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 (QP), 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 QP in the three sub-bottom layers. The QP values for the upper two layers with P-wave velocity (VP) < 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 QP within seafloor sediment. The basement layer (VP approximately 4 km/s) has a much higher QP value of 349, which is comparable to the value estimated for crustal layers exposed onshore. This QP 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 QP, low seismic attenuation in the basement layer reflects tectonic stability of the inner wedge of the accretionary margin. Our first estimates of QP in the present study provide a strong basis for future studies of seismic structure and strong ground motion prediction.
Highlights
The Kumano Basin is a forearc basin along the Nankai subduction zone (Figure 1a)
We interpret that the high Q of P waves (QP), low seismic attenuation in the basement layer reflects the tectonic stability of the inner wedge of the accretionary margin (Wang and Hu 2006)
The QP structure in the Kumano Basin, which is a forearc basin along the Nankai Trough located in the rupture area of the 1944 Tonankai earthquake (Mw 8.1), was derived from a long-offset walk-away vertical seismic experiment
Summary
The Kumano Basin is a forearc basin along the Nankai subduction zone (Figure 1a). Beneath the basin, destructive earthquakes occur along the plate boundary (Ando, 1975). Seismic surveys have been made to characterize the subseafloor structure around the fault, such as the threedimensional (3-D) geometry of the fault, the geometry of other active faults in the outer wedge or of the megathrust (e.g. Moore et al 2007), and the detailed P-wave velocity (VP) distribution (Nakanishi et al 2008; Park et al 2010; Kamei et al 2012). It is expected that reliable Q values of the sub-seafloor formation can be derived from an analysis of seismic waveform data, which are less affected by the strong attenuation of shallow seafloor sediments than those obtained by conventional ocean bottom or sea surface instruments. We estimate the QP structure of the Kumano Basin based on a spectral analysis of seismic data collected by the VSP and show for the first time the depth variation of seismic attenuation beneath the Nankai forearc. A spectrum of a seismogram can be expressed as a convolution of 1) the instrumental response, 2) the source
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