Uttarkashi district in Uttarakhand has been shaken by a series of earthquakes recently — nine in just six days. The first five quakes hit between January 24 and 25, causing Himalayan rocks to tumble from the landslide-prone Varunavat hills. These tremors were mild, with magnitudes between 2 and 3 on the Richter scale, but they were enough to force people rush out of their homes in panic.
What’s causing these repeated tremors in Uttarkashi?
Scientists say this belt of the Himalayas is under stress, with energy building up beneath the Earth's surface. Since this region hasn’t experienced a major earthquake in a long time, experts fear that a powerful one might be due soon.
Uttarkashi had last witnessed a devastating earthquake in 1991 in the Garhwal region, that killed over 700 people. The district lies in Zone IV of India’s seismic map, meaning it is highly prone to earthquakes.
Geologists have classified these repeated quakes as an "earthquake swarm". Swarms are commonly observed in volcanic regions like Japan, Iceland, and central Italy, often preceding volcanic eruptions. While the Himalayas are not volcanic, the underlying tectonic movements could be triggering a similar swarm-like behavior.
How the USGS defines earthquake swarms
An earthquake swarm is a sequence of small earthquakes with no identifiable mainshock. However, according to the United States Geological Survey(USGS), we still have much to learn about earthquake swarms and their underlying physical processes.
According to the USGS, an earthquake swarm is a sequence of small tremors occurring in a specific area over a short period, without a single large earthquake.
Earthquake swarms are different from the more common mainshock-aftershock pattern. In a typical earthquake sequence, a large quake (mainshock) is followed by aftershocks that gradually decrease in intensity over time. However, in a swarm, multiple tremors of similar magnitudes may occur without a clear pattern, and their activity might even increase instead of fading away.
Is a 'Big One' on the way?
Seismologists have long warned that a major earthquake is overdue in this belt of the Himalayas. The Indian and Eurasian tectonic plates are constantly colliding, moving at a rate of 50-52 mm per year. This movement creates stress, causing cracks in the Earth's crust, which leads to mild earthquakes. These small tremors in Uttarkashi and Chamoli originate from depths of 10 to 20 kilometers beneath the surface.
Experts have cautioned that while small tremors help release some energy, they don’t eliminate the risk of a major quake. Some believe this frequent seismic activity could be nature’s way of warning us to avoid excessive human intervention in seismically active zones.
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