No longer a safe haven

21.05.2014 | News

By:  Peter Rüegg

Until now, Bhutan has been considered a seismic gap that has been spared from devastating large earthquakes that shook all other parts of the Himalayas. This has to be revisited now: In river terraces, a team of geophysicists found first evidence that two heavy earthquakes occurred in Bhutan within the past 1000 years. In an interview with ETH news, György Hetényi from the Swiss Seismological Service (SED) explains the worrying findings.

György Hetényi of Swiss Seismological Service: "Bhutan isn't safe from large devastating earthquakes." (Photo: Peter Rüegg / ETH Zürich)  
György Hetényi of Swiss Seismological Service: "Bhutan cannot be considered as a seismic gap or a relatively safe place anymore." (Photo: Peter Rüegg / ETH Zürich)

With colleagues from France and Bhutan, you just published a new paper about the earthquake situation in Bhutan. What's the key finding?
By studying deformed and offset river deposits in the south-central part of the country, we were able to look back several thousand years in the geological and seismological records. The height and age of these deposits point towards two very large earthquakes of magnitude 8 or larger in Bhutan in the last millennium: one after 1150 and one after 1570. These events had significant devastating effects.

Why did the team led by your colleague Théo Berthet dig into river deposits? Are there no historical records?
Paleoseismology can go back much further than historical seismology which is based upon people’s notes. These notes date back only a few hundred years, e.g. monks at monasteries who reported earthquakes. In the historical record of Bhutan, we know of only one record of one major event that happened in 1713, documented by an eyewitness report from a child. Location and magnitude are completely unknown. As large earthquakes do not occur often, it’s important to combine short term and long term records.

Why are the new findings important?
They are important because we know now that the Bhutan Himalayas can also generate devastating earthquakes of magnitude 8 or higher. So far, the Bhutan Himalayas was conceived as a seismic gap. During the last 75 years in which seismicity has been measured instrumentally, large earthquakes haven’t been recorded for Bhutan. The largest event captured was of magnitude 6.7. With our study we conclude that the behavior of this region is similar to Nepali Himalayas, known to be able to generate major earthquakes.

What are the implications of these findings?
This study changes the interpretation of the seismicity and seismic hazard in the region. We also find that the convergence rate across the Himalayas of Bhutan over the past few thousand years is the same as in Nepal. The Eastern Himalayas are particular. To the south of Bhutan, there is the Shillong plateau with an average elevation of 1600 m, that likely hosted a strong earthquake in 1897. So far, we assumed that the deformation between the Indian subcontinent and the Tibetan plateau could focus earthquakes along the Shillong plateau and therefore Bhutan remains in the shadow. The country was considered to be a relatively safe place to live in. Now we are sure that earthquakes also focus along the fringe of the Bhutanese Himalaya. Both the fault along the entire Himalayan front including Bhutan and the fault at the Shillong plateau can be dangerous. Bhutan cannot be considered as a seismic gap or a relatively safe place anymore.

shillong  
The assumption that earthquakes focus mostly along the Shillong plateau rather than on Bhutan has been proven wrong. (Image: wikipedia)

What are the consequences, e.g. for the Bhutanese administration?
In Bhutan, the existing building code is based on engineering studies and seismic hazard estimation with values from India. The assumption for this code was that the largest event is of lower magnitude. Our study implies that seismic hazard is significantly higher than previously thought. As the time of earthquakes cannot be predicted, the local government should update guidelines on how to build houses and enforce their application more vigorously than before. There should be local studies to gather more data so the code can be updated accordingly. Values can no longer be taken from India anymore.

What are the implications for the people?
The population has to be better informed about seismic hazard. Students in schools should learn more about what to do when an earthquake strikes and also afterwards, and how to organize rescue services. Bhutan has started well with educational measures, but the way is long and good intentions have to be maintained.

You and your team installed and operate a temporary network of seismometers in Bhutan (cf. ETH Life). Is it still in place and for how long will you continue to run this network?
Currently, we are operating a network of seismometers in Bhutan to listen to actual earthquakes. From January 2013 to last month (April 2014) we had 38 stations operating in the frame of a scientific project which focuses on the structure of the Earth beneath the region of Bhutan, i.e. how does India collide with Tibet to build the Himalayas and what is the related earthquake activity in the country. We also analyse the noise generated by landslides to see whether we can can we detect them and if yes determine their size. 13 of the stations will now remain for one additional year in the field to continue to monitor seismicity. In the long term, we would like to establish a permanent seismological observatory in Bhutan. This is quite a challenge as the necessary funding cannot come from the ordinary ways, so we are looking for a tailored solution.

About the interviewee
György Hetényi (34 years old) is a senior research associate at the Swiss Seismological Service and is leading the «GANSSER» project in Bhutan for the Swiss Seismological Service and ETH Zurich. He did his doctorate on the evolution and deformation of the Himalayas.

Reference

Berthet T, Ritz J-C, Ferry M, Pelgay P, Cattin R, Drukpa D, Braucher R, Hetényi G: Active tectonics of the eastern Himalaya: New constraints from the first tectonic geomorphology study in southern Bhutan Geology, May 2014, v. 42, p. 427-430, first published on March 31, 2014, doi:10.1130/G35162.1

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