Scientists have identified a 2,291 sq km area in Uttarakhands Chamoli district as the most seismically active zone in Uttarakhand Himalayas after they found that it had the highest density of Earthquakes and landslides over the study period of eight years. The flash floods in February that killed 200 people in Chamoli had also occurred in an area within this zone.
Researchers at the Dehradun-based Wadia Institute of Himalayan Geology (WIHG) studied the Main Central Thrust (MCT) zone, a 2,200 km-long geological fault along the Himalayan belt which demarcates the lesser Himalayas from the higher Himalayas. The zone has been touted as the geographically weakest part of the Himalayas since all earthquake-related activity in the Himalayas have happened in and around a 10km periphery of MCT, according to scientists.
For the study, researchers looked at the part of MCT which fell in Uttarakhand, a total area of 19,000 sq km. The study, published in Current Science journal, analysed data of earthquakes and landslides in the region between July 2007 and October 2015. Data from seismographs was collected to study earthquakes while landslide data was procured by high resolution satellite images as well as images on the Google Earth platform.
The study found 2,260 earthquakes of magnitude ranging between 1.5 and 5.1 in the study area. Among these, 769 tremors were of magnitude ranging between 1.5 and 2.0, 1,335 between 2.1 and 3.0, 129 between 3.1 and 4.0 and 27 between 4.1 and 5.0.
The data highlighted that smaller earthquakes of magnitude less than 3.0 which were leading to frequent landslides were clustered in certain areas. So scientists categorised them into three main clusters: Uttarkashi earthquake rupture zone (3,179 sq km), Chamoli earthquake rupture zone (2,291 sq km) and Munsyari zone cluster (4,220 sq km). These clusters were further studied.
The Uttarkashi earthquake rupture zone which had witnessed a massive earthquake in 1991 that had claimed over 700 lives had recorded 145 earthquakes and 300 landslides during the study period. The Munsyari cluster recorded 269 earthquakes and 401 landslides.