Limits of the seismogenic zone in the epicentral region of the 26 December 2004 great Sumatra-Andaman earthquake:Results from seismic refraction and wide-angle reflectionsurveys and thermal modeling

摘要

The 26 December 2004 Sumatra earthquake (M_w = 9.1) initiated around 30 km depthand ruptured 1300 km of the Indo-Australian—Sunda plate boundary. During the Sumatra-OBS (ocean bottom seismometer) survey, a wide-angle seismic profile was acquiredacross the epicentral region. A seismic velocity model was obtained from combined traveltime tomography and forward modeling. Together with reflection seismic data from theSeaCause II cruise, the deep structure of the source region of the great earthquake isrevealed. Four to five kilometers of sediments overlie the oceanic crust at the trench, andthe subducting slab can be imaged down to a depth of 35 km. We find a crystallinebackstop 120 km from the trench axis, below the fore-arc basin. A high-velocity zone atthe lower landward limit of the ray-covered domain, at 22 km depth, marks a shallowcontinental Moho, 170 km from the trench. The deep structure obtained from the seismicdata was used to construct a thermal model of the fore arc in order to predict the limits ofthe seismogenic zone along the plate boundary fault. Assuming 100°-150°C as itsupdip limit, the seismogenic zone is predicted to begin 5-30 km from the trench. Thedowndip limit of the 2004 rupture as inferred from aftershocks is within the 350°-450°Ctemperature range, but this limit is 210-250 km from the trench axis and is muchdeeper than the fore-arc Moho. The deeper part of the rupture occurred along the contactbetween the mantle wedge and the downgoing plate.