global geography #12 study materialgeneral studies ias help.pdf

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This post was republished to IAS Help at 21:04:43 01/02/2010

Global Geography #12 | Study

Material::General Studies | IAS Help

Category Global Geography

EARTHQUAKES

Overview

An earthquake is a result of a sudden release of energy in the earths crust that creates

seismic waves

Earthquakes are recorded with a seismograph and are reported on a magnitude on the

Richter scale.

In general, earthquakes of magnitude less than 3 are imperceptible, and more than 7

cause serious damage

The intensity of an earthquake can also be measured on the Modified Mercalli (MM) scale.

The MM scale quantifies the effect an earthquake has on humans, natural objects and

man-made structures

The shaking caused by earthquakes can result in landslides, and in volcanic activity as well.

When a large earthquake occurs in the oceans, the ocean floor can suffer sufficient

displacement to cause a tsunami

Earthquakes are usually caused by rupture of geological faults, but can also be caused by

volcanic activity, landslides, mine blasts and nuclear experiments

The point of initial rupture of an earthquake is called its hypocentre, while the point on

the surface directly above it is called the epicentre

Earthquakes that occur under the ocean and of high magnitude can generate tsunamis (eg

2004 Indian Ocean tsunami) The most powerful earthquake ever recorded is the Valdivia earthquake in Chile in 1960. It

measured 9.5 on the Richter scale

Mechanism of action

Earthquakes can occur anywhere within the earth where there is stored elastic energy

sufficient enough to drive fault propagation along a fault plane

Tectonic plates move past each other smoothly only if there are no irregularities and

asperities. Most plate boundaries do have asperities and this leads to stick-slip behaviour

Once the boundary has locked into a relative stable position, continued relative motion

between the plates leads to increased stress and stored strain energy

This continues until the stress rises sufficiently to break through the relative stable position,

suddenly sliding over the locked position of the fault and thereby releasing the stored

energy

The energy is released as a combination of elastic seismic waves, frictional heating of the

surface and cracking of rock, thereby causing an earthquake

This process of gradual build up of stress and sudden release of energy in the form of

earthquakes is called elastic-rebound theory

It is estimated that less than 10 % of the total energy of an earthquake is radiated as

seismic energy. Most of the earthquakes energy is used to power fracture growth or is

converted as heat generated by friction

Occurrence of earthquakes
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Minor earthquakes occur nearly constantly.Most of these happen in places like California

and Alaska in the US, as well as in Guatemala, Chile, Peru, Indonesia, Iran, Pakistan, Turkey,

Greece, Italy, Japan and New Zealand. Larger earthquakes occur less frequently

However, in general, earthquakes can occur almost anywhere (even away from plate

boundaries)

The relationship between frequency and intensity of earthquakes is roughly exponential i.e.for instance, there are roughly 10 times as many earthquakes of magnitude 4 as of

magnitude 5

Most of the worlds earthquakes occur in Pacific Ring of Fire seismic belt. Massive

earthquakes occur along other plate boundaries too, such as the Himalayas

Induced seismicity

While most earthquakes occur due to natural movement of the earths tectonic plates,

human activity can produce earthquakesas well

Four main human activities that contribute to earthquakes include

o Large dams

o

Drilling and injecting liquids into wells

o Coal mining

o Oil drilling

For instance, the 2008 Sichuan earthquake in China is believed to have been caused by the

Zipingpu dam which caused the pressure of a nearby

fault to fluctuate, increasing the movement of the

fault and the magnitude of the earthquake

Earthquakes and volcanic activity

Earthquakes often occur in volcanic areas

They are caused both by tectonic faults and themovement of magma in volcanoes

Such earthquakes can serve as early warning of

impending volcanic eruptions.Eg: Mount St Helens eruption of 1980 (USA)

Seismic waves

Seismic waves are waves of force that travel through the earth

Earthquakes produces different types of seismic waves that travel through the earth at

different velocities:

o P waves (Pressure or Primary waves): they are longitudinal waves that travel

fastest through solids, and are therefore the first waves to appear on a seismogramo S waves (shear or secondary waves): transverse waves that travel slower than P

waves. They do not exist in fluids such as air or water

o Surface waves (Rayleigh waves and Love waves):slower than P and S waves, but

have much larger amplitude. These surface waves cause most damage during an

earthquake

The propagation velocity of the seismic waves depends on density and elasticity of the

medium

In solid rock, P waves travel at about 6-7 km/s (within the mantle about 13 km/s), while S

waves travel at about 2-3 km/s (mantle 9 km/s)

Measuring and locating earthquakes
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Earthquakes can be recorded at great distances, since seismic waves travel through the

whole of the earths interior

The absolute magnitude of a quake is reported on the Moment Magnitude scale, while

perceived magnitude is reported on the Modified Mercalli (MM) scale. The Richter scale is

another scale that measures the absolute magnitude - it is no longer used in academic circles

but is still used in popular parlance. As a rule of thumb,the distance to the earthquake epicentre is the number of seconds

between the P and S waves multiplied by 8

Major earthquakes

S. No. Date Location Magnitude

1 1960 Valdivia, Chile 9.5

2 Dec 2004 Sumatra, Indonesia 9.3

3 1964 Alaska, USA 9.2

4 1952 Kamchatka, Russia 9.0

5 1700 Cascadia Subduction Zone (Pacific Ocean rim) 9.0

LAST WEEK: SEMI-VOLCANIC ACTIVITY~~~~~NEXT WEEK: OCEANS
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