3-D Seismic Refraction Tomography in Sumatra Subduction Zone

A seismic refraction survey was conducted as part of the major UK and international project to image the 3-D structures and the seismic velocity of the Sumatra subduction zone. The 3-D seismic refraction tomography mainly focusses on the two segment boundaries identified by the earthquake ruptures in 2004 and 2005. High quality seismic datasets (refraction, reflection, gravity and magnetics) were collected in the two survey areas on the vessel R/V Sonne in 2008. The northern area, around the island of Simeulue, is about 196 km long and 185 km wide. 50 Ocean Bottom Seismometers (OBS) were deployed in this area, and 10462 air-gun shots were fired along 1550 km of profiles. 47 OBSs were then installed near the island of Nias, in an area of 246 km long and 180 km wide, and 9134 shots were fired on 1408 km of profiles. During the OBS deployment, air-gun shooting, and OBS recovery, high resolution swathe bathymetry data were recorded, and XBT data were collected in each OBS deployment location. Gravity data were also recorded during the whole survey and magnetics data collected during the air-gun shooting. The 3-D refraction tomography successfully sampled the two survey areas. Refractions from the oceanic and continental crust are clear and easy to pick, and refractions from the mantle lithosphere are also visible at some locations at an offset up to 150 km, which enables us to image the deeper structures of the Sumatra subduction zone. A tomographic inversion program JIVE-3D (Hobro et al. 2003) will be applied to the refraction/reflection travel times to invert them into a minimum-structure velocity model. The high resolution bathymetry will be smoothed and put into the model as a known interface. The XBT data will be used to calibrate the acoustic velocity in the water. During the shooting period, several earthquakes of magnitude 5.0 and above occurred near the survey area, which also provide extra information for the inversion. The well resolved 3-D structure models obtained will give insight into the possible rupture barriers causing the observed segmentation of the subduction zone.