represent a small amount of observable damage. At this level doors would rattle, dishes break and weak or poor plaster would crack. As the level rises toward the larger numbers, the amount of damage increases considerably. The top number, 12, represents total damage.

Regional & Whole-Earth Structure

2007-09-02T01:45:00.000-07:00

Seismology is the study of earthquakes and the Earth using seismic waves. From recordings of earthquake-generated waves, information about the earthquake source may be derived, including its magnitude, location, time of occurrence, depth, and its orientation and movement on the fault. Teleseismic waves (waves coming from distant earthquakes) provide information about the entire Earth structure (crust, mantle and core). Seismic waves generated by a controlled source are used to image the Earth's crustal structure. These images reveal the depth and area of basin, fault networks, and the physical properties of rocks. Seismic imaging techniques have analogs in medical science. Waves transmitted (refracted) directly through the earth produce a "catscan"-type image, while waves reflected back to the surface from layer boundaries or faults produce a "sonogram"-type image. Traveltime observations and variations in amplitude, frequency, and waveform are combined to produce a model of the geologic structure. The model must be consistent with other geophysical and geological data, such as measurements of the strength of gravity, measurements of the strength of the magnetic and electric fields, and laboratory measurements of the speed of seismic waves in rock samples.The related field of strong motion seismology uses waves from large earthquakes to study the earthquake source in detail, predict the strength of future shaking, establish safer building codes and improve seismic engineering design.

A Brief History of Seismology to 1910

2007-09-02T01:31:00.000-07:00

Would you believe that giant snakes, turtles, catfish, or spiders live underneath the ground, and it is their movements that create earthquakes? Maybe you wouldn't, but your ancestors did. Ancient peoples had many fanciful explanations for earthquakes, usually involving something large and restless living beneath the earth's surface.

Aristotle was one of the first to attempt an explanation of earthquakes based on natural phenomena. He postulated that winds within the earth whipped up the occasional shaking of the earth's surface.

Empirical observations of the effects of earthquakes were rare, however, until 1750, when England was uncharacteristically rocked by a series of five strong earthquakes. These earthquakes were followed on Sunday, November 1, 1755, by a cataclysmic shock and tsunami that killed an estimated 70,000 people, leveling the city of Lisbon, Portugal, while many of its residents were in church. This event marks the beginning of the modern era of seismology, prompting numerous studies into the effects, locations, and timing of earthquakes.

Prior to the Lisbon earthquake, scholars had looked almost exclusively to Aristotle, Pliny, and other ancient classical sources for explanations of earthquakes. Following the Lisbon earthquake, this attitude was jettisoned for one that stressed ideas based on modern observations. Cataloging of the times and locations of earthquakes and studying the physical effects of earthquakes began in earnest, led by such people as John Michell in England and Elie Bertrand in Switzerland.

The hundred or so years following the Lisbon earthquake saw sporadic but increasing studies of earthquake phenomena. These efforts were often spurred on by earthquake catastrophes, such as the 1783 Calabrian earthquakes that killed 35,000 people in the southern toe of Italy.

As communication between various parts of the world became more common, earthquake observations from throughout the world could be combined. Following an earthquake in Chile in 1822, the author Maria Graham reported systematic changes in the elevation of the Chilean coastline. Observations of coastline changes were confirmed following the 1835 Chilean earthquake by Robert FitzRoy, captain of the H.M.S. Beagle, while Charles Darwin was onshore examining the geology of the Andes.

In the 1850s, 60s, and 70s, three European contemporaries made cornerstone efforts in seismology. Robert Mallet, an engineer born in Dublin who designed many of London's bridges, measured the velocity of seismic waves in the earth using explosions of gunpowder. His idea was to look for variations in seismic velocity that would indicate variations in the properties of the earth. This same method is still used today, for example in oil field exploration. Robert Mallet was also one of the first to estimate the depth of an earthquake underground.

At the same time as Mallet was setting off explosions of gunpowder in England, Alexis Perrey, in France, was making quantitative analyses of catalogs of earthquakes. He was looking for periodic variations of earthquakes with the seasons and with lunar phases. And in Italy, Luigi Palmieri invented an electromagnetic seismograph, one of which was installed near Mount Vesuvius and another at the University of Naples. These seismographs were the first seismic instruments capable of routinely detecting earthquakes imperceptible to human beings.

