A simulated view of Palghar district, Maharashtra, as seen from above the Arabian Sea. Photo: Google Earth.
Since November 2018, the village of Dhundhalwadi in Palghar district, north of Mumbai, has been unsettled by rumblings emanating from two to six kilometres underground. These seemingly unending Earth tremors continue to this day, and have alarmed local residents and piqued the curiosity of seismologists and geologists. Sensors have reported earthquakes of magnitude up to 3.8.
Dhundhalwadi is experiencing what is known as an earthquake swarm, a sequence of seismic activity with no clear peak (mainshock), and which is localised to one area. A recent study by researchers around India, including the National Institute of Seismology, has found one potential explanation for the swarm that draws a link between the monsoons, groundwater circulation and rock deformation.
The Himalaya arc, along which the Indian and Asian tectonic plates collide, is the country’s most seismically active region. However, earthquakes do occur in the interior of peninsular India as well. India’s continental crust appears as a cohesive block but is actually quite heterogeneous in strength, and is crisscrossed with old faults, a legacy of its geological assembly from the Archaean period (4 to 2.5 billion years ago) onwards. Many of prominent earthquakes have occurred when recent stresses reactivated these old faults, such as in Koyna (1967), Jabalpur (1997) and Bhuj (2001).
Satellite images show the Palghar swarm’s earthquakes to be aligned along fractures that run N-S, sets of which riddle the west coast. The most well-known of these structures are the West Coast Fault and the Panvel Flexure. These fractures and faults originated when the rocky outer layers of the Indian western margin was pulled apart during Deccan volcanism about 66 million years ago and later when the microcontinent of Seychelles broke away. Some may have formed subsequent to these events due to other stresses. This western region falls in seismic zone 3, i.e. is capable of hosting earthquakes as strong as magnitude 6.
The pattern of arrival of primary waves at seismic receiving stations yields valuable information about the forces and direction of geological movement. If the crust ruptures due to it being stretched, then seismic waves transmitted in the direction of the movement will have had dilatational first motion: vibrations in the direction of the wave’s propagation, like sound from a shockwave. Just such a pattern was recorded by seismographs installed near Dhundhalwadi, indicating that there is faulting of the type known as a ‘normal slip’. This means that the crust subsided. A similar analysis of an earthquake from this swarm published in the March 2020 issue of Current Science shows ‘strike slip’ motion, which indicates horizontal motion of blocks along a near-vertical fault.
More information about ground deformation was collected from a synthetic aperture radar mounted on the Sentinel 1 satellite, launched by the European Space Agency in 2014. Data from the radar from October 2018 – before the swarm – was compared with May 2019. Even a tiny change in the relative height of the crust between the two dates would have changed the travel time of radar-waves bouncing between the surface and the satellite-mounted sensor.
This comparison showed that the ground around Dhundhalwadi had indeed subsided by an average of around 3 cm. Interestingly, the deformation estimated using seismic signals, such as the primary waves, is much less than 3 cm.
The implication of this disparity is that most of the ground movement did not trigger a seismic signal. Rather, the crust’s subsidence in the area of the Palghar swarm was aseismic, likely brought about by rock creep, an imperceptibly slow movement of rocks along a fault surface. Additionally, the persistent swarm of quakes means that energy is being released continuously – like a mug of tea cooling off rather than an explosion. The researchers see this as a sign that despite what the tremors might suggest, Palghar may not experience a big earthquake.
Now, this evidence of subsidence has posed something of a puzzle. The India-Asia collision has been transmitting compressional forces all across peninsular India. So the expected ground motion during an earthquake is to uplift along a reverse or thrust fault. The Bhuj, Jabalpur and Latur earthquakes were all due to movement along a thrust fault. (The Koyna earthquake of 1967 was due to subsidence but it is seen as something of an aberration.) What is the source of the force driving the subsidence?
One answer is tensile forces originating due to lithospheric flexure. This refers to the bending of the crust in response to a load being added or removed. Along India’s western margin, erosion has removed a considerable thickness of the Deccan lava. Scientists feel that the high topography, or elevation, of the Western Ghat escarpment is maintained by a crust bobbing up in response to this erosional unloading. As this region flexes up, E-W tensile forces get focused along the old N-S oriented faults and fractures. However, the new study concludes that since the topography of the Western Ghats east of the Palghar area is quite subdued, flexure is an unlikely source of tensile stress.
The researchers suggest a different answer: local stresses linked to groundwater circulation driving the slip and subsidence. For one, the tremors began in November 2018, giving enough time for groundwater recharged by the monsoons to percolate to a depth of 2-6 km. The researchers point out that some independent work using magnetotellurics, a geophysical method used to infer subsurface electrical conductivity, also indicates the presence of fluids. At these depths, fluid movement may have facilitated subsidence by lubricating fault surfaces and initiative the collapse of cavities. The tremors continued until April 2019.
After a brief interlude, enhanced earthquake activity resumed in July 2019, suggesting a connection with the resumed monsoons. Scientists have noticed a similar correlation between monsoons and seismicity in regions north of Palghar.
Some geologists have voiced an alternative analysis, that groundwater-linked seismicity should cease a couple of months after the monsoons and shouldn’t persist through the year, as it has in Palghar, thus keeping open the debate on whether tectonic forces are stressing these faults to critical levels. We also need to better understand how changes in pore pressure due to fluctuating groundwater load across seasons might be affecting stress levels along faults.
What could be causing cavities to form in the hard granite rock that occurs where these earthquakes originate? There is a clue not from this study but in rock recovered from boreholes 1.5-1.9 km deep at sites across the Koyna fault zone, about 320 km south of Palghar. The rock in this fault zone is shattered, made up of irregular fragments set in a fine clayey matrix and is traversed by fractures, passageways for groundwater to flow through. A preexisting fault below the Palghar area might be susceptible to reactivation by collapse of fracture cavities and also by aseismic creep due to soft clay on the fault surface.
More broadly, studies like the present one explore the varied geological circumstances in which earthquakes can occur and highlight different techniques seismologists use as they try to better understand the origin of earthquakes. Our planet, as it is, continues to come up with surprises. The area around Latur in Maharastra used to be considered a low seismic risk zone. But one early morning in 1993, the earth shook so violently as to cause immense and widespread grief, issuing a cruel reminder of our imperfect knowledge of the ways in which the ground beneath us keeps shifting in response to forces both great and small.