International audienceThe Sumatra subduction zone is one of the most seismically active subduction zones. Although there have been three Mw ≥ 8.4 earthquakes in the region, including the disastrous 2004 Mw=9.2 Sumatra-Andaman earthquake, a 500 km long patch around Mentawai Islands is still locked andcould produce a large megathrust earthquake. If the rupture propagates to the subduction front, as it most likely occurred during the 2004 earthquake, it may lead to a devastating tsunami. Here, we present high-resolution reflection seismic data from the Sumatra locked zone that shows thesubduction interface down to 20 km depth. The seismic data also show that the wedge is composed of two layers: a shallow layer formed by mixed to landward vergent thrusts, termed as pop-ups, and a deeper layer showing sub-horizontal reflectors. The lower layer is most probably formed by duplexes, whose roof serves as a pseudo-décollement for the mixed to landward thrust systems. Based on the seismic results, we perform mechanical modeling in order to understand the formation of these structures and to retrieve the associated frictional properties. We first show that the activation of the pseudo-décollement requires (1) either a sudden increase of effective friction along the plate interface or an irregular geometry of the plate interface, (2) a lower effective friction along the pseudo-décollement than along the plate interface. We then show that low effective frictional values (≤ 0.1) are required to reproduce the observed frontal structures. The low effective friction along the pseudo-décollement could either be due to the presence of a long-term high pore pressure layer or to dynamic weakening associated with earthquakes. Since similar structures are present in the 2010 tsunami earthquake area, we favor the dynamic weakening hypothesis. According to the mechanical modeling, if the next rupture propagates up to the toe rupturing the three most frontal pop-up structures, we could expect at least 5.5 to 9.2 m of frontal horizontal displacement and a frontal uplift of 2 to 6.6 m along the frontal thrusts. This would amplify the uplift of the water column and, as a consequence, generate a large tsunami

Southern Sumatra forearc is classified as a compressional accretionary type. The existence of the accretionary wedge has a significant role in sediment deformation. This sediment deformation may contribute to generating higher tsunami generation. A two-dimensional crustal deformation modeling was performed to analyze the influence of the accretionary wedge. Furthermore, oceanic crust thickness variations are also simulated in this modeling as the incoming plate’s role in surface deformation is poorly understood. The crustal deformation modeling consists of two types of earthquake period: interseismic period and coseismic period. Various geometry features were applied for each earthquake period. The crustal deformation modeling itself is based on the finite element method. The result shows that there are different effects on the geometrical model being used. The pattern and amount of surface deformation also depend on at which the fault is activated. Extended geometry models such as three-dimensional modeling will be required for further study.

The assumption of a homogeneous elastic half-space model is widely used to model the earth’s deformation. However, the homogeneous assumption would not accurately reflect the complexity of the shallow crust. We performed a 3D coseismic deformation model using the finite element method and referred to the 2010 Mentawai earthquake. The 2010 tsunami earthquake was located at the Mentawai segment, which is a part of the accretionary wedge in the Sumatra subduction zone. This active accretionary wedge is identified as the most complicated structure on earth and lies along the Sumatra subduction zone, at which most destructive earthquakes happen in this region. We examined the impact of the accretionary wedge geometry and material properties by considering the wedge as a single different property separated from the continental plate. Various geometrical features, such as topography and wedge dimension, as well as physical properties, were simulated. Those features are then observed for their responses on the surface deformation. The topography affected the magnitude of the horizontal deformation up to 10% but only the pattern of the vertical deformation. The wedge dimension seems to have an insignificant influence on the surface deformation compared to the topography. Different physical properties of the accretionary wedge affect not only the magnitude of the horizontal deformation up to 40% but also the orientation. The direction of the lateral movement is seemingly affected by the material under the GPS station and by the source. On the other hand, the variations in the physical properties resulted in discrepancies of 0.5 meters in the vertical deformation near the source. These results indicated that regional physical property information and geometrical features are critical in estimating coseismic deformation, leading to more accurate slip inversion and earthquake and tsunami hazard prediction, particularly in regions with significant inhomogeneity.