The Earth and its Interior
Long time ago, a large collection of material masses coalesced and formed the Earth. Large amount of heat was generated by this fusion, and slowly as the Earth cooled, the heavier and denser materials sank to the center and the lighter ones rose to the top. The differentiated Earth consists of the Inner Core , the Outer Core , the Mantle and the Crust .
Causes of Earthquakes
An Earthquake is a series of underground shock waves and movements on the earth’s surface caused by natural processes within the earth’s crust.
Plate Tectonics Movements
- The convective flows of Mantle material cause the Crust and some portion of the Mantle, to slide on the hot molten outer core.
- This sliding of Earth’s mass takes place in pieces called Tectonic Plates.
- The surface of the Earth consists of seven major tectonic plates (North American Plate, South American Plate, Antarctic Plate, African Plate, Australian Plate, Eurasian Plate and Pacific Plate) and many smaller ones.
- These plates move in different directions and at different speeds from those of the neighbouring ones.
- Sometimes, the plate in the front is slower; the plate behind it comes and collides (and mountains are formed) : Convergent Boundary
- On the other hand, sometimes two plates move away from one another (and rifts are created) : Divergent Boundary
- In another case, two plates move side-by-side, along the same direction or in opposite directions : Transform Boundary
TYPES OF INTER-PLATE BOUNDARIES
- Convergent – where crust is destroyed as one plate dives under another.
- Divergent – where new crust is generated as the plates pull away from each other.
- Transform – where crust is neither produced nor destroyed as the plates slide horizontally past each other.
Rocks are made of elastic material, and so elastic strain energy is stored in them during the deformations that occur due to the gigantic tectonic plate actions that occur in the Earth. But, the material contained in rocks is also very brittle. Thus, when the rocks along a weak region in the Earth’s Crust reach their strength, a sudden movement takes place there , opposite sides of the fault (a crack in the rocks where movement has taken place) suddenly slip release the large elastic strain energy stored in the interface rocks. For example, the energy released during the 2001 Bhuj (India) earthquake is about 400 times (or more) that released by the 1945 Atom Bomb dropped on Hiroshima!!
ELASTIC REBOUND THEORY
The sudden slip at the fault causes the earthquake, a violent shaking of the Earth when large elastic strain energy released spreads out through seismic waves that travel through the body and along the surface of the Earth. After the earthquake is over, the process of strain build-up at this modified interface between the rocks starts all over again .
Earth scientists know this as the Elastic Rebound Theory.
Earthquakes can be described by use of two distinctly different scales of measurement demonstrating magnitude and intensity.
Earthquake magnitude or amount of energy released is determined by use of a seismograph, and instrument that continuously records ground vibrations.
A scale developed by a seismologist named Charles Richter mathematically adjusts the readings for the distance of the instrument from the epicenter, known as the Richter Scale.
A second type of scale, the earthquake intensity scale, measures the effects of an earthquake where it occurs. The most widely used scale of this type was developed in 1902 by Mercalli, an Italian seismologist. The scale was extended and modified to suit modern times. Called the Modified Mercalli Scale, it expresses the intensity of earthquake effects on people.
|Zone V||Very high risk quakes of Magnitude 8 and greater|
|Zone IV||High Risk Quakes upto Magnitude 7.9
|Zone III||Moderate Risk Quakes upto Magnitude 6.9|
|Zone II||Seismic Disturbances upto Magnitude 4.9|
REGION FALLING IN VARIOUS ZONES OF THE COUNTRY
|Damage risk and Intensity||Region|
|Earthquake Very high damage risk zone||The entire North-east, including all the seven sister states, the Kutch district, parts of Himachal and Jammu & Kashmir, and the Andaman and Nicobar islands. These areas may experience|
|Earthquake High damage risk zone||Parts of the Northern belt starting from Jammu and Kashmir to Himachal Pradesh. Also including Delhi and parts of Haryana. The Koyna region of Maharashtra is also in this zone.|
|Earthquake Moderate damage risk zone||A large part of the country stretching from the North including some parts of Rajasthan to the South through the Konkan coast, and also the Eastern parts of the country.
|Earthquake Low damage risk zone||These two zones are contiguous, covering parts of Karnataka, Andhra Pradesh, Orissa, Madhya Pradesh, and Rajasthan, known as low risk earthquake zones.
- The primary hazards associated with earthquakes are fault displacement and ground shaking.
- Secondary hazards include ground failure, liquefaction, landslides and avalanches, and tsunamis and seiches.
- Fault Displacement and ground shaking
- Landslides and avalanches
- Seismic vibrations may cause settlement beneath buildings when soils consolidate or compact. Certain types of soils, such as alluvial or sandy silts are more likely to fail during an earthquake.
- Liquefaction is a type of ground failure which occurs when saturated soil loses its strength and collapses or becomes liquefied. During the 1964 earthquake in Niigata, Japan, ground beneath buildings that were earthquake resistant became liquefied, causing the buildings to lean or topple down sideways.
- Another type of ground failure that may result from earthquakes is subsidence or vertically downward earth movement caused by reduction in soil water pressure.
- Tsunami is a Japanese word meaning “harbor wave”. Tsunamis are popularly called tidal waves but they actually have nothing to do with the tides. These waves, which often affect distant shores, originate from undersea or coastal seismic activity, landslides, and volcanic eruptions.
- Physical Damage – Damage occurs to human settlements, buildings, structures and infrastructure, especially bridges, elevated roads, railways, water towers, water treatment facilities, utility lines, pipelines, electrical generating facilities and transformer stations. Aftershocks can do much damage to already weakened structures.
- Casualties – The casualty rate is often high, especially when earthquakes occur in areas:
- a) Of high population density, particularly when streets between buildings are narrow and buildings themselves are not earthquake resistant, and/ the ground is sloping and unstable;
- b) Where adobe or dry-stone construction is common with heavy upper floors and roofs.
- Public health – multiple fracture injuries and number of severely and moderately injured is the most widespread problem, breakdown in sanitary conditions pose a threat and fear of epidemic due to large deaths.
- Water supply – severe problems due to failure of the water supply and distribution network and storage reservoirs.
- Transport network – severely affected due to failure of roads and bridges, turns in railway track alignment, failure of runway.
- Electricity and Communication – all links affected. Tower collapse, transponders collapse, transformers collapse.
- Food Supplies – food distribution and marketing systems may be disrupted. Irrigation works may be damaged. In areas where earthquakes give rise to flooding or a tsunami strikes, food stocks and standing crops may be lost.
- Community preparedness
- Public Education
- Post Disaster Assistance Needs
- Earthquake Response Plan