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Abstract:
In the central to western Nankai Trough, seismic reflection surveys were conducted in 2018-2020. Compiled seismic dataset was used for picking BSRs (bottom-simulating reflectors). BSR-derived heat flow is calculated from the depth and the temperature at BSR, and the average thermal conductivity between seafloor and BSR.
Heat flow is highest near the trough axis off Muroto (near the central Nankai Trough). In the forearc region, heat flow varies between 50-70 mW/m^2, but it is lowest in the forearc off Hyuga-Nada, westernmost portion of the Nankai Trough. On the forearc area off Muroto, the topography is characterized by a large landward embayment including the trough axis and deformation front. Within 20 km landward from the deformation front, heat flow is ~80 mW/m^2 in this embayed area, whereas it is 40-60 mW/m^2 on either side of embayment. Further landward, we found a low heat flow (~30 mW/m2) region above the subducted seamount. We propose that the heat flow is affected by the subduction of seamount. In the Hyuga-nada forearc region off eastern Kyushu, the Kyushu-Palau Ridge (KPR) is obliquely subducting toward N30W since several Ma B.P. Across KPR we observed a‘bowl-shape’negative heat flow anomaly with its width ~50 km. The heat flow outside is ~45 mW/m^2, whereas it is ~25 mW/m^2 above the subducted KPR.
We discuss possible mechanisms of variable heat flow around the subducted seamount with implication for the earthquake likelihood in the shallow portion of seismogenic zone.
