Hamada Yohei, Kitamura Manami, Yamada Yasuhiro, Sanada Yoshinori, Sugihara Takamitsu, Saito Saneatsu, Moe Kyaw, Hirose Takehiro
Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, 200 Monobe-otsu, Nankoku, Kochi, 783-8502, Japan.
Research and Development Center for Ocean Drilling Science, Japan Agency for Marine-Earth Science and Technology, 3173-25, Showa-machi, Kanazawa-ku, Yokohama-city, Kanagawa, 236-0001, Japan.
Sci Rep. 2018 Feb 14;8(1):2622. doi: 10.1038/s41598-018-20870-8.
A new method for evaluating the in situ rock strength beneath the seafloor is proposed and applied to the Nankai Trough accretionary prism. The depth-continuous in situ rock strength is a critical parameter for numerous studies in earth science, particularly for seismology and tectonics at plate convergence zones; yet, measurements are limited owing to a lack of drilled cores. Here, we propose a new indicator of strength, the equivalent strength (EST), which is determined only by drilling performance parameters such as drill string rotational torque, bit depth, and string rotational speed. A continuous depth profile of EST was drawn from 0 to 3000 m below the seafloor (mbsf) across the forearc basin and accretionary prism in the Nankai Trough. The EST did not show a significant increase around the forearc basin-accretionary prism boundary, but it did show a clear increase within the prism, ca. below 1500 mbsf. This result may indicate that even the shallow accretionary prism has been strengthened by horizontal compression derived from plate subduction. The EST is a potential parameter to continuously evaluate the in situ rock strength during drilling, and its accuracy of the absolute value can be improved by combining with laboratory drilling experiments.
提出了一种评估海底原位岩石强度的新方法，并将其应用于南海海槽增生楔。深度连续的原位岩石强度是地球科学众多研究中的关键参数，特别是对于板块汇聚带的地震学和构造学研究而言；然而，由于缺乏钻孔岩芯，相关测量受到限制。在此，我们提出一种新的强度指标——等效强度（EST），它仅由钻柱旋转扭矩、钻头深度和钻柱转速等钻进性能参数确定。在南海海槽的弧前盆地和增生楔区域，绘制了从海底以下0至3000米（mbsf）的EST连续深度剖面。EST在弧前盆地 - 增生楔边界附近未显示出显著增加，但在增生楔内部（约在1500 mbsf以下）确实呈现出明显增加。这一结果可能表明，即使是浅部的增生楔也已受到板块俯冲产生的水平挤压而得到强化。EST是在钻进过程中连续评估原位岩石强度的一个潜在参数，通过与实验室钻进实验相结合，其绝对值的准确性可以得到提高。
