[S-CG40] Science of slow-to-fast earthquakes

[SCG40-P13] Outer rise normal fault controls on shallow decollement heterogeneity, prism evolution, and relationships to shallow slip mode in the Japan trench

*Christine Regalla¹, Emily Schottenfels¹, Aubrey LaPlante¹, Yasuyuki Nakamura² (1.Northern Arizona University, 2.Japan Agencey for Marine Earth-Science and Technology)

Keywords:Japan trench, shallow seismogenic slip, outer rise faults

Here, we investigate how the outer-rise normal fault offset and incoming plate sediment thickness influence the flux of sediment and the physical properties and slip mode of the shallow decollement at the Japan trench in the region that hosted the M_w 9 2011 Tohoku earthquake. We use JAMSTEC high-resolution seismic reflection and bathymetric data across the Japan trench to map decollement geometry and frontal prism deformation in response to the subduction of outer rise normal faults in the subducting plate. First, seismic data reveal small scale (5-10 km) variations in decollement position and sediment flux that appear to be controlled by the ratio of outer rise fault throw to incoming plate sediment thickness. We find that fault throws less than sediment thickness promote sediment accretion and a non-planar, compositionally homogenous decollement, whereas fault throws larger than sediment thickness lead to sediment subduction or erosion, and a planar, compositionally heterogeneous decollement. Second, bathymetric data show that the frontal prism is pervasively deformed by thrust and normal faults (1-10m spacing) that likely reflects multiphase deformation in response to the subduction of horst and graben. Third, bathymetry data also reveal graben-like topographic lows on the overriding plate frontal prism that coincide with subducted graben on the incoming plate. We suggest these basins can form in repose to slip on an undulating décollement that steps over and down into subducted graben and deforms the overlying prism, and continued slip on subducted normal faults that offsets the frontal prism and plate boundary décollement. Finally, we find that along-strike variations in overriding plate morphology, apparent in spatial variations in the topographic basins, correspond to overriding plate variations in frontal prism evolution and sediment flux, incoming plate variations of sediment thickness and outer rise fault throw, and potentially to different styles of megathrust rupture behavior during the 2011 Tohoku and 1896 Sanriku earthquakes.