Magnitude

The Indian Institute of Remote Sensing (IIRS) report on Deformation Analysis showed that the central, eastern, and southern (high altitude) parts of the Joshimath town displayed a higher rate of land displacement, about 12 cm per year, as compared to other areas.

IIRS analysed satellite images of Joshimath between June 2019 and February 2022 and gave a slew of reasons for “subsidence” primarily, “toe-cutting” by Alaknanda river which flows at the toe/base of Joshimath hillside; slope instability as a result of seepage of local drainage water in the soil; loose and unconsolidated moraine materials of the slope (due to old landside); and flash flood events in and around the area in recent past.

“This resulted in the development of cracks in the ground as well as houses in Joshimath town,” the report said and added there was a sudden increase in deformation from November 9, 2022 to January 19, 2023 because of increase in underground land displacement in the vertical direction primarily due to higher flow of underground water.

Geological reasons

The report on to study geological and geotechnical subsurface strata at Joshimath by National Institute of Geophysical Institute, Hyderabad, said the Joshimath slope reported over 6 cm displacement during the December 2022-January 2023 period, with peak subsidence regions reporting fissures of up to one metre in length horizontally and vertically.

The report said the loose material covering solid rock is not uniform across Joshimath, with its maximum thickness seen around rivulets and streams. The ground fissures have been mostly observed in the zones with thick cover.

A similar study by IIT Roorkee said that Joshimath has loose soil having low shear strength parameters even at higher depth. “The low soil resistance , land subsidence and high-rise building overloads may aggravate the slope failures”, the report added.

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What may further aggravate the ground situation, the report said, was the Main Central Thrust (MCT) fault-line that passes close to the south of Joshimath at Helang, making the rocks structurally weak and sheared. As such, the area around Joshimath town is covered by a thick layer of overburden material (a very old landslide) and it has been witnessing gradual sinking for a long time, which was formally reported first by Mishra Committee of 1976, the report said.

The Wadia Institute of Himalayan Geology (WIHG) said cracks may increase as Joshimath falls within high seismic zone. Between January 13 to April 12, 2023, WIHG’s seismic network recorded 16 micro-earthquakes of maximum magnitude 1.5 within a distance of 50 km of Joshimath. Some higher magnitude earthquakes including one of 5.4 on Richter scale on January 24, 2023, were recorded within a distance of 100 km. “The present and past seismic activity has a similar trend of increasing seismicity to the south and southwest mainly concentrated around the epicentre of the Chamoli earthquake,” the report said.

Human impact

The report on “Safety assessment of buildings in Joshimath” by Central Building Research Institute (CBRI) pointed out that the main cause of cracks in houses and land were due to “excessive vibration” in the ground on account of traffic; CBRI studied 2,364 buildings. The report also documented around 40 ground fissures with a width of up to 300 mm and a depth of 3 to 4 metres. CBRI stated that 37% of the total buildings were “usable”, 42% needed “further assessment”, 20% were “unusable”, and 1% “need to be demolished”.

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While most of the reports blamed the topography of Joshimath for cracks, the Central Water Commission report on hydrology pointed to the large number of constructions in the spring zone of Joshimath town as the main reason . “As large number of pilgrims for Chardham Yatra and tourists come every year, to provide accommodation, ample number of hotels, homestays are constructed in the area,” the report said, claiming that this “stopped the flow” of spring water, resulting in it seeping out in JP residential colony.

Delinking NTPC

The National Institute of Hydrology, Roorkee’s report source of water gushing in JP Colony ruled out its connection with National Thermal Power Corporation hydro-projects in the area. Instead, it attributed the phenomena to a linkage between “subsidence” and subsurface water in form of numerous springs in the western part of Joshimath. The report said the topography (cascade of flat and steep terrain from top to bottom) and geological setting create an unfavourable condition for the development of permanent water channels to dispose water coming from upper reaches and any blockade can create underground storage, aggravating subsidence.

The Geological Survey of India found 81 ground cracks, of which 42 were recent and mostly in “excessive rate” of water flow zone. It attributed them to presence of a thick, loose, unconsolidated debris mass on the Joshimath slope due to land sinking for last four to five decades and underground seepage natural lines. The report said water sample tests from JP Colony and NTPC’s Tapovan-Vishnugad Hydel Project didn’t match.

Uttarakhand Disaster Management Secretary Dr Ranjit Kumar Sinha said that it on the basis of these reports that the state’s Post Disaster Needs Assessment (PDNA) was prepared. The report is now with the Union government, he added. “The report will also be used in stabilisation of the town. It has also been shared with the Public Works Department for further action.”