Teleseismic double-difference relocation of earthquakes along the Sumatra-Andaman subduction zone using a 3-D model

We have extended the double-difference seismic tomography method to teleseismic distances with 3-D ray tracing conducted through nested regional-global velocity models and applied the method to relocate seismicity from the Sumatra-Andaman region before and after the great earthquakes of 2004 and 2005. We tested the algorithm's accuracy using both independent local data and an alternate relocation method and found good agreement between the results. The use of depth phases, differential times, and a realistic 3-D velocity model improves the accuracy and precision of epicenters and focal depths, systematically shifting them perpendicular to the trench and shallower, respectively. The relocations refine the location of the megathrust and other faults, the patterns of aftershocks, and their relation to slip during the two great earthquakes. In addition, the relocations reveal several discrete features not readily discernible in the scatter of teleseismic catalogs, including an arcuate, narrow band of earthquakes presumed to define the updip rupture limit in the 2005 event and a lineation at depth tracing the subduction of the Investigator Fracture Zone. When viewed in conjunction with tomography results, the geometry and structural features of the subduction zone are revealed in unprecedented detail.