Heat flow in the Nankai forearc, SW Japan, derived from BSR and drilling: Possible effect ofseamount subduction on earthquakes

In the central to western Nankai Trough. a series of dense seismic reflection surveys were conducted in 2018-2020 (Nakamura et al., 2022 GRL). They show impressive topographic features of the subducting plate boundary, including a subducting seamount in the Hyuga-Nada region and off Cap Muroto. Compiled seismic dataset was used for picking BSRs (bottom-simulating reflectors), which define a boundary between the hydrate-rich formation above and a gas-bearing layer below. The heat flow values are calculated from these BSR depths and the average thermal conductivity between the seafloor and BSR. The short wavelength variations are filtered out and the obtained heat flow values are regionally averaged ones. The result was then merged with the existing heat flow data (surface, borehole and BSR-derived) in this area.

Heat flow is highest near the trough axis off Cape Muroto (near the central Nankai Trough), which is interpreted as the fluid seepage along the decollement. In the forearc region, heat flow varies between 50-70 mW/m^2, On the forearc area off Muroto, the bathymetry 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 (~30mW/m^2) region above the subducted seamount. We propose that the heat flow is affected by the subduction of seamount.

In Hyuga-Nada forearc, the westernmost portion of the Nankai Trough region off eastern Kyushu, the Kyushu-Palau Ridge (KPR) is obliquely subducting toward N30W since several Ma B.P. Heat flow marks a sharp contrast at KPR; to the east it is 50-100mW/m2 to the east and 25-40mW/m2 to the west. The transition from high to low heat flow occurs in only 20 km across KPR. Higher heat flows of 100 mW/m2 to the east are located near the axis of Nankai Trough, similar to those reported off Muroto. Lower heat flow to the west is attributed to the subduction of older West Philippine Basin. Near the KPR we observed a ‘bowl-shape’ negative heat flow anomaly; heat flow outside is ~45mW/m2, whereas it is ~25 mW/m2 above the subducted KPR.

We hypothesize these local low heat flow close to subducted seamounts in 2 models. The first is that the subducted seamount was already cooled down by the downward fluid flow (recharge) after it was formed as the opening of Shikoku Backarc basin (15-25 Ma). The second is that the seamount subduction caused a stress contrast between the leading and trailing sides, encouraging a poroelastic fluid flow in the sediment above seamount. Through numerical simulations we found either model is possible to explain the observation. However, considering a frequent occurrence of low-frequency tremors around the seamount in Hyuga-Nada, we favor the latter model, because the fluid flow can reduce the effective stress, leading to the occurrence of seismic activities. We further discuss these mechanisms.