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Report on Earthquake:

o Causes of Earthquakeo Various Seismic zones in Indiao Epicenter, Seismic focus, Richter Scaleo Seismographo Protection against Earthquake

E A R T H Q U A K E

Pooja

IG-1117

Class - 8th B

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What Causes Earthquakes?IntroductionWhat causes earthquakes? Earthquakes are vibrations inside the Earth that follow the release of energy that has built up inside rocks. Rocks fracturing, volcanoes erupting, and man made explosions can all release the energy stored in the rocks.

Loma Prieta earthquake damage to homes in Marina District, USGS

P waves and Love wavesAll earthquakes produce earthquake waves. There are four types of earthquake waves. P waves are also know as primary waves that travel through all parts of the Earth. Love waves are only produced by the most powerful earthquakes. They travel around the surface of the Earth.

Earthquake zones and megathrust earthquakesEarthquakes occur in earthquake zones. These zones form around crustal plates. As the plates smash into other plates, move apart and subduct they create earthquake faults. Megathrust earthquakes are associated with subduction zones where powerful earthquakes cause tsunamis.

Great earthquakesThe moment magnitude scale was developed after the 1960 Chile earthquake and the 1964 Alaska earthquake. These were the two most powerful earthquakes of the 20th Century.

New Madrid and Haiti earthquakesThe New Madrid earthquakes occurred in 1811 and 1812 were very powerful quakes that rang bells in Boston. The Haiti earthquake occurred in an area that has large numbers of very powerful earthquakes.

What causes earthquakesEarthquakes waves radiate outward in all directions from the earthquake focus. The focus is where the rocks first break apart during an earthquake.Earthquake faults are places where two crustal plates are moving together, apart or slipping past each other. Normal faults occur when plates are moving apart and one section of land moves downward.

earthquake, trembling or shaking movement of the earth's surface. Most earthquakes are minor tremors. Larger earthquakes usually begin with slight tremors but rapidly take the form of one or more

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violent shocks, and end in vibrations of gradually diminishing force called aftershocks. The subterranean point of origin of an earthquake is called its focus; the point on the surface directly above the focus is the epicenter. The magnitude and intensity of an earthquake is determined by the use of scales, e.g., the

moment magnitude scale, Richter scale Richter scale , measure of the magnitude of seismic waves

from an earthquake. Devised in 1935 by the American seismologist Charles F. Richter (1900–1985) and

technically known as the local magnitude scale, it has been superseded by the moment magnitude

scale,

..... Click the link for more information. , and the modified Mercalli scale.

Causes of Earthquakes

Most earthquakes are causally related to compressional or tensional stresses built up at the margins of

the huge moving lithospheric plates that make up the earth's surface (see lithosphere

lithosphere , brittle uppermost shell of the earth, broken into a number of tectonic plates. The

lithosphere consists of the heavy oceanic and lighter continental crusts, and the uppermost portion of the

mantle.

..... Click the link for more information. ). The immediate cause of most shallow earthquakes is the

sudden release of stress along a fault fault, in geology, fracture in the earth's crust in which the rock

on one side of the fracture has measurable movement in relation to the rock on the other side. Faults on

other planets and satellites of the solar system also have been recognized.

..... Click the link for more information. , or fracture in the earth's crust, resulting in movement of the

opposing blocks of rock past one another. These movements cause vibrations to pass through and around the earth in wave form, just as ripples are generated when a pebble is dropped into water. Volcanic eruptions, rockfalls, landslides, and explosions can also cause a quake, but most of these are of only local extent. Shock waves from a powerful earthquake can trigger smaller earthquakes in a distant location hundreds of miles away if the geologic conditions are favorable.

See also plate tectonics plate tectonics, theory that unifies many of the features and

characteristics of continental drift and seafloor spreading into a coherent model and has revolutionized

geologists' understanding of continents, ocean basins, mountains, and earth history.

..... Click the link for more information. .

Seismic Waves

There are several types of earthquake waves including P, or primary, waves, which are compressional and travel fastest; and S, or secondary, waves, which are transverse, i.e., they cause the earth to vibrate perpendicularly to the direction of their motion. Surface waves consist of several major types and are called L, or long, waves. Since the velocities of the P and S waves are affected by changes in the density and rigidity of the material through which they pass, the boundaries between the regions of the earth known as the crust, mantle, and core have been discerned by seismologists, scientists who deal with the

analysis and interpretation of earthquake waves (see earth earth, in geology and astronomy, 3rd

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planet of the solar system and the 5th largest, the only planet definitely known to support life.

Gravitational forces have molded the earth, like all celestial bodies, into a spherical shape.

Seismographs (see seismology seismology , scientific study of earthquakes and related

phenomena, including the propagation of waves and shocks on or within the earth by natural or

artificially generated seismic signals.

are used to record P, S, and L waves. The disappearance of S waves below depths of 1,800 mi (2,900 km) indicates that at least the outer part of the earth's core is liquid.