The foregoing work set the stage for the late 1800s and early 1900s, when many fundamental advances in seismology would be made. In Japan, three English professors, John Milne, James Ewing, and Thomas Gray, working at the Imperial College of Tokyo, invented the first seismic instruments sensitive enough to be used in the scientific study of earthquakes.

In the United States, Grove Karl Gilbert, after studying the fault scarp from the 1872 Owens Valley, California earthquake, concluded that the faults were a primary feature of earthquakes, not a secondary one. Until his time, most people thought that earthquakes were the result of underground explosions and that faults were only a result of the explosion, not a primary feature of earthquakes.

Also in the United States, Harry Fielding Reid took Gilbert's work one step further. After examining the fault trace of the 1906 San Francisco earthquake, Reid deduced that earthquakes were the result of the gradual buildup of stresses within the earth occurring over many years. This stress is due to distant forces and is eventually released violently during an earthquake, allowing the earth to rapidly rebound after years of accumulated strain.

The late 1800s and early 1900s also saw scientific inquiry into earthquakes begun by Japanese researchers. Seikei Sekiya became the first person to be named a professor in seismology; he was also one of the first people to quantitatively analyse seismic recordings from earthquakes. Another famous Japanese researcher from that time is Fusakichi Omori, who, among other work, studied the rate of decay of aftershock activity following large earthquakes. His equations are still in use today.

The twentieth century has seen an increased interest in the scientific study of earthquakes, too involved to discuss here. It should be noted, however, that research into earthquakes has broadened and contributions now come from numerous areas affected by earthquakes, including Japan, the United States, Europe, Russia, Canada, Mexico, China, Central and South America, New Zealand, and Australia, among others.

Some Common Myths about Earthquakes

2007-08-29T08:22:00.000-07:00

Can animals predict earthquakes?

The earliest reference we have to unusual animal behavior prior to a significant earthquake is from Greece in 373 BC. Rates, weasels, snakes, and centipedes reportedly left their homes and headed for safety several days before a destructive earthquake. Anecdotal evidence abounds of animals, fish, birds, reptiles, and insects exhibiting strange behavior anywhere from weeks to seconds before an earthquake. However, consistent and reliable behavior prior to seismic events, and a mechanism explaining how it could work, still eludes us. Most, but not all, scientists pursuing this mystery are in China or Japan.

Can some people sense that an earthquake is about to happen? (Earthquake sensitive)?

There is no scientific explanation for the symptoms some people claim to have preceding an earthquake, and more often than not there is no earthquake following the symptoms.

Is there earthquake weather?

In the 4th Century B.C., Aristotle proposed that earthquakes were caused by winds trapped in subterranean caves. Small tremors were thought to have been caused by air pushing on the cavern roofs, and large ones by the air breaking the surface. This theory lead to a belief in earthquake weather, that because a large amount of air was trapped underground, the weather would be hot and calm before an earthquake.
However, there is no connection between weather and earthquakes. They are the result of geologic processes within the earth and can happen in any weather and at any time during the year. Earthquakes originate miles underground. Wind, precipitation, temperature, and barometric pressure changes affect only the surface and shallow subsurface of the Earth. Earthquakes are focused at depths well out of the reach of weather, and the forces that cause earthquakes are much larger than the weather forces. Earthquakes occur in all types of weather, in all climate zones, in all seasons of the year, and at any time of day. Sometimes, we are asked: "Do earthquakes change the weather in any way? Earthquakes themselves do not cause weather to change. Earthquakes, however, are a part of global tectonics, a process that often changes the elevation of the land and its morphology. Tectonics can cause inland areas to become coastal or vice versa. Changes significant to alter the climate occur over millions of years, however, and after many earthquakes.

