Nashik-Palghar Tremors: Cause For Alarm

Heavy rain causes water to percolate deep into the rock, and where there are fractures, that water fills the pores and builds up what we call pore water pressure.

That pressure can widen existing cracks, and eventually the rock gives way -- which surfaces as tremors.'

Nashik and Palghar are experiencing an earthquake swarm, with similar episodes recorded in Palghar in 2019 and 2023.

Hydro-seismicity could be the probable trigger: Monsoon rainwater can seep deep into fractured basalt, raise pore water pressure and widen existing cracks, causing tremors.

The region is in Seismic Zone 3, where moderate earthquake activity is expected; Mumbai, Nashik and Palghar all fall under this zone.

The current tremors, reaching around magnitude 4.3-4.8, are below the broad design range of magnitude 6-6.5 that structures in Zone 3 are expected to account for.

The immediate risk is less about the swarm itself and more about buildings that have not been designed according to India's seismic safety code, IS 1893.

 

Nashik and Palghar have been shaking, quite literally, for over a fortnight now. Since the last week of July, residents across Dindori, Peth, Surgana and the neighbouring talukas have counted close to thirty tremors, the sharpest touching magnitude 4.3 near Ozar on August 8.

Palghar's Jawhar and Vikramgad -- beautiful hill stations -- belts have had their own share too, with some nights bringing two jolts within the hour.

None has caused serious damage so far, though a handful of homes have reported hairline cracks, and the state government has since ordered a fresh seismic risk assessment for the two districts, with Chief Minister Devendra Fadnavis asking additional institutions to join a study that was already under way.

It isn't the first time these talukas have rattled. Palghar went through similar spells in 2019 and again in 2023, and on each occasion scientists reached for much the same explanation: An earthquake swarm, driven less by tectonic plates grinding against one another and more by monsoon rainwater working its way into cracked basaltic rock.

To make sense of what is happening beneath the Deccan Plateau, and whether Mumbai's high-rises have anything to worry about, we spoke to Professor Deepankar Choudhury of IIT Bombay, a geotechnical earthquake engineer who also sits on the BrihanMumbai Municipal Corporation's high-rise building committee as an expert member and the chairman of the technical advisory committee on disaster resilience and capacity building, being a member of the State Disaster Management Authority, government of Maharashtra.

'The Deccan Plateau is sits on relatively stable, hard basaltic rock. But 'relatively stable' doesn't mean earthquake-free'

We have seen an unusual cluster of earthquakes in Nashik and Palghar over the last fortnight -- more than sixteen of them, most quite small.

Should people read this as a normal earthquake swarm, or is something more unusual under way? And is there a precedent for this kind of activity, in India or elsewhere?

If you look at India alone, there are some precedent. Palghar went through something very similar in 2023, and before that in 2019. So this region has actually been witnessing this kind of activity of earthquake swarm for a while now.

What we're seeing are relatively small-magnitude tremors, and when they cluster together like this, we call it an earthquake swarm.

And what exactly counts as a swarm? : Essentially, it's a set of minor to moderate shaking events, typically in the magnitude range of 3.5 to 4.5, occasionally touching 5. Now, Nashik and Palghar both lie in the Deccan Plateau, which is largely basaltic -- igneous rock.

To understand why earthquakes happen here at all, it helps to know that there are broadly two kinds, depending on where and how they form: Plate-boundary earthquakes and fault-based earthquakes.

Plate-boundary earthquakes occur because the Earth's crust is made up of several plates, and these are constantly moving relative to one another -- this isn't a static picture at all.

Where two plates meet, there are three possibilities: They can converge, they can diverge, or one can slide past the other, which we call a transform boundary.

Depending on which of these is happening, you get different kinds of earthquakes, and these can be of higher magnitude.

Our own Indian plate, for instance, is being sub-ducted (one tectonic plate is forced sideways and downward beneath another plate into the Earth's mantle during a collision) under the Eurasian plate, and that collision is what gave us the Himalaya.

That entire belt is a plate boundary, and unsurprisingly, it's one of India's most earthquake-prone zones in terms of historical, large-magnitude events.

The Deccan Plateau is a different story. It sits on relatively stable, hard basaltic rock. But 'relatively stable' doesn't mean earthquake-free. Even within this hard rock, there can be weak zones that develop into faults over time -- cracks that form for a variety of reasons.

What kind of reasons: One of them is what we call hydro-seismicity -- and this, I believe, is possibly what's may be behind the Nashik and Palghar recent tremors. It's a phenomenon that typically shows up during and just after the monsoon.

Heavy rain causes water to percolate deep into the rock, and where there are fractures, that water fills the pores and builds up what we call pore water pressure. That pressure can widen existing cracks, and eventually the rock gives way -- which surfaces as tremors.

Without a thorough geological and seismological investigation, one can't say this with certainty, but going by the pattern of past earthquakes here, it's a strong probable cause.

'If designed as per code, no major damage should happen.' So should these earthquakes not be a cause for worry on the Deccan Plateau?

I wouldn't put it quite like that. It can be a cause for worry -- if the buildings standing on this ground haven't been designed properly to handle it.

You're also on the BMC's high-rise building committee. Is there any alarm among experts there? : I'm actually coming straight from a committee meeting as we speak. We work within the seismic code we have in this country -- IS 1893. Part 1 of that code divides India into four seismic zones. Zone 2 is the least hazardous, though that doesn't mean it's earthquake-free, and Zone 5 is the most hazardous seismically.

Which regions fall in Zone 5? : Largely the Himalayan belt -- the northeast, the India-Nepal border areas -- and also Kutch region in Gujarat, where the devastating 2001 Bhuj earthquake struck.

Zone 4 is comparatively lower risk but still counts as hazardous. Then there's Zone 3, lower still, and that's where Mumbai, Nashik and Palghar all fall.

What does Zone 3 mean in practice : It means a moderate level of earthquake activity can be expected. We measure this in two ways -- intensity-based scales, like the Medvedev-Sponheuer-Karnik (MSK-64) or the Modified Mercalli Intensity scale, and magnitude-based scales, like the Richter scale.

In Zone 3, intensity can reach up to about 7 on the MMI scale.

And in magnitude terms : That would correspond to roughly 5.5 to 6, even up to 6.5 on the Richter scale -- that's the broad correlation seismology gives us between the two measures. So, going by our code, every structure in Zone 3, including those in Nashik and Palghar, ought to be designed to withstand something in that range -- magnitude 6 to 6.5.

The current swarm has topped out at around 4.3 to 4.8. So if a quake of that size hits a building that hasn't accounted for the Indian design code's seismic force provisions, damage is possible. But if it has been designed as per code, no major damage should happen.