Damage Caused by Earthquakes

The effects of an earthquake are strongest in a broad zone surrounding the epicenter. Surface ground cracking associated with faults that reach the surface often occurs, with horizontal and vertical displacements of several yards common. Such movement does not have to occur during a major earthquake; slight periodic movements called fault creep can be accompanied by microearthquakes too small to be felt. The extent of earthquake vibration and subsequent damage to a region is partly dependent on characteristics of the ground. For example, earthquake vibrations last longer and are of greater wave amplitudes in unconsolidated surface material, such as poorly compacted fill or river deposits; bedrock areas receive fewer effects. The worst damage occurs in densely populated urban areas where structures are not built to withstand intense shaking. There, L waves can produce destructive vibrations in buildings and break water and gas lines, starting uncontrollable fires.

Damage and loss of life sustained during an earthquake result from falling structures and flying glass and objects. Flexible structures built on bedrock are generally more resistant to earthquake damage than rigid structures built on loose soil. In certain areas, an earthquake can trigger mudslides, which slip down

mountain slopes and can bury habitations below. A submarine earthquake can cause a tsunami

tsunami , series of catastrophic ocean waves generated by submarine movements, which may be

caused by earthquakes, volcanic eruptions, landslides beneath the ocean, or an asteroid striking the

earth. , a series of damaging waves that ripple outward from the earthquake epicenter and inundate

coastal cities.

Major Earthquakes

On average about 1,000 earthquakes with intensities of 5.0 or greater are recorded each year. Great earthquakes (magnitude 8.0 or higher) occur once a year, major earthquakes (magnitude 7.0–7.9) occur 18 times a year, strong earthquakes (magnitude 6.0–6.9) 10 times a month, and moderate earthquakes (magnitude 5.0–5.9) more than twice a day. Because most of these occur under the ocean or in underpopulated areas, they pass unnoticed by all but seismologists. Moderate to strong earthquakes can cause more significant destruction if they occur closer to the earth's surface. Notable earthquakes have occurred at Lisbon, Portugal (1755); New Madrid, Mo. (1811 and 1812); Charleston, S.C. (1886); Assam, India (1897 and 1950); San Francisco (1906); Messina, Italy (1908); Gansu, China (1920); Tokyo, Japan (1923); Chile (1960); Iran (1962); S Alaska (1964); Managua, Nicaragua (1972); Guatemala (1976); Hebei, China (1976); Mexico (1985); Armenia (1988); Luzon, Philippines (1990); N Japan (1993); Kobe, Japan (1995); Izmit, Turkey (1999); central Taiwan (1999); Oaxaca state, Mexico (1999); Bam, Iran (2003); NW Sumatra, Indonesia (2004); Sichuan, China (2008); S Haiti (2010); Chile (2010); South Island, New Zealand (2010, 2011); and NE Japan (2011). The Lisbon, Chilean, Alaskan, Sumatran, and NE Japan earthquakes were accompanied by significant tsunamis.

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Twelve of the twenty largest earthquakes in the United States have occurred in Alaska. Most of the largest in the continental United States have occurred in California or elsewhere along the Pacific Coast, but the three New Madrid earthquakes (1811–12) also were among the largest continental events, as was the Charleston, S.C., earthquake (1886). On Good Friday 1964, one of the most severe North American earthquakes ever recorded struck near Anchorage, Alaska, measuring 8.4 to 8.6 in magnitude. Besides elevating some 70,000 sq mi (181,300 sq km) of land and devastating several cities, it generated a tsunami that caused damage as far south as California. Other recent earthquakes that have affected the United States include the Feb., 1971, movement of the San Fernando fault near Los Angeles. It rocked the area for 10 sec, thrust parts of mountains 8 ft (2.4 m) upward, killed 64 persons, and caused damage amounting to $500 million. In 1989, the Loma Prieta earthquake above Santa Cruz shook for 15 seconds at an magnitude of 7.1, killed 67 people, and toppled buildings and bridges. In Jan., 1994, an earthquake measuring 6.6 with its epicenter in N Los Angeles caused major damage to the city's infrastructure and left thousands homeless.

earthquake

Sudden shaking of the ground caused by a disturbance deeper within the crust of the Earth. Most earthquakes occur when masses of rock straining against one another along fault lines suddenly fracture and slip. The Earth's major earthquakes occur mainly in belts coinciding with the margins of tectonic plates. These include the Circum-Pacific Belt, which affects New Zealand, New Guinea, Japan, the Aleutian Islands, Alaska, and the western coasts of North and South America; the Alpide Belt, which passes through the Mediterranean region eastward through Asia; oceanic ridges in the Arctic, Atlantic, and western Indian oceans; and the rift valleys of East Africa. The “size,” or magnitude, of earthquakes is usually expressed in terms of the Richter scale, which assigns levels from 1.0 or lower to 8.0 or higher. The largest quake ever recorded (Richter magnitude 9.5) occurred off the coast of Chile in 1960. The “strength” of an earthquake is rated in intensity scales such as the Mercalli scale, which assigns qualitative measures of damage to terrain and structures that range from “not felt” to “damage nearly total.” The most destructive quake of modern times occurred in 1976, when the city of Tangshan, China, was leveled and more than 250,000 people killed

earthquake a sudden release of energy in the earth's crust or upper mantle, usually caused by movement along a fault plane or by volcanic activity and resulting in the generation of seismic waves which can be destructive

