Part 1: Report by Wadia Institute of Himalayan Geology finds very small seismic activities as contributing factor
A report by eight agencies investigating the causes of landslides in Joshimath has been made public after almost eight months. This is the first of a five-part series by Down To Earth, starting with the report by Wadia Institute of Himalayan Geology.
The insitute’s ten-member team left for Joshimath on January 10, 2023. The team formally started a LiDAR survey from February 23, 2023. It surveyed the geological condition, structural construction, temporary construction, active tectonics, etc between Pipalkoti and Tapovan.
The team made a map with details of cracked buildings, subsidence and joints along the banks of Pipalkoti, Joshimath and Tapovan.
Wadia’s team has not clarified what happened on the intervening night of January 2-3, 2023, which caused rapid land subsidence in Joshimath city. However, the paper mentioned that small earthquakes keep occurring in Joshimath.
Joshimath region of Uttarakhand falls in the highest seismic zone of India, it said. The 1999 Chamoli earthquake of magnitude 6.6 had its epicentre near and south of Joshimath. Seismic activity is greater where the Himalayan seismic belt passes through the region.
The Main Himalayan Thrust — a fault surface under the Himalaya Range within the upper crust is the major tectonic fault driving seismic activity, the team found. This is why the institute established a nearby seismic network of 11 stations in and around Joshimath to monitor micro earthquakes of magnitude less than 1.0.
The network was linked to the online seismic station at Central Recording Station, Dehradun for continuous extraction of seismic data and continuous monitoring.
From January 13 to April 12, the seismic network recorded 16 micro earthquakes of maximum magnitude 1.5 within a distance of 50 kilometres from Joshimath, according to the report. These had epicentres within the upper crust at a depth of 10 km.
The epicentre of the earthquake was mainly south and south-west of Joshimath, where the Vakrita Thrust (VT) and Munsiyari Thrust (MT) tectonic faults are present.
These present earthquakes and past seismic activity show a similar trend of increasing seismicity in the south and south-west, mainly concentrated around the Chamoli epicentre. Therefore, this seismic activity of micro earthquakes is normal in this area.
However, some mild and high intensity earthquakes were recorded within a distance of 100 km around Joshimath, which are of a slightly higher scale than the surrounding areas in the Himalayas, the team said. The highest intensity earthquake is of 5.4 magnitude — which occurred on January 24, 2023 — was at a distance of 100 km south-east of Joshimath in the western part of Nepal.
On observing recent cracks on the roads and walls of houses on the slopes of Joshimath during the study, the team found that the geometry of most of the cracks is similar — meaning cracks were propagated from top to bottom or are still propagating.
The upper part of the cracks being wider than the lower part indicates that these cracks were formed due to pressure.
Joshimath city is located on a slope on which no in situ rock has been observed so far, the report said. Rather, the slopes consist of loose boulders of varying sizes and soil content. The boulders are made of garnet mineral and gneiss, a type of metamorphic rock.
The institute’s team collected data through electrodes at seven different locations in Joshimath. These include Parsari, two spots in Manohar Bagh, Singhdhar, JP Colony, Sunil Gaon and Marwari (below JP Colony).
A consistent bedrock layer exists at a depth of 15 metres in Parasari, the team found. No sign of water leakage was found here. Boulder were also found under the ground in Sunil Gaon. No leakage area was found here also.
In the first site of Manohar Bagh, an area with heavy water flow was observed at a depth of 5 metres to 35 m along the north-west facing slope near Tower 1 from Auli Road, the team said.
At the other site of Manohar Bagh, water leakage was observed at a depth of 5-10 m parallel to Auli Road. Isolated fragments of water seepage were also present in Singhdhar, while a large stone was present in the middle of the section to a depth of 25 m.
In Jaypee Colony, water leakage up to a depth of 10 m was clearly seen, along with boulders.
Water leakage was observed in isolated parts in Marwari, but only to a very shallow depth of 4-5 m. Based on these facts, the report said, “It can be said that the seepage water is natural and not affected by anthropogenic activities.”
It is worth noting that local residents claimed that the water coming out of various locations was from the Tapovan-Vishnugad water plant, which had entered the tunnel due to last year’s floods.
Since Joshimath is located on a steep slope, where there is a lot of dense concrete construction as well as dense forests in some parts, airborne LiDAR of altitude model should be adopted for mapping the area, Wadia scientists advised in their report. The technology appears to be the most suitable and viable option.
Because Uttarakhand is a mountainous state that frequently experiences mountain disasters, it is critical that the state conduct LiDAR topographic mapping, the researchers added. This will allow planners and decision makers to use ground topography and surface models when planning management and mitigation.
The report also highlighted the importance of monitoring by Survey of India and the upgradation of data not only in Uttarakhand but also in other Himalayan locations.
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The reports of the agencies investigating the Joshimath sinking have been made public after nearly eight months. Photo: Sunny Gautam / CSE.png)
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