The Sumatra subduction zone: seismicity, velocity structure and seismic anisotropy

On September 12 2007, an Mw 8.4 earthquake occurred within the southern section of the Mentawai segment of the Sumatra subduction zone, where the subduction thrust had previously ruptured in 1833 and 1797. Following the 2007 rupture, a temporary local network was installed in the Mentawai region between December 2007 and October 2008 to record the aftershocks. Additionally, a second network was installed in central Sumatra between April 2008 and February 2009. In this study the data obtained from the Mentawai network were used to determine 2D and 3D Vp and Vp/Vs models, first motion polarity focal mechanisms and accurate hypocentre locations. In addition to this, shear wave splitting (SWS) measurements from both networks were used to determine the type, amount and location of anisotropy. This has enabled us to obtain a detailed image of the structure of the subduction zone, ascertain the down-dip limit of the seismogenic zone and determine the deformation occurring. The forearc islands are characterized by a low Vp (4.5-5.8 km/s) and a high Vp/Vs ratio (>2.0), suggesting that they consist of fluid-saturated sediments. The down-going slab is clearly distinguished by a dipping region of high Vp (8.0 km/s), which can be traced to ~50 km depth, with an increased Vp/Vs ratio (1.75 to 1.90) beneath the forearc islands and the western side of the forearc basin, suggesting hydrated oceanic crust. Beneath the slab, a ~150 km thick layer of sub-slab anisotropy has developed due to the oceanic asthenosphere being entrained by the subducting slab. Two clusters of seismic activity are found within the ~25-30 km thick overriding crust. The location of the first cluster confirms that the Mentawai Fault is active and may accommodate backthrust movement, while the second cluster suggests a backthrust may be present on the eastern side of the forearc basin. Local SWS measurements suggest that in the overriding plate, adjacent to the Sumatran Fault, a layer of anisotropy has formed from fault-parallel aligned fractures and minerals. Beneath the forearc, a shallow continental Moho o