Sesmic zone in IndiaBasic Geography and Tectonic FeaturesIndia lies at the northwestern end of the Indo-Australian Plate, which encompasses India, Australia, amajor portion of the Indian Ocean and other smallercountries. This plate is colliding against the hugeEurasian Plate (Figure 1) and going under the EurasianPlate; this process of one tectonic plate getting underanother is called subduction. A sea, Tethys, separatedthese plates before they collided. Part of thelithosphere, the Earth’s Crust, is covered by oceansand the rest by the continents. The former can undergosubduction at great depths when it converges againstanother plate, but the latter is buoyant and so tends toremain close to the surface. When continents converge,large amounts of shortening and thickening takes

place, like at the Himalayas and the Tibet.eThree chief tectonic sub-regions of India are the

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mighty Himalayas along the north, the plains of theGanges and other rivers, and the peninsula. TheHimalayas consist primarily of sediments accumulatedover long geological time in the Tethys. The Indo-Gangetic basin with deep alluvium is a greatdepression caused by the load of the Himalayas on thecontinent. The peninsular part of the country consistsof ancient rocks deformed in the past Himalayan-likecollisions. Erosion has exposed the roots of the oldmountains and removed most of the topography. Therocks are very hard, but are softened by weatheringnear the surface. Before the Himalayan collision,

several tens of millions of years ago, lava flowed

across the central part of peninsular India leavinglayers of basalt rock. Coastal areas like Kachchh showmarine deposits testifying to submergence under thesea millions of years ago.Prominent Past Earthquakes in IndiaA number of significant earthquakes occurred inand around India over the past century (Figure 2).Some of these occurred in populated and urbanizedareas and hence caused great damage. Many wentunnoticed, as they occurred deep under the Earth’ssurface or in relatively un-inhabited places. Some ofthe damaging and recent earthquakes are listed inTable 1. Most earthquakes occur along the Himalayanplate boundary (these are inter-plate earthquakes), buta number of earthquakes have also occurred in the

peninsular region (these are intra-plate earthquakes).

r Table 1: Some Past Earthquakes in India: Date Event Time Magnitude Max.Intensity Deaths

16 June 1819 Cutch 11:00 8.3 VIII 1,50012 June 1897 Assam 17:11 8.7 XII 1,5008 Feb. 1900 Coimbatore 03:11 6.0 X Nil4 Apr. 1905 Kangra 06:20 8.6 X 19,00015 Jan. 1934 Bihar-Nepal 14:13 8.4 X 11,00031 May 1935 Quetta 03:03 7.6 X 30,00015 Aug. 1950 Assam 19:31 8.5 X 1,53021 Jul. 1956 Anjar 21:02 7.0 IX 11510 Dec. 1967 Koyna 04:30 6.5 VIII 20023 Mar. 1970 Bharuch 20:56 5.4 VII 3021 Aug. 1988 Bihar-Nepal 04:39 6.6 IX 1,00420 Oct. 1991 Uttarkashi 02:53 6.6 IX 76830 Sep. 1993 Killari (Latur) 03:53 6.4 IX 7,92822 May 1997 Jabalpur 04:22 6.0 VIII 3829 Mar. 1999 Chamoli 12:35 6.6 VIII 63

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26 Jan . 2001 Bhuj 08:46 7.7 X 13,805

.re the Seismic Zones in India?

The Richter Magnitude Scale ( in short Richter Scale)

Seismic waves are the vibrations from earthquakes that travel through the Earth; they are recorded on

instruments called seismographs. Seismographs record a zig-zag trace that shows the varying

amplitude of ground oscillations beneath the instrument. Sensitive seismographs, which greatly

magnify these ground motions, can detect strong earthquakes from sources anywhere in the world.

The time, locations, and magnitude of an earthquake can be determined from the data recorded by

seismograph stations.

The power of an earthquake is expressed in terms of magnitude on a scale called Richter scale.

The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of

Technology as a mathematical device to compare the size of earthquakes. The magnitude of an

earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs.

Adjustments are included for the variation in the distance between the various seismographs and the

epicenter of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and

decimal fractions. For example, a magnitude 5.3 might be computed for a moderate earthquake, and a

strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale,

each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an

estimate of energy, each whole number step in the magnitude scale corresponds to the release of

about 31 times more energy than the amount associated with the preceding whole number value.

At first, the Richter Scale could be applied only to the records from instruments of identical

manufacture. Now, instruments are carefully calibrated with respect to each other. Thus, magnitude

can be computed from the record of any calibrated seismograph.

Earthquakes with magnitude of about 2.0 or less are usually called microearthquakes; they are not

commonly felt by people and are generally recorded only on local seismographs. Events with

magnitudes of about 4.5 or greater - there are several thousand such shocks annually - are strong

enough to be recorded by sensitive seismographs all over the world. Great earthquakes, such as the

1964 Good Friday earthquake in Alaska, have magnitudes of 8.0 or higher. On the average, one

earthquake of such size occurs somewhere in the world each year. The Richter Scale has no upper

limit. Recently, another scale called the moment magnitude scale has been devised for more precise

study of great earthquakes.

