Tsunami generated by Great Sumatra and Andaman earthquake in 2004 with Mw of 9.3 is greater than the size of the earthquake magnitude. The southern 400 km rupture was a fast slip and northern 900 km rupture was a slow slip. Time window is inadequate to alert the public about the generation of ocean wide tsunami for earthquakes > Mw 8.5. To compare the size of the tsunami caused by this earthquake with other great earthquakes tsunami magnitude , ‘Mt’, and body wave ‘ mb ‘ surface wave , ‘Ms’, moment magnitude ,‘Mw’ and seismic duration ,‘T’ of earthquake are considered. The larger differences between ‘Mt’ 9.1 and ‘Mw 9.3 ’ and the variation between ‘mb 7.25 and Mw 9 3 can be accredited to the abnormal nature of source of slow faulting or submarine slide. Multiple focal mechanisms in subduction zone and uplift of western and submergence of eastern margins of Nicobar –Andaman islands appears to have slipped 10 m can be accounted for by seismic model with time scale of ~ 1 hour. Nevertheless, no such strong seismic waves’ were observed in aftershock zone. But satellite observations of tsunami waves in Bay of Bengal after 2 to 3 hours of the rupture, constrain on the slip distribution in the aftershock zone. This aftershock zone is directly perpendicular to that tsunami waves that stoke along coasts of Sri Lanka, India and Thailand. Huge methane gas hydrate deposits reported in off shore of Andaman. Triggering of 2004 earthquake increased the pore pressure of the gas hydrate, free sediment gas; seepage –mud volcanoes and unroofed sediments and initiated slope instability and submarine landslides in consequent to that catastrophic ocean wide tsunami devastated Indian Ocean countries in 2004.
