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Aftershock Investigation in the Andaman-Nicobar Islands: An Antidote to Public Panic?

Central Geophysics Division
Geological Survey of India
Kolkata 700016,India
niom_mishra2005{at}yahoo.co.in
(O.P. M.)

View larger version (54K): [in this window] [in a new window]   Figure 1. Tectonic map of the study area with the 26 December 2004 Sumatra-Andaman mainshock (Mw 9.3) shown by a black asterisk (MS) and four (1-4) big aftershocks (Mw 6.3) denoted by black asterisks and their corresponding CMT solutions (from the U. S. Geological Survey) marked with 1 to 4. The black-and-white shed in the CMT projections denote compressive and tension zones, respectively. The open triangles denote seismograph locations (from south to north: Car Nicobar; Hutbay; Port Blair; Rangat; Diglipur; and Narcondum) installed by Geological Survey of India following the mainshock. Volcanic zones (BV: Barren volcano; NV: Narcondum, and BT: Baratang mud volcano) are denoted by black triangles. The study region is demarcated by a rectangle in the insert map of India.

View larger version (71K): [in this window] [in a new window]   Figure 2. (A) Graph showing aftershocks recorded until 16 March 2005 at different seismograph stations as shown by their alphabetical codes (PBR: Port Blair; CNB: Car Nicobar; HTB: Hutbay; RGT: Rangat; DGP: Diglipur; NCD: Narcondum). (B-D) Graphs showing temporal variations of aftershocks at different seismograph stations from 6 to 31 January 2005. The star denotes the occurrence of the mainshock (Mw 9.3).

View larger version (79K): [in this window] [in a new window]   Figure 3. (A) Epicenter map of aftershocks located by the multiple-station method in this study. The variation in size of circles denotes the variation in magnitude of aftershocks (see magnitude scale) and the variations in color of the circles denote variations in depths of aftershocks (see depth scale). The black star denotes the mainshock epicenter. The white triangle denotes the location of the Baratang mud volcano. The black triangles denote Barren and Narcondum volcanoes. The white boundary zones denote the aftershock clusters or swarms around the volcanic zones. WAF represents the West Andaman fault. (B) The entire study area is divided into 10 (1-10) different blocks for studying b-values (table 1), and composite fault-plane solutions of aftershock clusters (table 2 and figure 4). (C) Epicenter map of aftershocks recorded by teleseismic network ascribed to USGS/IRIS. The black triangles denote Barren and Narcondum volcanoes while the black star denotes the Sumatra-Andaman mainshock. The question marks denote anticipated aftershock gap that does not exist in figure 3(A).

View larger version (110K): [in this window] [in a new window]   Figure 4. (A) Photo showing mud eruption at Baratang on 28 December 2004 in Middle Andaman. Arrow indicates location of the mud crater. (B) Ten blocks same as shown in figure 3(B) with aftershock clusters at 0-15,16-30, > 31 km depths. (C) Composite fault-plane solutions denoted by numeric values in parentheses (table 2). Photo by O. P. Mishra.

View larger version (99K): [in this window] [in a new window]   Figure 5. Lava eruption (bright areas) from Barren Island volcano through several craters. The photos were taken from different directions at a distance of about 300 m from the volcanic source. Arrows indicate locations of lava craters. Photos by O. P. Mishra.

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