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1 Central Geophysics Division
Geological Survey of India
Kolkata 700 016, India
(O.P.M., J.R.K., O.P.S.)
2 Geophysics Division, Eastern
Region
Geological Survey of India
Kolkata 700091, India
(G.K.C., D.G.)
* Present address: Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan.
Six three-component short-period digital seismograph (four Reftek and two
Kinemetrics) stations were established in different parts of the Andaman–
Nicobar Islands following the 26 December 2004 devastating Sumatra–Andaman
mainshock (Mw 9.3). Here, we analyze about 18,000
aftershocks (M 
3.0) recorded from 6 January to 16 March 2005 to
better understand the seismotectonics of the region. A sudden burst of
aftershock activity with irregular trend in the month of January 2005 was
observed at almost all seismograph stations. The estimate of P- value =
0.9532 from aftershocks (M 4.5) is near to normal value of 1.0,
which suggests a slow decay sequence of aftershock with complex and nonuniform
stress change in a fault system (creep effects and history-dependent stress
changes). The frequency–magnitude relation of the aftershocks followed the
power law with average b-value of 0.7723 and it varies from 0.49 to
1.03, indicating the compressive stress state of the region and its
heterogeneous structure with possible variations in frictional conditions along
the fault. The distribution of the located aftershocks by a multistation method
shows a north–south-trending aftershock cluster in an area of about 800 x 300
km2, which reflects an approximate rupture dimension of the mainshock
beneath the Andaman–Nicobar Islands. Most of aftershocks occurred in a depth
range of 5–65 km. The determination of composite fault-plane solutions of the
best located aftershock clusters at three different depth ranges (0–15,
16–30, and >31 km) in the ten blocks of the region suggests that the mainshock
rupture propagated through normal, reverse, and strike-slip earthquakes. The
areas in the vicinity of Barren and Narcondum volcanic zones show a dominantly
normal fault mechanism due to predominant tensional forces, suggesting the
facilitation of brittle failure in the weakened crust by the process of
underheating. The zone near Baratang mud volcano is associated predominantly
with thrust fault at depth of 0–30 km, suggesting a high compressive force
beneath the Baratang that ejected a huge amount of mud and slurry materials to
the surface after the 26 December 2004 Sumatra–Andaman earthquake
(Mw 9.3). Here, we propose that the relocation of our large
aftershock dataset using converted sP-depth-phase technique for precise
depth control is needed, and its sharing with global earthquake data can render
more information on the subduction dynamics and geodynamical processes of the
region.
This article has been cited by other articles:
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  O. P. Mishra, O. P. Singh, G. K. Chakrabortty, J. R. Kayal, and D. Ghosh Aftershock Investigation in the Andaman-Nicobar Islands: An Antidote to Public Panic? Seismological Research Letters, November 1, 2007; 78(6): 591 - 599. [Full Text] [PDF]
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 S. L. Bilek, K. Satake, and K. Sieh Introduction to the Special Issue on the 2004 Sumatra-Andaman Earthquake and the Indian Ocean Tsunami Bulletin of the Seismological Society of America, January 1, 2007; 97(1A): S1 - S5. [Full Text] [PDF]
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