Stressed state of the Earth’s crust in the western region of the Sunda subduction zone before the Sumatra-Andaman earthquake on December 26, 2004

Abstract

Reconstruction of the parameters of stress tensor based on focal mechanisms of earthquakes occurring before the Sumatra‐Andaman earthquake (SAE) showed that deviatory stresses and all-round compression in the western Sunda subduction zone were distributed nonuniformly. Nonuniform state of the stress in this region is caused by the active sectors of the Andaman‐Nicobar and Central Sumatra dextral strike-slip fractures. It was found that peculiarities in the development of the seismic fracture are caused by the revealed inhomogeneity of stresses. The fracture, which originated in the regions with the greatest level of deviatory stresses and internal elastic energy, spread to the north and south along the subduction zone with different velocities. This is related to different values of the earthquake efficiency during the spreading of the fracture into the region of greater or smaller stresses. The maximal velocity of the development of the fracture front is confined to the lowest stress area, which is located to the north of the fracture initiation. Correspondingly, the epicenter of the earthquake near Nias Island ( M w = 8.7) in March 2005 was located to the south of the epicenter of the December 26, 2004, earthquake in the same high stress region.