The Richter Scale is not used to express damage. An earthquake in a densely populated area which

results in many deaths and considerable damage may have the same magnitude as a shock in a

remote area that does nothing more than frighten the wildlife. Large-magnitude earthquakes that occur

beneath the oceans may not even be felt by humans.

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The Richter scale is a standard scale used to compare earthquakes. It is a logarithmic scale, meaning that the numbers on the scale measure factors of 10. So, for example, an earthquake that measures 4.0 on the Richter scale is 10 times larger than one that measures 3.0. On the Richter scale, anything below 2.0 is undetectable to a normal person and is called a microquake.

Meaning of Epicenter:

An epicenter is the point on Earth that is just above where an earthquake starts

out from. This is where a fault begins and usually causes the most damage. There

have been times where this area was not where the most damage occurred during

an earthquake though, like in 2002 when an earthquake hit Denali, Alaska.

Or

The epicenter or epicentre is the point on the Earth's surface that is directly above

the hypocenter or focus, the point where an earthquake or underground explosion

originates.

epicentre

ˈɛpɪsɛntə/nounnoun: epicenter

1. 1.the point on the earth's surface vertically above the focus of an earthquake.o the central point of something, typically a difficult or unpleasant situation."the epicentre of labour militancy was the capital itself"Origin

late 19th cent.: from Greek epikentros ‘situated on a centre’, from epi ‘upon’ + kentron‘centre’.

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What is an EarthquakeFocus and Epicenter?

IntroductionWhere is the earthquake focus? The focus of an earthquake is the point where the rocks start to fracture. It is the origin of the earthquake. The epicenter is the point on land directly above the focus.

Focus of an Earthquake, USGS

Focus of an earthquakeThe focus is also called the hypocenter of an earthquake. The vibrating waves travel away from the focus of the earthquake in all directions. The waves can be so powerful they will reach all parts of the Earth and cause it to vibrate like a turning fork.

Epicenter of an earthquakeDirectly above the focus on the Earth's surface is the earthquake epicenter. Earthquake waves start at he focus and travel outward in all directions. Earthquake waves do not originate at the epicenter.

News stories about earthquakesMost news stories on earthquakes will list the epicenter of an earthquake and then tell how deep the earthquake was from the epicenter. Great earthquakes that occur in subduction zones may give an earthquake focus but they actually break along hundreds of kilometers. The 1960 Chilean earthquake broke along 800 kilometers of the fault line.

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Richter scale used for shallow-focus earthquakesShallow-focus earthquakes occur between 0 and 40 miles deep. Shallow-focus earthquakes are much more common than deep-focus earthquakes. Crustal plates moving against each other produce most of the shallow-focus earthquakes here on Earth. These earthquakes are generally smaller and scientists use the Richter scale when measuring these earthquakes.

Energy released by shallow focus earthquakesShallow-focus earthquakes are much more dangerous than deep-focus earthquakes. They release 75% of all the energy produced by earthquakes each year. They are crustal earthquakes that are smaller than deep-focus earthquakes.

Deep-focus earthquakes use moment magnitude scaleDeep-focus earthquakes occur 180 miles or more below the Earth's surface. These earthquakes occur in island arc or deep ocean trenches where one plate is slipping over another in subduction zones. Great earthquakes where one plate is slipping over another plate in a subduction zone trigger deep-focus earthquakes. They are the largest earthquakes and scientists use the moment magnitude scale to measure them.

Fault TypesFault surfaces are surfaces along which rocks move under, over, or past each other. Rocks may get “stuck” along the fault surface, causing a build-up of strain energy, and resulting in an earthquakewhen the rocks break free of each other. There are 3 types of stress that can affect rocks, resulting in

3 different types of faults:

1. Tension pulls rocksapart resulting innormal faults

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2. Compression squeezes rocktogether resulting in reversefaults

3. Shear stress causes rocksto slide past each otherresulting in strike-slip

faults

FoldingDuring mountain building processes rocks can undergo folding as well as faulting.

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Sometimes rocks deform ductilely, particularly if they are subjected to heat and pressure. Atelevated temperature and pressure within the crust, folds can form from compressional forces.Entire mountain rages, like the Appalachians, have extensive fold systems.The conditions of whether a rock faults or folds vary with temperature, pressure, rockcomposition, and strain rate. In the same location, some rocks can fold while others fault. Oftenfolding is just a precursor to faulting.

Z-fold in schist with white felsic dike (hammer forscale). Near Lake Murray, S.C. Photo courtesy of K.McCarney-Castle

Large fold in outcrop (geologists for scale). NearOakridge, Tennessee, Appalachian Mtns. Photo courtesyof K. McCarney-Castle.

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Seismic WavesSeismic waves are generated by the release of energy during an earthquake. They travelthrough the earth like waves travel through water.The location within the Earth where the rock actually breaks is called the focus of theearthquake. Most foci are located within 65 km of the Earth‟s surface; however, some havebeen recorded at depths of 700 km. The location on the Earth‟s surface directly above the focusis called the epicenter.The study of seismic waves and earthquakes is called seismology, which is a branch ofgeophysics.

