Earthquake Swarm in Palghar, Maharashtra

Why in news?

The Palghar district in northern Maharastra has been witnessing an unusual frequency of earthquakes since November, 2018.

What happened recently?

Dahanu town in Maharashtra’s Palghar district has been hit by some 30 low-intensity earthquakes since November 2018.
Palghar was rocked by a series of minor earthquakes (6 tremors) on a single day recently.
The magnitudes of the quakes ranged between 3 and 4.1 on the Richter scale.
Many aftershocks of lesser magnitude have also been observed in the area.
Data collected so far point to an “earthquake swarm”.

What is an “earthquake swarm”?

It is a series of many (sometimes thousands) low magnitude earthquakes without a discernible main shock.
They occur in a localised region and over a period of time ranging from days, weeks to even months, without a clear sequence of foreshocks, main quakes and aftershocks.
When seismic energy piles up inside the Earth and is released in small amounts from certain points, such a series of earthquakes can occur.
Sometimes, these are also accompanied by acoustic or sound emissions.

What is the case with the Deccan region?

In India, sequences of low-intensity quakes are common in areas that have been hit previously. E.g. Saurashtra in Gujarat, Koyna in Maharashtra
But they are also seen in areas without a history of seismic activity and, swarms are normal in peninsular India.
Notably, no mining activity has been found and there are no small reservoirs too to explain the cause for the quakes.
Deccan Plateau is not an earthquake-prone zone because of its hard rock crust, as seismic waves travel faster in hard rocks which helps the tremors dissipate faster.
But there is also loose soil which makes the waves stay longer, release more energy and cause more damage.
Earthquake swarms are not limited to the Peninsula. In 2016, a series of 58 earthquakes were recorded in the Rampur area of Himachal Pradesh.
This Himalayan swarm was later attributed to low strength of the earth’s crust in the area which could not hold the tectonic energy.

What is the need for caution?

Many major earthquakes in the region have been preceded by earthquake swarm activity as foreshocks.
E.g. Latur and Koyna earthquakes in 1993 and 1967 respectively

It is not certain yet if the current quakes had been caused by seismic activity, hydro-seismicity due to water percolation post-monsoon, or magmatic activity in the region.
So without more data, it would be premature to draw a definite conclusion.
Also, identifying the quakes as a swarm would suggest there is little threat of a deadlier one hitting in the near future.
The possibility of either the quakes now subsiding or of a big one coming is not ruled out yet.
Significantly, kachcha homes, about a third of the buildings in areas hit by the quakes, would be especially vulnerable in a large quake.

So a more closer and scientific look into the causes of the earthquakes is essential now.

The district authorities must draw up a community disaster management plan, with Standard Operating Procedures.

Source: Indian Express, Down to Earth