Maharashtra’s Palghar district has experienced thousands of small quakes since November last year, raising fears of a larger one. Fortifying the district for such an event is going to be challenging, reports Priyanka Pulla

When the earthquakes first began in November 2018, the children of Palghar’s Nareshwadi Learning Centre were terrified. The youngest, aged 5-7 years, burst into tears. Five months later, the tremors continue, but the children are used to it, much to the chagrin of the school coordinator, Vivek Siyaram Patel. He would like them to stay alert and dash out of their classrooms every time the ground shakes. That’s what they have been taught to do.

But when the largest quake occurred on March 1, its noise was like a large Diwali bomb, "the bigger kids ran out but the younger children kept sitting inside. It’s like they were used to it,” says Mr. Patel.

Like thousands of residents of Palghar, a district on Maharashtra’s coastline, Mr. Patel and the rest of the staff at Nareshwadi, a residential school for underprivileged children, aren’t sure if their buildings will withstand further shaking. Cracks have already appeared in the school’s dining hall. A few of the tall metal rods that support the hall’s sloping roof have bent with each quake. Unsure of whether everything will come down with the next tremor, some 220 of the school’s boarders now sleep in tents.

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The tremors have been numerous. Since November, hundreds of earthquakes, ranging between 1 and 4.3 on the local magnitude scale, or ML (broadly equivalent to the widely used moment magnitude, or Mw), have struck Palghar district, 150 km north of Mumbai. Around 18 of Palghar’s villages, with 63,000 residents, have borne the brunt because of their proximity to the epicentres.

These earthquake “swarms”, as clusters of small quakes are called, have pushed Hyderabad’s National Geophysical Research Institute (NGRI) and Delhi’s National Centre for Seismology (NCS) to install seismometers in the region. These devices will help measure the magnitude of the quakes. The district administration, meanwhile, is scrambling to provide tents for those who are too scared to live in their homes; some 1,300 community tents have been distributed. Simultaneously, a team from the Indian Institute of Technology (IIT), Bombay, headed by structural engineer Ravi Sinha, is training Palghar’s engineers and masons to assess the safety of constructions in the quake hotspots.

Much of this panic could have been avoided if existing building regulations had been better enforced. Long before these temblors began, Palghar was classified as Zone 3 on India’s seismic zoning map, which means it can expect quakes measuring up to 6-6.5 on the Mw scale; for comparison, the devastating 1993 Latur quake measured 6.1 Mw. So, local laws require all constructions in Palghar to be designed for such quakes. They must comply with the National Building Code and tick multiple boxes of earthquake-resistant design.

As things stand, repeated battering by the swarms have left several homes with cracks, and a handful with collapsed walls. A preliminary assessment in Dahanu and Talasari subdivisions, the worst hit locations, has identified 1,750 damaged houses. While these are mostly in rural areas, where the enforcement of seismic codes is poorer, urban areas may not be better off.

According to IIT Bombay’s analysis of the 2011 Housing Census data, 10 lakh people in Palghar live in potentially weak homes. Unless these structures are upgraded, the region will likely suffer loss of life in the event of a larger quake.

The chances of a large quake (upto 6.5 Mw) are no longer remote, according to seismologists. Even though the initial suspicion was that the swarms were a temporary phenomenon linked to rains, NGRI seismologists have argued that the real reason is tectonic activity along a geological fault in Palghar. “If it is related to a fault, we cannot be complacent that quakes will continue to be small,” says Mr. Sinha. Moreover, given that the epicentres are in rural areas, “it is likely that a disproportionately large fraction of buildings will be damaged with relatively low shaking,” he adds.

Mahesh Gollapudi and Dhiraj Kumar Singh, project assistants at Hyderabad's National Geophysical Research Institute, collect data from a seismometer in Gagodi village, Palghar. | Photo Credit: Prashant Nakwe

Searching for the cause

Several residents of Dahanu don’t remember prior earthquakes in the region. “This is the first time I am experiencing it,” says the principal of the Nareshwadi Learning Centre, Babasaheb N. Pawar, who has been in Palghar for 30 years. Then there’s the noise, described sometimes as a deep rumbling and others as a large bomb. “My heart would leap out of my body,” says Mr. Patel. “Can you imagine the condition of the little children?”

Still, despite the absence of tremors in Palghar’s memory, the Deccan peninsula has a history of swarms. This phenomenon, thought to be triggered by rains, is called hydro-seismicity. Even though the tremors seem tectonic — the kind that occurred in Bhuj, Latur or Nepal — they aren’t. While both involve geological faults — cracks in the earth’s crust along which rocks can move — that’s where the similarity ends. The movement of the rocks that causes earthquakes is triggered by very different mechanisms.