Two types of seismic waves are generated at the earthquake focus:1. Body waves spread outward from the focus in all directions.2. Surface waves spread outward from the epicenter to the Earth‟s surface, similar toripples on a pond. These waves can move rock particles in a rolling motion that very fewstructures can withstand. These waves move slower than body waves

Seismic Waves: Epicenter locationAlthough S-waves, P-waves and surface waves all start out at the same time, they travel atdifferent speeds. The speed of a traveling seismic wave can be used to determine the location of an

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earthquake epicenter.A seismograph records the arrival time and the magnitude of horizontal and verticalmovements caused by an earthquake. The arrival time between different seismic waves is used tocalculate the travel time and the distance from the epicenter.The difference in arrival time between primary waves and secondary waves is used to calculatethe distance from the seismograph station to the epicenter.It is crucial that seismic waves are recorded by three different seismograph stations in order toestimate the location of the epicenter (see next slide.)Standard 8-3.3: Infer an earthquake’s epicenter from seismographic data.

This example shows seismic waves arriving at different times at twoseismograph stations. Station B isfarther away from Station A so thewaves take longer to reach StationB. Primary waves arrive first,followed by secondary waves, andthen surface waves.

Earthquake classification scalesEarthquakes can be very destructive at the Earth‟s surface. The magnitude of an earthquake is ameasure of how destructive it is. Basically the magnitude corresponds to how much energy isreleased.

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The Richter Scale is used to express earthquake magnitude on the basis of the height (amplitude)of the largest line (seismic wave, P or S) on a seismogram. The Richter scale was originallydeveloped for earthquakes in Southern California. The utility of this scale was its ability to accountfor decreased wave amplitude with increased distance from the epicenter. Richter‟s scale is also alogarithmic scale.Today, a standard magnitude scale is used, Seismic Moment, which more accurately representsthe energy released in an earthquake, especially large magnitude events.The majority of earthquakes are minor and have magnitudes of 3-4.9 on the Richter scale. Thesecan be felt, but cause little or no damage, and there are about 55,000 of these earthquakes each year.Thousands of earthquakes are recorded every day with magnitudes < 3.0 but are almost neverfelt.The Mercalli scale is different from the Richter scale because it measures the intensity of howpeople and structures are affected by the seismic event. In essence, it measures damage. It is muchmore subjective and uses numbers ranging from 1 (no damage) to 12 (total destruction).

Seismograph

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measure Seismograph are instruments that motions of the ground, including those of seismic

waves generated by earthquakes, volcanic eruptions, and otherseismic sources. Records of seismic waves

allow seismologists to map the interior of the Earth, and locate and measure the size of these different sources.

The word derives from the Greek σεισμός, seismós, a shaking or quake, from the verb σείω, seíō, to shake;

and μέτρον, métron, measure and was coined by David Milne-Home in 1841, to describe an instrument

designed by Scottish physicist James David Forbes.[1]

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.[2]

The concerning technical discipline is called seismometry,[3] a branch of seismology.

Basic principles

A simple seismometer that is sensitive to up-down motions of the earth can be understood by visualizing a

weight hanging on a spring. The spring and weight are suspended from a frame that moves along with the

earthʼs surface. As the earth moves, the relative motion between the weight and the earth provides a measure

of the vertical ground motion. If a recording system is installed, such as a rotat- ing drum attached to the frame,

and a pen attached to the mass, this relative motion between the weight and earth can be recorded to produce

a history of ground motion, called a seismogram.

Any movement of the ground moves the frame. The mass tends not to move because of its inertia, and by

measuring the movement between the frame and the mass, the motion of the ground can be determined.

Early seismometers used optical levers or mechanical linkages to amplify the small motions involved, recording

on soot-covered paper or photographic paper. Modern instruments use electronics. In some systems, the mass

is held nearly motionless relative to the frame by an electronic negative feedback loop. The motion of the mass

relative to the frame is measured, and the feedback loop applies a magnetic or electrostatic force to keep the

mass nearly motionless. The voltage needed to produce this force is the output of the seismometer, which is

recorded digitally. In other systems the weight is allowed to move, and its motion produces a voltage in a coil

attached to the mass and moving through the magnetic field of a magnet attached to the frame. This design is

often used in the geophones used in seismic surveys for oil and gas.

Professional seismic observatories usually have instruments measuring three axes: north-south, east-west, and

the vertical. If only one axis can be measured, this is usually the vertical because it is less noisy and gives

better records of some seismic waves.

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The foundation of a seismic station is critical.[4] A professional station is sometimes mounted on bedrock. The

best mountings may be in deep boreholes, which avoid thermal effects, ground noise and tilting from weather

and tides. Other instruments are often mounted in insulated enclosures on small buried piers of unreinforced

concrete. Reinforcing rods and aggregates would distort the pier as the temperature changes. A site is always

surveyed for ground noise with a temporary installation before pouring the pier and laying conduit. Originally,

European seismographs were placed in a particular area after a destructive earthquake. Today, they are

spread to provide appropriate coverage (in the case of weak-motion seismology) or concentrated in high-risk

regions (strong-motion seismology).[5]

Teleseismometers

A low-frequency 3-direction ocean-bottom seismometer (cover removed). Two masses for x- and y-direction can be seen,

the third one for z-direction is below. This model is a CMG-40TOBS, manufactured by Güralp Systems Ltd and is part of

theMonterey Accelerated Research System.

