30-07-2014, 12:41 PM
DIRECT GPS MEASUREMENT OF KOYNA DAM DEFORMATION DURING EARTHQUAKE
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Abstract
On June 8th, 2005, the Koyna region in Western Maharashtra, India was affected
with a felt earthquake of magnitude M 4.2. This earthquake was followed by an aftershock of
M 3.6. For monitoring the deformation of the dam, a GPS receiver was set up on the top of
the dam by the Indian Institute of Technology Bombay (IITB) and was continuously
operating, even during the time of the earthquakes. The GPS Group at the IITB has been
working over 5 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 the GPS Technology in deformation measurement
analysis. The data collected by the receiver on June 8th, 2005, was used to analyse the
movement of the dam due to the earthquakes. Analysis of the data just before and after the
earthquakes occurrence showed a displacement of 2.3 cm in the northwest direction. After this
period, a displacement of 2.1 cm in the opposite direction (southeast) was observed i.e. the
station attained its original position. These results reflect the movement and the response of
the dam body to the occurrence of the mainshock and its aftershocks. This result indicates the
high stability of Koyna Dam. Keywords: Koyna Dam, earthquake, GPS measurements,
Displacement.
Introduction
Koyna Dam, a rubble concrete dam and the reservoir formed went under construction in 1962
and were completed in 1963. 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 11th, 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 in the world as it is one of the few sites where earthquakes of magnitude
greater than 5 continue to occur [1, 2].
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 area
and the behaviour of the dam body, using geodetic GPS technique. It includes establishing
dense GPS network in the identified seismically active area, its repeat observations, detailed
Earthquake Information
On June 8th, 2005, at 3 : 02 : 10.7 am IST, an earthquake with M 4.2 (focal depth 10 km)
occurred about 20 km southeast of Koyna Dam, Western Maharashtra, India. The latitude and
longitude of this earthquake was recorded as 17º 14.6’ and 73º 46.7’ respectively. This
earthquake was felt throughout the Koyna region and was felt by the GPS team too. This
earthquake was followed by an aftershock of M 3.6 (focal depth 8 km) that occurred at 4 : 34 :
21.2 am (i.e., 1.5 hours after the main shock) at a latitude 17º 15.7’ and longitude 73º 44.5’.
The information of the earthquakes was collected from the Koyna Dam Authorities.
GPS Data Analysis
The GPS measurements at Koyna Dam area including the dam body itself are usually carried
out two – three times per year by the GPS team using Trimble 4000SSI and Trimble 5700
dual frequency geodetic GPS receivers. During the latest campaign in June 2005, an
earthquake with M 4.2 followed by its aftershock of M 3.6 occurred on June 8th, 2005. A GPS
receiver was set up the previous day on the top of the dam (Figure 1) and was continuously
operating even during the time of the earthquakes. After the occurrence of these earthquakes,
the attention was focused to study the co-seismic movement of the dam body due to these
earthquakes and was done by analysing the GPS data collected before and after the
earthquakes. Data collected 2.5 hours before the earthquakes and the 2.5 hours including the
main shock and after shock was considered to study the co-seismic movement of the dam. The
data collected in next 2.5 hours after the earthquakes were also considered to study the
response of the dam after the earthquakes. The collected data was processed using Bernese
v.4.2 Software [4]. To get very high accurate results, three of IGS stations BAHR in the
Arabian plate, LHAS in the Eurasian plate and IISc in the Indian plate, were used in addition
to the Koyna Dam station (Figure 2).
Change in Displacement of Dam Station
Analysis of the periods 2.5 hours before and 2.5 hours including the earthquakes shows a
displacement of about 2.3 cm (Table 3) in the northwest direction (Figure 4). This is due to
the co-seismic effect of the main shock of M 4.2 and its aftershock M 3.6, which occurred
southeast of the Koyna Dam (Figure 4). Tectonic features including the Koyna seismic zone
and location of the earthquakes are also shown (Figure 4) [5, 6].
For detecting the change in the movement of deformation of the dam, analysis between the
results obtained from the data collected 2.5 hours including the earthquakes and the next 2.5
hours after the earthquakes were carried out. The analysis shows a displacement of 2.1 cm
(Table 4) with a southeast direction (Figure 4). The similarity in the value of displacements
and their opposite direction of movement indicates that the dam body rebounds to its original
position and is therefore a case of forced oscillations of an over-damped system (dam). This
movement in different directions might be due to the high elastic property of the dam body.
Conclusion
This is probably the first case in India that the co-seismic displacement of a dam body was
estimated using GPS data recorded during the time of occurrence of an felt earthquake and its
aftershock.
The value of displacement of the dam obtained from GPS of 2.3 cm observed during the time
of the earthquakes indicates that effect of the waves on the dam is small. This is probably due
to the fact that the Koyna dam has rubble and concrete foundation that might be providing a
damping to the seismic waves.
The return of the displacement of the dam to its original position after the earthquakes reflects
the high response value of the dam foundation and this means the dam body is stable and not
affected by the earthquakes. It might be because of these reasons that the dam did not suffer
much damage due the December 11, 1967 earthquake of M 6.3.
It is highly recommended that continuously operating GPS deformation monitoring systems
should be set up on all major dams and bridges in India, for monitoring deformations
associated with earthquakes, in real time and should be interpreted in combination with
response of dams and bridges.