EARTHQUAKE

EARTHQUAKE

An earthquake is the shaking of the earth. It is a natural event. It is caused due to the release of energy, which generates waves that travel in all directions.

Types of earthquakes

• Tectonic earthquakes: The most common ones are the tectonic earthquakes. These are generated due to the sliding of rocks along a fault plane.
• Volcanic Earthquakes: A special class of tectonic earthquake is sometimes recognized 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.

Causes of earthquakes 

1. Fault Zones

• Immediate cause: Most shallow earthquakes are due to the sudden release of stress along a fault rupture (crack) in the earth’s crust.
  • Reason: Sudden slipping of rock formations along fault rupture happens due to the constant change in volume and density of rocks due to intense temperature and pressure in the earth’s interior.
• Magnitude: The longer the length and the wider the width of the faulted area, the larger the resulting magnitude.
• Spatial variation:
  • Convergent boundary: The longest earthquake ruptures along thrust faults (convergent boundary) are approximately 1,000 km.
  • Transform fault: The longest earthquake ruptures on strike-slip faults (transform fault) are about half to one-third as long as the lengths along the thrust fault.
  • Divergent boundary: The fault ruptures along normal faults (divergent boundary) are shorter.

2. Plate tectonics’ Explanation of Earthquakes: Earthquakes are a consequence of inter-plate interaction.

• Due to force: Along the convergent boundary, compressive forces crush and cause earthquakes, while along the divergent boundary, tensile forces stretch and snap the earth, creating earthquakes.
• Due to motion: Earthquakes can also be due to plate motions and the friction of lithospheric plates while they drift along the asthenosphere.
• Location of focal point: Divergent boundaries and convergent boundaries between two continental plates will only have shallow focus earthquakes. Intermediate and deep focus earthquakes are almost entirely limited to ocean floor subduction.

3. Volcanic activity: Volcanic activity also can cause an earthquake.

• Less severe: The earthquakes of volcanic origin are generally less severe and more limited in extent than those caused by fracturing of the earth’s crust.
• Release of energy: Earthquakes in volcanic regions are caused by the consequent release of elastic strain energy both by tectonic faults and the movement of magma in volcanoes.
  • Such earthquakes can serve as an early warning of volcanic eruptions, as during the 1980 eruption of Mount St. Helens.
• Location: There is a clear correspondence between the geographic distribution of volcanoes and major earthquakes, particularly in the Circum-Pacific Belt and along oceanic ridges.

4. Human Induced Earthquakes: It refers to typically minor earthquakes and tremors that are caused by human activity like mining, large-scale petroleum extraction, artificial lakes (reservoirs), nuclear tests, etc.

5. Reservoir-induced seismicity

• Pressure of water column: The pressure offered by a column of water in a large and deep artificial lake alters stresses along an existing fault or fracture. Also, the percolation of water weakens the soil structure and lubricates the faults.
• Activities in the area: Loading and unloading of water can significantly change the stress. This significant change in stress can lead to a sudden movement along the fault or fracture.
  • For Example: The 6.3 magnitude 1967 Koynanagar earthquake occurred near the Koyna Dam reservoir in Maharashtra and claimed more than 150 lives. There have been several earthquakes of smaller magnitude since then.
  • For Example: The 2008 Sichuan earthquake, which caused approximately 68,000 deaths, is another possible example. It is believed that the construction and filling of the Zipingpu Dam may have triggered the earthquake.

EARTHQUAKE SWARMS

Earthquake swarms are generally defined as a sequence of events closely clustered in time and space without a single outstanding shock. There are a series of low-magnitude earthquakes that occur in a localized region over a period of time ranging from days, weeks, to even months.

Features

• Differentiated from earthquakes: They are differentiated from earthquakes succeeded by a series of aftershocks.
• Piling up of energy: When seismic energy piles up inside the Earth and is released in small amounts from certain points, such a series of earthquakes can occur.
• An early warning: Sometimes, the rumblings of the Earth are also accompanied by acoustic or sound emissions. The situation cannot be taken lightly as many earthquakes have been preceded by earthquake swarm activity.

