Earthquake

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.
Naturally occurring earthquakes Fault typesMost 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.
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.
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.
Size and frequency of occurrenceSmall earthquakes occur nearly constantly around the world in places like California and Alaska in the U.S., as well as in Chile, Peru, Indonesia, Iran, the Azores in Portugal, New Zealand, Greece and Japan.[3] Large earthquakes occur less frequently, the relationship being exponential; for example, roughly ten times as many earthquakes larger than magnitude 4 occur in a particular time period than earthquakes larger than magnitude 5. In the (low seismicity) United Kingdom, for example, it has been calculated that the average recurrences are:
an earthquake of 3.7 - 4.6 every year an earthquake of 4.7 - 5.5 every 10 years an earthquake of 5.6 or larger every 100 years. The number of seismic stations has increased from about 350 in 1931 to many thousands today. As a result, many more earthquakes are reported than in the past because of the vast improvement in instrumentation (not because the number of earthquakes has increased). The USGS estimates that, since 1900, there have been an average of 18 major earthquakes (magnitude 7.0-7.9) and one great earthquake (magnitude 8.0 or greater) per year, and that this average has been relatively stable. In fact, in recent years, the number of major earthquakes per year has actually decreased, although this is likely a statistical fluctuation. More detailed statistics on the size and frequency of earthquakes is available from the USGS.
Most of the world's earthquakes (90%, and 81% of the largest) take place in the 40,000-km-long, horseshoe-shaped zone called the circum-Pacific seismic belt, also known as the Pacific Ring of Fire, which for the most part bounds the Pacific Plate. Massive earthquakes tend to occur along other plate boundaries, too, such as along the Himalayan Mountains.
Effects/impacts of earthquakesShaking and ground ruptureShaking 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 liquefactionSoil 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 impactsEarthquakes 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.

Volcanoes

The build-up of molten rock in a volcano before it erupts is like the gases in a shaken bottle of champagne. If the amount of gas in a volcano’s magma is high, the inevitable release leads to massive explosions. The amount of gas inside magma—molten rock—is one of the most important indicators determining how violent an eruption will be. The viscosity, or thickness, of magma is another important factor. Under ground, gases remain suspended under pressure in the magma, but when magma rises to the lower pressures of the surface, the gases expand. Volcanoes with less gaseous and more fluid magma usually have less violent eruptions because the small amount of gas easily escapes from the lava into the air. Thick, sticky magma, on the other hand, slows down the escape of gases and may also block a volcano’s main vent. When the gases are finally released, they burst out of the lava in furious and turbulent blasts. These explosive eruptions are characterized by large clouds of flying rock particles, rather than lava flows.
Volcanic Products Volcanoes emit a variety of substances, with varying degrees of force. These substances are lava, pyroclastic material, ash, and gases. Lava is magma that reaches the surface. This liquefied rock is many times hotter than boiling water and glows bright yellow, orange, and red. Lava may erupt in explosive bursts, like giant fountains, or flow gently down the slopes of a mountain. Lava can leave a volcano from the top vent or emerge from vents along the sides.
Except for the molten rock that lands back inside the main crater to continue bubbling, all lava eventually cools and solidifies. Some lava cools quickly, on or near the volcano, but more fluid lava may travel for miles before slowly congealing into rock. Over time, solidified lava from different eruptions steadily increases the size and height of the volcano. All fragments thrown into the air by a volcanic eruption are called pyroclastics. During a more violent eruption, the force of the blast sends superhot gas and millions of pieces of lava into the air. These particles are classified as bombs, cinders, or ash, depending on their size and shape. Small pieces of lava, which solidify almost immediately, form slivers of volcanic glass. Together with rocks blown from the sides of a volcano, the entire collection of ejected material becomes a hot, fast-moving cloud of rock and ash. These flows can travel at great speed down the flanks of a volcano and into surrounding areas, causing extensive destruction. In 1902 the eruption of Mount Pelée, on the island of Martinique, created this type of pyroclastic blast and destroyed the town of Saint-Pierre, killing about 30,000 people.
