Seismogenic Zone Structures Revealed by Improved 3‐D Seismic Images in the Nankai Trough off Kumano

Abstract

To reveal the detailed deformation structures related to plate subduction in the Nankai Trough, we applied up‐to‐date technologies to improve our 3‐D seismic images. This region is dominated by a megasplay fault system that consists of a coseismic out‐of‐sequence thrust branching from the plate interface and separating the inner and outer accretionary prism. The 3‐D seismic volume was acquired off Kumano in 2006 as a preliminary site survey as part of the Nankai Trough Seismogenic Zone Experiment. The preprocessed data quality was improved by recovering the broadband responses and by better attenuating multiple reflections and noise. New reflection images were then produced using prestack time migration and prestack depth migration with updated velocity models. The new velocity model suggests the possible existence of a high‐velocity zone just above the megasplay fault, which might indicate petrophysical alteration in the seismogenic zone. The fault geometry with spatial dip angle variation and the overburden velocities are important factors for further estimating the force distribution along the coseismic fault. Deformation structures newly imaged beneath the Kumano Basin and dipping layers above the megasplay fault imply complex thrusting and possible fluid flow paths within the inner prism. Fine‐scale deformation features are clarified in the shallow areas from the outer prism to the transition zone, which are useful for reconstructing the accretionary prism development. A low‐reflectivity zone, including an isolated layered block, may be originally underthrusted sediments that have been remobilized during later strike‐slip faults along the edge of Kumano Basin.

GO_3D_OBS – The Nankai Trough-inspired benchmark geomodel for seismic imaging methods assessment and next generation 3D surveys design (version 1.0)

“…The land-ward part is formed by large deformed stacked thrusts sheets (solid blue lines in Figure 2(a)) corresponding to the old accretionary prism which extends sea-ward into large out-of-sequence thrusts acting as a backstop now. Similar inner-wedge structures can be found in various subduction zones (Dessa et al, 2004;Raimbourg et al, 2014;Shiraishi et al, 2019;Collot et al, 2008;Cawood et al, 2009;Contreras-Reyes et al, 2010), as well as modelled with analogue sand-box experiments (Gutscher et al, 1998(Gutscher et al, , 1996. The outer-wedge (red shaded polygon in Figure 2(a)) contains the sedimentary prism adapted from Kington and Tobin (2011) and Kington (2012) (Figure 2 Finally, the frontal prism is fed by the layers of incoming sediments (black lines in Figure 2(a)) -relatively thick in the area of trench and thin over the volcanic ridge.…”

Structural control and system-level behavior of the seismic cycle at the Nankai Trough

“…Therefore, we expect different behaviors in a sediment-thick Lesser Antilles subduction zone and a sediment-starved northern Chile margin, for example. The fabric of the upper plate also varies greatly within the Nankai Trough, with a mega-splay fault bounding the accretionary prism from the forearc offshore the Kii Peninsula (Park et al 2002), with many reverse faults occupying the entire outer wedge (Park et al 2010;Kamei et al 2012;Tsuji et al 2014Tsuji et al , 2015Tsuji et al , 2017Shiraishi et al 2019). We therefore anticipate important along-strike variations with different rupture styles in the accretionary region even within the Nankai Trough.…”

Weak Faults at Megathrust Plate Boundary Respond to Tidal Stress

“…Based on Ps -P times of 6.1 and 7.6 s, a Vp model (Shiraishi et al 2019), and an empirical relation between Vp and Vs (Brocher 2005), two Ps waves are reflected from the megasplay fault at a depth of 7 km and the top of the oceanic crust at a depth of 9 km. For example, using a Vp of 3.70 km s -1 between the megasplay fault and the top of the oceanic crust (Shiraishi et al 2019), we obtain a Vs of 2.02 km s -1 from the empirical relation (Brocher 2005). Using the differential travel time of 1.5 s, we obtained a thickness of 1.96 km between the two reflections.…”