Reanalysis of Aftershocks of the 2004 Great Sumatra Earthquake in the Andaman-Nicobar Islands Region Using Temporary Local Network and Global Arrival Time Data

The Geological Survey of India, following the Mw 9.0 2004 Sumatra-Andaman Islands great earthquake, deployed six three-component short-period digital seismograph stations along islands of the Andaman-Nicobar region. The arrival time data from this temporary local network, for the period 6 January through 16 March 2005, are combined with arrival time data reported globally to the USGS/NEIC for events between 8-15 deg North and 91-95 deg East. The combined arrival time data for this period result in significantly improved epicenters and focal depths for more than 160 earthquakes in the region, including 40 events that have Harvard CMT solutions. The reduced uncertainties in the epicenters and depths of these events (on the order of 15 and 10 km, respectively) provide additional details about the subducting Indian plate and foster improved interpretation of the focal mechanism data. The revised earthquake dataset reveals a downdip width of ~200 km for the aftershock zone of the 2004 Mw 9.0 earthquake in the Andaman-Nicobar Islands region. Nearly all of the aftershocks are located along a shallow eastward dipping plane of earthquakes that range in focal depth from 7 to 50 km. A complex mix of normal, reverse and strike-slip faulting characterize earthquakes along the trenchward part of the plane. Shallow depth normal fault earthquakes that were triggered along the Andaman back-arc spreading center illustrate the complex and variable nature of seismicity following this great earthquake.