Geological Survey of India
The Bhutan and the Garhwal Himalayan regions witnessed strong to moderate
earthquakes on September 21, 2009
An earthquake of magnitude (Mw 6.3) rocked the Bhutan Himalaya on September 21,
2009 at 08:53:05 (UTC), which was located at the Centroid depth of about 34 km beneath the
Bhutan-Assam (India) border with Latitude of 27.35N and Longitude of 91.42E (USGS & IMD).
The epicenter of the Bhutan earthquake is found to be located at about 135 km NNW of
Guwahati, the capital city of Assam, India and about 180 km away from Thimpu, a capital city
of Bhutan. The shaking was felt by the people of Guwahati, Kolkata, North Bengal and NE states
of India at around 14:25 (IST), and also by Bangladesh, and Southwest China.
The permanent Seismic Observatories of Geological Survey of India recorded the 21st
September 2009 Bhutan earthquake and corroborated the location within the acceptable range of
tolerance due to single station method (Table 1). The occurrence of the 21st September 2009
Bhutan earthquake near Assam border is not surprising because of complex seismotectonic
settings of the entire Bhutan Himalayan region where past damaging earthquakes have already
been reported. The present earthquake epicenter location falls very near to the 1713 and 1947
Bhutan earthquakes that damaged Bhutan and Parts of Assam (Ambraseys and Jackson, 2003,
Curr. Sci., 84, 571 – 582). This observation suggests that the 21st September 2009 Bhutan
earthquake may be associated with the previous ruptured zones of the two past damaging
earthquakes of the 1713 and the 1947, where the present failure occurred through thrust faulting
with northward dip. This observation supports the convergence of the subducting Indian plate
with overriding Tibetan plate. The zone is under compressive stress due to prevalence of
collisional - subduction tectonics in the region with several regional faults, besides unknown /
hidden seismogenic causative sources, which need to be deciphered using the present state-ofthe-art techniques of geosciences.
Just about one hour later, the Garhwal Himalaya also witnessed a moderate earthquake
(Mw 4.7) at about 09:53:47 (UTC) beneath Uttarkashi, Uttarakhand, India (Latitude: 30.9N;
79.10E) at a depth of about 13 km (USGS). This is the same region where the 1991 Uttarkashi
earthquake (M 6.3) rocked the area having strong lateral and depth-ward structural
heterogeneities in Primary seismic velocity (Vp). The low-Vp zone imaged at depths between 15
km and 21 km in around the 21st Uttarkashi earthquake (Mw 4.7) by Mishra et al (2005, GSI
Spcl Pub. 85, 337, 2005) using travel times generated from the aftershocks of the 1991
Uttarkashi earthquake and background seismicity suggests the existence of plane of detachment
above which this moderate earthquake occurred.
Contributed by Geo-Seismology Division, CGD, GSI (CHQ), Kolkata
Geological Survey of India
The frequent occurrence of such earthquakes in different parts of the Himalayan region is
suggestive of strain release because of complicated geotectonic settings and intriguing
evolutionary history of the entire Himalaya, where background seismicity is well corroborated
with the seismogenically active tectonics existing in the region.
TABLE 1
Earthquake
Origin
Time
Latitu
de (N)
Longitude
(E)
Magnitude
(Mw)
Depth
(km)
The Bhutan
Earthquake
21.09.09
27.35
91.42
6.2
10 km
(Fixed)
08:53:05
Source
USGS
Centroid
depth =
34 km
Do
21.09.09
27.30
91.50
6.3
8 km
(Fixed)
IMD
28.659
91.287
6.6
30 km
GSI
(Fixed)
Nagpur
Observt.
08:53:04
Do
21.09.09
08:52:58
Do
21.09.09
27.50
91.39
6.4
08:52:23
The
Uttarkashi
21:09:09
30.90
79.10E
4.7
30 km
GSI
(Fixed)
(Agartala
Observt.
13 km
USGS
09:53:47
Earthquake
Contributed by Geo-Seismology Division, CGD, GSI (CHQ), Kolkata