Sunday, April 26, 2015

know about Earthquakes

  • An earthquake is a result of a sudden release of energy in the earth’s 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 earthquake’s energy is used to power fracture growth or is converted as heat generated by friction

Occurrence of earthquakes

  • 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 world’s 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 earth’s tectonic plates, human activity can produce earthquakes as well
  • Four main human activities that contribute to earthquakes include
    • Large dams
    • Drilling and injecting liquids into wells
    • Coal mining
    • 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 the movement 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:

    • 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 seismogram
    • 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

    • 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)
  • Earthquakes can be recorded at great distances, since seismic waves travel through the whole of the earth’s 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

What is an earthquake and what causes them to happen?
An earthquake is caused by a sudden slip on a fault. Stresses in the earth's outer layer push the sides of the fault together. Stress builds up and the rocks slips suddenly, releasing energy in waves that travel through the earth's crust and cause the shaking that we feel during an earthquake. An earthquake occurs when plates grind and scrape against each other.

What is a fault?

Faults are fractures in Earth's crust where rocks on either side of the crack have slid past each other. Sometimes the cracks are tiny, as thin as hair, with barely noticeable movement between the rock layers. But faults can also be hundreds of miles long.
At what depth do earthquakes occur?
Earthquakes occur in the crust or upper mantle, which ranges from the earth's surface to about 800 kilometers deep (about 500 miles).
What is "surface rupture" in an earthquake?
Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. Not all earthquakes result in surface rupture.
How are earthquakes measured?
The familiar Richter scale (which is not a physical device but rather a mathematical formula) is no longer widely used by scientists to report an earthquake's size. Today, an earthquake's size is typically reported simply by its magnitude, which is a measure of the size of the earthquake's source, where the ground began shaking. While there are many modern scales used to calculate the magnitude, the most common is the moment magnitude, which allows for more precise measurements of large earthquakes than the Richter scale. A network of geological monitoring stations, each with instruments that measure how much the ground shakes over time called seismographs allow scientists to calculate an earthquake's time, location and magnitude. Seismographs record a zigzag trace that shows how the ground shakes beneath the instrument. Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world.
How are quakes classified?
Based on their magnitude, quakes are assigned to a class. An increase in one number, say from 5.5 to 6.5, means that a quake's magnitude is 10 times as great. The classes are as follows:

Great: Magnitude is greater than or equal to 8.0. A magnitude-8.0 earthquake is capable of tremendous damage.

Major: Magnitude in the rage of 7.0 to 7.9. A magnitude-7.0 earthquake is a major earthquake that is capable of widespread, heavy damage.

Strong: Magnitude in the rage of 6.0 to 6.9. A magnitude-6.0 quake can cause severe damage.

Moderate: Magnitude in the rage of 5.0 to 5.9. A magnitude-5.0 quake can cause considerable damage.

Light: Magnitude in the rage of 4.0 to 4.9. A magnitude-4.0 quake is capable of moderate damage.

Minor: Magnitude in the rage of 3.0 to 3.9.

Micro: Magnitude less than-3.0. Quakes between 2.5 and 3.0 are the smallest generally felt by people.

After an earthquake strikes, its magnitude is continuously revised as time passes and more stations report their seismic readings. Several days can pass before a final number is agreed upon.
What are the seismic zones of India?
The Geological Survey of India (GSI.) first published the seismic zoning map of the country in the year 1935. With numerous modifications made afterwards, this map was initially based on the amount of damage suffered by the different regions of India because of earthquakes. Color coded in different shades of the color red, this map shows the four distinct seismic zones of India. Following are the varied seismic zones of the nation, which are prominently shown in the map:

Zone - II: This is said to be the least active seismic zone

Zone - III: It is included in the moderate seismic zone

Zone - IV: This is considered to be the high seismic zone

Zone - V: It is the highest seismic zone

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