In hydro-seismicity, heavy rains seep into the top layers of earth, compressing the rock beneath and increasing the pressure inside the rock pores. “It’s a bit like a baby elephant sitting on a mattress,” says Kusala Rajendran, a seismologist at Bengaluru’s Indian Institute of Science.

According to one estimate, for each 10 m rise in groundwater level, the pore pressure increases by 1 bar (bar is a unit of pressure, equal to 100,000 pascals). When this happens in the vicinity of existing geological faults, the pressure can destabilise them. Another way for rain to trigger quakes is if the water enters faults that are sealed and inactive. “Here, the water can lubricate the clayish contact surface of the fault, causing it to slip,” she explains.

Such hydro-seismicity has struck the peninsula before. In October 2017, residents of Hyderabad’s Borabanda suburb experienced small quakes for over a month. They began after intense rains, says D. Srinagesh, who heads NGRI’s seismology observatory, and measured less than 1 ML each, a tiny wobble. “But they sounded like Diwali hydrogen bombs. People grew panicky and were running helter-skelter,” he recalls. When the rains ended, the tremors stopped. Similar phenomena had occurred in Andhra Pradesh’s Nellore in 2015, Saurashtra’s Talala in 2007 and 2011, and Madhya Pradesh’s Khandwa district.

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Such quakes have distinct characteristics — they are small, shallow, noisy, and typically end after the monsoon water drained. Small means they never exceed 4 in magnitude, making them relatively harmless. Shallow implies that they originate from within 5 km of the earth’s crust. This also makes them audible, in contrast to intermediate and deep earthquakes, which emerge from beyond 60 km.

The NCS, the first body to set up 4 seismometers in Palghar in December, opines that the swarms could well be hydro-seismicity. Its calculations show a shallow depth of 3-4 km. “We are going by experience here,” says Vineet K. Gahalaut, NCS director. “We have seen several such swarms in western and central India.” But he cautions that tectonic activity cannot be ruled out.

In contrast, NGRI, which also began monitoring the Palghar quakes in January, is convinced that the quakes are tectonic, triggered by the same forces that cause planetary tectonic plates to drift over a ductile layer of the mantle. The NGRI now has six seismometers in Palghar, and its data suggest that most quakes are emerging from between 6 km and 15 km, too deep for rain-related seismicity.

Another factor that has swayed the NGRI’s assessment is how long the quakes have continued. “They should have died down by now,” says Mr. Srinagesh. “Usually, they have a lifespan of a month or two.” But the Palghar tremors have continued for nearly five months, at the rate of 30-35 per day. Further, their magnitude has risen, with the largest ML 4.3 quake occurring on March 1.

The differences between the NGRI’s and the NCS’ depth estimates could be due to a number of reasons. To calculate depth, scientists use the timings at which seismic waves arrive at their seismometers. Then, based on assumptions of the wave velocity, and distance of each seismometer from the epicentre, they estimate the depth that best fits their data. This means that the calculations have error margins, and depend a lot on assumptions made. Since neither NCS nor NGRI have published their data, it’s hard to say why their estimates differ. “I haven’t seen their data, so I cannot comment on the accuracy. But in general, for geophysical problems, there is no unique solution,” says Mr. Rajendran.

Mr. Sinha, however, has seen NGRI’s data and is inclined to buy its arguments. The district administration too is working on the assumption that the quakes are not mere hydro-seismicity. “I have reasons to believe that what the NGRI is saying is correct,” he says.

The implications of this are significant. While rain-related seismicity peters away quickly, tectonic quakes can be large and destructive. If so, the current swarms could merely be foreshocks before a bigger temblor. It has happened before. In the year before the 1993 Latur earthquake, NGRI recorded several small earthquakes in the Killari region. No one thought they would end in a large one, given the historical lack of seismicity there, but they did. If the same happened in Palghar today, it would find itself unprepared.

Fortifying Palghar

This is why Mr. Sinha’s team is working on a training module for Palghar’s engineers, so that they can assess which buildings are the most vulnerable. Mr. Sinha has his task cut out. Prima facie, many structures in the district aren’t earthquake-resistant. Nevertheless, to strengthen every such home in the district is too massive an exercise. “It’s expensive, not just monetarily, but also in terms of available human resources. There are only so many engineers who can be deployed in Palghar to do this, without affecting the governance of the entire State,” he says. For comparison, a World Bank-funded project to repair and reconstruct 225,000 houses in Latur cost about $220 million and took four years.

So, Mr. Sinha’s strategy is to triage: find the most vulnerable of the vulnerable houses. These will be reconstructed, while others will merely be strengthened. “It is possible to do things so that without demolishing and reconstructing a house, we can strengthen it to the extent that it doesn’t collapse. It may still collapse partially, though,” he says. For example, traditional houses made of irregular stones, or “random rubble”, caused heavy casualties in both the Latur and the 2001 Bhuj quakes. Their walls are made of two vertical layers, or wythes, of stones. When tremors hit, these poorly bonded wythes separate and fall apart. So, one fortifying technique is to add long “through stones” at intervals along the wall’s length. These hold the wythes together when shaking occurs.

