Description: On 12 September 2007, an Mw8.4 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. Traveltime data obtained from a temporary local seismic network, deployed between December 2007 and October 2008 to record the aftershocks of the 2007 event, was used to determine two-dimensional (2-D) and three-dimensional (3-D) velocity models of the Mentawai segment. The seismicity distribution reveals significant activity along the subduction interface and within two clusters in the overriding plate either side of the forearc basin. The downgoing slab is clearly distinguished by a dipping region of highVp (8.0 km/s), which can be a traced to ∼50 km depth, with an increased Vp/Vs ratio (1.75 to 1.90) beneath the islands and the western side of the forearc basin, suggesting hydrated oceanic crust. Above the slab, a shallow continental Moho of less than 30 km depth can be inferred, suggesting that the intersection of the continental mantle with the subducting slab is much shallower than the downdip limit of the seismogenic zone despite localized serpentinization being present at the toe of the mantle wedge. The outer arc islands are characterized by low Vp (4.5–5.8 km/s) and high Vp/Vs (greater than 2.0), suggesting that they consist of fluid saturated sediments. The very low rigidity of the outer forearc contributed to the slow rupture of the Mw 7.7 Mentawai tsunami earthquake on 25 October 2010.

Description: The Sumatran margin suffered three great earthquakes in recent years (Aceh-Andaman 26 December 2004 Mw = 9.1, Nias 28 March 2005 Mw = 8.7, Bengkulu 12 September 2007 Mw = 8.5). Here we present local earthquake data from a dense, amphibious local seismic network covering a segment of the Sumatran margin that last ruptured in 1797. The occurrence of forearc islands along this part of the Sumatran margin allows the deployment of seismic land-stations above the shallow part of the thrust fault. In combination with ocean bottom seismometers this station geometry provides high quality hypocentre location for the updip end of the seismogenic zone in an area where geodetic data are also available. In this region, the Investigator Fracture Zone (IFZ), which consists of 4 sub-ridges, is subducted below the Sunda plate. This topography appears to influence seismicity at all depth intervals. A well-defined linear streak of seismicity extending from 80 to 200 km depth lies along the prolongation of closely spaced IFZ sub-ridges. More intermediate depth seismicity is located to the southeast of this string of seismicity and is related to subducted rough oceanic seafloor. The plate interface beneath Siberut Island which ruptured last in 1797 is characterised by almost complete absence of seismicity.

Description: Great subduction earthquakes exhibit segmentation both within the rupture of individual events and in the long term history of the margin. The 2004 December 26 Aceh-Andaman and 2005 March 28 Nias event in northern Sumatra are two of the largest earthquakes in recent years, with both co- and post-seismic displacements constrained in unprecedented detail. Using aftershock locations from a temporary seismic array in the boundary region between both events and waveform modelling of large aftershocks, we demonstrate that the vast majority of aftershocks in the study region occur on the plate interface within a narrow band ( 20 km) seaward of the outer arc high. Comparing the seismicity distribution to the co- and post-seismic displacements, we infer that the seismic band marks the transition between the seismogenic zone and stable sliding. The location of the band and therefore the transition appears to be correlated with the ∼500 m bathymetry contour. This close correspondence is disrupted at the boundary between the two great earthquakes, where the transition to seismogenic behaviour occurs further landward by ∼25 km. To the west of Simeulue, where seafloor bathymetry throughout the forearc is deeper than 500 m, the seismic band terminates abruptly and the focus of aftershock activity is found near the trench. The seismic efficiency of afterslip varies dramatically along strike: the segment below the Banyak islands, in the gap between the two main asperities of the Nias earthquake, accommodates a much larger proportion of afterslip seismically.

Description: On 12 September 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. Traveltime data obtained from a temporary local seismic network, deployed between December 2007 and October 2008 to record the aftershocks of the 2007 event, was used to determine two-dimensional (2-D) and three-dimensional (3-D) velocity models of the Mentawai segment. The seismicity distribution reveals significant activity along the subduction interface and within two clusters in the overriding plate either side of the forearc basin. The downgoing slab is clearly distinguished by a dipping region of high Vp (8.0 km/s), which can be a traced to ∼50 km depth, with an increased Vp/Vs ratio (1.75 to 1.90) beneath the islands and the western side of the forearc basin, suggesting hydrated oceanic crust. Above the slab, a shallow continental Moho of less than 30 km depth can be inferred, suggesting that the intersection of the continental mantle with the subducting slab is much shallower than the downdip limit of the seismogenic zone despite localized serpentinization being present at the toe of the mantle wedge. The outer arc islands are characterized by low Vp (4.5–5.8 km/s) and high Vp/Vs (greater than 2.0), suggesting that they consist of fluid saturated sediments. The very low rigidity of the outer forearc contributed to the slow rupture of the Mw 7.7 Mentawai tsunami earthquake on 25 October 2010.

