3D Interpretation of Active Thrust Systems, Nankai Accretionary Wedge: Relationship Between In-Sequence and Out-of Sequence Thrusts

At the Nankai Trough subduction zone large tsunamigenic earthquakes occur with a recurrence interval of 100 to 200 years. One key parameter governing the severity and characteristics of tsunamis is the amount and location of slip near the sea floor at the up-dip end of the megathrust fault system. As a result, the structure in the uppermost 10 km has received close attention, including a recently acquired 3D seismic survey. A major apparent out of sequence thrust (OOST), termed the mega-splay thrust fault, has been identified underlying the Kumano Forearc Basin and approaching the surface at the outer arc high. This mega-splay system may accommodate a large amount of plate convergence in the form of tsunamigenic slip in great earthquakes (>8). Working with new industry-quality 3D seismic reflection data, we focused on the structural differences between the landward out of sequence mega-splay thrust system and the seaward in-sequence thrusts of the outer prism, to unravel their evolution as the prism has developed. In the upper most few kilometers, the OOST branches into 3 to 4 major splays and numerous minor ones with complex three-dimensional geometry. In places, these shallow splays are high-amplitude reflectors and juxtapose thrust sheets of contrasting internal reflectivity character. Numerous apparently in-sequence faults of the outer Accretionary wedge are also well imaged. Detailed 3D seismic interpretations sheds light on the transition from outer wedge faulting to OOST's of the mega-splay. Interpretation and images of the complex geometry of the faults across this transition will be presented. Determining the slip history of the out of sequence thrusts is of particular interest to understand how tsunamigenic slip has occurred during earthquakes, and how the complex fault patterns in the prism have evolved over time.