Modern and ancient seismogenic out‐of‐sequence thrusts in the Nankai accretionary prism: Comparison of laboratory‐derived physical properties and seismic reflection data

Abstract

To investigate characteristics of a seismogenic out‐of‐sequence thrust (OOST) imaged as a strong reflection on seismic profiles in the Nankai accretionary prism, we determined acoustic properties of discrete samples from an fossil Nobeoka OOST outcrop under confining pressures, and compared the acoustic properties with those of an active OOST in the Nankai accretionary prism. We observed anisotropy of velocity and attenuation in the hanging wall of Nobeoka OOST attributed to foliation of pelitic‐phyllite. In contrast, the footwall is composed of brittlely deformed, chaotic shales and fine sandstones, and velocities in the footwall are lower than those in the hanging wall. Amplitude variation with offset (AVO) modeling utilizing contrasts in P‐ and S‐wave velocities and densities between the hanging wall and footwall of the Nobeoka OOST indicates that fractures filled with overpressured fluid likely account for angle‐dependent reflection amplitudes of the active OOST in the Nankai Trough.