Large earthquake due in central Himalaya
Indian researchers say a future quake will release energy trapped along the Main Himalayan Thrust.
doi:10.1038/nindia.2018.147 Published online 20 November 2018
A massive seismic deformation resulting from a 7.8 magnitude earthquake that hit the Himalayan nation of Nepal in 2015 could trigger a similar temblor in future, releasing the remnant trapped strain in the region, according to a new study1.
Three years back in April, the ‘Gorkha earthquake’ and its aftershocks only partially released the accumulated strain along the Main Himalayan Thrust (MHT), a fault line between the Indian and Eurasian tectonic plates whose collision is believed to have given birth to the Himalaya. The earthquake with its epicentre in the Gorkha district of Nepal was mainly confined to the north of the rupture.
Researchers have now identified a region west of the earthquake's epicentre that houses the unreleased energy locked up. "This could spawn a future large earthquake in central Himalaya akin in magnitude to the 2015 Gorkha event," the researchers say in the report.
K. M. Sreejith at the Space Applications Centre, Ahmedabad, and co-workers analysed two decades of data from a network of 36 Global positioning System (GPS) stations using a geodetic method called InSAR (Interferometric synthetic aperture radar). The method generates maps of surface deformation or digital elevation along a track passing through the epicentral region of the 2015 earthquake. Additionally, ‘b-values’ derived from past seismicity data were used to identify zones of stress accumulation. The ‘b-value’ is a parameter of seismic hazard used to estimate the recurrence rates of large earthquakes.
According to their report, "total strain rate and b-values together suggest the presence of two patches of high strain accumulation zone (called asperity), one towards east and the other west of the 2015 Gorkha epicenter." The high strain zone in the east has already ruptured during the Gorkha earthquake although with no surface manifestation, but its western counterpart, with unreleased strain energy, is yet to be visited by an earthquake.
"This asperity to the west of the Gorkha epicentre requires special mention in view of the seismic hazard of the region," Sreejith told Nature India."Assuming a 3 meter average slip, the western asperity has the potential to generate an earthquake of magnitude around 7.6, close to that of 2015 Gorkha event," he said.
He, however, noted their work should not be mistaken for an earthquake prediction. "We can't say anything about when the next earthquake could occur. At the same time, our study clearly suggests that earthquake(s) of a magnitude similar to or greater than that of 2015 Gorkha earthquake are due for the region."
The warning comes close on the heels of another study2 pointing to the next major earthquake in the Garhwal–Kumaun segment of Northwest Himalayas, partly located in India and partly in western Nepal. The region has not experienced a great earthquake since 1505. Vineet Gahalaut, director of the National Centre for Seismology in New Delhi, led this study analysing five years (2013-2018) of continuous GPS measurements of crustal deformation from 28 sites in the Garhwal–Kumaun Himalaya and the adjoining Indo-Gangetic plains. Their study suggests "strong seismic coupling" underneath the region, meaning that the fault below the Garhwal–Kumaun segment is locked and capable of accumulating stress.
The width of this coupled region is estimated to be about 85 km. According to Gahalaut, the strongly coupled region must have now accumulated a "slip"— a relative displacement of formerly adjacent points on opposite sides of a fault – of more than 7 metres since 1505 – enough to produce a great earthquake in the region.
Gahalaut said Sreejith and his team have developed capabilities to process InSAR data from the Himalayan region. "If correct, the asperity identified by them to the west of the Gorkha earthquake epicentre could be the region of the next great or major earthquakes," he told Nature India. However, the robustness of predictions of ‘high strain’ need thorough checks since there are several estimates of coupling on the MHT region. Gahalaut said he is also intrigued about what stopped the Gorkha earthquake's partial rupture to propagate to the South and West (while it did travel to the East).
"All these models bring out the fact that the central Himalaya is most likely to enter into a phase of active earthquake activity," Chittenipattu Puthenveettil Rajendran, a paleoseismology expert at the Jawaharlal Nehru Centre for Advanced Scientific Research in Bengaluru told Nature India. "The lack of earthquakes commensurate with the strain accumulation (along the MHT) is baffling. But this situation cannot continue for long."
1. Sreejith, K. M. et al. Audit of stored strain energy and extent of future earthquake rupture in central Himalaya. Sci. Rep. 8, 16697 (2018) doi: 10.1038/s41598-018-35025-y
2. Yadav, R. K. et al. Strong seismic coupling underneath Garhwal–Kumaun region, NW Himalaya, India. Earth Planet. Sci. Lett.(2019) doi: 10.1016/j.epsl.2018.10.023