NRSC identifies landslide-shaped subsidence zone in Joshimath; satellite imagery-based report rings alarm bells

A National Remote Sensing Centre report has sounded the alarm bells for the central and Uttarakhand state authorities to act quickly on the Joshimath subsidence.

The Hyderabad-based ISRO institute reported a rapid subsidence of 5.4 cm, recorded in a span of just 12 days, confined to the central part of the town in Chamoli, Garhwal.

The rapid event, NRSC scientists said, was triggered between December 27, 2022 and January 8, 2023. "Possibly on January 2, as per the eyewitness reports," the scientists said.

“A subsidence zone resembling a generic landslide-shaped was identified, tapered at the top and fanning out at base. Crown of the subsidence is located near Joshimath-Auli road at a height of 2180 m,” the NRSC report informed.

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The Narsingh temple and army helipad, two major landmarks, were shown as falling within the subsidence area.

The areal extent of subsidence has also increased, NRSC said in its preliminary findings on the geological event that has involved a slew of agencies, departments including the PMO in mitigation efforts.

Joshimath or Jyotirmath, perched at an altitude of 1,875 metres, is the gateway to one of the holiest Hindu pilgrim centres -- Badrinath and also leads to several scenic Himalayan trekking trails and skiing point -- Auli.

The town itself is famous for the Adi Shankaracharya Mutt and Narsingh temple, where the Badrinath idol is brought every winter for worship when the temple in higher altitude is closed for six months.

The rapid subsidence comes following the slow subsidence of up to 9 cm within the Joshimath town recorded over a period of seven months between April and November, 2022.

Speaking to DH, a senior NRSC scientist said that “they cannot comment or predict if the damage would expand to neighboring houses and areas or if the town faces the danger of collapse.”

“The town came up on the debris of several landslides, and the soil's load bearing capacity is less. The slopes are steep and unstable. Drainage systems, infrastructure built-up around could also have played a part in subsidence,” the scientist said.

“We hope our satellite imagery based findings help in the multi-departmental investigations, remedial actions on the ground. Not to alarm anyone but yes, our report brings out a cause for concern,” a NRSC scientist involved in the study told DH.

The findings are based on Sentinel-1 satellite Synthetic Aperture Radar imagery (descending pass) to identify the possible location and extent of land subsidence in long and short time intervals. The identified subsidence zone was correlated with new Cartosat-2S satellite data acquired by ISRO on January 7 and 10, 2023.

NRSC said it will update the results further after the analysis of temporal InSAR to identify landslide kinematics.

Due to the landslide-related creep, cracks have appeared in over 700 houses apart from roads, hotels, hospitals, other buildings in Joshimath.

Authorities are evacuating the locals in houses marked as unsafe for residing and are razing several structures posing danger.