January 10, 2025
THIMPHU – The 7.1 magnitude earthquake that hit Shigatse region of Tibet, China on January 7 triggered two avalanches from the Table Mountain above the Bechung Glacier in Lunana.
While the glacier’s gentle slope absorbed the avalanches and prevented them from reaching Bechung Glacial Lake, such events are potential triggers for dangerous Glacial Lake Outburst Floods (GLOFs).
The tremors of the Shigatse earthquake were felt across the neighbouring countries, including Bhutan.
Gasa, the nearest dzongkhag to the epicentre, experienced the maximum intensity of four on the Modified Mercalli Intensity (MMI) scale, which measures how strongly people feel the tremors.
Lunana, home to numerous glaciers, including the dangerous Thorthomi Lake, are particularly at risk from seismic activity due to its glacial topography.
Glacial lakes such as Bechung, Rapstreng, Thorthomi, and Lugye Tsho are located below the Table Mountain.
“The vibrations destabilise glaciers, triggering avalanches,” said the executive glaciologist of National Centre for Hydrology and Meteorology (NCHM), Phuntsho Tshering. “Fortunately, this avalanche occurred on the Bechung Glacier side, which has a slope capable of absorbing the impact. However, Bechung Glacier Lake is rapidly forming, raising concerns about how long it can withstand the growing mass.”
Communities in Lunana were alerted as soon as the earthquake occurred.
Following the earthquake, the NCHM staff at Lunana, along with a team from the headquarters, were sent to assess the lake sites.
They observed no major disturbances at Thorthomi, Rapstreng, and Bechung glacial lakes. However, minor moraine slides on the inner moraine walls of the Thorthomi-Rapstreng barrier were observed.
The team could not assess Luggye glacial lake since it is inaccessible in winter.
The Thorthomi-Rapstreng moraine dam, critical for preventing a GLOF, has been deteriorating over the years. A collapse of the dam could result in the combined outflow of Thorthomi and Rapstreng lakes, potentially releasing 53 million cubic metres of water in a worst case scenario, according to a NCHM study. This is why Thorthomi is considered the most dangerous lake.
Following recommendations from NCHM, there has been ongoing efforts to permanently relocate the residents of Thangza and Tenchoe villages in Lunana, which are nearer to Thorthomi Lake, to safer, higher elevations at Damthangkha, Bayza-Gangjuk, and Dungbiteng since last year.
The NCHM team will continue monitoring the glacial lakes and has advised downstream communities to remain vigilant.
The tremors were mild in other parts of the country, with intensity ranging between zero and three. Western Bhutan experienced an intensity of two on the MMI scale.
In terms of ground shaking (called Peak Ground Acceleration or PGA), an intensity of four on the MMI scale is equal to a ground movement of about 15 centimetres per second squared, or 0.0153 times the force of gravity (0.0153g).
The current building codes in the country are designed to withstand significantly stronger ground shaking, up to 0.36g, ensuring structural safety for most constructions.
The Department of Geology and Mines, in collaboration with the GovTech Agency, is working to upgrade Bhutan’s earthquake monitoring systems, which is expected to be completed by the end of March.
The upgrade will include a web-based platform for real-time earthquake intensity information, allowing the public to report perceived shaking.
Currently, there are 13 seismic monitoring stations and 223 intensity metres across the country to measure earthquakes.