SEISMOTECTONIC MODELS OF THE THREE RECENT DEVASTATING

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SEISMOTECTONIC MODELS OF THE THREE RECENT DEVASTATING Powered By Docstoc
					RECENT LARGE EARTHQUAKES IN INDIA WITH SPECIAL REFERENCE TO
THE 2004 TSUNAMI EARTHQUAKE : A SEISMOTECTONIC PERSPECTIVE

J.R. KAYAL

Geological Survey of India, 27, J. L. Nehru Road,
Calcutta – 700 016, India, email : jr_kayal@hotmail.com



         During the last decade (1993-2004), five devastating earthquakes, the
Killari 1993 (mb 6.3), Jabalpur 1997 (mb 6.0), Chamoli 1999 (mb 6.3), Bhuj
2001 (Mw 7.7) and the Sumatra-Andaman 2004 earthquake (Mw 9.3) caused
several damages and large casualties in various parts of India.         These
earthquake sequences are well studied by the Geological Survey of India; the
results are highlighted here.

The 1993 Killari Earthquake (mb 6.3)

         The September 30, 1993 Killari earthquake (mb 6.3) occurred in the
Deccan province of central India; the maximum intensity VIII was
estimated. The Killari earthquake and its 150 well located aftershocks were
confined to a shallower depth (0-15 km), a common type of Stable
Continental Region (SCR) seismicity. The main shock occurred by reverse
faulting at a depth of 6 km; the deeper (6-15 km) aftershocks also occurred
by reverse faulting. The shallower aftershocks, (0-<6 km), on the other
hand, occurred by right-lateral strike-slip faulting. The Killari earthquake
sequence is explained by a fault interaction model. Seismic tomography
study revealed a detailed structure of the source area; the main shock




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occurred at the contact zone of a high velocity and low velocity zones
(Kayal and Mukhopadhyay, 2002).

The 1997 Jabalpur Earthquake (mb 6.0)

         The May 22, 1997 Jabalpur earthquake (mb 6.0), maximum intensity
VIII, occurred at the base of the Narmada Rift Basin, within the SCR at a
depth of 35 km by reverse faulting with a left-lateral strike-slip motion.
Only about 25 aftershocks were recorded that occurred at depth 35-40 km in
the lower crust. The fault-plane solution of the aftershocks also reveals
reverse faulting with left-lateral strike-slip motion.   The south dipping
Narmada South Fault, the southern margin fault of the Narmada Rift Basin,
was activated by reverse faulting (Kayal, 2000). Seismic tomography study
of the source area could not be done due to meagre aftershock data.

The 1999 Chamoli Earthquake (mb 6.3)

         The March 28, 1999 Chamoli earthquake (mb 6.3), maximum intensity
VIII, occurred by thrust faulting on the interplate thrust (Plane of
Detachment) at a shallower depth (21 km) in the western Himalaya,
continent-continent collision zone. The aftershocks were triggered at the
seismogenic faults to the south of the Main Central Thrust by thrust as well
as by strike slip motion (Kayal et al., 2003). Seismic tomography revealed
the seismogenic high velocity structure at the fault end.        The major
thrust/fault zones are well reflected in the seismic images (Mukhopadhyay
and Kayal, 2003).




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The 2001 Bhuj Earthquake (Mw 7.7)

         The most recent devastating Bhuj earthquake (MW = 7.7) of January
26, 2001 is one of the rarest and largest events that occurred in SCR of the
world. The maximum intensity reached X (MSK scale). This is the second
largest event in the western margin of Peninsular India continental region,
after the 1819 Kutch earthquake of MW = 7.8. The 2001 Bhuj earthquake is
another example of a deeper paleo-rift zone earthquake which occurred at a
depth of 25 km in the Kutch Rift Basin. The fault-plane solution of the main
shock and the aftershocks show complicated seismogenic structures. The
observations suggest a fault interaction model, which illustrates that the
main shock originated at the base of the paleo-rift zone by reverse faulting;
the rupture propagated along NE as well as along NW. The aftershocks
occurred by left-lateral strike-slip motion along the NE trending fault,
compatible with the main shock solution, and by pure reverse to right-lateral
strike-slip motion along the NW trending conjugate fault; these are not
compatible with the main shock solution (Kayal et al., 2002a). Seismic
tomography study revealed high Vp, low Vs and high Vp / Vs in the source
area, which indicate that the source area is a fluid-filled fractured rock -
matrix that triggered the main shock (Kayal et al., 2002b).

