When it comes to earthquakes, the kingdom of Bhutan is an anomaly. Despite being surrounded by countries regularly shaken by seismic activity, the small kingdom—nestled between India and China—has been seemingly free of large temblors over the last 500 years. Now, by piecing together historical and tectonic records, an international collaboration of European and Bhutanese researchers says it has solved the mystery.
Scientists have long wondered about the apparent lack of seismic activity in Bhutan. One theory is that the region is truly aseismic, meaning that it’s incapable of producing large earthquakes. Another possibility is that the relative calm is simply an information gap, a lack of knowledge among the international scientific community about the country’s geological history. After all, Bhutan only began opening its borders to foreigners in the 1970s, and travel to the country is still restricted.
To search for evidence of historical earthquakes in Bhutan, geophysicist György Hetényi of the University of Lausanne in Switzerland and his collaborators examined published accounts of Bhutanese history and original documents written by monks and temple builders. “Instrumental records reflect all significant earthquakes that happen now,” he says. “But 100 years ago, we only had a few seismographs working on carbon paper rotating on cylinders and drums, so before 1900 we're better off using historical records, if there are any.”
A team member and Bhutanese historian, Karma Phuntsho of the Shejun Agency for Bhutan's Cultural Documentation and Research in Thimphu, studied original historical texts—including biographies of monks—written in Classical Tibetan. One biography noted a child’s account of a large temblor in the spring of 1714: “An earthquake pulverized all houses and huts in every direction. I was sleeping beneath my mother’s bed, and when our house collapsed it killed my kind mother outright. My father and a group of other survivors dug me out from under the rubble of earth, stones, and wood the next day.”
Another record, a biography of a temple builder, noted that “a major earthquake” struck the country on “the twentieth day of the third month of the Wood Male Horse year.” Taking into account potential calendar conversion errors, this date corresponds to roughly 4 May 1714. “The Earth shook about thirty times that day alone … the aftershocks continued for about a month,” according to the text.
Based on these historical documents and others, Hetényi and his colleagues reconstructed that event: A large earthquake capable of destroying buildings had occurred on or around 4 May 1714. The researchers then gathered geological evidence to support the written records. They dug trenches in Bhutan to look for tectonic faults that showed evidence of movement. Hetényi and his team found that an earthquake had occurred along one fault between 1642 and 1836, and they associated this event with the 1714 earthquake.
With the historical and geological data in hand, Hetényi and his team ran computer simulations on where the earthquake could have taken place, and how large it could have been. The researchers estimated that the quake’s magnitude had been between 7.5 and 8.5, comparable to the Nepal earthquake in 2015 that killed nearly 9000 people. They also determined that its hypocenter—the point of origin of the earthquake where the fault begins to rupture—fell within a strip 240 kilometers long and 70 kilometers wide. “This earthquake occurred in an area that we thought had no earthquakes,” Hetényi says.
Thus it appears that Bhutan is not immune to earthquakes, and that the country’s apparent aseismicity was just an information gap, the team reported late last month in Geophysical Research Letters. “This study makes a compelling case that a significant Himalayan earthquake occurred in Bhutan in 1714,” says Susan Hough, a seismologist with the U.S. Geological Survey in Pasadena, California, who was not involved in the work.
The findings also confirm that the entire area of the Himalayas is capable of producing large earthquakes like the magnitude-7.8 quake that struck Nepal in 2015. For the millions of people who live in and near Bhutan, “[we now have a] better earthquake hazard assessment of the region,” Hetényi says.