Maharashtra has recorded a sharp cluster of small earthquakes over the past fortnight, with Nashik and Palghar witnessing repeated tremors in a pattern experts describe as an “earthquake swarm”. On Friday alone, five tremors were recorded in and around Nashik, adding to growing concern over the unusual occurence of seismic activity.

Data reviewed by shows that at least 29 tremors were recorded across Maharashtra between July 30 and August 14. Of these, 24 were in and around Nashik, four in Palghar and one in Nagpur. The tremors ranged from magnitude 2 to 4.3 and occurred at depths of between 5 km and 8 km.

The largest tremor during this period was a magnitude 4.3 earthquake recorded in Nashik on August 8 at a depth of 5 km.

The activity has been particularly pronounced on some days. On August 6, Nashik recorded six tremors between 5.41 am and 5.54 pm. Four more tremors were recorded on August 8, while another four were reported on August 10.

“There is indeed an alarming rise in the number of small earthquakes. However, this is not uncommon,” Professor Deepankar Choudhury, an earthquake specialist and civil engineering professor at IIT Bombay, told .

What is an earthquake swarm?

An earthquake swarm refers to a series of small earthquakes occurring in a localised area over a relatively short period. Unlike a conventional earthquake, there may be no dominant quake followed by aftershocks and tremors felt during an earthquake swarm typically remain shallow with its depth ranging between 3.5 km to 5 km under the surface. Such activity can last for days, weeks or even months. Scientists say the current activity in Nashik and Palghar is consistent with this pattern.

Choudhury, earthquake specialist who also serves as chairman of Maharashtra’s technical advisory committee on disaster resilience and capacity building and a member of the State Disaster Management Authority, said earthquake swarms are typically reported during and post monsoon season and are, in Deccan Plateau region, a result of ‘hydro-seismicity’.

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“Nashik falls under the larger Deccan Plateau region which is home to mostly basaltic rock. Fractures or cracks can exist in such basaltic rock at any time. As monsoon commences, these pores and cracks in the rock get filled with water. With time, this generates pore pressure development. This means that pressure gets developed due to the water ingress inside the cracks of the rocks. When it builds up to high pressure, it releases its energy in the form of small tremors,” explained Choudhury.

The recent activity is not without precedent. Palghar witnessed a similar series of earthquakes in September 2023, while another swarm was reported in the region in February 2019.

Nashik and Palghar fall under seismic Zone III under the Indian Standard IS 1893, Part 1, indicating a moderate earthquake risk. Choudhury said earthquakes in the region could broadly be linked either to movement of tectonic plates or to faults within the earth’s crust. The Deccan Plateau, however, is not located on a major plate boundary and is considered relatively stable.

How much of a risk is it for cities like Mumbai?

Experts said the current tremors should be monitored but do not necessarily indicate that a major earthquake is imminent.

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“Owing to its shallow depth, earthquake swarms do not travel very far as it attenuates faster. This means that the impact of Nashik earthquakes will not travel large distances up to Mumbai,” Choudhury said.

However, the repeated tremors underline the need for buildings and infrastructure in vulnerable areas to comply with earthquake-resistant construction standards. Experts said adherence to the provisions of IS 1893 would help buildings and infrastructure withstand earthquakes of moderate intensity.