EARTHQUAKES-A TYPE OF DIASTROPHISM
An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth’s lithosphere that creates seismic waves.
The release of energy occurs along a fault. A fault is a sharp break in the crustal rocks. Rocks along a fault tend to move in opposite directions. As the overlying rock strata press them, the friction locks them together. However, their tendency to move apart at some point of time overcomes the friction. As a result, the blocks get deformed and eventually, they slide past one another abruptly. This causes a release of energy, and the energy waves travel in all directions.
The point where the energy is released is called the focus of an earthquake, alternatively, it is called the hypocentre. The energy waves travelling in different directions reach the surface. The point on the surface, nearest to the focus, is called epicentre.
TYPES OF FAULTS ASSOSCIATED WITH EARTHQUAKES
There are three main types of fault, all of which may cause an interplate earthquake: normal, reverse (thrust) and strike-slip.
Normal and reverse faulting are examples of dip-slip, where the displacement along the fault is in the direction of dip and movement on them involves a vertical component.
Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary. Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary. Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other; transform boundaries are a particular type of strike-slip fault.
Reverse faults, particularly those along convergent plate boundaries are associated with the most powerful earthquakes, megathrust earthquakes, including almost all of those of magnitude 8 or more.
TYPES OF EARTHQUAKES
- The most common ones are the tectonic earthquakes. These are generated due to sliding of rocks along a fault plane.
- A special class of tectonic earthquake is sometimes recognised as volcanic earthquake. However, these are confined to areas of active volcanoes.
- In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquakes.
- Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes.
- The earthquakes that occur in the areas of large reservoirs are referred to as reservoir induced earthquakes.
Earthquake shaking and damage is the result of three basic types of elastic waves.
Two of the three propagate within a body of rock. The faster of these body waves is called the primary or P wave. Its motion is the same as that of a sound wave in that, as it spreads out, it alternately pushes (compresses) and pulls (dilates) the rock. These P waves are able to travel through both solid rock, such as granite mountains, and liquid material, such as volcanic magma or the water of the oceans.
The slower wave through the body of rock is called the secondary or S wave. As an S wave propagates, it shears the rock sideways at right angles to the direction of travel. If a liquid is sheared sideways or twisted, it will not spring back, hence S waves cannot propagate in the liquid parts of the earth, such as oceans and lakes.
The third general type of earthquake wave is called a surface wave, reason being is that its motion is restricted to near the ground surface. Such waves correspond to ripples of water that travel across a lake.
Surface waves in earthquakes can be divided into two types.
The first is called a Love wave. Its motion is essentially that of S waves that have no vertical displacement; it moves the ground from side to side in a horizontal plane but at right angles to the direction of propagation. The horizontal shaking of Love waves is particuly damaging to the foundations of structures
The second type of surface wave is known as a Rayleigh wave. Like rolling ocean waves, Rayleigh waves wave move both vertically and horizontally in a vertical plane pointed in the direction in which the waves are travelling.
The earthquake events are scaled either according to the magnitude or intensity of the shock.
The magnitude scale is known as the Richter scale. The magnitude relates to the energy released during the quake. The magnitude is expressed in absolute numbers, 0-10.
The intensity scale is named after Mercalli, an Italian seismologist. The intensity scale takes into account the visible damage caused by the event. The range of intensity scale is from 1-12.
It’s important not to confuse an earthquake’s magnitude with its intensity. They are different measures. And while the Richter scale is widely quoted, the modern magnitude measurement uses a different scale. Magnitude is a quantitative measure of the size of an earthquake, it says, while intensity is a qualitative measure of the shaking at a given location.
Two scales are commonly used for intensity, the Modified Mercalli Intensity scale and the MSK scale, both of which classify earthquakes from I (least perceptible) to XII (most severe). These readings are based on factors such as how people perceive the shaking. The same earthquake will have different intensity readings at different places; the farther one moves away from the epicentre, the less intense the shaking
EARTHQUAKE ZONES IN INDIA
Each zone indicates the effects of an earthquake at a particular place based on the observations of the affected areas and can also be described using a descriptive scale like Modified Mercalli intensity scale or the Medvedev–Sponheuer–Karnik scale.
Zone 5 covers the areas with the highest risks zone that suffers earthquakes of intensity MSK IX or greater. The IS code assigns zone factor of 0.36 for Zone 5. Structural designers use this factor for earthquake resistant design of structures in Zone 5. The zone factor of 0.36 is indicative of effective (zero period) level earthquake in this zone. It is referred to as the Very High Damage Risk Zone. The region of Kashmir, the Western and Central Himalayas, North and Middle Bihar, the North-East Indian region, the Rann of Kutch and the Andaman and Nicobar group of islands fall in this zone.
