Relevance – Palghar district (in Maharashtra) has seen thousands of small earthquakes since November 2018, but the March 01, 2019 quake measuring 4.3 on the moment magnitude (Mw) (a measure of earthquake magnitude) scale was the first time the magnitude crossed 4 here.
What is a earthquake swarm?
As on February 6, a seismometer located around 70 km from Palghar and operated by Gujarat’s Institute of Seismological Research (ISR), recorded 74 quakes, with 26 measuring between 1 and 1.9 on the moment magnitude scale (Mw), 39 measuring 2 and 2.9, and nine measuring over 3, according to ISR director Sumer Chopra. This pattern of several small earthquakes occurring in a brief time-window is called an earthquake swarm. All such quakes are similar and unlike conventional earthquakes, there is nothing like a foreshock or mainshock. Swarms are common in peninsular India and mostly harmless because of their low magnitude. Generally most earthquakes in such swarms are very shallow i.e. they originate from within a few kilometres (generally 4 to 5kms) below the surface.
In general, quakes are caused by geological faults, or cracks in the earth’s crust across which rocks get displaced. However, in “hydro-seismicity,” which is hypothesised as the reason for swarms in peninsular India, water from heavy rainfall enters small fractures in rocks. This raises the pressure within them. With every 10 metre rise in groundwater, pore pressure increases by 1 bar. This pressure is released in earthquake swarms. Generally, such activity typically starts in June and dies down in December.
Scientists, however, are divided on the primary cause of these earthquakes in Palghar. Some believe it is related to groundwater levels, and others attribute it to tectonic activity. If high groundwater is the cause, the quakes may remain small. This phenomenon, called hydroseismicity, is common across the Deccan plateau. However, another hypothesis is that intra-plate tectonic forces could be generating stresses along faults in Palghar. Such a mechanism is believed to be behind the 1993 Latur earthquake. If intra-plate activity is the cause, Palghar could see larger, more destructive quakes.
The Palghar quakes have continued into February, indicating that tectonic activity is at play. A few of the temblors originated from 15 km underground, deeper than most hydroseismic quakes.
The disaster mitigation procedures should be set into motion. Palghar falls in Zone 3 of the seismic zoning map developed by the Bureau of Indian Standards. This means that buildings here must be able to withstand earthquakes of intensity 5.5-6.5 on the Medvedev-Sponheuer-Karnik (MSK) scale. Intensity is a qualitative measure of how people experience earthquakes, rather than the energy released, which is measured by the magnitude scale. In earthquakes measuring 5.5-6.5 on the MSK scale, people are frightened and run outdoors, and heavy furniture can move. Buildings that follow the BIS codes are likely to survive swarms and even larger quakes. So, it is crucial for the code to be implemented stringently.