Colette's Grand Detour

Bhutan sits on the Main Himalayan Thrust (MHT) where the India and Eurasia plates collide (same as Nepal). This plate-boundary fault system is capable of very large earthquakes of Mw 8+. However the seismic hazard is not uniform across Bhutan. The southern part of Bhutan, nearer the Himalayan front has the highest earthquake hazard because the MHT is at its shallowest here and so shaking attenuates here less before reaching the surface. Soil conditions in the south can also amplify shaking. However, in terms of risk, urban centers and populated valleys like Paro and Thimphu are also at highest risk due to significant property and people exposure.

The 1714 Bhutan earthquake is among the strongest known events effecting Bhutan and its magnitude is estimated at ~Mw 8.1.There have been 2 major earthquakes in the last 15 years that have caused significant damage in Bhutan. In 2009, a Mw 6.1 earthquake hit Eastern Bhutan, destroying over 4000 houses and damaging monasteries and schools. In 2011, the Sikkim-Bhutan earthquake, centered in nearby Sikkim, India but felt strongly in Western Bhutan, had a Mw of 6.9 and triggered landslides in highly populated areas like Paro and Thimphu.

Smaller earthquakes of Mw5-6 happen regularly across the country. However, Bhutan’s seismic record is relatively sparse and older events are poorly documented, meaning that many earthquakes likely go underreported. Some literature also notes that Bhutan lies in a ‘seismic gap’, a stretch of the Himalayas that has not ruptured and created an earthquake for a long time. Some interpret this as potential for stored strain on the fault and potential for a forthcoming large event to release that strain.

Bhutan has operated a national seismic monitoring network since 2017. It has 6 seismic stations and 20 intensity meters distributed across all 20 dzongkhags (provinces). This has allowed researchers to build a local earthquake catalog and have a better understanding of how seismicity attenuates locally. This is helping develop more detailed hazard mapping for the country which can inform better design and planning of infrastructure. However, with just 6 stations, many smaller or remote events will go undetected or have poor location accuracy, especially in rugged terrain. The country has recently secured grants to enhance its seismic monitoring network.

Many houses and public buildings across Bhutan, but particularly in rural areas are built from stone masonry, unreinforced masonry or vernacular construction like rammed earth and local stone. These buildings are vulnerable to collapse even under moderate shaking conditions. Vernacular buildings are particularly at risk due to their brittle behaviour and weak connections.

For modern buildings constructed after 1997, Bhutan uses the seismic standards developed and used by India, which are well tested. However, currently the whole country is treated under the same ‘zone’ in terms of the level of seismic design required, which likely ignores some areas that are at higher risk and require higher design levels. The Indian code is also based on Indian seismicity, geology and data, and this may not account for Bhutan’s local conditions.

The mountainous terrain means that secondary effects from shaking such as landslides, river damming, and glacial outbursts would magnify losses and complicate any disaster response. Many key access points like roads and bridges cross steep terrain and river valleys. Shaking-induced landslides would severe connectivity and hamper relief efforts.

Bhutan faces some unique challenges with regards to managing its earthquake risk. It has a complex terrain and many valleys and terraced hillsides are prone to liquefaction and slope failure, complicating rescue and reconstruction. There are technical constraints such limited seismic data and monitoring, and a shortage of technical expertise such as structural engineers and seismologists. And compliance of building codes remains uneven due to cost and a lack of training. Bhutan is a small developing country with budget limitations, which limit its capability to retro-fit older structures and mean it would largely be dependent on external aid from India and international agencies in the event of a large earthquake.