Saturday
March 24, 2012
KUENSEL
ENVIRONMENT
*
17
This is the fourteen article of a 16-part series on the GLOF research and mitigation project between May 2009 and March 2012. The articles will highlight latest findings on glacier, glacial lakes
condition and natural hazards in the Bhutan Himalayas. Experts from the department of geology and mines (DGM), Japan International Cooperation Agency (JICA) and Japan Science and
Technology Agency (JST) are involved with the project.
Bhutan needs an active fault map to reduce seismic disaster
A
n earthquake is
considered to trigger GLOFs, and
active faults are
potential sources
of earthquakes. Do you think
that a mega earthquake of the
same size as the 2011 Great East
Japan earthquake could occur in
Bhutan?
I need to say ‘yes.’ The
tectonic condition of Bhutan is
similar to that of the northern
Japan (Figure 1). The 2011
earthquake broke along the tip
of the plate boundary between
the Pacific and Eurasia plates
under the sea. Bhutan is located
at a Himalayan range, which was
formed as a result of the head-on
collision between the Indian and
Eurasia plates. In fact, four mega
earthquakes (over Mw=7.5) occurred during the last 100 years
along the Himalayan front of
Nepal and India. It has been by
pure chance that Bhutan has not
suffered seismic disaster by the
mega earthquakes.
Unfortunately we cannot
predict the precise timing of an
earthquake in advance, but we can
know where the mega earthquake
tends to occur. Any earthquake is
generated by the fault movement
when the rock in subsurface slides
suddenly to release the strain of
the plate motion. Any fault usually
remains quiet until the fault movement occurs. In case of the fault
movement with large slip at shallow subsurface, it is accompanied
with the large to mega earthquake,
and surface deformation and
surface rupture occurs. Thus finding out the active fault trace is to
recognise the location of the large
to mega earthquakes.
As part of the GLOF research
project, I firstly stayed at Thimphu
for three weeks to find out the
fault trace, using the air photos,
with Jamyang Chophel, one of the
project members of the DGM. According to the active fault survey,
topographic interpretation by the
photos is the fundamental and indispensable process of the survey.
Our photo-interpretation recognised many fault traces (red lines
in figure 2) in southern Bhutan.
When I came to Bhutan
again, we surveyed the active
fault in the field to get geological
evidence of fault movement. The
photo in Figure 3 (left) is an outcrop on the left bank of Sarpang
river. The contact between gravel
layer and rock, which should be
smooth originally, has been tilt
back to upstream and warping
strongly, producing a fault scarp
with several meters height on the
surface. Those strange features
were not made by regular river
system, but by fault movement
shown in the cartoon (figure 3,
right). Moreover, a higher scarp
with over 20m in height is continuing from the fault scarp on
the same trace. It suggests that
the fault movement has occurred
repeatedly along the same trace,
and it will occur in future again.
Although we cannot prevent
seismic disaster by a mega earthquake we can aim to reduce the
damage as much as possible.
The fault movement generates
not only heavy shaking but also
rupture and deformation on
the surface like the outcrop in
Sarpang. When the fault moves,
and if the infrastructures such
as hydro power stations are built
away from the fault line, there
will be less impact from shaking,
and the damage caused by the
rupture and deformation will be
minimised. Thus finding out the
location of the fault trace directly
contributes to the reduction of
the seismic disaster.
In Japan, geoscientists
provide the detailed map of fault
traces as ‘active fault map’. The
public can consider the location
of the traces before new construction. I believe that making
the active fault map of Bhutan is
the first step to reduce the damage caused by seismic disaster.
During the project implementation period, we could locate fault
traces only in southern Bhutan,
but it is necessary to continue
developing an active fault mapping for the whole of to Bhutan.
Dr. Yasuhiro KUMAHARA (left)
Associate professor of Gunma
University,
Geomorphologist
Mr. Jamyang Chophel (right)
Geophysical Survey Engineer,
DGM, BHTUAN
Topographic cross sections of Northern Japan and
Bhutan with plate motion and plate boundary. The
2011 Great East Japan earthquake occurred along
the plate boundary and generated severe tsunami.
In Bhutan the plate boundary is emerged on the land
surface along its southern margin. If a plate-boundary
type earthquake occurs, Bhutan will tremble severely,
because Bhutan is located closer to the plate
boundary than northern Japan.
Figure 2: Active fault map of Himalayan frontal area of Bhutan.
Figure 3: (Left) Photo of outcrop of deformation of gravel bed due to active faulting in Sarpang. This outcrop is located in the southern
margin of the Himalayan range, implied that the fault is regarded as the tip of the plate boundary.
(Right) The cartoon shows before and after the fault movement interpreted by our field observation of the outcrop.