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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) < 2.7 km/s are 34 and 57. These values are almost identical to those obtained in the North Atlantic, suggesting the broad consistency of Q P within seafloor sediment. The basement layer (V P approximately 4 km/s) has a much higher Q P value of 349, which is comparable to the value estimated for crustal layers exposed onshore. This Q P value is higher than the value previously assumed in a simulation of strong ground motion associated with megathrust earthquakes along the Nankai margin. We interpret that the high Q P, low seismic attenuation in the basement layer reflects tectonic stability of the inner wedge of the accretionary margin. Our first estimates of Q P in the present study provide a strong basis for future studies of seismic structure and strong ground motion prediction.

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