EarthquakeAn earthquake (also known as a quake, tremor or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.

Earthquakes are measured using observations from seismometers. The moment magnitude is the most common scale on which earthquakes larger than approximately 5 are reported for the entire globe. The more numerous earthquakes smaller than magnitude 5 reported by national seismological observatories are measured mostly on the local magnitude scale, also referred to as the Richter scale. These two scales are numerically similar over their range of validity. Magnitude 3 or lower earthquakes are mostly almost imperceptible or weak and magnitude 7 and over potentially cause serious damage over larger areas, depending on their depth. The largest earthquakes in historic times have been of magnitude slightly over 9, although there is no limit to the possible magnitude. The most recent large earthquake of magnitude 9.0 or larger was a 9.0 magnitude earthquake in Japan in 2011 (as of March 2014), and it was the largest Japanese earthquake since records began. Intensity of shaking is measured on the modified Mercalli scale. The shallower an earthquake, the more damage to structures it causes, all else being equal.

Different types of Faults

1. Where the crust is being pulled apart, normal faulting occurs, in which the overlying (hanging-wall) block moves down with respect to the lower (foot wall) block.

2. Where the crust is being compressed, reverse faulting occurs, in which the hanging-wall block moves up and over the footwall block – reverse slip on a gently inclined plane is referred to as thrust faulting.

3. Crustal blocks may also move sideways past each other, usually along nearly-vertical faults. This ‘strike-slip’ movement is described as sinistral when the far side moves to the left, and dextral, when the far side moves to the right.

4. An oblique slip involves various combinations of these basic movements, as in the 1855 Wairarapa Fault rupture, which included both reverse and dextral movement.

Faults can be as short as a few metres and as long as 1000km. The fault rupture from an earthquake isn’t always a straight or continuous line. Sometimes there

can be short offsets between parts of the fault, and even major faults can have large bends in them.

Different Instruments to Measure an earthquakeSeismometer

Seismometers are instruments that measure motions of the ground, including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources. Records of seismic waves allow seismologists to map the interior of the Earth, and locate and measure the size of these different sources.

Seismograph

Seismograph is another Greek term from seismós and γράφω, gráphō, to draw. It is often used to mean seismometer, though it is more applicable to the older instruments in which the measuring and recording of ground motion were combined than to modern systems, in which these functions are separated. Both types provide a continuous record of ground motion; this distinguishes them from seismoscopes, which merely indicate that motion has occurred, perhaps with some simple measure of how large it was.

Earthquake safety tips

Earthquakes are a common occurrence, rumbling below Earth's surface thousands of times every day. But major earthquakes are less common. Here are some things to do to prepare for an earthquake and what to do once the ground starts shaking.

Safety Tips

Have an earthquake readiness plan. Consult a professional to learn how to make your home sturdier, such as

bolting bookcases to wall studs, installing strong latches on cupboards, and strapping the water heater to wall studs.

Locate a place in each room of the house that you can go to in case of an earthquake. It should be a spot where nothing is likely to fall on you.

Keep a supply of canned food, an up-to-date first aid kit, 3 gallons (11.4 liters) of water per person, dust masks and goggles, and a working battery-operated radio and flashlights.

Know how to turn off your gas and water mains.

If Shaking Begins

Drop down; take cover under a desk or table and hold on. Stay indoors until the shaking stops and you're sure it's safe to exit. Stay away from bookcases or furniture that can fall on you. Stay away from windows. In a high-rise building, expect the fire alarms and

sprinklers to go off during a quake. If you are in bed, hold on and stay there, protecting your head with a pillow. If you are outdoors, find a clear spot away from buildings, trees, and power

lines. Drop to the ground. If you are in a car, slow down and drive to a clear place. Stay in the car until the

shaking stops.

After an Earthquake1. Check for injuries to your family and your surrounding neighbors. Do not attempt to move seriously injured persons unless they are in danger of further injury.2. Check for fires or fire hazards.3. If indoors, check the structural aspects of the building, if any part of the structure appears to be unsafe, evacuate the building until a more detailed inspection can be made. Buildings that are damaged by the main shock could receive additional damage from aftershocks.4. Wear shoes in all areas near debris or broken glass.5. Do not touch downed power lines or objects touched by the downed wires.6. Prepare for an after shock.