The 2004 Sumatra-Tsunami Earthquake (MW 9.3)

         The December 26, 2004 Sumatra-Andaman earthquake (MW 9.3) is
the fourth largest event (M>9.0) in the world during the last 100 years. It
occurred by thrust faulting on the interplate thrust of the subducting Indian




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plate and overriding Burma platelet. The main shock rupture, ~1200 km
long, propagated from north of Sumatra to Andaman – Nicobar Islands; the
slow rupture generated the Tsunami which killed about 300,000 people. The
epicentre of the earthquake is located at 3.90N and 94.260E with a focal
depth at 28 km (USGS). The past significant earthquakes in this zone are
the 31 December, 1881 (M 7.9), 26 June, 1941 (M 7.7), 20 January, 1982 (M
6.3) and 14 September, 2002 (Mw 6.5).

         This mega seismic event (Mw 9.3) triggered giant tsunamis that
devastated the coastal regions of Indonesia, Malaysia, Thailand, Sri Lanka,
India, Maldives and even the east coast of Africa. The impact of the tsunami
was quite severe in the coasts of Andaman and Nicobar group of Islands,
Tamil Nadu, Andhra Pradesh, Pondicherry and Kerala. The Air-base in the
Car-Nicobar island was totally devasted by the tsunami, and killed about 200
people. Macroseismic survey was carried out by different teams of GSI in
North Andaman, Middle Andaman, South Andaman, Havelock, Hut Bay and
also in the Nicobar group of Islands.       A maximum intensity VIII was
recorded in the Andaman Islands.

         The mega thrust event was followed by an intense aftershock activity
spreading over an area extending between 30-140N along the Andaman –
Nicobar – Sumatra Island arc region.        The aftershocks are distributed
northwards from the epicentre of the main shock suggesting a unilateral
rupture propagation. The aftershock area covers a length of about 1200 km
and a width of about 200 km, in a ‘banana’ shape that indicates the main
shock rupture area.       The national network (IMD) recorded almost all



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aftershocks M>5.0; about 350 were recorded till 31.01.2005.             The US
Geological Survey (USGS) located about 500 aftershocks M>4.5 till January
31, 2005. The GSI deployed six temporary seismograph stations in the
Andaman and Nicobar Islands and also in the Narkunda (volcanic) islands.
About 20,000 aftershocks (M>3.0) were recorded till end of March, 2005.
About 1000 aftershocks (M>3.0) are located by the GSI network till January
31, 2005. The aftershocks are still continuing, frequency of occurrences is,
however, reduced now. Fault plane solutions of the aftershocks suggest
predominant thrust faulting in the fore arc region, and normal/strike slip in
the back arc region, consistent with the regional tectonics. The oblique
subduction of the Indian plate beneath the Burmese plate is partitioned into
thrust faulting along the plate-interface involving slip directed perpendicular
to the trench, and strike-slip faulting much east of the trench with slip
directed parallel to the trench axis.


References :

Kayal, J.R., 2000. Seismotectonic study of the two recent SCR earthquakes
         in central India, J. Geol. Soc. India, 55, 123-138.

Kayal, J.R. and Mukhopadhyay, S., 2002. Seismic tomography structure of
         the 1993 Killari earthquake source area, Bull. Seism. Soc. Am. 92(5),
         2036-2039.

Kayal, J.R., De, R., Saginaram, Srirama, B.V. and Gaonkar, S.G., 2002a.
         Aftershocks of the 26 January, 2001 Bhuj earthquake in western India
         and its seismotectonic implications, J. Geol. Soc. India, 59, 395-417.



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Kayal, J.R., Zhao, D., Mishra, O.P., De, R. and Singh, O.P., 2002b. The
         2001 Bhuj earthquake: Tomography evidence for fluids at hypocentre
         and its implications for rupture nucleation, Geophys. Res. Lett. 29
         (24), 51-54.

Kayal J.R., Sagina Ram, Singh, O.P., Chakraborty, P.K. and Karunakar, G.,
         2003.       Aftershocks of the March, 1999 Chamoli Earthquake and
         Seismotectonic Structure of the Garhwal Himalaya, Bull. Seism. Soc.
         Am, 93(1), 109-117.

Mukhopadhyay, S. and Kayal, J.R., 2003. Seismic tomography structure of
         the 1999 Chamoli earthquake source area in the Garhwal Himalaya,
         Bull. Seism. Soc. Am, 93(4), 1854-1861.




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