Earthquake Facts

2007-08-27T08:08:00.000-07:00

The largest recorded earthquake in the world was a magnitude 9।5 (Mw) in Chile on May 22, 1960।
Before electronics allowed recordings of large earthquakes, scientists built large spring-pendulum seismometers in an attempt to record the long-period motion produced by such quakes। The largest one weighed about 15 tons. There is a medium-sized one three stories high in Mexico City that is still in operation.
Before electronics allowed recordings of large earthquakes, scientists built large spring-pendulum seismometers in an attempt to record the long-period motion produced by such quakes। The largest one weighed about 15 tons. There is a medium-sized one three stories high in Mexico City that is still in operation.
The East African Rift System is a 50-60 km (31-37 miles) wide zone of active volcanics and faulting that extends north-south in eastern Africa for more than 3000 km (1864 miles) from Ethiopia in the north to Zambezi in the south। It is a rare example of an active continental rift zone, where a continental plate is attempting to split into two plates which are moving away from one another.
The first "pendulum seismoscope" to measure the shaking of the ground during an earthquake was developed in 1751, and it wasn't until 1855 that faults were recognized as the source of earthquakes।
Moonquakes ("earthquakes" on the moon) do occur, but they happen less frequently and have smaller magnitudes than earthquakes on the Earth। It appears they are related to the tidal stresses associated with the varying distance between the Earth and Moon. They also occur at great depth, about halfway between the surface and the center of the moon.
Although both are sea waves, a tsunami and a tidal wave are two different unrelated phenomenona। A tidal wave is a shallow water wave caused by the gravitational interactions between the Sun, Moon, and Earth. A tsunami is a sea wave caused by an underwater earthquake or landslide (usually triggered by an earthquake) displacing the ocean water.
The greatest mountain range is the Mid-Ocean Ridge, extending 64,374 km (40,000 mi) from the Arctic Ocean to the Atlantic Ocean, around Africa, Asia, and Australia, and under the Pacific Ocean to the west coast of North America। It has a greatest height of 4207 m (13,800 ft) above the base ocean depth.
The world's greatest land mountain range is the Himalaya-Karakoram। It countains 96 of the world's 109 peaks of over 7317 m (24,000 ft). The longest range is the Andes of South America which is 7564 km (4700 mi) in length. Both were created bythe movement of tectonic plates.
It is estimated that there are 500,000 detectable earthquakes in the world each year। 100,000 of those can be felt, and 100 of them cause damage.
It is thought that more damage was done by the resulting fire after the 1906 San Francisco earthquake than by the earthquake itself.

Can we cause earthquakes? Is there any way to prevent earthquakes?

2007-08-21T08:56:00.000-07:00

Earthquakes induced by human activity have been documented in a few locations in the United States, Japan, and Canada। The cause was injection of fluids into deep wells for waste disposal and secondary recovery of oil, and the use of reservoirs for water supplies. Most of these earthquakes were minor. The largest and most widely known resulted from fluid injection at the Rocky Mountain Arsenal near Denver, Colorado. In 1967, an earthquake of magnitude 5.5 followed a series of smaller earthquakes. Injection had been discontinued at the site in the previous year once the link between the fluid injection and the earlier series of earthquakes was established. (Nicholson, Craig and Wesson, R.L., 1990, Earthquake Hazard Associated with Deep Well Injection--A Report to the U.S. Environmental Protection Agency: U.S. Geological Survey Bulletin 1951, 74 p.)

Other human activities, even nuclear detonations, have not been linked to earthquake activity. Energy from nuclear blasts dissipates quickly along the Earth's surface. Earthquakes are part of a global tectonic process that generally occurs well beyond the influence or control of humans. The focus (point of origin) of earthquakes is typically tens to hundreds of miles underground. The scale and force necessary to produce earthquakes are well beyond our daily lives. We cannot prevent earthquakes; however, we can significantly mitigate their effects by identifying hazards, building safer structures, and providing education on earthquake safety.

Deadliest earthquakes on record

2007-08-20T10:38:00.001-07:00

Rank	Name	Date	Location	Fatalities	Magnitude	Comments
1	"Shaanxi"	January 23, 1556	Shaanxi, China	830,000	8
2	"Tangshan"	July 28, 1976	Tangshan, China	255,000 (official)	7.5	Estimated death toll as high as 655,000.
3	"Aleppo"	August 9, 1138	Aleppo, Syria	230,000		Death toll disputed as first mention of 230,000 dead was in the 15th century.
3	"Indian Ocean"	December 26, 2004	Off west coast northern Sumatra, Indonesia	230,000	9.3	Deaths from earthquake and tsunami.[3]
5	"Damghan"	December 22, 856	Damghan, Iran	200,000
	"Gansu"	December 16, 1920	Ningxia-Gansu, China	200,000	8.6	Major fractures, landslides.
	"Tsinghai"	May 22, 1927	Tsinghai, China	200,000	7.9	Large fractures.
8	"Ardabil"	March 23, 893+	Ardabil, Iran	150,000
9	"Great Kantō"	September 1, 1923	Kantō, Japan	143,000	7.9	Great Tokyo fire.
10	"Ashgabat"	October 6, 1948	Ashgabat, Turkmenistan	110,000	7.3
11	"Kashmir"	October 8, 2005	Kashmir & N.W.F.P, Pakistan	100,000 (estimated) , 80,000 (official)	7.6 or 7.8	3.5 million people homeless, 100,000 feared dead

