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

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	SUMMARY

 
Public panic prevailed in every part of the Andaman and Nicobar Iislands of India following the megathrust Sumatra earthquake (M_w 9.3) on 26 December 2004. In this article, we present a very brief analysis of our continuous three-month (January-March 2005) monitoring and recording of aftershock data following the main earthquake to show how this endeavor reduced public panic and constituted an important ingredient to a disaster management program for the Andaman-Nicobar region. Monitoring was conducted using six short-period three-component temporary digital seismograph stations set up in different parts of the Andaman and Nicobar islands. Our findings demonstrate that 1) there was no aftershock gap zone as recorded by the far-distant seismographic network, hence negating the possibility of immediate strong quakes in the Andaman and Nicobar islands; 2) there was no strong shaking at full moon (26 January 2005) due to tidal stresses, although the rate of aftershock activity increased by about 31% from the events of the preceding day; 3) there is a north-south trending 850 x 350-km² rupture area beneath the Andaman and Nicobar islands; and 4) eruptions of mud at the Baratang volcanic zone and lavas at the Barren volcanic zone occurred because of strong shaking due to the mainshock and a series of aftershock clusters within 100 km of the individual volcanic zones. These eruptions may continue for a couple of years, until the aftershock sequence ceases. We show that this new dataset from the local seismographic network constituted an important factor in reassuring the coastal people and islanders about imminent dangers from future earthquakes and tsunamis, and we propose geochemical tests of the erupted mud samples from the Baratang volcanic zone, with geophysical exploration followed by drilling, to ascertain the presence of oil and gas reserves and the phenomenon of gas seepage under the land . . . [Full Text of this Article]

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