Continuous depth profile of the rock strength in the Nankai accretionary prism based on drilling performance parameters

Yohei Hamada,Saneatsu Saito,Takehiro Hirose,Kyaw Moe,Yoshinori Sanada,Manami Kitamura,Yasuhiro Yamada,Takamitsu Sugihara

doi:10.1038/s41598-018-20870-8

Yohei Hamada, Saneatsu Saito + Show 6 more

Open Access

https://doi.org/10.1038/s41598-018-20870-8

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Abstract

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.

Highlights

The in situ continuous strength of the crust is one of the most fundamental and vital information in various research fields of earth sciences
The proposed method was applied to drilling data obtained from Integrated Ocean Drilling Program (IODP) Site C0002 at the Nankai Trough, which is the main site for deep drilling to the seismogenic zone, to evaluate the continuous in situ strength in the Nankai accretionary prism
The riser drilling method was applied in the vicinity of non-riser holes to penetrate the accretionary prism beneath the forearc basin, and this method will be used to penetrate the megathrust in the future

Summary

Introduction

The in situ continuous strength of the crust is one of the most fundamental and vital information in various research fields of earth sciences. The proposed method was applied to drilling data obtained from IODP Site C0002 at the Nankai Trough, which is the main site for deep drilling to the seismogenic zone, to evaluate the continuous in situ strength in the Nankai accretionary prism. This method allows estimation of the strength without assuming uncertain in situ conditions (e.g., stress and fluid pressure conditions) or using special techniques. The continuous strength was evaluated by applying previously obtained drilling parameters, which enabled us to clarify the strength profile extracted from the limited experimental strength data

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