Be Mains Ready

Approach

• Introduce the term Earthquake Swarms by giving a context.
• Explain Earthquake Swarms.
• Mention possible reasons for the phenomena.
• Briefly discuss traditional earthquakes.
• List how they are different from earthquakes caused by the movement of tectonic plates.
• Write about distribution of Earthquakes.

Introduction

• Recently Dahanu town in Maharashtra’s Palghar district was hit by a series of low-intensity earthquakes, called as earthquake swarm.
• Earthquake swarm is a series of low magnitude earthquakes without a distinguished main shock, that occur in a localised region and over a period of time ranging from days, weeks to even months.

Body

• Earthquake swarm has been considered to be "an exceptional phenomena" which differs from the normal earthquakes. Dominant hypotheses that explain the occurrence of the swarm include the immediate influence of magma and groundwater (ex. intrusion into the fault), peculiar inhomogeneity of crustal structure and so on.
• When seismic energy piles up inside the earth and is released in small amounts from certain points, a series of earthquakes or swarms can occur.
• Sometimes, these rumblings of the earth are also accompanied by acoustic or sound emissions.
• Seismic activities can be classified into either a major quake followed by aftershocks, or an earthquake swarm.

Traditional Earthquakes

• Traditional Earthquakes are caused by disturbances in the interior of the earth and other causes. Primary causes of earthquakes are:
  
  • Tectonic Movements: Tectonic forces create tension and pressure and when the stress tends to be more than what the rocks of the earth can bear, the rocks break and are displaced from their state of equilibrium realising huge amount of energy.
  • Volcanic Eruptions: The volcanic eruptions are often very violent and cause vibrations in the earth's crust. E.g.: The Krakatoa that erupted in 1883 became the cause of a violent earthquake.
  • Man-made: Deep mining, underground nuclear tests, reservoir induced seismicity (RIS), extraction of fossil fuels, groundwater extraction, etc. may also release massive energy to cause tremors in the earth's crust.

Difference

• In general, within major earthquakes a maximum-scale quake is followed by a number of minor aftershocks. Meanwhile, the swarm earthquake is a series of quakes, similar in scale to the mainshock.
• The pace of subsidence in earthquake swarms is gradual relative to aftershocks in normal earthquakes, and the seismic activities are protracted.

Distribution

• Earth’s major earthquakes occur mainly in belts coinciding with the margins of tectonic plates.
  
  • Circum-Pacific Belt: The most important earthquake belt is the Circum-Pacific Belt, which affects regions around the Pacific Ocean, i.e. New Zealand, New Guinea, Japan, the Aleutian Islands, Alaska, and the western coasts of North and South America.
  • Alpine Belt (Himalayas and Alps): A second belt, known as the Alpine Belt (Himalayas and Alps), mid-world mountain belt (Alpine Belt) extends parallel to the equator from Mexico across the Atlantic Ocean, the Mediterranean Sea from Alpine-Caucasus ranges to the Caspian, Himalayan mountains and the adjoining lands. This zone has folded mountains, large depressions and active volcanoes.
  • Oceanic Ridges: There also are connected belts of seismic activity, mainly along oceanic ridges, including those in the Arctic Ocean, the Atlantic Ocean, and the western Indian Ocean, and along the rift valley of East Africa.

Conclusion

• Earthquake swarms are often assumed to result from an intrusion of fluids into the seismogenic zone, causing seismicity patterns which significantly differ from aftershock sequences. However, neither the temporal evolution nor the energy release of earthquake swarms is generally well understood.
• The recent swarms in Maharashtra cannot be taken lightly as many major earthquakes in the region have been preceded by earthquake swarm activity. E.g.: The Latur and Koyna earthquakes in 1993 and 1967 killed almost 10,000 and 200 people respectively hit after series of swarms.Hence, required concern and preparation needs to be accorded to these swarms.