Description: The Sumatran margin hosted three great earthquakes in the last decade (Aceh-Andaman 26 December 2004 Mw = 9.2, Nias 28 March 2005 Mw = 8.7, Bengkulu 12 September 2007 Mw = 8.5), two of which were associated with significant loss of life. Yet, part of the margin near the northern Mentawai islands remains unbroken, and the historical record suggests that only half of the accumulated tectonic strain might have been released by the Bengkulu earthquake in 2007. Local earthquake tomography techniques are applied on manually picked arrival times from a local network that operated between April 2008 and February 2009. The network consists of 52 continuously recording land stations and 10 OBS covering 325 km of the active margin between 1.5_S and 1.5_N. This segment of the Sumatran margin ruptured last in 1797 and is characterised by the Investigator Fracture zone which subducts beneath the Sumatran mainland near the southern termination of the 2005 Nias event. The subduction of this aseismic ridge appears to influence seismicity down to 200 km depth. We present 2D and 3D velocity models including focal mechanism based on first motion polarities. The Vp and Vs velocity models resolve the structure of the forearc including the downgoing slab, the sedimentary basins and the volcanic arc down to depths of 75 km. The Vp velocity beneath the outer arc islands is between 5-6km/s while the region between the Batu Islands and the mainland shows considerably lower Vp velocities of _4.5 km/s. At depths less than 40 km, the upper part of the subducting slab has a velocity of _7 km/s and is located beneath the outer arc islands at a depth of _25km, dipping at a shallow angle.

Description: The Sumatra subduction zone is located on the eastern side of the Sunda Arc between the Sunda Strait and the Andaman Islands, where the Indo-Australian plate is subducting beneath the Eurasian plate. An important tool in understanding the style and geometry of deformation within a subduction zone is the measurement of seismic anisotropy, through observations of shear wave splitting, which provides information about the mantle flow. In Sumatra two temporary seismic networks were deployed within the Mentawai and Northern region, between December 2007 and October 2008 and April 2008 and February 2009, respectively. Here we use new splitting measurements from SKS and local S phases from these networks, to characterize the type and amount of anisotropy within the region. High quality SKS splitting results obtained from 16 stations show a coherent fast direction of NNW (350°) to SSE (170°) with delay times of 1.0 to 3.0s. The observations do not conform to the standard classification of trench parallel and trench perpendicular, instead they are trench oblique, approximately parallel to the direction of motion of the subducting Indo-Australian plate. The magnitude and direction of splitting suggests that the anisotropy is due too entrained flow in the asthenosphere beneath the subducting lithosphere. Local S wave splitting measurements were obtained from 85 rays, generated by earthquakes at focal depths of 15 km-200 km and recorded across 39 stations. The polarization of the fast shear wave is trench parallel along the islands and can be contributed to shape preferred orientation of cracks in the top of the subducting slab and overriding crust. In the forearc a rotation in fast direction to trench perpendicular is observed, with a clear positive correlation between the distance the ray has traveled in the mantle and delay time, suggesting anisotropy originates from entrained flow within the mantle wedge. In the Sumatra fault region and the back arc, events originating at the plate interface show both trench perpendicular and trench parallel fast directions with delay times of up to 0.4 s, while shallow events on the Sumatra fault indicate a clear fault parallel direction with delay times of 0.1 s to 0.18 s. This pattern of shear wave splitting suggests layers of different anisotropy, one deeper layer due to entrained flow within the Sumatran mantle wedge causing trench perpendicular direction, and the second shallower one within the overriding crust due to shape preferred orientation possibly caused by the Sumatran Fault.

Description: Great subduction earthquakes exhibit segmentation both within the rupture of individual events and in the long term history of the margin. The 2004 December 26 Aceh-Andaman and 2005 March 28 Nias event in northern Sumatra are two of the largest earthquakes in recent years, with both co- and post-seismic displacements constrained in unprecedented detail. Using aftershock locations from a temporary seismic array in the boundary region between both events and waveform modelling of large aftershocks, we demonstrate that the vast majority of aftershocks in the study region occur on the plate interface within a narrow band (〈∼20 km) seaward of the outer arc high. Comparing the seismicity distribution to the co- and post-seismic displacements, we infer that the seismic band marks the transition between the seismogenic zone and stable sliding. The location of the band and therefore the transition appears to be correlated with the ∼500 m bathymetry contour. This close correspondence is disrupted at the boundary between the two great earthquakes, where the transition to seismogenic behaviour occurs further landward by ∼25 km. To the west of Simeulue, where seafloor bathymetry throughout the forearc is deeper than 500 m, the seismic band terminates abruptly and the focus of aftershock activity is found near the trench. The seismic efficiency of afterslip varies dramatically along strike: the segment below the Banyak islands, in the gap between the two main asperities of the Nias earthquake, accommodates a much larger proportion of afterslip seismically.