EARTHQUAKE

- (also known as a quake, tremor or temblor) is the result of a sudden release of energy in

theEarth'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.

- a sudden release of energy in the earth's crust or upper mantle, usuallycaused by movement a

long a fault plane or by volcanic activity andresulting in the generation of seismic waves whi

ch can be destructive.

Causes:

1. Fault Movement

Fault - is a planar fracture or discontinuity in a volume of rock, across which there has been

significant displacement along the fractures as a result of earth movement. Large faults within

the Earth's crust result from the action of plate tectonic forces, with the largest forming the

boundaries between the plates, such as subduction zones or transform faults. Energy release

associated with rapid movement on active faults is the cause of most earthquakes.

A fault line is the surface trace of a fault, the line of intersection between the fault plane and

the Earth's surface.

Types of fault:

o Normal

- In a normal fault, the block above the fault moves down relative to the block below

the fault. This fault motion is caused by tensional forces and results in extension.

[Other names: normal-slip fault, tensional fault or gravity fault] EX., Sierra

Nevada/Owens Valley; Basin & Range faults

o Reversed

- In a reverse fault, the block above the fault moves up relative to the block below the

fault. This fault motion is caused by compressional forces and results in shortening.

A reverse fault is called a thrust fault if the dip of the fault plane is small. [Other

names: thrust fault, reverse-slip fault or compressional fault] EX., Rocky Mountains,

Himalayas

o Strike-slip fault

- In a strike-slip fault, the movement of blocks along a fault is horizontal. If the block on the far side of the fault moves to the left, as shown in this animation, the fault is called left-lateral. If the block on the far side moves to the right, the fault is called right-lateral. The fault motion of a strike-slip fault is caused by shearing forces. Examples: San Andreas Fault, California; Anatolian Fault, Turkey [Other names: trans-current fault, lateral fault, tear fault or wrench fault.]

o Transformed

- A transform fault is a type of strike-slip fault wherein the relative horizontal slip is

accommodating the movement between two ocean ridges or other tectonic

boundaries. Additional animations on seafloor spreading and transform faults are

available from Tanya Atwater.

o Oblique

- Oblique-slip faulting suggests both dip-slip faulting and strike-slip faulting. It is

caused by a combination of shearing and tension or compressional forces. Nearly all

faults will have some component of both dip-slip (normal or reverse) and strike-slip,

so defining a fault as oblique requires both dip and strike components to be

measurable and significant.

2. Volcanism

-A volcano tectonic earthquake is an earthquake induced by the movement (injection or

withdrawal) of magma.[1] The movement results in pressure changes in the rock around where the

magma has experienced stress. At some point, the rock may break or move. The earthquakes

may also be related to dike intrusion and may occur as earthquake swarms.[2] An example is

the 2007–2008 Nazko earthquake swarm in central British Columbia, Canada.

Other types of seismic activity related to volcanoes and their eruptions are long period seismic

waves, which are from sudden sporadic movement of magma, which is blocked from moving

due to a blockage. Another is a harmonic tremor, which is steady movement of magma, deep in

the mantle.

3. Induced Siesmicity

Induced seismicity refers to typically minor earthquakes and tremors that are caused

by human activity that alters the stresses and strains on the Earth's crust. Most induced

seismicity is of a low magnitude. A few sites regularly have larger quakes, such asThe

Geysers geothermal plant in California, which in the past 5 years has averaged 2 M4 events

and 15 M3 events every year.

There are many ways in which induced seismicity has been seen to occur. In the past several

years, some energy technologies that inject or extract fluid from the Earth, such as oil and

gas extraction and geothermal energy development, have been found or suspected to cause

seismic events. Some energy technologies also produce wastes that may be managed through

disposal or storage by injection deep into the ground. For example, waste water from oil and

gas production and carbon dioxide from a variety of industrial processes may be managed

through underground injection.

Ex, Artificial Lakes, Mining, Extraction of fossil fuels, Groundwater extraction, Enhanced

geothermal systems

The epicenter, epicentre / ̍ ɛ p ɪ s ɛ n t ər /  or epicentrum[1] is the point on the Earth's surface that is directly above thehypocenter or focus, the point where an earthquake or underground explosion originates.

An earthquake's hypocenter is the position where the strain energy stored in the rock is first released, marking the point where thefault begins to rupture.[1] This occurs directly beneath the epicenter, at a distance known as the focal or hypocentral depth.[1]

A seismic shadow zone is an area of the Earth's surface where seismographs cannot detect an earthquake after its seismic waves have passed through the Earth. When an earthquake occurs, seismic waves radiate out spherically from the earthquake's focus. The primary seismic waves are refracted by the liquid outer core of the Earth and are not detected between 104° and 140° (between approximately 11,570 and 15,570 km or 7,190 and 9,670 mi) from the epicenter. The secondary seismic waves cannot pass through the liquid outer core and are not detected more than 104° (approximately 11,570 km or 7,190 mi) from the epicenter.

