Advanced analysis of satellite images revealed that the town of Joshimath, which sits on the ruins of an ancient landslide, has been sinking about 10 centimeters every year since 2018. The Aristotle University of Thessaloniki (AUTh) and CNRS-EOST (Centre National de la Recherche Scientifique/University of Strasbourg) have published an analysis by remote sensing and landslide experts indicating a large gravitational instability in Joshimath over the past four years. flight.
The geological conditions of this landslide are very complex and it is clear that it is an ancient landslide complex that is only marginally stable as mentioned in the Mishra Committee Report of 1976. In addition, there have been several instances of rapid development at Joshimath which were not optimal , while no attention was paid to proper drainage and excavation activities.
Surface displacement rates are potentially exacerbated by urbanization
“The city is located on steep slopes and was built on unconsolidated soils known to be unstable for several decades,” AUTh and CNRS-EOST say in their consolidated report, which ran a time series of displacement in the wider Joshimath area since January. 2018 to 31 December 2022. It suggests that surface displacement rates are potentially exacerbated by urbanization in the last four years, including drainage disruption, uncontrolled water discharge and slope undercutting.
The analysis shows that three parts of the landslide complex are currently moving, with the highest values moving towards the bottom of the slope. The movement boundaries coincide with the edges of the landslide blocks identifiable from Google Earth. We would expect the greatest damage to the building to occur around these edges. So it is really interesting to compare the above diagram with the map posted on Twitter by Thiyagarajan J (@jThiyagu) showing the location of the shelters damaged by the landslide:-
#JoshimathIsSinking! These custom maps show the city of #Joshimath with markers around the areas where buildings are affected (as reported in print).
From the tunnel, the nearest area is Parsari ward (500 m) and the farthest area is Marwadi ward (2600 m), approx. @rajbhagatt @rajatpTOI — (@jThiyagu) January 8, 2023
There is a strong coincidence to suggest that these analyzes are beginning to elaborate on the nature of the landslide problem in Joshimath.
The movement of landslide complexes such as this is very complex and we would expect velocity changes to occur over time. Thus, the interpretation of data with short time series will require great caution. The bottom line, however, is that this slope is unlikely to achieve permanent stability in its current physical state without significant engineering intervention. The current crisis will subside at some point, but the underlying chronic problem, whatever the cause, is likely to remain. This does not mean that all is lost, the immediate crisis will pass, but action will likely be required.
Satellites reveal new details
Interferometric Synthetic Aperture Radar (InSAR) is a technique for mapping Earth deformation using radar images of the Earth’s surface that are collected from orbiting satellites. Two radar images of the same area that were collected at different times from similar locations in space can be compared to each other.
Recent developments have been monitored by the Surface motion MAPPING (SNAPPING) service for the Sentinel-1 mission on the Geohazards Exploitation Platform (GEP) to map ground deformation that covers a large spatial area. SNAPPING provides average velocities as well as the entire displacement time series (that is, the evolution of motion over time) for each PSI (Persistent Scattering Interferometry).
PSI is a powerful remote sensing technique that measures the major displacement of the Earth’s surface over time. This radar-based technique also belongs to the group of differential synthetic aperture interferometric radars (DinSAR).