Like lava, pyroclastic material raining down on a volcano eventually compacts into solid layers that build up the volcano’s bulk. Some eruptions actually reduce the height of a volcano, because they are so powerful that they literally blow the top of the volcano off. In 1883 the cataclysmic explosion of Krakatau in Indonesia destroyed most of the island, which had been formed by the volcano.
Volcanoes often spew great quantities of ash many kilometres into the air. This fine ash can drift for thousands of kilometres, falling on distant lands, yet the smallest particles of dust may remain suspended in the atmosphere for months. The uprush of gas and vapours from the Krakatau eruption reportedly carried fine ash to a height of 27 kilometres (17 miles). In addition to creating colourful sunsets for many months afterwards, the vapour and ash clouds can have long-lasting effects on the atmosphere and climate.
Steam and other gases such as carbon dioxide, hydrogen, carbon monoxide, and sulphur dioxide continuously escape from the surface of lava. Volcanic areas can emit harmful gases in immense quantities. In 1986 a volcanic lake in northern Cameroon released toxic gases that killed more than 1,700 people.
The danger to life posed by active volcanoes is not limited to the eruption of molten rock or showers of ash and cinders. Disastrous mudflows are an equally serious hazard. One triggered by a small eruption that melted ice and snow on Ruiz Peak volcano in Colombia claimed more than 25,000 lives in 1985, one of the worst volcanic disasters in the 20th century. Some mudflows may occur long after an eruption is over, when heavy rains saturate loose volcanic debris. In addition, eruptions near glaciers can melt vast quantities of ice, resulting in damaging floods. Iceland occasionally suffers these massive floods, known there as Jökulhlaup. Volcanic Landforms The shapes of volcanoes vary according to the types of particles thrown from the volcano during eruptions. The beautifully symmetrical cones of Mount Fuji in Japan and Mayon in the Philippines are examples of strato-volcanoes, or composite volcanoes. This type of volcano emits a combination of lava and pyroclastic material. The mixture allows the successive layers to solidify and support additional mass. Strato-volcanoes are the highest and steepest volcanoes in the world. Volcanoes that consist predominantly of pyroclastic materials are called cinder cones. These mountains, such as Capulin Mountain in New Mexico (USA), are easily eroded and usually do not reach great heights. Shield volcanoes, on the other hand, are predominantly lava-based landforms that have gradual slopes and wide bases, because they release fluid lava slowly. These volcanoes can create huge landforms. Mauna Loa and Mauna Kea on the island of Hawaii (The Big Island) are classic examples: Mauna Kea has a base on the ocean floor more than 200 kilometres (120 miles) wide. Under certain circumstances, instead of issuing from a central vent, lava pours out along cracks, or fissures, that may extend for several kilometres across the land surface. Flows of this sort have created thick sheets of basalt covering thousands of square kilometres. The Deccan Plateau in India, which covers more than 500,000 square kilometres (200,000 square miles), was formed in this way. The Columbia Plateau in the northwest United States is another example. In modern times, fissure eruptions on a smaller scale have been observed in Iceland and Hawaii.
