4.4. Seismo-tectonics, Volcanism, Tsunami
There
are two earthquake events that have been recorded in the area just south of the
project area in the recent past.
The
first record belongs to the year 1938. Its epicentre was supposed to be in 07.50
N, 79.00 E, which is just south of the project region. Its magnitude is
questionable but is thought to be 5.8. Its intensity has been recorded as VII.
The
second record is from 1993. Its epicenter was at 06.818 N, 78.301 E. Its depth
(D) was identified to be 10.0 km; Its Body Wave Magnitude (Mb) was 5.2; Its
Surface Wave Magnitude (Ms) was 4.7.
There is
a record that an earthquake which originated at the Car Nicobar islands on 31
December 1881 (with an Mw 7.9) had generated a tsunami in the Bay
of Bengal and this had been felt at Pamban. This fact stresses the importance
of studying the history of the earthquakes in an area roughly around 300 km
around.
The 26th
June 1941 earthquake of Andaman Islands (12.500 N, 92.500 E) (Mw7.7) had also
generated a tsunami and this had reached Chennai.
The
first recorded Bay of Bengal earthquake is on 28th January 1679. It
is suspected to have originated at the Andaman Islands.
The more
recent Pondicherry earthquake of 26th September 2001 had its epicenter
in the ocean crust (11.945 N, 80.227 E, Mw 5.4.
NEERI
EIA mentions the role of tectonics in relation to the reduction in the sea bed
depth in Gulf of Mannar area. Let us quote this passage in some detail:
‘Recent
depth contour map of 1999 has been compared with bathymetry map of 1975 (that
is a 24 year period); it reflects that the sea floor level has decreased along
the coastal areas and around the islands in the study area. It might be due to
the emergence of land or lowering of sea (due to tectonism) and sediment
deposit…. The average depth reduction of sea floor along the coast has been
estimated as 0.51m over this period. .. The annual sediment deposit on the GOM
sea floor is about 0.001m/year (Basanta Kumar Jena 1997), or 0.024m for a period
of 24 years. As found from the present study, the decrease of depth for the
period of 24 years is about 0.51m. Sedimentation accounts for about 0.024m
reduction in the total of 0.5m from clearance depth. The remaining 0.486 m
reduction in depth may be due to emerging of land or lowering of sea level (by
tectonic activities). Based on the above data, the rate of emerging of land or
lowering of sea level can be estimated as 0.02m/year.” ( NEERI EIA p.2.26)
Let us
quote from GRK Murthy et al. (1994) at length to get an idea about the tectonic
setup of the GOM depression and also about the submerged volcanoes present in
it.
‘The
international Indian Ocean Expedition of 1975 had collected a good deal of
magnetic and gravity data on the southern part of the Gulf of Mannar depression
to understand the tectonics and evolution of the basin. (Udintsev, 1975)
Nainini and Talwani (1982) and Kahle et al (1976) have brought out the
isostatic gravity anomaly map of the Mannar depression. Sastri et al (1973,
1981), Prabakar and Zutshi (1993) have studied the structure and basement
morphology from seismic information of the adjoining areas.’
‘According
to Eremenko and Gagelganz (1966), the Ind-Ceylon trough/GOM is considered to
belong to the Mesozoic-Cenozoic group of basins. The regional alignment of
tectonic features is NE-SW, parallel to the Eastern Ghat trend. They have
opined that the basement has a horst-graben configuration resulting from faults
with considerable throw. The depth of the basement is nearly 1 to 6 km in all
the three depressions (the other two are the Palk and the Cauvery depressions),
but the thickness of the sediments is slightly less in the Mannar depression.
Kumar (1983) has reported the presence of basltic rocks from the borehole
drilled in the northern part of the Mannar depression (emphasis
ours).’
‘The
bathymetric contours (NHO Chart, 1974) clearly reveal the physiography of the
GOM. A broad bathymetric trough trending nearly NE-SW between water depths 1000
and 3000 m is noticed. The bathymetric data along AA’ also indicate a broad
trough between water depths 2000 and 2500 m. the profile also indicates a
channel like feature of about 15-20 km width lying within the trough. It is
interesting to note that the maximum and minimum of the anomaly are lying over
the flanks on either side of the axis of the trough.’
‘Due to
complex horst graben structure of the sub basins in the GOM, we attempted to
explain the total observed anomaly assuming uniform magnetization in the modeling.
The basement depth at 4 km obtained from spectral analysis is in agreement with
the observations of Eremenko and Gagelganz (1966). But the modeling indicated a
relief in the depth of 5 to 11 km to the top of the anomalous body. If this is
assumed to be the basement structure then the model indicates the presence of a
sediment thickness of about 7 to 8 km to the top of the anomalous body. If this
is assumed to be the basement structure then the model indicates the presence
of a sediment thickness of about 7 to 8 km in the central portion of the
profile AA’. However there is no evidence of the presence of sediment thickness
of this order in the area. This suggests the possibility of an intrusive body
within the basement. Carl (1966) has reported a shallow earthquake of magnitude
less than 7, very close to AA’. The location of the epicenter of this
earthquake lies over the northward extension of the fractures trending in N-S
direction. Volcanoes with underwater summits were also reported
(Udintsev,1975 and Sastri et al., 1981) in this area and are shown in the above
figure. This suggests the presence of volcanic vents in the area. So the
anomalous body inferred from magnetics could be related to such volcanic vents.
The high susceptibility value of 0.02 cgs suggests that the body could be more
basic in nature.’
‘The
spectral analysis of the marine magnetic data along AA’ across the Gulf of
Mannar reveals two depths at about 4 and 9 km from the sea surface. The two
dimensional model under the constraint of spectral depth shows the presence of
an anomalous body within the basement. The depth to the top of the body varies
from 5 to 11 km from the sea surface with a regional relief, of about 6 km.
This relief coincides with a broad gravity low. The reported volcanic
vents in the vicinity suggest the possibility of the anomalous body and the
volcanic vents in the vicinity may together indicate the presence of a major
tectonic structural feature in the Gulf of Mannar.’