Earthquake: Types and Mechanism
An Earthquake is a tectonic movement caused by endogenetic (originating within the earth) thermal conditions inside earth's interior which is transmitted through the surface layer of the earth.
What is an Earthquake?
It is shaking or trembling of the earth surface causing energy to release suddenly. An earthquake can range from a faint tremor to a wild motion capable of shaking building apart. Minor tremors caused by gentle waves of vibration within the earth s crust occur every few minutes. Major earthquakes, usually caused by movement along faults, can be very disastrous particularly in a densely populated area.
Earthquakes themselves may cause only restricted damage in the regions of occurrence, but their aftershocks can be very catastrophic.An aftershock is an earthquake of the smaller magnitude that occurs after the main shock. They are also known to cause Tsunami waves.
Nearly 54% of land area in India is prone to earthquakes. Earthquakes are by far the most unpredictable and highly destructive of all the natural disasters. It not only damages and destroys the settlements, infrastructure etc. but also result in loss of lives of men and animals.
Focus and epicentre of an earthquake
Focus - The point within the earth s crust where an earthquake originates is called the focus. It is also referred as seismic focus or hypocenter. It generally lies within the depth of 60 kilometres in the earth crust.
Epicentre - The point vertically above the focus on the earth s surface is known as epicenter . Earthquake travel in the form of the longitudinal wave from the focus to epicentre. The intensity is the highest at the epicentre. That is why the maximum destruction occurs at and around the epicentre. The intensity of vibrations decreases as one moves away from the epicentre.
Causes of earthquakes
Folding, faulting and displacement of rock strata, upwarping and downwarping of crust are some of the main causes of earthquakes. Some examples of this type of earthquakes are the San Francisco earthquakes of California in 1906, the Assam earthquakes of 1951, the Bihar earthquakes of 1935.
The violent volcanic eruptions put even the solid rocks under great stress. It causes vibrations in the earth s crust. But, these earthquakes are limited to the areas of volcanic activity such as the circum-pacific ring of fire and the Mid-Atlantic Ridge. An important example of this type of earthquake includes the earthquake preceding the eruption of Mauna Loa volcano of Hawaii Island in 1868.
Forces within the earth
Gaseous expansion and contraction within the earth can cause stress in strata which build up over time and is released suddenly in the form of earthquakes.
An earthquake may also be caused due to landslide and collapse of cave or mines etc. which can cause a sudden release in energy.
These may range from dam building, mining, dredging, road building, drilling etc.
Types of earthquakes
- Tectonic earthquakes - The most common ones are the tectonic earthquakes. These are generated due to sliding of rocks along a fault plane.
- Volcanic Earthquakes - A special class of tectonic earthquake is sometimes recognised as a volcanic earthquake. They are generated due to violent volcanic eruptions. However, these are confined to areas of active volcanoes.
- Collapse Earthquakes - In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquakes.
- Explosion Earthquakes - Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes.
- Reservoir-induced Earthquakes - The earthquakes that occur in the areas of large reservoirs are referred to as reservoir-induced earthquakes.
On the basis of focus, earthquakes can be classified as a shallow focus ( focus located at depth of up to 50 km), intermediate focus ( 50 - 250 km depth) and deep focus earthquakes (foci depth of up to 700 km).
Mechanism of tectonic earthquake
- Tectonic earthquakes are the most common earthquakes occurring on earth surface.
- They occur due to movement of tectonic plates past each other which builds up stress along the fault line.
- A fault is a sharp break in the crustal rocks.
- Rocks along a fault tend to move in opposite directions.
- The friction between the plates locks them together due to which they cannot glide past each other.
- However, at some point of time, their tendency to move apart overcomes the friction and they slide past one another abruptly.
- This causes a sudden release of energy along the fault, and the energy waves radiate in all directions.
Earthquake waves are energy waves, generated by the sudden release of energy from within the earth surface during an earthquake. The velocity of waves changes as they travel through materials with different densities. The denser the material, the higher is the velocity. Their direction also changes as they reflect or refract when coming across materials with different densities.
Earthquake waves are basically of two types body waves and surface waves.
- Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Hence, the name body waves.
- Travelling through the interior of the earth, body waves arrive before the surface waves emitted by an earthquake.
- These waves are of a higher frequency than surface waves.
- The body waves interact with the surface rocks and generate a new set of waves called surface waves.
- These waves move along the surface.
- The surface waves are the last to report on a seismograph.
- Surface waves are considered to be the most destructive waves.
- They cause displacement of rocks, and hence, the collapse of structures occurs.
Types of Body waves
There are two types of body waves. They are called P and S-waves.
- These are also called primary waves .
- P-waves move faster and are the first to arrive at the surface.
- The P-waves are similar to sound waves.
- They travel through gaseous, liquid and solid materials.
- P-waves vibrate parallel to the direction of the wave.
- This exerts pressure on the material in the direction of the propagation.
- Hence, it creates density differences in the material leading to stretching and squeezing of the material.
- These are called secondary waves.
- S-waves arrive at the surface with some time lag.
- They can travel only through a solid medium.
- S-waves vibrate perpendicular to the direction of the wave in the vertical plane.
- Hence, they create troughs and crests in the material through which they pass.
Shadow Zone of earthquakes
- When an earthquake occurs, earthquake waves radiate out spherically from the earthquake's focus. Earthquake waves get recorded in seismographs located at far off locations.
- A shadow zone is an area of the Earth's surface where seismographs do not detect any earthquake waves.
- For each earthquake, there exists an altogether different shadow zone.
- As P waves are refracted by the liquid outer core, the shadow zone of P-waves appears as a band around the earth between 103 and 142 away from the epicentre.
