Seismicity Within the West Sumatra Subduction Zone

Oblique subduction along the western Sunda arc results in strain partitioning along the Sumatra margin. The oblique 65 mm/yr convergence is partitioned into 45 mm/yr of thrust motion accommodated along the megathrust and 11-28 mm/yr of dextral strike slip motion along the Sumatra Fault. Past seismicity along the Sumatra-Andaman margin indicates that previous large earthquakes (1797, 1833, 1861, 2004, 2005) along Sumatra margin, resulted from shallow thrust faulting below the fore-arc. The Sumatra Fault extends for 1900km from the Sunda Strait to the Andaman sea across Sumatra, creating a plate sliver known as the Sunda fore-arc. The obliquity of the subduction zone and increase in slip rate as you move northwest wards along the fault (6mm/yr at the Sunda strait to 25 mm/yr at the equator, increasing to to 50 mm/yr in the Andaman sea) results in stretching of the Sunda fore-arc. As only two thirds of the trench parallel motion is taken up by the Sumatran Fault in southern Sumatra, one third of the motion must be taken up elsewhere. Within the Sunda fore-arc, east of the Mentawai Islands at the boundary between the fore-arc ridge and the fore-arc basin lies the 600km long Mentawai fault. Originally it was suggested that the Mentawai fault was a large strike slip fault but more recent studies indicate that this may not be the case. Here, data is presented from a seismic network located within the Mentawai region of Sumatra between November 2007 and October 2008, recording the the post seismic activity of the Mw 8.5 Bengkulu 2007 earthquake including the Mw 7.2 event in February 2008. From the continuous data, arrival times of compressional (P) and shear waves (S) for 1037 events (12 600 P picks and 6 285 S picks) were manually picked, with a high quality subset of 386 events being selected for a simultaneous inversion to determine accurate hypocentres, a 1D velocity model for the region and station corrections. Hypocentre locations in the first half of the survey (November 2007 to April 2008) exhibit a high level of aftershock activity from the 2007 and 2008 earthquakes focused between the North and South Pagai islands, 100km from the trench at depths of 10-30km. Two large clusters of activity are seen 175km and 200km from the trench, within the offshore fore-arc between the mainland and the islands, which coincide with the SW and NE boundaries of the fore-arc basin. The first cluster probably corresponds to the Mentawai fault but the second cluster that extends from the slab into the overriding crust does not appear to correspond with any previously mapped faults. In the second half of the survey (April 2008 to October 2008) as the aftershock rate declines significantly, seismic activity is more evenly spread on the subduction interface.