Structure of the central Sumatran subduction zone revealed by local earthquake travel-time tomography using an amphibious network

Abstract. The Sumatran subduction zone exhibits strong seismic and tsunamogenic potential with the prominent examples of the 2004, 2005 and 2007 earthquakes. Here, we invert travel-time data of local earthquakes for v_p and v_p∕v_s velocity models of the central Sumatran forearc. Data were acquired by an amphibious seismometer network consisting of 52 land stations and 10 ocean-bottom seismometers located on a segment of the Sumatran subduction zone that had not ruptured in a great earthquake since 1797 but witnessed recent ruptures to the north in 2005 (Nias earthquake, M_w = 8.7) and to the south in 2007 (Bengkulu earthquake, M_w = 8.5). The 2-D and 3-D v_p velocity anomalies reveal the downgoing slab and the sedimentary basins. Although the seismicity pattern in the study area appears to be strongly influenced by the obliquely subducting Investigator Fracture Zone to at least 200 km depth, the 3-D velocity model shows prevailing trench-parallel structures at depths of the plate interface. The tomographic model suggests a thinned crust below the basin east of the forearc islands (Nias, Pulau Batu, Siberut) at  ∼ 180 km distance to the trench. v_p velocities beneath the magmatic arc and the Sumatran fault zone (SFZ) are around 5 km s⁻¹ at 10 km depth and the v_p∕v_s ratios in the uppermost 10 km are low, indicating the presence of felsic lithologies typical for continental crust. We find moderately elevated v_p∕v_s values of 1.85 at  ∼ 150 km distance to the trench in the region of the Mentawai Fault. v_p∕v_s ratios suggest an absence of large-scale alteration of the mantle wedge and might explain why the seismogenic plate interface (observed as a locked zone from geodetic data) extends below the continental forearc Moho in Sumatra. Reduced v_p velocities beneath the forearc basin covering the region between the Mentawai Islands and the Sumatra mainland possibly reflect a reduced thickness of the overriding crust.