Q P structure of the accretionary wedge in the Kumano Basin, Nankai Trough, Japan, revealed by long-offset walk-away VSP Seismology

Abstract

We determined the seismic attenuation structure of the Kumano Basin, a forearc basin in the central part of the Nankai subduction zone. Despite its importance for understanding the physical condition of the Earth's interior and seismic wave propagation processes, the attenuation factor Q has been poorly estimated in the crustal layers of the offshore areas of Nankai because severe attenuation occurring in the seafloor sediments prevents the reliable estimation of Q from conventional active source seismic surveys. In the present study, we derive Q values from the diminishing rate of the high-frequency contents of seismic energy during propagation through sub-seafloor layers. The records of vertical seismic profiling acquired at approximately 1,000 m below the seafloor, which have fewer effects from shallow attenuation, enabled us to elucidate depth variation of Q of P waves (Q P), the attenuation factor of P waves, down to approximately 8 km below the seafloor. Assuming that the frequency dependence of Q is small and using a previously obtained P-wave velocity structure model for the basin, we inverted the fall-off rate of the spectral ratios at various shot-receiver distances to obtain Q P in the three sub-bottom layers. The Q P values for the upper two layers with P-wave velocity (V P)∈