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The scale of the exercise is currently restricted to the 18 quake hotspots, although a larger temblor would spread farther. Around 90 local engineers will be trained. They will scan buildings and inform occupants about safety.

Public structures, such as schools and hospitals, will be the top priority, with the government paying for their strengthening. “The idea is that, in the event of a real Zone 3 earthquake, the government’s own capacity should not be degraded,” says Mr. Sinha. Private constructions, on the other hand, will have to fund themselves. Only low-income groups will get financial help through the Pradhan Mantri Awas Yojana.

There isn’t much time. Once the monsoon arrives, the government tents may not hold up. So, the assessments must be completed earlier. At the end of it, some of Palghar’s residents may learn that their homes are safe enough to return to, says Mr. Sinha. Others may have to upgrade their houses, while still others will have to rebuild altogether.

The compliance gap

Palghar’s unpreparedness is typical of large parts of India today. Close to 60% of the country lies in Zones 3, 4 and 5, which means these areas can experience moderate to severe quakes. These places legally require all construction to comply with the seismic codes developed by the Bureau of Indian Standards. Yet, a swarm of small temblors has disrupted life in Palghar.

Why has compliance been below par? The reasons are complex. First, the worst affected areas are villages, where there is a shortfall of trained engineers. So, even though local laws require home owners to consult a licensed structural engineer and meet NBC requirements, it often doesn’t happen. Says Aseemkumar Gupta, Secretary of the State’s Rural Development and Panchayati Raj Department, “It would be wrong on my part to say that for every building constructed in rural Palghar, there is a structural consultant applying his mind about Zone 3 earthquake issues. It doesn’t happen for 99% of the small houses.”

In urban areas, the problems are different. Many constructions are slums built without municipal permissions. Others may follow seismic codes on paper, but not in reality, given the added costs. And municipal bodies do not have the resources to police everyone.

It was to tackle such issues that the National Disaster Management Authority (NDMA) published earthquake management guidelines in 2007. The ambitious document recommended wide-ranging measures such as training engineers, improving enforcement, and raising public awareness.

But implementation has been uneven because it isn’t a small task, says Kamal Kishore, an NDMA member. Between the 2001 and 2011 Censuses, for example, close to 3 crore new brick masonry houses came up in India’s earthquake-prone regions. To ensure compliance of all of them would need many more engineers than municipal bodies have at present. “You simply can’t do it overnight,” Mr. Kishore says. “Even countries that have turned over their entire building stock successfully have taken not years, but decades.”

Rare, but deadly

There is another obstacle in the way of earthquake preparedness: quakes are rare, despite their deadliness. Further, intra-plate earthquakes, which occur deep inside the peninsula — such as the Latur, Bhuj and even Palghar quakes — are even more infrequent, compared to Himalayan ones. This makes it likelier that peninsular inhabitants will be unaware of the region’s seismic history. “A large number of people have forgotten about the 1993 Maharashtra earthquake. It’s been more than 25 years and a whole generation has turned over since then,” says Mr. Kishore. Not knowing how much damage an earthquake can wreak can take away motivation to spend money on seismic compliance.

Still, globally, there is a growing realisation that earthquakes do more than kill humans; they cause mass migrations, job losses, and economic stagnation. Mr. Sinha cites New Zealand’s Canterbury earthquakes in 2010 and 2011, which measured 6.3 Mw and 7 Mw. Because the region was known to be situated on a fault, most buildings were appropriately designed. As a result, casualties were fewer than they would have been otherwise.

The problem was that even though most structures survived, they subsequently became unusable. “They were designed to be safe against fatalities, but not designed to be stronger than that,” says Mr. Sinha. “So they were not safe for people to return after the quakes.” As a result, Christchurch, until then a thriving manufacturing hub, had to demolish over 600 commercial buildings, and cordon off sections for days. According to a 2015 paper in the International Journal of Disaster Risk Reduction, 6,000 businesses were displaced by the cordon. The New Zealand Treasury estimated the capital cost of the quakes to be equal to 20% of the country’s GDP, a significant hit.

For Palghar’s residents, though, GDP is the farthest thing on the mind. It’s the everyday anxiety of not knowing if their homes will collapse, and of sleeping in tents, that gnaws. “We suffered a lot in the initial days,” says Patel. When the tremors began, schoolchildren slept under the open sky during the chilly winter nights. Now, they have tents and bunk beds, but it would be nice just to get back to their rooms.