Abstract
A swarm activity occurred east off Nicobar Islands about a month after the 2004 Sumatra-Andaman earthquake. We discussed three issues related to the swarm activity, i.e., (1) How can the spatial distribution of strike-slip events and normal fault events be explained? (2) Why was the swarm activity triggered east off Nicobar Islands? (3) What is the mechanism of one month delay of the swarm activity? In order to address these issues, we relocated the hypocenters of the swarm activity using the Modified Joint Hypocenter Determination (MJHD) method and investigated the spatial distribution of fault plane solutions. As a result, we found that the spatial distribution of strike-slip events and normal fault events can be explained by Riedel shears in the region between West Andaman Fault (WAF) and Sumatra Fault System (SFS). We calculated the spatial pattern of the change of the Coulomb Failure Function (ΔCFF) which suggests that the swarm activity was promoted by the coseismic slip and the afterslip of the Sumatra-Andaman earthquake. One month delay of the swarm activity may be explained by the upward migration of fluid due to the change of bulk strain at the lower end of the mainshock fault.