Seismic waves are waves of energy that travel through the Earth's layers, and are a result of

an earthquake, explosion, or a volcano that gives out low-frequency acoustic energy. Many other

natural and anthropogenic sources create low amplitude waves commonly referred to as ambient

vibrations. Seismic waves are studied by geophysicists called seismologists. Seismic wave fields

are recorded by a seismometer, hydrophone (in water), or accelerometer.

The propagation velocity of the waves depends on density and elasticity of the medium. Velocity

tends to increase with depth, and ranges from approximately 2 to 8 km/s in the Earth's crust up to

13 km/s in the deep mantle.

P-waves are a type of body wave that is the first wave to arrive to the seismograph,

called seismic waves in seismology, that can travel through a continuum. The continuum is made

up of gases (as sound waves), liquids, or solids, including the Earth. P-waves can be produced

by earthquakes and recorded by seismographs. The name P-wave is often said to stand either

for primary wave, as it has the highest velocity and is therefore the first to be recorded;

or pressure wave,[1] as it is formed from alternatingcompressions and rarefactions.

In isotropic and homogeneous solids, the mode of propagation of a P-wave is

always longitudinal; thus, the particles in the solid have vibrations along or parallel to the

travel direction of the wave energy.

A type of elastic wave, the S-wave, secondary wave, or shear wave (sometimes called

an elastic S-wave) is one of the two main types of elastic body waves, so named because they

move through the body of an object, unlike surface waves.

The S-wave moves as a shear or transverse wave, so motion is perpendicular to the direction of

wave propagation. The wave moves through elastic media, and the main restoring force comes

from shear effects. These waves do not diverge, and they obey the continuity equation for

incompressible media.

Measurement

- A seismic scale is used to calculate and compare the severity of earthquakes.

- Two fundamentally different but equally important types of scales are commonly used by

seismologists to describe earthquakes. The original force or energy of an earthquake is

measured on a magnitude scale, while the intensity of shaking occurring at any given point

on the Earth's surface is measured on an intensity scale.

- 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.

- 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.

- The concept of Earthquake Duration Magnitude - originally proposed by Bisztricsany[1] in

1958 using surface waves only - is based on the realization that on a recorded

earthquake seismogram the total length of the seismic wavetrain - sometimes referred to as

the CODA - reflects its size. Thus larger earthquakes give longer seismograms [as well as

stronger seismic waves] than small ones. The seismic wave interval measured on the time

axis of an earthquake record - starting with the first seismic wave onset until the

wavetrain amplitude diminishes to at least 10% of its maximum recorded value - is referred

to as "earthquake duration". It is this concept that Bisztricsany first used to develop

his Earthquake Duration Magnitude Scale employing surface wave durations.

TSUNAMI

A tsunami is a series of ocean waves that sends surges of water, sometimes reaching heights of over 100 feet (30.5 meters), onto land. These walls of water can cause widespread destruction when they crash ashore.

These awe-inspiring waves are typically caused by large, undersea earthquakes at tectonic plate boundaries. When the ocean floor at a plate boundary rises or falls suddenly it displaces the water above it and launches the rolling waves that will become a tsunami.

Most tsunamis, about 80 percent, happen within the Pacific Ocean’s “Ring of Fire,” a geologically active area where tectonic shifts make volcanoes and earthquakes common.

Tsunamis may also be caused by underwater landslides or volcanic eruptions. They may even be launched, as they frequently were in Earth’s ancient past, by the impact of a large meteorite plunging into an ocean.

Tsunamis race across the sea at up to 500 miles (805 kilometers) an hour—about as fast as a jet airplane. At that pace they can cross the entire expanse of the Pacific Ocean in less than a day. And their long wavelengths mean they lose very little energy along the way.

In deep ocean, tsunami waves may appear only a foot or so high. But as they approach shoreline and enter shallower water they slow down and begin to grow in energy and height. The tops of the waves move faster than their bottoms do, which causes them to rise precipitously.

A tsunami’s trough, the low point beneath the wave’s crest, often reaches shore first. When it does, it produces a vacuum effect that sucks coastal water seaward and exposes harbor and sea floors. This retreating of sea water is an important warning sign of a tsunami, because the wave’s crest and its enormous volume of water typically hit shore five minutes or so later. Recognizing this phenomenon can save lives.

A tsunami is usually composed of a series of waves, called a wave train, so its destructive force may be compounded as successive waves reach shore. People experiencing a tsunami should remember that the danger may not have passed with the first wave and should await official word that it is safe to return to vulnerable locations.

Some tsunamis do not appear on shore as massive breaking waves but instead resemble a quickly surging tide that inundates coastal areas.

The best defense against any tsunami is early warning that allows people to seek higher ground. The Pacific Tsunami Warning System, a coalition of 26 nations headquartered in Hawaii, maintains a web of seismic equipment and water level gauges to identify tsunamis at sea. Similar systems are proposed to protect coastal areas worldwide.

Tsunami vs. Tidal Waves

Tidal waves are waves created by the gravitational forces of the sun or moon, and cause changes in the level of water bodies. Tsunami is also a series of water waves that are caused because of the displacement of large bodies of water, but due to seismic disturbances.

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