On shaky ground, Himachal Pradesh records 40 quakes this year

Experts say could be precursor to bigger “seismic event”; most tremors that struck the state this year had their epicentre in Chamba and Mandi districts

Forty earthquakes have struck Himachal Pradesh so far this year, which experts say could be the precursor to a bigger “seismic event”.

A majority of the quakes that struck the state this year had their epicentre in Chamba and Mandi, the area north of the Main Boundary Thrust (MTB). While Chamba recorded 15 mild quakes, 10 had their epicentre in Mandi district. (Representative photo)

“Tremors are the result of the release of stress accumulating beneath the surface. The tectonic shift of the Himalayan plates causes them. The plates shift about 3cm annually. Smaller quakes release energy but there is no surety of a bigger one in the future,” says Puneet Guleria, the state geologist at the Himachal Pradesh industries and mining department.

A majority of the quakes that struck the state this year had their epicentre in Chamba and Mandi, the area north of the Main Boundary Thrust (MTB). While Chamba recorded 15 mild quakes, 10 had their epicentre in Mandi district.

A tremor of 4.1 magnitude rattled Mandi district at 9.32pm on November 16. Its epicentre was near Jogindernagar, 27km north-west of Mandi town, at a depth of 5km. The quake was felt in neighbouring Kullu, Hamirpur and Kangra districts. No loss of life or damage to property was reported, but people rushed out of their houses in panic.

This was the third earthquake to be felt in Himachal in November. A mild intensity quake of less than 3-magnitude struck Kangra and Chamba on November 5 and 6, respectively .

In severe seismic zone

The Himalayas by virtue of being the youngest mountain range are seismically active. The activity is closely associated with the faults and folds that trend normal or oblique to the main Himalayan trend, which leads to the under-thrusting of the blocks.

Himachal Pradesh falls in zones V and IV as per the seismic map of India. Zone V covers areas liable to seismic intensity 9 and above in the Modified Mercalli Scale and is the most severe seismic zone referred to as the very high damage risk zone. Zone IV covers areas liable to seismic intensity 8 and is second in severity.

The five districts of Chamba, Hamirpur, Kangra, Kullu and Mandi have 53% to 98% area in zone V and the remaining districts fall in zone IV.

1,300 quakes in 100 years

A study carried out on seismicity in Himachal Pradesh finds that the seismicity in the frontal region is mainly controlled by the Main Boundary Thrust (MBT) to the north of Kangra and the Himalayan Frontal Thrust (HFT) in the south, separating the Punjab plains from the Upper Shivaliks.

“After perusing the epicentre data plotted, it has been found that Himachal Pradesh has been rocked by more than 1,300 earthquakes during the last 100 years,” says SS Randhawa, principal scientific officer at the State Centre for Climate Change in Shimla. Of them, 141 quakes were of the magnitude of 3 to 3.9 on the Richter Scale, 22 quakes had a magnitude of 4 to 4.9, 43 with a magnitude of 5 to 5.9, seven earthquakes had a magnitude between 6 to 6.9 and only one had a magnitude of 8. That was the Kangra earthquake of 1905.

Two major clusters of quakes

The study reveals that there are two major clusters where the majority of the earthquakes occurred. One of them is to the north of Dharamshala along the Dhauladhar range in Kangra and Chamba districts, whereas the other is to the east of Sundernagar valley and along the right bank of the Sutlej river.

The areas to the south of the MBT up to the Punjab plains show the scattered nature of past epicentres but they follow prominent trends of lineaments along the streams originating from the Dhauladhar range.

All these streams follow a linear trend and are structurally controlled, which may have witnessed the epicentres of past quakes with a magnitudes of more than 5 on the Richter scale along the Pong Dam area in the Kangra valley.

The areas to the north of the MBT along the Dhauladhar range in Kangra and Chamba districts, and the areas to the east of the Sundernagar valley show dense and orthogonal pattern of past epicentres, which reflects that whatever stress/strain is being accumulated within the earth’s crust is being released in the form of micro earthquakes and thus chances for having a major quake in this part of the state are reduced to some extent but can’t be overruled, Guleria said.

In the areas to the south of the MBT, past epicentres are scattered in nature and some of the major earthquakes are on the transverse lineaments along the major streams originating from the Dhauldahar range. Thus, this part of the state falling within the Kangra valley, Una valley and along the Seer Khad in Hamirpur, Mandi and Bilaspur and Sirmaur districts is a seismic gap where although the major tectonic lines dominate the energy is not being released in the manner as it is on the northern part of the state. These areas can be termed as future locations for having earthquakes in the state.