doi:10.1038/nindia.2016.16 Published online 1 February 2016
Heavy extraction of ground water in the Indo-Gangetic basin for over five decades contributed to the devastating magnitude 7.8 earthquake in Nepal in April 2015, a new study suggests1.
The connection has been found by researchers at the National Geophysical Research Institute in Hyderabad and the National Centre for Seismology (NCS) in New Delhi, under India's Ministry of Earth Sciences.
While seismic activity along a plate boundary is largely driven by plate tectonics, various surface or subsurface processes — natural or man-made — like reservoir impoundment, underground mining, and groundwater extraction can also promote long-term fault slip and modulate seismicity, says Vineet Gahalaut, director of NCS and one of the authors.
The researchers carried out a simulation study using Gravity Recovery and Climate Experiment data and hydrological models. Gahalaut told Nature India that, according to the study, groundwater extraction in the Ganga basin modulates the stress accumulation process on the Main Himalayan Thrust (MHT), beneath the Himalayan arc where earthquakes originate.
Groundwater usage in the Indo-Gangetic plains exceeds replenishment of aquifers, leading to substantial reduction in the crustal mass, Gahalaut said. "This crustal unloading causes a significant component of horizontal compression which adds to the secular inter-seismic compression on the MHT."
The Indo-Gangetic plain covering about 250 million hectares of fertile plains of most of the northern and eastern part of India is probably the most intensely irrigated region in South-East Asia. The report quoting earlier work said that groundwater depletion during 2002-2008 amounted to a crustal unloading of about 22,000 gigatons per year distributed over the 1000-km length of the basin.
"Taking into account this estimate of groundwater depletion in the Ganga basin, we computed the rate of stress change on the MHT," the report said. Assuming that the current rate of groundwater loss is applicable at least since 1960 — the time of green revolution in India — the researchers estimate that the crustal unloading process contributed about 7% of the secular inter-seismic stress change", implying that the resulting stress accumulation process in the Nepal Himalaya was "significantly influenced by ground water unloading."
Interestingly, according to the report, the groundwater unloading rate is about six times more in the Ganga basin in comparison to that in the San Joaquin Valley, California. "The Nepal case under our analysis is also similar to that of the magnitude 5.1 earthquake which occurred in Lorca, southeast Spain, wherein the role of extensive groundwater extraction causing crustal unloading was implicated."
"Present study implies that the 2015 Nepal earthquake and probably all earthquakes occurring on the MHT beneath the Himalayan arc are influenced (by human activities) related to groundwater extraction in the Indo-Gangetic plains," the report concludes.
C.P. Rajendran, a geophysicist at the Jawaharlal Nehru Centre for Advanced Scientific Research in Bengaluru says the study is a refreshingly out-of-the-box approach to understand the triggering of the 2015 April 25 Nepal earthquake."But the validation of this model requires crucial evidence that would suggest linkages of extraction of groundwater since 1960 and the rate of change of seismicity in the Himalaya during the last 50 years."
Rajendran points out there is no evidence to suggest the incidences of 2015-type earthquakes in the Himalaya have picked up in the last 50 years and "we also lack details of near-source groundwater changes to compliment the far-field data."