Generally, the areas having trap rock or basaltic rock are prone to earthquakes.
This zone is called the High Damage Risk Zone and covers areas liable to MSK VIII. The IS code assigns zone factor of 0.24 for Zone 4 Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Sikkim, the parts of Indo-Gangetic plains (North Punjab, Chandigarh, Western Uttar Pradesh, Terai, North Bengal, Sundarbans) and the capital of the country Delhi fall in Zone 4. In Maharashtra, the Patan area (Koynanagar) is also in zone no-4. In Bihar the northern part of the state like Raxaul, Near the border of India and Nepal, is also in zone no-4.
This zone is classified as Moderate Damage Risk Zone which is liable to MSK VII. and also 7.8 The IS code assigns zone factor of 0.16 for Zone 3.
This region is liable to MSK VI or less and is classified as the Low Damage Risk Zone. The IS code assigns zone factor of 0.10 (maximum horizontal acceleration that can be experienced by a structure in this zone is 10% of gravitational acceleration) for Zone 2.
Since the current division of India into earthquake hazard zones does not use Zone 1, no area of India is classed as Zone 1.
DISASTER MANAGEMENT ACT, 2005
The Act lays down institutional, legal, financial and coordination mechanisms at the national, state, district and local levels. These institutions are not parallel structures and will work in close harmony. The new institutional framework is expected to usher in a paradigm shift in DM from relief-centric approach to a proactive regime that lays greater emphasis on preparedness, prevention and mitigation.
Institutional Framework under the DM Act
National Disaster Management Authority (NDMA)
The NDMA, as the apex body for disaster management, is headed by the Prime Minister and has the responsibility for laying down policies, plans and guidelines for DM (and coordinating their enforcement and implementation for ensuring timely and effective response to disasters) .
The guidelines will assist the Central Ministries, Departments and States to formulate their respective DM plans. It will approve the National Disaster Management and DM plans of the Central Ministries/Departments. It will take such other measures as it may consider necessary, for the prevention of disasters, or mitigation, or preparedness and capacity building, for dealing with a threatening disaster situation or disaster. The general superintendence, direction and control of National Disaster Response Force (NDRF) are vested in and will be exercised by the NDMA. The National Institute of Disaster Management (NIDM) works within the framework of broad policies and guidelines laid down by NDMA
At the State level, the SDMA, headed by the Chief Minister, will lay down policies and plans for DM in the State. It will, inter alia approve the State Plan in accordance with the guidelines laid down by the NDMA.
The DDMA will be headed by the District Collector, Deputy Commissioner or District Magistrate as the case may be, with the elected representative of the local authority as the Co-Chairperson.
National Disaster Response Force (NDRF)
For the purpose of specialised response to a threatening disaster situation or disasters/emergencies both natural and man-made such as those of Chemical, Biological, Radiological and Nuclear origin, the Act has mandated the constitution of a National Disaster Response Force (NDRF). The general superintendence, direction and control of this force shall be vested in and exercised by the NDMA.
THE NATIONAL CENTER FOR SEISMOLOGY
IT is an office of the Indian Ministry of Earth Sciences. The office provides earthquake surveillance and hazard reports to governmental agencies. It includes three divisions:
Earthquake Monitoring & Services,
Earthquake Hazard & Risk Assessment,
Geophysical Observation System
National Centre for Seismology launches ‘India Quake’ – An App for Earthquake Parameter Dissemination
National Centre for Seismology (NCS) operates national seismological network with 84 stations. These stations are connected to NCS headquarter through VSAT for real time data communication. In the event of an earthquake NCS locates them using data from its network and disseminate earthquake parameters to all the concerned government department and other stake holders through SMS, email and fax. However this causes some delay in dissemination and also restricts the number of recipients.
To overcome this, a Mobile App has been developed by the NCS for automatic dissemination of earthquake parameter (location, time and magnitude) after the occurrence of earthquakes. The App will make information dissemination faster with no restrictions on the number of recipients. Any citizen can download this App and get the real time earthquake location information on his/her mobile.
Other than scientific and administrative benefits of the App, it will help in reducing panic amongst people during an earthquake. For example, if an earthquake occurs in Hindukush region, Afghanistan and is strongly felt in Delhi, then people in Delhi will know in less than 2 minutes that the earthquake has actually occurred in Afghanistan and not in Delhi.
RELEVANT CHAPTER IN NCERT-