- S waves cannot pass through the liquid outer core and are not detected beyond 103 . Thus, the entire zone beyond 103 is referred as Shadow zone of S-waves
- The shadow zone of S-wave is much larger than that of the P-waves.
- It is also a little over 40 percent of the earth surface.
Measurement of earthquakes
The earthquake events are scaled either according to the magnitude or intensity of the shock.
- Richter scale - 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.
- Mercalli scale - 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.
Earthquake-prone zones in India
Over 55% of the land area in India is vulnerable to earthquakes. Some of the most vulnerable states are Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Sikkim, and the Darjiling and subdivision of West Bengal and all the seven states of the northeast.
Apart from these regions, the central-western parts of India, particularly Gujarat and Maharashtra have also experienced some severe earthquakes.
Bureau of Indian Standards, based on the past seismic history, grouped the country into four seismic zones, viz. Zone II, III, IV and V. Of these, Zone V is the most seismically active region, while zone II is the least. The current division of India into earthquake-prone zones does not use Zone I.
The Modified Mercalli (MM) intensity, which measures the impact of the earthquakes on the surface of the earth, broadly associated with various zones, is as follows:
|Seismic Zone||Description||Intensity on MM scale|
|Zone II||Low intensity zone||VI (or less)|
|Zone III||Moderate intensity zone||VII|
|Zone IV||Severe intensity zone||VIII|
|Zone V||Very severe intensity zone||IX (and above)|
Areas covered under different Seismic Zones
Zone - V covers 10.79% area of the country. It comprises entire northeastern India, parts of Jammu and Kashmir, Himachal Pradesh, Uttaranchal, Rann of Kutch in Gujarat, part of North Bihar and Andaman & Nicobar Islands.
Zone - IV covers 17.49% area of the country. It comprises remaining parts of Jammu and Kashmir and Himachal Pradesh, National Capital Territory (NCT) of Delhi, Sikkim, Northern Parts of Uttar Pradesh, Bihar and West Bengal, parts of Gujarat and small portions of Maharashtra near the west coast and Rajasthan.
Zone III covers 30.79% area of the country. It comprises Kerala, Goa, Lakshadweep islands, remaining parts of Uttar Pradesh, Gujarat and West Bengal, Parts of Punjab, Rajasthan, Madhya Pradesh, Bihar, Jharkhand, Chhattisgarh, Maharashtra, Orissa, Andhra Pradesh, Tamil Nadu and Karnataka.
Zone - II covers 40.93% area of the country. It comprises major parts of peninsular region. Karnataka Plateau falls in this zone.
Distribution of earthquakes in the world
The occurrence of an earthquake is a phenomenon of almost every part of the world. But, there are two well-defined belts where they occur more frequently. These belts are the Circum-Pacific belt and the Mid-world mountain belt.
The Circum Pacific Belt comprises the western coast of North and South America; the Aleutian Islands and island groups along the eastern coasts of Asia such as Japan and Philippines. As it encircles the Pacific Ocean from end to end, it is named as such. The earthquakes in this belt are associated with the ring of mountains and volcanoes. It is estimated that about 68 % of earthquakes of the world occur in this belt alone.
The Mid-world Mountain Belt extends from the Alps with their extension into the Mediterranean the Caucasus and the Himalayan region and continues into Indonesia. About 21 % of total earthquakes of the world originate in this belt. Remaining 11 % occur in the other parts of the world.
Consequences or effects of the earthquakes
Damage to property: when an earthquake occurs, all buildings from cottage to palaces and stronger skyscrapers are greatly damaged or totally destroyed. Earthquakes also cause a great deal of infrastructural damage. Underground pipelines and railway lines are damaged or broken. Dams on river collapse, resultant floods cause havoc.
Loss of lives: Duration of tremors of an earthquake is normally of only a few seconds, but thousands of people may die in this short period. More than 25,000 people died in Gujarat earthquake of 2001. Earthquakes also cause the death of wildlife and result in a destruction of their habitat.
Floods: Flood may result as an indirect consequence of an earthquake due to dam or levee failure.
Changes in river courses: Sometimes river channels are blocked or their courses are changed due to the impact of the earthquake.
Tsunamis: Tsunamis are extremely high sea wave caused by an earthquake. It wreaks havoc on settlement of coastal areas. It sinks large ships. The effect of a tsunami would occur only if the epicentre of the tremor is below oceanic waters and the magnitude is sufficiently high. Tsunamis are waves generated by the tremors and not an earthquake in itself.
Soil liquefaction: Soil liquefaction occurs when, because of the shaking, water-saturated granular material (such as sand) temporarily loses its strength and transforms from a solid to a liquid. Soil liquefaction may cause rigid structures, like buildings and bridges, to tilt or sink into the liquefied deposits.
Cracks and fissures: Sometimes cracks and fissures develop in roads railway tracks, and fields, making them useless. The well known San Andreas Fault formed during the earthquake of San Francisco (California).
Landslides and Avalanches: landslides and avalanches may be triggered due to an earthquake.
Fires: Earthquakes can cause fires by damaging electrical power or gas lines. it may also become difficult to stop the spread of a fire once it has started.
Prediction of earthquake
Earthquake can occur at any time of the year, day or night. Its impact is very sudden. There are no warning signs of earthquakes. Extensive and sincere research has been conducted in the forecast or prediction of an earthquake.
For the first time in India, a system to detect earthquakes and disseminate warnings was installed in Uttarakhand, in 2015. It issues warnings 1-40 seconds before earthquakes of magnitude 5 occur. All sensors under this system that warn of earthquakes are based on the detection of P and S waves generated during an earthquake. The P wave, which is harmless and travels faster than the S wave, is detected by the sensors for advance warning.
IIT Roorkee is conducting research to develop first of its kind sensors to be deployed in all seismic prone major cities in North India.