Some enormous, craterous basins called calderas, at the top of long-dormant or extinct volcanoes, form when a massive explosion forces the upper part of a volcano to collapse. Some of these calderas eventually fill with water, forming deep lakes, such as the picturesque Crater Lake in the northwest US. States of Volcanic Activity Volcanoes can be active, dormant, or extinct. Active volcanoes have erupted in a relatively recent period. There are more than 500 active volcanoes on continents or islands; thousands more exist under the oceans. Many active volcanoes are in the Ring of Fire, a zone of seismic and volcanic activity that encircles the Pacific Ocean. Izalco Volcano, in El Salvador, has been erupting since 1770. Other active volcanoes include Stromboli in the Aeolian Islands near Sicily, and Cotopaxi in the Andes of Ecuador. Dormant volcanoes are those that have not erupted for many years, but have the potential to erupt again. The eruption that follows prolonged dormancy is usually violent, as was the explosion in 1980 of Mount Saint Helens in the northwest US, after 123 years of inactivity. The massive eruption in 1991 of Mount Pinatubo, in the Philippines, came after six centuries of dormancy. Extinct volcanoes have not erupted in thousands of years and show no signs of doing so in the future. Mount Kenya, the second highest mountain in Africa, is an extinct volcano. Edinburgh Castle, in Scotland, sits on top of an extinct volcano. Creation of Volcanoes Most active volcanoes ultimately derive their energy from processes associated with the theory of plate tectonics. Volcanoes tend to coincide with major plate boundaries, though some, like the Hawaiian Islands, formed over hot spots in the earth's surface far from plate boundaries. At subduction zones, where one plate moves beneath the other, the subducted plate is dragged downwards into the earth's mantle until it reaches a depth where high temperatures partially melt the rock. The resulting magma then rises along vertical fissures and reaches the surface through a volcanic vent. Volcanoes along the Andes in South America and the Cascade Range in North America are examples of volcanoes that formed on continental crust overlying subduction zones. When fissures open up on the seafloor, volcanic islands form as a result, such as Japan and the Philippines. At divergent plate boundaries, where two plates move away from each other, magma wells up along the linear boundary. Iceland is a volcanic land mass on top the Mid-Atlantic Ridge, a divergent plate boundary. New additions along this ridge, such as the island of Surtsey, still continue to be created. A third type, known as transform boundaries, exists when two plates slide alongside each other. The interaction of plates at a transform boundary, such as the San Andreas Fault in the western United States, does not normally lead to volcanic activity. Hot Spots Hundreds of hot spots exist around the world. These are areas in the lithosphere that are underlain by unusually hot magma. This heat causes partial melting of the lithosphere, eventually leading to volcanic activity. The Hawaiian Islands are a classic example of a volcanic grouping formed over one hot spot. Over thousands of years, as the Pacific Plate inched its way in a northwest direction, the stationary hot spot underneath the plate successively created volcanoes above it. Several of these volcanoes reached the ocean’s surface, forming the Hawaiian Islands. As the plate continued to move, volcanoes, embedded in the plate, travelled away from the source of magma and eventually became extinct. This hot spot still continues to create new volcanoes. Thus, the islands are progressively younger from the northwest to the southeast. Several volcanoes in the chain are still quite active, and new underwater volcanoes are forming to the southeast of Hawaii as the Pacific Plate continues to move over the hot spot.
Humans and Volcanoes Volcanoes are an important aspect of many cultures. One of the most famous and beautiful volcanoes in the world is Mount Fuji in Japan, which last erupted in 1708. According to legend, Mount Fuji arose from the plain during a single night in 286 BC. Geologically, the mountain is much older than the legend asserts. Certain religious sects regard the mountain as a sacred place. Thousands of pilgrims from all parts of the country visit Mount Fuji annually, and numerous shrines and temples are on its slopes. Mount Fuji is also revered in Japanese literature and art. Volcanoes, when not causing mass destruction, can actually benefit humans. For example, they may provide extremely fertile land for crops and forests. Vineyards and orchards now cover the lower slopes of Mount Vesuvius, which destroyed the town of Pompeii in AD 79 in a pyroclastic explosion. Higher up, oaks and chestnut trees grow. Volcanoes, when inactive, can also provide areas for sightseeing, hiking, and camping, and many have become parks. Tourism often results from continuous or recent volcanic eruptions. Many people visit Hawaii Volcanoes National Park to view the spectacular lava flows from a safe distance.
Scientific Inquiry Geologists and volcanologists, who specifically study volcanoes, attempt to increase our knowledge of volcanoes and try to predict when eruptions will occur. Volcanic earthquakes and changes in the shape of volcanoes are two signals of impending eruptions. Like earthquakes, however, volcanoes can be unpredictable, and those who live in their vicinity are constantly at risk.