Reasons why earthquake swarms take place

• Observation: Swarms are observed in volcanic environments, hydrothermal systems, and other active geothermal areas, according to geophysicists.
• Regions in India: In India, sequences of low-intensity quakes are common in areas that have seen it previously, like Saurashtra in Gujarat and Koyna in Maharashtra, but they are also seen in areas without a history of seismic activity.
  • Reasons: They are the result of seismic activity, hydro-seismicity due to water percolation post-monsoon, or magmatic activity in the region.
    • Ex: In the Rampur area of Himachal Pradesh, the earthquake swarm was later attributed to low strength of the earth’s crust in the area which could not hold the tectonic energy, etc.

DISTRIBUTION

1. Circum-Pacific belt

• Area Covered: It includes Western side of North and South America and countries like Indonesia, Philippines, Japan, etc.
• Magnitude: This belt registers about 80% of earthquakes.
• Reason: The earthquake proneness is because of subduction boundaries, both Continental-Oceanic and Oceanic-Oceanic.
• Types: This belt experiences all 3 types of earthquakes, i.e., shallow, intermediate, and deep.

2. Trans-Eurasian mountain belt

• Area covered: This belt extends from the Mediterranean Sea through Southwest Asia and the Himalayas into the South East Asia, where it meets the Circum-Pacific belt.
• Magnitude: This belt registers about 15% of earthquakes of the world.
• Types: This belt experiences Shallow and intermediate earthquakes.
• Reason: Earthquake proneness is because of convergent boundary interaction—Continental-Continental and Continental-Oceanic.

3. Mid-oceanic ridges & East Africa rift Valley system

• Area Covered: The third zone of earthquake is associated with the Global system of mid-oceanic ridges with an extension to the East African rift Valley system.
• Types: This belt registers only shallow earthquakes.
• Reason: Ridges are prone to earthquakes because of transform boundary interaction along the section where these regions have got sliced off.

4. Intra plate earthquakes

• Areas: These are those earthquakes which occur in the interior of the plate away from the plate margin, like for example the earthquake in Gujarat in India.
• Reason: Intraplate earthquakes are because of the reactivation of patient faults in the body of the plates. The activation is because of the stress induced in the plate as a result of boundary interaction.

DISTRIBUTION OF EARTHQUAKE ZONE IN INDIA

1. Affected rate: About 59% of the land area in India is vulnerable to earthquakes.
2. Vulnerable Areas: Some of the most vulnerable states are Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Sikkim, and the Darjeeling 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.
3. Seismic zones: BIS, 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.

• Zone V: Very High Risk Zone, Area liable to shaking Intensity IX (and above).
• Zone IV: High Risk Zone, Intensity VIII.
• Zone III: Moderate Risk Zone, Intensity VII.
• Zone II: Low Risk Zone, Intensity VI (and lower).
• About 59 percent of the land area of India is liable to seismic hazard damage.

EFFECTS OF THE EARTHQUAKES

1. Socio-Economic effect

• 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.
  • For Example: More than 25,000 people died in the Gujarat earthquake of 2001.
• Damage to property and infrastructure: Underground pipelines and railway lines are damaged or broken.
• Loss of biodiversity: Earthquakes also cause the death of wildlife and result in a destruction of their habitat.

2. Geographic effect

• Changes in river courses: Sometimes river channels are blocked or their courses are changed due to the impact of the earthquake.
  • For example: They can cause submergence and emergence of landforms along coastal regions, for example, the Coastline of Kutch.
• 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).

3. Disaster effect

• Floods: It may result as an indirect consequence of an earthquake due to dam or levee failure.

• Tsunamis: Tsunamis are extremely high sea waves caused by an earthquake. It wreaks havoc on the settlement of coastal areas. Tsunamis are waves generated by the tremors and not an earthquake in itself.
• 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.