The 5 most powerful recorded earthquakes

1.22 May 1960 – Chile

Magnitude 9.5The world's most powerful earthquake left 4,485 people dead and injured and 2 million homeless after it struck southern Chile in 1960. The port of Puerto Saavedra was destroyed in the ensuing tsunami, which caused $550m worth of damage in Chile and killed a further 170 people as five-metre waves hit the coasts of Japan and the Philippines. A day later Volcán Puyehue in Chile's lake district spewed ash 6,000m into the air in an eruption that lasted for several weeks.

2.28 March 1964 – Prince William Sound, Alaska

Magnitude 9.2The Gulf of Alaska was devastated by the Prince William Sound earthquake that caused landslides in Anchorage and raised parts of outlying islands by as much as 11 metres. The resulting tsunami reached heights of 67 metres as it swept into the shallow Valdez inlet and

was responsible for most of the 128 deaths and $311m worth of damage. The massive water displacement was felt as far away as the Louisiana Gulf coast and registered on tidal gauges in Puerto Rico.

3.26 December 2004 – Off the west coast of northern Sumatra

Magnitude 9.1The deadliest tsunami in history was felt in 14 countries across Asia and east Africa, triggered by a "megathrust" as the Indian tectonic plate was forced beneath the Burmese plate. Indonesia was the worst affected with an estimated 170,000 of the nearly 230,000 dead. With many of the victims' bodies missing, the eventual death toll took a month to establish. Some the world's poorest communities lost more than 60% of their fishing and industrial infrastructure.

4.4 November 1952 – Kamchatka

Magnitude 9The volcanic Russian peninsula was near the epicentre of the quake, but it was the Hawaiian islands that took the brunt of the tsunami that caused a million dollars' worth of damage as

waves scoured the coasts, ripping boats from their moorings and, in Honolulu harbour, lifting a cement barge before throwing it down on to a freighter. No deaths were recorded, unless you count the six cows lost by one unfortunate Oahu farmer, who was left cursing an event that had occurred more than 3,000 miles away

5.13 August 1868 – Arica, Peru (now part of Chile)

Magnitude 9Hawaii also felt the force of the tsunami created by this pacific basin earthquake, but here the destruction was just as heavy in South America with the city of Arequipa destroyed and 25,000 killed. The quake was felt as far away as La Paz in Bolivia. Four hours after the first shocks, waves as high as 16 metres inundated the coast and carried one US gunboat two miles inland to rest precariously on the edge of a 60m cliff.

Tsunamialso known as a seismic sea wave, is a series of water waves caused by the displacement of a large volume of a body of water, generally an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater explosions (including detonations of underwater nuclear devices), landslides, glacier calvings, meteorite impacts and other disturbances above or below water all have the potential to generate a tsunami.

Seismicity

Tsunami can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the Earth's crustal deformation; when these

earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position.[26]More specifically, a tsunami can be generated when thrust faults associated with convergent or destructive plate boundaries move abruptly, resulting in water displacement, owing to the vertical component of movement involved. Movement on normal faults will also cause displacement of the seabed, but the size of the largest of such events is normally too small to give rise to a significant tsunami.

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ScienceSubmmited By :John Caling

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IntensityThe intensity is a number (written as a Roman numeral) describing the severity of an earthquake in terms of its effects on the earth's surface and on humans and their structures. Several scales exist, but the ones most commonly used in the United States are the Modified Mercalli scale and the Rossi-Forel scale. There are many intensities for an earthquake, depending on where you are, unlike the magnitude, which is one number for each earthquake. Tells us how much a certain are was shaken when the earthquake reached that area.

Instruments that measures Intensity

Mercalli intensity scale

The Mercalli intensity scale is a seismic scale used for measuring the intensity of an earthquake. It measures the effects of an earthquake, and is distinct from the moment magnitude   usually reported for an earthquake (sometimes misreported as the Richter magnitude), which is a measure of the energy released. The intensity of an earthquake is not totally determined by its magnitude.

