Earthquakes in Bhutan: past, present, future
The intent of this article is to provide a brief introduction to the study of earthquakes, and then
summarize our understanding of earthquakes in the Kingdom of Bhutan; past, present and future.
Studying a planet in motion
Earthquakes are part of a natural process, the expression of enormous forces that are generated and
released as a mosaic of distinct rock masses or “plates” float on our planet’s surface and grind
against one another. Our understanding of these phenomena, known as “plate tectonics”, was born
in the early 20th century with the work of Alfred Wegener and other scientists. Looking at maps of
the continents, they observed that these shapes seemed to fit together like pieces in a puzzle. There
were other clues such as similarities in geology and fossils, signs of a shared history in plant and
animal life among these now distant lands.
Seismology, the study of earthquakes, has become a world-wide endeavour because lessons learned
in one region can help us to understand and prepare for earthquakes everywhere. For example, we
can compare the behaviour of earthquakes occurring in several locations around the Pacific Ocean
(Japan, Alaska, Oregon, Chile, and others) and in Sumatra. This is possible because they all have a
similar fault structure, called a “subduction zone”, wherein the oceanic plate is colliding with and
dipping beneath the continental plate. The Himalayas and the Alps are another example; both are
collisional mountain belts, so here too seismologists are studying behaviour common to these areas.
Seismometers are precise instruments that were developed to measure the frequency, duration, and
orientation of earthquake shaking. In recent decades, arrays of seismometers have been established
in all corners of the globe to detect seismic activity and provide data valuable for a variety of
research interests. From these measurements, we know that earthquakes are occurring all the time,
with typically hundreds of small events recorded each day worldwide.
Some of these smaller events can be felt by people and animals in local areas, but they are modest in
duration and cause minor to no damage.
Moderate to large earthquakes can shake an entire country and, while they are not so frequent,
most people living in the Himalayas experience at least one such event in their lifetime. These events
can impact communities through injuries and loss of life, damage to manmade infrastructure, and
impacts to the environment, such as landslides.
Finally, catastrophic earthquakes, such as the recent event in Nepal, are sadly also part of this natural
process in some areas. They have the potential to take many lives and cause widespread destruction,
but the time between them is so long that many communities cannot remember such events even
from their ancestors’ stories.
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Past
Knowledge of past earthquakes can help us understand the type, size, location, and frequency of
events that can be expected in the future. But how much information is available for the study of
tectonic history that spans thousands or even tens of thousands of years?
The source of data that can reach furthest back in time is the study of individual faults by
investigating geologic evidence. One approach is to excavate a trench across a known fault and try to
identify the relative displacement of geologic layers that occurred during a past earthquake. This can
provide information on the approximate size and duration of the event. The study of terraced river
deposits can also provide information about ancient earthquakes. Such analyses were performed in
the region of Gelephu and Sarpang by a French-Bhutanese team. They explored surface features of
the fault along which the Himalayas thrust over the India plate, supposedly producing rare but
catastrophic events. The results show that in the past millennium two catastrophic events have
happened, with magnitudes possibly exceeding M8.
A fascinating aspect of seismology is the search for historical documents that can offer data about
the frequency and intensity of past events. In the region of Bhutan, historical records of earthquakes
include books and written notes by officials, monks and citizens. For example, there is excellent
documentation for the great Shillong Plateau earthquake (Magnitude M8.1) in 1897, which was felt
across Bhutan. However, older records of earthquakes are rare, possibly due to fires that ravaged
monasteries and Dzongs. One earthquake that is believed to have occurred in Spring of 1713, was
described by a 3 or 4 year old child, Shakya Rinchen, who later became the Ninth Je Khenpo. The
event was large, if not catastrophic, but neither the location nor the magnitude can be reconstructed
from this single note.
With the advent of seismometers the international community has catalogued about 10 to 15
medium to large earthquakes (M6) in or near Bhutan over the last century. One could say there is
roughly one M6 event every 10 years, but this is an average: some decades had several events, some
decades had no event at all. Most Readers will remember the September 2011 earthquake in Sikkim,
and the September 2009 earthquake near Mongar. Seismologists evaluated the relative size of these
events, and assigned the Sikkim event a magnitude M6.9, and a magnitude M6.1 for the Mongar
event. While the Sikkim event was larger, the Mongar earthquake caused more damage in Bhutan
because its source was shallower and nearer Bhutanese cities.
Although the available information on past earthquakes is limited, it is clear that earthquakes of all
sizes can happen in Bhutan. So the question is not if, but when will the next large event occur?
Currently, the average time between large events is not well known; however, there are ways to
learn more about this.
Present
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 2 years. The network
spanned from Phuentsholing to Gasa, from Samdrup Jongkhar to beyond Trashiyangtse, and at
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several locations in the centre of the country. We have realized this project in frame of a
collaboration between ETH Zürich and the Department of Geology and Mines (lead by Dowchu
Drukpa), with the kind support of Helvetas Bhutan.
The stations operated from early 2013 to late 2014. During this time, seismometers located outside
Bhutan detected only 3 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. But when one drives around the country and
asks people about their latest shaking experience, most people recite recent memories.
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 parts 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 ten 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.
Future
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
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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, longterm 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
c/o HELVETAS Swiss Intercooperation, Thimphu
Travis Munson, Geotechnical Engineer, Portland, Oregon, USA
Published in two parts on May 9 and May 16, 2015
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