On
June 8, 2005, the Koyna region in Western Maharashtra, India,
was rocked by a magnitude 4.2 earthquake. This earthquake
was followed by an aftershock of M 3.6. The locations of the
two earthquakes were about 20 km SE of the dam situated in
Koyna. For monitoring the deformation of the dam, a GPS receiver
was set up on the top of the dam by Indian Institute of Technology
Bombay (IITB), which was continuously operating even during
the time of occurrence of the earthquakes. The GPS group of
IITB has been working over six years in the field of deformation
measurements and analysis of Koyna Dam under a research project
funded by the Department of Science and Technology, Government
of India. A GPS network has been established for this purpose,
and observed periodically in order to investigate the potential
of GPS Technology in deformation measurement analysis. The
data collected by the GPS receiver on June 8, 2005 was used
to analyze the movement of the dam due to the earthquakes.
The analysis of the data collected just before and after the
earthquake occurrences showed a displacement in the NW direction
with a certain magnitude of deflection. After the occurrence
of the earthquakes, the dam was observed to attain its original
position by shifting in the opposite direction (SE) with the
same degree of deflection as observed during the time of the
earthquakes. These results reflect the reflex movement of
the dam body and its response to the occurrence of the main
shock and its aftershock. These results indicate the high
stability of the Koyna Dam.
The
construction of Koyna Dam, a rubble concrete dam and the reservoir
formed by it, began in 1956 and was completed in 1961. The
height of the dam is 85 m, with 892 sq. km of catchment area.
The dam and the reservoir are located on the Indian Peninsular
shield, one of the oldest continental blocks on the Earth's
surface. Prior to the year 1962, this shield was referred
to as a stable rock. But the December 11, 1967 earthquake
of M 6.3 in the Koyna region contradicted all these beliefs.
However, the dam in the region withstood this significant
seismic activity without much damage. The seismicity associated
with the Koyna reservoir is believed to be unique as it is
the only known location in the world, where seismicity began
after the start of impoundment of the reservoir, and has persisted
for more than 40 years (Rajendran and Harsh, 2000; and Manake
and Kulkarni, 2002).
Dam
and crustal deformation studies using GPS are being carried
out in this active region of Koyna by the GPS team of Indian
Institute of Technology Bombay (IITB), under a research project
funded by the Department of Science and Technology (DST),
Government of India. The objective of this project is to study
the seismologically disturbed region of Koyna, and the behavior
of the dam body using geodetic GPS technique. It includes
establishing the dense GPS network in the identified seismically
active area, taking its repeat observations, detailing GPS
data processing using scientific software, estimating the
parameters of deformation, and developing methodologies for
checking the stability of the region. A GPS network comprising
31 stations was established on the dam and surrounding regions,
which observed periodically over 14 campaigns from December
2000 to May 2006 (each field work spanning two to three weeks).
Of the 31 stations, 12 stations are established on the dam
body, and the rest in the area surrounding the dam. Of the
12 stations on the dam, one GPS station is set up on the top
most and center of the dam structure and is observed continuously
throughout the day and campaigns using suitable GPS receiver.
The remaining stations are generally observed for 6-8 h (Kulkarni
et al., 2004; and Kulkarni et al., 2006).
The
aim of this paper is to analyze the deformation of the dam
due to earthquakes that occurred during the GPS campaign in
June, 2005. This was done by processing and analyzing the
continuously recorded GPS data collected before and after
the earthquakes. The objective is to detect the coseismic
movement of the dam body. |