[S-CG45] Science of slow-to-fast earthquakes

[SCG45-P23] Influence of outer-rise faults on sediment flux, décollement heterogeneity, and slip mode at the Japan trench

*Christine Regalla¹, Emily Schottenfels¹, Yasuyuki Nakamura² (1.Northern Arizona University, 2.Japan Agency for Marine Earth Science and Technology)

Keywords:japn trench, shallow slip, decollement heterogeneity, accretion and tectonic erosion

Outer-rise faults formed due to plate flexure at subduction zones generate systems of horsts and grabens that are common structural features on the incoming plate. When these structures are subducted at shallow depths, they are considered a first order control on the mass flux of sediment accreted or subducted and the material properties and roughness of the shallow plate interface. Here, we investigate how the outer-rise normal fault offset and incoming plate sediment thickness influence the flux of sediment and the physical properties of the shallow decollement at the Japan trench in the region that hosted the M_w 9 2011 Tohoku earthquake. Using structural and litho-stratigraphic mapping of high-resolution seismic reflection data collected by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), we demonstrate that sediment flux is heterogeneous and can vary between sediment accretion, sediment subduction, and frontal tectonic erosion over length-scales as short as 5 km. Sediment accretion occurs when horsts are subducted, whereas all modes of deformation can occur when grabens are subducted. This suggests that a fault throw threshold may control modes of deformation at the frontal prism. Our results show that sediment accretion via imbricate thrust faulting is promoted in regions of the incoming plate with low relief outer-rise fault offsets and a thick incoming plate sediment section. In contrast, frontal tectonic erosion may occur in regions with high relief outer-rise faults and a relatively thin incoming sediment section. These variable modes of mass flux have important implications for the geometry and composition of the shallow décollement in the Tohoku earthquake slip region. In regions where sediment accretion occurs, the décollement may localize in frictionally weak clays near the base of the incoming sediment section and undulate with the topography of the subducted horsts and grabens. In contrast, frontal tectonic erosion of the frontal prism may occur when the décollement is possibly planar and cuts across subducting graben such that prism sediments and incoming plate sediments are locally subducted. Spatial variations in these properties appear to correlate to different modes of shallow slip, suggesting that the combination of sediment thickness and fault throw may be an important control on shallow megathrust mechanics.