News

The Himalayan foothills are sinking

K. S. Jayaraman

doi:10.1038/nindia.2020.16 Published online 5 February 2020

The sub‐Himalayan region – better known as the Himalayan foothills – and the Indo‐Gangetic Plain (IGP) are sinking, while the surrounding areas rise up, a new multi-institutional study1 has found. The subsidence and uplift are due to variations in seasonal groundwater apart from tectonic forces, the report says.

Researchers found this previously unnoticed effect of hydrological mass in sub‐Himalaya and IGP by analysing satellite data from the Global Positioning System (GPS) and Gravity Recovery And Climate Experiment (GRACE). The GRACE satellites, launched by the US in 2002, monitor changes in water and snow stores on the continents, thereby making for a new way to study terrestrial hydrology.

A schematic illustration of the study area in a GPS and GRACE satellite perspective view.

© Saji, A. P. et al.

In the Himalaya, seasonal water from glaciers as well as monsoon precipitation play a key role in deformation of the crust and the seismicity associated with it. "In addition to tectonic and non-tectonic forcing, unsustainable consumption of groundwater influences the subsidence rate," Sunil Sukumaran, head of marine geology and geophysics at Cochin University of Science and Technology (CUST), told Nature India

Sukumaran, one of the authors of the study, says crustal deformation due to water storage and surface load variations are relatively difficult to quantify using global models. The combined use of GPS and GRACE data has made it possible to quantify the variations of hydrologic mass, the authors say.

To decipher the ground deformation because of hydrological mass variations in the Himalaya and North India, Sukumaran's Ph.D student Ajish P. Saji at Indian Institute of Geomagnetism, Mumbai, and other colleagues analysed GPS observations from 50 sites along with GRACE data from 2004 to 2015. The other institutions involved in the study were the Space Applications Centre, Ahmadabad, Wadia Institute of Himalayan Geology, Dehradun and Andhra University, Visakhapatnam.

The authors say that the GPS-derived vertical deformation, after correcting for the hydrological effects, suggests that the sub-Himalaya and IGP are undergoing subsidence as the surrounding areas get uplifted. They say the GRACE correction reflects a 12% reduction in the rate of the subsurface slip (meaning how fast the two sides of a fault are slipping relative to one another) over the Main Himalayan Thrust (MHT) due to hydrological variations and human activities. The MHT  periodically releases accumulated strain built up from the collision of the India-Eurasia plates.

Taking out the effect of hydrological loading (derived from GRACE) from GPS measurements provides a better estimate of the tectonic load, says Vineet Gahalaut, senior principal scientist at the National Geophysical Research Institute in Hyderabad. However, the low resolution of GRACE data poses a challenge, particularly in the Himalayan terrain with large spatial variation in hydrological conditions even in short distances, he points out.

Also, coverage of GPS, which is basically a point measurement, is still very sparse to accurately represent the spatial variation of deformation. "We only hope that with time, these challenges will be overcome," he says.

From a tectonic point of view, the finding that this subsidence inhibits the slip on the Himalayan thrust plain by about 12% is based on a 2-D dislocation model. "The robustness of such models depends on how reliable the input parameters are," says Chittenipattu Rajendran, a geophysicist at the Jawaharlal Nehru Centre for Advanced Scientific Research in Bangalore.


References

1. Saji, A. P. et al. Surface deformation and influence of hydrological mass over Himalaya and North India revealed from a decade of continuous GPS and GRACE observations. J. Geophys. Res. Earth 125, e2018JF004943 (2020) doi: 10.1029/2018JF004943