Fortunately, it is not necessary to “wait” for more large earthquakes to progress the study of seismicity in Bhutan. Rather, careful analysis of smaller earthquakes that occur much more frequently can reveal important characteristics of the tectonic structure and overall seismic behaviour.
This technique requires a network of finely tuned sensors located near the earthquake sources to allow precise estimation of the epicentre location and the earthquake magnitude. While Bhutan does not currently have the network required for this work, a Swiss-Bhutanese team recently operated a temporary network of 38 seismological stations across Bhutan for two years. The network spanned from Phuentsholing to Gasa, from SamdrupJongkhar to beyond Trashiyangtse, and at several locations in the centre of the country.
The stations operated from early 2013 to late 2014. During this time, seismometers located outside Bhutan detected only three events in Bhutan and about a dozen in the surrounding region. The local network in Bhutan detected several hundred earthquakes in Bhutan and a few thousand in the surrounding region. This dramatic increase in the number of events that could be detected is simply due to the fact that the seismological stations were near the earthquakes.
The largest local events that occurred during the study were around magnitude M4: one on 29 October 2013 near Mongar, at a relatively shallow depth of about 17 km, and another on 6 June 2013, deep under Lunana (about 77 km depth). There were a dozen earthquakes of similar size or larger in the region surrounding Bhutan.
Most of the detected earthquakes were too small to be felt by people. However, the results show that there was approximately one event per week that could be felt by people living in Bhutan. This is much more frequent than what appears in the media.
The detected earthquakes were not equally distributed across the country. Most events were along a line connecting Phuentsholing with Samtse and reaching Gangtok (Sikkim). There were many events in the larger region of Mongar, where the 2009 earthquake occurred. There were a group of events east of Punakha; and there were also events in other part of the country, typically at least a few in each region.
So what is the benefit of observing and counting frequent smaller earthquakes, when it is the rare, large earthquakes that are our primary concern? Studying small events is very useful because seismologists have identified the following general trend between the number of small and large earthquakes. If we observe a certain number of M6 earthquakes, there will be approximately 10 times more M5, 100 times more M4 and 1000 times more M3 earthquakes in the same region and same time period. In other words: if, on average in Bhutan, a M6 event happens every 10 years as suggested by the past century’s record, then a M5 should come every year (on average), and 10 M4 events should come each year (on average).
However, comparing results of this brief study of detailed earthquake behaviour with the more general but incomplete historical record, we see there is still some discrepancy regarding the frequency and magnitude of events that should be expected in Bhutan. This uncertainty is due primarily to the relatively short duration of the study compared to the repeat time of seismic events, and the variability of earthquake behaviour from year to year and decade to decade. One conclusion is sure: more observations, over a longer period of time, are necessary to characterize the recurrence interval of small and large events in Bhutan.
Earthquakes cannot be predicted. The next felt event may happen today, in a week or in a few months. The next catastrophic event may hit today, or it may hit in years or decades from now. We cannot prevent them from happening, but we can strive to become ready for them. In parts of the world where seismic risks are recognized, governments and individuals are educating themselves concerning the serious hazards of earthquakes. Building codes and policy is being modified to consider such hazards in the design of infrastructure, particularly for essential facilities and lifelines for public safety.
A critical step in this process is the development of an informed seismic model. For this effort, long-term observations of earthquakes in Bhutan are important because: (1) the longer we observe, the more certain we can be that the distribution of small to medium earthquakes is representative of their long term pattern; (2) a more complete list of smaller earthquakes makes it possible to better estimate how often the larger events may happen on average; (3) a long running and complete earthquake catalogue is the fundamental element of a quantitative earthquake hazard map.
Today there is no permanent seismological network operating in Bhutan, but everyone agrees that such a network is highly needed. The core of the network should be very sensitive devices that are able to detect even the smallest events that people do not feel. These sensors (also called “broadband seismometers”) should provide data from all over the country to build up the earthquake catalogue of the next years and decades. In parallel, sensors that are less sensitive but able to record strong shaking during larger events should be installed. These sensors (so called “accelerometers”) will provide the necessary information to estimate the potential damage. This is especially important in larger towns and at critical infrastructure sites such as hydro power plants.
Today, although there is no permanent network in Bhutan, there are ongoing efforts to establish one, which is laudable. There are multiple successful seismological observatories around the world that demonstrate good practices for this work. First, a single institution should operate the entire Bhutan Seismological Observatory: a uniform station network, a matching data centre, reliable telecommunications, and an analysis and alerting service.
Second, the Observatory needs permanent staff, knowledgeable on all technical details, from station maintenance to earthquake localization routines and interpretation. At a minimum, the Observatory requires six full-time technical staff with guaranteed long term jobs and commitment. Third, ideally an international collaboration should be envisaged: to procure and install the stations and infrastructure, to adequately train and support the staff, and to remain available for remote assistance for several years, as and when required, until the Observatory is able to run independently.
Bhutan’s expenditures for such an Observatory would be a very small fraction of the damage caused by the 2009 Mongar earthquake, and hence a good investment priority for Bhutan in the light of the situation outlined above
An Observatory will not be an easy garden to build, but some seeds have already been planted. Continued efforts on this front will determine what is harvested for generations to come.
Contributed by Dr. György Hetényi,
Geophysicist, ETH Zürich, Switzerland Travis Munson, Geotechnical Engineer, Portland, Oregon, USA