Study of the thermal structure of the seismogenic zone along the Nankai margin through heat flow measurements in shallow sea areas

1. 我们使用弹出式仪器对沉积物的温度分布和底部水温进行了长期监测。南开边缘17个站点连续记录5 ~ 17个月。从沉积物温度记录中去除底水温变化的影响，可以得到6个水深相对较浅的站点的热流值。我们还表明，通过分析普通深海探测器测量的沉积物温度曲线，结合长期的底部水温记录，可以确定浅海的热流。将该方法应用于三个站点的数据，成功地确定了热流。我们评估了用普通探头可以测量热流的最小水深和根据长期底水温记录确定热流所需的弹出式仪器的监测时间长度。海底水温的变化也与物理海洋学数据进行了比较。温度变化的周期性可能与表面电流的弯曲有关（Kuroshio）。热流测量也在深海区域使用普通探头进行。结合上述浅海地区的观测结果和已有的资料，可以在基伊半岛东南部（熊野附近）建立一个横跨南开海槽的热流剖面。考虑沉积作用对大洋板块的影响和吸积棱镜中流体和沉积物的流动，计算了熊野外缘和四国东部外缘南开俯冲带的温度结构。通过将计算的表面热流与观测数据进行比较，估计了沿板边界的摩擦加热（剪切应力）。

1. We conducted long-term monitoring of temperature profiles in sediments and bottom water temperature using pop-up type instruments. Continuous records for 5 to 17 months were obtained at 17 stations along the Nankai margin.2. Reducing the effect of bottom water temperature variation from the sediment temperature records, we could obtain heat flow values at six stations with relatively shallow water depths. We also showed that it is possible to determine heat flow in shallow sea by analyzing sediment temperature profiles measured with ordinary deep-sea probes in combination with long-term bottom water temperature records. This new method was applied to the data at three stations and heat flow was successfully determined.3. We evaluated the minimum water depth at which heat flow can be measured with ordinary probes and the length of monitoring time with pop-up type instruments necessary for heat flow determination based on long-term bottom water temperature records. Variations in bottom water temperature were also compared with physical oceanography data. The periodicity in the temperature variations might be related to meandering of the surface current (Kuroshio).4. Heat flow measurements were also made in deep sea areas using ordinary probes. Combining the obtained data with the results in shallow sea areas mentioned above and the existing data, we could construct a heat flow profile across the Nankai Trough in the area southeast of the Ki-i peninsula (off-Kumano).5. The temperature structure of the Nankai subduction zone was calculated in the off-Kumano area and in the areas off eastern Shikoku, taking account of effects of sedimentation on the incoming oceanic plate and fluid and sediment flows in the accretionary prism. We estimated the frictional heating (shear stress) along the plate boundary by comparing the calculated surface heat flow with the observed data.