The modern broadband seismograph can record a very broad range of frequencies. It consists of a small "proof

mass", confined by electrical forces, driven by sophisticated electronics. As the earth moves, the electronics

attempt to hold the mass steady through a feedback circuit. The amount of force necessary to achieve this is

then recorded.

In most designs the electronics holds a mass motionless relative to the frame. This device is called a "force

balance accelerometer". It measuresacceleration instead of velocity of ground movement. Basically, the

distance between the mass and some part of the frame is measured very precisely, by a linear variable

differential transformer. Some instruments use a linear variable differential capacitor.

That measurement is then amplified by electronic amplifiers attached to parts of an electronic negative

feedback loop. One of the amplified currents from the negative feedback loop drives a coil very like

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a loudspeaker, except that the coil is attached to the mass, and the magnet is mounted on the frame. The

result is that the mass stays nearly motionless.

Most instruments measure directly the ground motion using the distance sensor. The voltage generated in a

sense coil on the mass by the magnet directly measures the instantaneous velocity of the ground. The current

to the drive coil provides a sensitive, accurate measurement of the force between the mass and frame, thus

measuring directly the ground's acceleration (using f=ma where f=force, m=mass, a=acceleration).

One of the continuing problems with sensitive vertical seismographs is the buoyancy of their masses. The

uneven changes in pressure caused by wind blowing on an open window can easily change the density of the

air in a room enough to cause a vertical seismograph to show spurious signals. Therefore, most professional

seismographs are sealed in rigid gas-tight enclosures. For example, this is why a common Streckheisen model

has a thick glass base that must be glued to its pier without bubbles in the glue.

It might seem logical to make the heavy magnet serve as a mass, but that subjects the seismograph to errors

when the Earth's magnetic field moves. This is also why seismograph's moving parts are constructed from a

material that interacts minimally with magnetic fields. A seismograph is also sensitive to changes in

temperature so many instruments are constructed from low expansion materials such as nonmagnetic invar.

The hinges on a seismograph are usually patented, and by the time the patent has expired, the design has

been improved. The most successful public domain designs use thin foil hinges in a clamp.

Another issue is that the transfer function of a seismograph must be accurately characterized, so that its

frequency response is known. This is often the crucial difference between professional and amateur

instruments. Most instruments are characterized on a variable frequency shaking table.

Strong-motion seismograph

Another type of seismometer is a digital strong-motion seismograph, or accelerograph. The data from such an

instrument is essential to understand how an earthquake affects manmade structures.

A strong-motion seismograph measures acceleration. This can be mathematically integrated later to give

velocity and position. Strong-motion seismometers are not as sensitive to ground motions as teleseismic

instruments but they stay on scale during the strongest seismic shaking.

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Other forms

A Kinemetrics seismograph, formerly used by the United States Department of the Interior.

Accelerographs and geophones are often heavy cylindrical magnets with a spring-mounted coil inside. As case

moves, the coil tends to stay stationary, so the magnetic field cuts the wires, inducing current in the output

wires. They receive frequencies from several hundred hertz down to 1 Hz. Some have electronic damping, a

low-budget way to get some of the performance of the closed-loop wide-band geologic seismographs.

Strain-beam accelerometers constructed as integrated circuits are too insensitive for geologic seismographs

(2002), but are widely used in geophones.

Some other sensitive designs measure the current generated by the flow of a non-corrosive ionic fluid through

an electret sponge or a conductive fluid through a magnetic field.

Modern recording

Today, the most common recorder is a computer with an analog-to-digital converter, a disk drive and an

internet connection; for amateurs, a PC with a sound card and associated software is adequate. Most systems

record continuously, but some record only when a signal is detected, as shown by a short-term increase in the

variation of the signal, compared to its long-term average (which can vary slowly because of changes in

seismic noise).

Interconnected seismometers

Seismometers spaced in an array can also be used to precisely locate, in three dimensions, the source of an

earthquake, using the time it takes for seismic waves to propagate away from thehypocenter, the initiating point

of fault rupture (See also Earthquake location). Interconnected seismometers are also used to detect

underground nuclear test explosions. These seismometer are often used as part of a large scale, multi-million

dollar governmental or scientific project, but some organizations, such as the Quake-Catcher Network, can use

residential size detectors built into computers to detect earthquakes as well.

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In reflection seismology, an array of seismometers image sub-surface features. The data are reduced to

images using algorithms similar to tomography. The data reduction methods resemble those of computer-aided

tomographic medical imaging X-ray machines (CAT-scans), or imaging sonars.

A world-wide array of seismometers can actually image the interior of the Earth in wave-speed and

transmissivity. This type of system uses events such as earthquakes, impact events or nuclear explosions as

wave sources. The first efforts at this method used manual data reduction from paper seismograph charts.