Largest earthquakes by magnitude

2007-08-20T10:35:00.000-07:00

Pos.	Date	Location	Magnitude
1	May 22, 1960	Valdivia, Chile (see: Great Chilean Earthquake)	9.5
2	December 26, 2004	Off west coast northern Sumatra, Indonesia (see: 2004 Indian Ocean earthquake)	9.31
3	October 16, 1737	Kamchatka, Russia (see: Kamchatka earthquakes)	9.3
4	March 27, 1964	Prince William Sound, Alaska, USA (see: Good Friday Earthquake)	9.2
5	March 9, 1957	Andreanof Islands, Alaska, USA	9.1
6	November 4, 1952	Kamchatka, Russia	9.0
7	January 26, 1700	Cascadia subduction zone (see: Cascadia earthquake)	9
8	January 31, 1906	Colombia-Ecuador	8.8
9	February 4, 1965	Rat Islands, Alaska, USA	8.7
10	November 25, 1833	Sumatra, Indonesia	8.7
11	November 1, 1755	Lisbon, Portugal (see: 1755 Lisbon earthquake)	8.7
12	March 28, 2005	Sumatra, Indonesia	8.5-8.7
13	December 16, 1920	Ningxia-Gansu, China	8.6
14	August 15, 1950	Assam-Tibet	8.6
15	December 16, 1575	Valdivia, Chile	8.5

Know the Terms

2007-08-15T20:34:00.000-07:00

Familiarize yourself with these terms to help identify an earthquake hazard:

Earthquake: A sudden slipping or movement of a portion of the earth’s crust, accompanied and followed by a series of vibrations.

Aftershock :An earthquake of similar or lesser intensity that follows the main earthquake.

Fault: The fracture across which displacement has occurred during an earthquake. The slippage may range from less than an inch to more than 10 yards in a severe earthquake.

Epicenter: The place on the earth’s surface directly above the point on the fault where the earthquake rupture began. Once fault slippage begins, it expands along the fault during the earthquake and can extend hundreds of miles before stopping.

Seismic Waves: Vibrations that travel outward from the earthquake fault at speeds of several miles per second. Although fault slippage directly under a structure can cause considerable damage, the vibrations of seismic waves cause most of the destruction during earthquakes.

Magnitude: The amount of energy released during an earthquake, which is computed from the amplitude of the seismic waves. A magnitude of 7.0 on the Richter Scale indicates an extremely strong earthquake. Each whole number on the scale represents an increase of about 30 times more energy released than the previous whole number represents. Therefore, an earthquake measuring 6.0 is about 30 times more powerful than one measuring 5.0.

Earth Quake

2007-08-14T04:15:00.000-07:00

An earthquake is the result of a sudden release of stored energy in the Earth’s crust that creates seismic waves. Earthquakes are accordingly measured with a seismometer, commonly known as a seismograph. The magnitude of an earthquake is conventionally reported using the Richter scale or a related Moment scale (with magnitude 3 or lower earthquakes being hard to notice and magnitude 7 causing serious damage over large areas)
At the Earth's surface, earthquakes may manifest themselves by a shaking or displacement of the ground. Sometimes, they cause tsunamis, which may lead to loss of life and destruction of property. An earthquake is caused by tectonic plates getting stuck and putting a strain on the ground. The strain becomes so great that rocks give way by breaking and sliding along fault planes.
Earthquakes may occur naturally or as a result of human activities. Smaller earthquakes can also be caused by volcanic activity, landslides, mine blasts, and nuclear tests. 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.
An earthquake's point of initial ground 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 become 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, the elastic strain energy is released and seismic waves are radiated, thus causing an earthquake. This process of strain, stress, and 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 and is converted into heat, or is released to friction. Therefore, earthquakes lower the Earth's available potential energy and raise its temperature, though these changes are negligible.
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 earthquake 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.
Earthquakes may also occur in volcanic regions and are caused there both by tectonic faults and by the movement of magma in volcanoes. Such earthquakes can be an early warning of volcanic eruptions.
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, and with some of the later earthquakes as damaging as the early ones.