The scale quantifies the effects of an earthquake on the Earth's surface, humans, objects of nature, and man-made structures on a scale from I (not felt) to XII (total destruction). Values depend upon the distance to the earthquake, with the highest intensities being around the epicentral area. Data gathered from people who have experienced the quake are used to determine an intensity value for their location. The Mercalli (Intensity) scale originated with the widely used simple ten-degree Rossi-Forel scale which was revised by Italian volcanologist, Giuseppe Mercalli in 1884 and 1906.

Rossi-Forel scale

The Rossi–Forel scale was one of the first seismic scales to reflect earthquake intensities. Developed by Michele Stefano Conte de Rossi of Italy and François-Alphonse Forel ofSwitzerland in the late 19th century, it was used for about two decades until the introduction of the Mercalli intensity scale in 1902.

The 1873 version of the Rossi–Forel scale had 10 intensity levels:

I. Microseismic tremor. Recorded by a single seismograph or by seismographs of the same model, but not by several seismographs of different kinds. The shock felt by an experienced observer.

II. Extremely feeble tremor. Recorded by several seismographs of different kinds. Felt by a small number of persons at rest.

III. Feeble tremor. Felt by several persons at rest. Strong enough for the direction or duration to be appreciable.

IV. Slight tremor. Felt by persons in motion. Disturbance of movable objects, doors, windows, cracking of ceilings.

V. Moderate tremor. Felt generally by everyone. Disturbance of furniture, ringing of some bells.

VI. Strong tremor. General awakening of those asleep. General ringing of bells. Oscillation of chandeliers, stopping of clocks, visible agitation of trees and shrubs. Some startled persons leaving their dwellings.

VII. Very strong tremor. Overthrow of movable objects, fall of plaster, ringing of church bells. General panic. Moderate to heavy damage buildings.

VIII. Damaging tremor. Fall of chimneys. Cracks in the walls of buildings. IX. Devastating tremor. Partial or total destruction of buildings. X. Extremely high intensity tremor. Great disaster, ruins, disturbance of the strata,

fissures in the ground, rock falls from mountains.

MagnitudeIt is a measure of earthquake size and is determined from the logarithm of the maximum displacement or amplitude of the earthquake signal as seen on the seismogram, with a correction for the distance between the focus and the seismometer. This is necessary as the closer the seismometer is to the earthquake, the larger the amplitude on the seismogram, irrespective of the size or magnitude of the event. Since the measurement can be made from P, S or surface waves, several different scales exist, all of which are logarithmic because of the large range of earthquake energies (for example a magnitude 6 ML is 30 times larger, in terms of energy than a magnitude 5 ML). The Richter local magnitude (ML) is defined to be used for 'local' earthquakes up to 600 km away, and is the magnitude scale used by BGS when locating UK earthquakes. Describes the total amount of energy that was released by the earthquake at the focus

Instruments that measures magnitudeRichter magnitude scale

The Richter magnitude scale (also Richter scale) assigns a magnitude number to quantify the energy released by an earthquake. The Richter scale is a base-10 logarithmic scale, which defines magnitude as the logarithm of the ratio of the amplitude of the seismic waves to an arbitrary, minor amplitude.

As measured with a seismometer, an earthquake that registers 5.0 on the Richter scale has a shaking amplitude 10 times that of an earthquake that registered 4.0, and thus corresponds to a release of energy 31.6 times that released by the lesser earthquake.

Moment magnitude scale

The moment magnitude scale (abbreviated as MMS; denoted as MW or M) is used by seismologists to measure the size of earthquakes in terms of the energy released. he magnitude is based on the seismic moment of the earthquake, which is equal to the rigidity of the Earth multiplied by the average amount of slip on the fault and the size of the area that slipped. The scale was developed in the 1970s to succeed the 1930s-era Richter magnitude scale (ML). Even though the formulae are different, the new scale retains the familiar continuum of magnitude values defined by the older one. The MMS is now the scale used to estimate magnitudes for all modern large earthquakes by the United States Geological Survey .