Modern digital seismograph records are better adapted to direct computer use. With inexpensive seismometer

designs and internet access, amateurs and small institutions have even formed a "public seismograph

network."[10]

Seismographic systems used for petroleum or other mineral exploration historically used an explosive and a

wireline of geophones unrolled behind a truck. Now most short-range systems use "thumpers" that hit the

ground, and some small commercial systems have such good digital signal processing that a few

sledgehammer strikes provide enough signal for short-distance refractive surveys. Exotic cross or two-

dimensional arrays of geophones are sometimes used to perform three-dimensional reflective imaging of

subsurface features. Basic linear refractive geomapping software (once a black art) is available off-the-shelf,

running on laptop computers, using strings as small as three geophones. Some systems now come in an 18"

(0.5 m) plastic field case with a computer, display and printer in the cover.

Small seismic imaging systems are now sufficiently inexpensive to be used by civil engineers to survey

foundation sites, locate bedrock, and find subsurface water.

Protection against Earthquakes

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HOW TO PROTECT YOURSELF DURING AN EARTHQUAKE...

OFFICIAL RESCUE TEAMS from the U.S. and other countries who have searched for trapped people in collapsed structures around the world, as well as emergency managers, researchers, and school safety advocates, all agree that "Drop, Cover, and Hold On" is the appropriate action to reduce injury and death during earthquakes. Methods like standing in a doorway, running outside, and "triangle of life" method are considered dangerous and are not recommended.

WHAT TO DO IMMEDIATELY WHEN SHAKING BEGINS

Your past experience in earthquakes may give you a false sense of safety; you didn't do anything, or you ran outside, yet you survived with no injuries. Or perhaps you got under your desk and others thought you overreacted. However, you likely have never experienced the kind of strong earthquake shaking that is possible in much large earthquakes: sudden and intense back and forth motions of several feet per second will cause the floor or the ground to jerk sideways out from under you, and every unsecured object around you could topple, fall, or become airborne, potentially causing serious injury. This is why you must learn to immediately protect yourself after the first jolt... don't wait to see if the earthquake shaking will be strong!

In MOST situations, you will reduce your chance of injury if you:

DROP down onto your hands and knees (before the earthquakes knocks you down). This position protects you from falling but allows you to still move if necessary.

COVER your head and neck (and your entire body if possible) under a sturdy table or desk. If there is no shelter nearby, only then should you get down near an interior wall (or next to low-lying furniture that won't fall on you), and cover your head and neck with your arms and hands.

HOLD ON to your shelter (or to your head and neck) until the shaking stops. Be prepared to move with your shelter if the shaking shifts it around.

Wherever you are, protect yourself! You may be in situation where you cannot find shelter beneath furniture (or low against a wall, with your arms covering your head and neck). It is important to think about what you will do to protect yourself wherever you are. What if you are driving, in a theater, in bed, at the beach, etc.? Step 5 of the Seven Steps to Earthquake Safety describes what to do in various situations, no matter where you are when you feel earthquake shaking.

WHY RESCUERS AND EXPERTS RECOMMEND   DROP, COVER, AND HOLD ON

Trying to moving during shaking puts you at risk:Earthquakes occur without any warning and may be so violent that you cannot run or crawl; you therefore will most likely be knocked to the ground where you happen to be. So it is best to drop before the earthquake drops you, and find nearby shelter or use your arms and hands to protect your head and neck. "Drop, Cover, and Hold On" gives you the best overall chance of quickly protecting yourself during an earthquake... even during quakes that cause furniture to move about rooms, and even in buildings that might ultimately collapse.

The greatest danger is from falling and flying objects: Studies of injuries and deaths caused by earthquakes over the last several decades show that you are

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much more likely to be injured by falling or flying objects (TVs, lamps, glass, bookcases, etc.) than to die in a collapsed building. "Drop, Cover, and Hold On" (as described above) will protect you from most of these injuries.

If there is no furniture nearby, you can still reduce the chance of injury from falling objects by getting down next to an interior wall and covering your head and neck with your arms (exterior walls are more likely to collapse and have windows that may break). If you are in bed, the best thing to do is to stay there and cover your head with a pillow. Studies of injuries in earthquakes show that people who moved from their beds would not have been injured if they had remained in bed.

You can also reduce your change of injury or damage to your belongings by securing them in the first place. Secure top heavy furniture to walls with flexible straps. Use earthquake putty or velcro fasteners for objects on tables, shelves, or other furniture. Install safety latches on cabinets to keep them closed. Instructions for how to "secure your space" are at www.daretoprepare.org.

Building collapse is less of a danger: While images of collapsed structures in earthquakes around the world are frightening and get the most attention from the media, most buildings do not collapse at all, and few completely collapse. In earthquake prone areas of the U.S. and in many other countries, strict building codes have worked to greatly reduce the potential of structure collapse. However, there is the possibility of structural failure in certain building types, especially unreinforced masonry (brick buildings), and in certain structures constructed before the latest building codes. Rescue professionals are trained to understand how these structures collapse in order to identify potential locations of survivors within "survivable void spaces."

