In 12 days, Joshimath sank 5.4 cm, reveals ISRO report

The Indian Space Research Organisation (ISRO) has recorded a “rapid subsidence” of 5.4 cm, over a period of 12 days, in Joshimath.

Satellite data-based preliminary results, compiled by the National Remote Sensing Centre (NRSC), ISRO, said the Uttarakhand hill town recorded the rapid subsidence between December 27, 2022 and January 8, 2023.

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The town witnessed a "rapid subsidence event" during the period, possibly on January 2, the NRSC said based on eyewitness reports. “...the areal extent of the subsidence has also increased. But it is confined to the central part of Joshimath town,” the Hyderabad-headquartered centre observed.

Earlier, a slow subsidence of 8.9 cm was recorded in the town, over a period of seven months – from April to November 2022.

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The NRSC processed imagery from Sentinel-1 SAR using the DInSAR (satellite-based remote sensing) technique to trace the location and extent of subsidence for the two periods. The subsidence zone was, later, correlated with new Cartosat-2S satellite data acquired by ISRO on January 7 and 10, the NRSC said.

It has identified a subsidence zone resembling a generic landslide shape (tapered top and fanning out at base), with the crown of the subsidence located near Joshimath-Auli Road, at a height of 2,180 metres.

Temporal InSAR (a technique that maps deformation of ground through radar images) is also being analysed.