The Structure of the Mentawai segment of the Sumatra subduction zone revealed by local earthquake travel time tomography

Within the last five years three large earthquakes greater than Mw 8 have occurred along the Sumatra Andaman margin, 2004 Mw=9.3, 2005 Mw=8.6 and 2007 Mw=8.5. The 2007 Bengkulu earthquake and subsequent Mw=7.9 event off the coast of Padang 12 hours later occurred within the southern section of the Mentawai segment where the fault previously ruptured in 1833 and 1797. The rupture areas of the 1833 and 1797 earthquakes indicate that permanent barriers to rupture propagation exist north of the Mentawai segment at Batu and south at Enggano. However the rupture pattern of the 2007 earthquake also suggests that non permanent barriers can develop; preventing complete failure of a segment. Between December 2007 and October 2008 a seismic array of 29 stations (19 three component and 10 broadband stations) was installed in western Sumatra between 0°S and 4°S on the islands of Enggano, the Mentawai Islands and the adjacent mainland to record the aftershock activity of the 2007 event. From the continuous data, arrival times of compressional (P) and shear (S) waves for 1037 events were manually picked. A high quality subset of 312 events (4535 P picks and 2644 S picks) that have a GAP of <180°, 10 or more P picks and 4 or more S picks was selected for a simultaneous inversion to determine, firstly, a 1D velocity model and subsequently a 2D and 3D velocity model for the region with accurate hypocentre locations. The final 1D velocity model has a velocity of 5.53 km/s at the surface with a sharp increase to 7.23 km/s at 20-22.5km depth representing the top of the subducting slab. At 50km depth the velocities reach 8.2km/s. The Vs model suggests a high Vp/Vs ratio at both shallower depths (<5km) and between 20 to 27.5km. The 2D tomography indicates that the continental Moho reaches a shallow depth of ~25-30km, agreeing with previous geophysical studies. The upper part of the subducting slab has a velocity of ~7km/s and is located beneath the outer arc islands at a depth of ~15-20km, dipping at a shallow angle. The velocity of the outer arc islands is between 5-6km/s with no high velocity bodies observed, indicating that the islands do not have a volcanic origin and are part of the accretionary prism. The Vp/Vs tomography shows that the Vp/Vs ratio varies significantly across the subduction zone. Beneath the islands, the western side of the forearc basin and within part of the subducting slab a high Vp/Vs ratio of up to 2.10 is observed. In contrast beneath the mainland the Vp/Vs ratio is considerably lower, 1.70-1.80. The seismicity distribution reveals significant activity along the subduction interface. Additionally two clusters either side of the forearc basin are observed. The western cluster extends from ~20km depth through the subducting slab to 35km depth, collocated within a region of high Vp/Vs, near the Mentawai Fault. The cluster on the eastern side of the forearc basin is at a shallow depth, 0-15km, and could be attributed to a previously unidentified thrust fault.