The main goal of "Drop, Cover, and Hold On" is to protect you from falling and flying debris and other nonstructural hazards, and to increase the chance of your ending up in a Survivable Void Space if the building actually collapses. The space under a sturdy table or desk is likely to remain even if the building collapses- pictures from around the world show tables and desks standing with rubble all around them, and even holding up floors that have collapsed. Experienced rescuers agree that successfully predicting other safe locations in advance is nearly impossible, as where these voids will be depends on the direction of the shaking and many other factors. (See "triangle of life" below.)

The ONLY exception to the "Drop, Cover and Hold On" rule is if you are in a country with unengineered construction, and if you are on the ground floor of an unreinforced mud-brick (adobe) building, with a heavy ceiling. In that case, you should try to move quickly outside to an open space. This cannot be recommended as a substitute for building earthquake-resistant structures in the first place!

WHAT RESCUERS AND EXPERTS *DO NOT* RECOMMEND YOU DO DURING AN EARTHQUAKE

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Based on years of research about how people are injured or killed during earthquakes, and the experiences of U.S. and international search and rescue teams, these three actions are not recommended to protect yourself during earthquakes:

DO NOT run outside or to other rooms during shaking: The area near the exterior walls of a building is the most dangerous place to be. Windows, facades and architectural details are often the first parts of the building to collapse. To stay away from this danger zone, stay inside if you are inside and outside if you are outside. Also, shaking can be so strong that you will not be able to move far without falling down, and objects may fall or be thrown at you that you do not expect. Injuries can be avoided if you drop to the ground before the earthquake drops you.

DO NOT stand in a doorway: An enduring earthquake image of California is a collapsed adobe home with the door frame as the only standing part. From this came our belief that a doorway is the safest place to be during an earthquake. True- if you live in an old, unreinforced adobe house or some older woodframe houses. In modern houses, doorways are no stronger than any other part of the house, and the doorway does not protect you from the most likely source of injury- falling or flying objects. You also may not be able to brace yourself in the door during strong shaking. You are safer under a table.

DO NOT get in the "triangle of life": In recent years, an e-mail has been circulating which describes an alternative to the long-established "Drop, Cover, and Hold On" advice. The so-called "triangle of life" and some of the other actions recommended in the e-mail are potentially life threatening, and the credibility of the source of these recommendations has been broadly questioned (see links at left).

The "triangle of life" advice (always get next to a table rather than underneath it) is based on several wrong assumptions:

buildings always collapse in earthquakes (wrong- especially in developed nations, and flat "pancake" collapse is rare anywhere);

when buildings collapse they always crush all furniture inside (wrong- people DO survive under furniture or other shelters);

people can always anticipate how their building might collapse and anticipate the location of survivable void spaces (wrong- the direction of shaking and unique structural aspects of the building make this nearly impossible) ; and

during strong shaking people can move to a desired location (wrong- strong shaking can make moving very difficult and dangerous).

Some other recommendations in the "triangle of life" e-mail are also based on wrong assumptions and very hazardous. For example, the recommendation to get out of your car during an earthquake and lie down next to it assumes that there is always an elevated freeway above you that will fall and crush your car. Of course there are very few elevated freeways, and lying next to your car is very dangerous because the car can move and crush you, and other drivers may not see you on the ground! A compilation of rebuttals from many organizations to these alternative recommendations, as well as news articles about the controversy, is listed at left. 

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PRACTICE THE RIGHT THING TO DO… IT COULD SAVE YOUR LIFE

You will be more likely to react quickly when shaking begins if you have actually practiced how to protect yourself on a regular basis. A great time to practice Drop, Cover, and Hold On is by participating in a Great ShakeOut Earthquake Drill (each October in most areas).

If you are at home:

Take shelter under a table and stay there till the shaking stops. Stay away from tall and heavy objects that may fall on you. If you are in bed, do not get up. Protect your head with the pillow.

If you are outdoor:

Find a clear spot, away from building, trees and overhead power lines. Drop to the ground.

If you are in a car or bus, do not come out. Ask the driver to drive slowly to a clear spot. Do not come out till the tremors stop.

Earthquake Short Summary

A natural phenomenon that cannot be predicted is anearthquake. The earth consists of three major layers, called the crust, the mantle and the core. The core is further divided into the inner core and the outer

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core. The mantle consists of semi-solid material above which the crust floats. The crust consists of oceans and continents. The crust is divided into several parts, called tectonic plates. The regions where one tectonic plate slides against another are referred to as fault zones, and these are the regions where an earthquake is likely to occur. Hence, these zones are referred to as seismic zones.

The place in the interior of the earth where an earthquake occurs is the focus, and the region on the surface of the earth that is the closest to focus is likely to experience the largest damage. This region is called the epicentre of the earthquake.

The instrument that measures the severity of an earthquake is a seismograph. It basically consists of a drum that rolls and a pendulum with a stylus that traces the waves of an earthquake on a sheet like a graph paper. The severity of an earthquake is measured on the Richter scale. A major earthquake measures 7 or more on the Richter scale.