Aftershocks and Seismotectonic Implications of the 13 September 2002 Earthquake (Mw 6.5) in the Andaman Sea Basin

A medium-size earthquake mainshock (Mw 6.5) occurred on 13 September 2002 (22 h 28 m 31 s UTC) in the Andaman Sea, about 20 km offshore Diglipur, north Andaman Island. The epicenter at 13.087° N and 93.112° E and centroid depth at 31 km were estimated by the U.S. Geological Survey (USGS). The main shock was a reverse-faulting event. Four digital seismograph stations were established in the Andaman Islands to monitor aftershocks. About 200 aftershocks (Md >=1.0) were recorded during a period from 19 September to 7 October 2002. The epicenter map shows a northwest-southeast-trending aftershock cluster in an area of about 140 x 70 km^2, which reflects the rupture area of the mainshock. The aftershocks occurred mostly at a depth range of 5-20 km, except one at 38 km. The moment tensor solutions (USGS) of the mainshock and the largest aftershock (Mw 5.8), which occurred 22 hr after the mainshock, revealed consistent reverse faulting. The northeast-dipping northwest-southeast-trending nodal plane, which is comparable with the aftershock trend, is inferred to be the fault plane. The northeast-southwest compressional stress (P axis), obtained by the fault-plane solutions, is compatible with the north-northeast movement of the Indian plate. The mainshock and the two well-located largest aftershocks, located by the USGS, occurred within the subducted plate. One composite fault-plane solution is obtained for the shallower aftershocks recorded by the temporary network. The solution shows normal faulting with a northeast-southwest tensional (T) axis. These aftershocks occurred off the subducted plate at a much shallower depth (<10 km) and are due to local tension in the overriding plate.