Elsevier

Tectonophysics

Volume 600, 17 July 2013, Pages 99-107
Tectonophysics

Stress field observation and modeling from the NanTroSEIZE scientific drillings in the Nankai Trough system, SW Japan

https://doi.org/10.1016/j.tecto.2013.04.009Get rights and content

Highlights

  • We demonstrate in-situ observations within the drill sites of NanTROSEIZE.

  • We model the stress state according to different slip models.

  • Modeled stress states are compared with in-situ observations.

  • The modeled stress states could more or less explain logging data.

Abstract

Many studies have investigated the stress state in terms of drilling projects in the Nankai Trough region, the potential area for next devastating earthquakes. To understand the stress state and the geologic properties in Nankai, several drilling projects have been conducted over time. Among these projects, the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) included studies regarding stress orientations and magnitude estimations by logging data from the Nankai Trough. Due to limitations in technology of drilling and the data processing, the actual stress orientation and the magnitude of the Nankai Trough at depth remains controversial. In this study, we discussed the stress state in each borehole and evaluated the stress state using various models. Our models evaluated stress states contributed by a slip deficit model in the Nankai Trough system and the coseismic dislocations of the two September 5th, 2004 earthquakes. Compared to the stress estimation from logging data, the simulated stress orientation could more or less explain the logging data at the NanTroSEIZE drilling boreholes. Based on these models, the stress tensor in Nankai can be extended from the boreholes to more dimensional area, even the locations where no drilling has been reached.

Introduction

The Nankai Trough is a subducting plate boundary that was formed by the Philippine Sea and the Eurasian Plates. The convergence rate between the two plates is 4.1–6.5 cm/year along the strike azimuthal direction of 300°–315° (Miyazaki and Heki, 2001) (Fig. 1). Over the past 1300 years, a well-documented history has reported tsunamis and ground shakings by great earthquakes in this region, including the M 8.2 1944 Tonankai and the M 8.3 1946 Nankaido earthquakes (Fig. 1) (Ando, 1975, Hori et al., 2004). More recently, two earthquakes with M 7.2 and 7.3, respectively, occurred in 2004 in the plate boundary (Fig. 1) (Bai et al, 2007). The continuous occurrence of large earthquakes indicated the importance of studies on seismic hazard mitigation. The understanding of a stress field in Nankai is one of the key factors for determining fault process and its mechanisms within the seismogenic zones of the Nankai Trough.

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a multistage project in the Nankai area (Fig. 1). The main goals of this project are to understand the behaviors of a mega-thrust as well as the physical properties of the subduction boundary and the aseismic–seismic transition. Drill sites were located in the Nankai margin from the Kumano Basin to the Shikoku Basin (Fig. 1). Throughout the project, specifying the different mechanical behaviors between the subducting basement high and the basement plain in terms of fault dynamics was expected.

In this study, we examine in-situ observations within the drill wells of NanTroSEIZE. In order to understand the mechanisms that control the stress state in the Nankai area, the stress states based on different sources are modeled. We considered the sources, which dominate stress regime in a regional scale, i.e. the slip deficit in the Nankai Trough system and the coseismic dislocations of the earthquake sequence near the surface trace of the Nankai Trench since September 5th, 2004. Below, we discuss the feasibility of each source by comparing the corresponding stress state models with observations.

Section snippets

Stress orientations and magnitudes in boreholes

For the majority of scientific drillings, resistivity images such as obtained using Formation Micro Imager (FMI) or Logging While Drilling (LWD) logging are direct methods for evaluating boreholes and stress concentration on the borehole walls (Zoback et al., 2003). The breakouts obtained from the image logs indicate the azimuth of the least horizontal principal stress (Zoback et al., 1985). Stress orientation can be indicated based on the principal stresses acting on borehole formation, rock

Optimally oriented planes (OOP) and Coulomb failure stress (CFS)

Using the Coulomb criterion and the constant apparent friction model (Harris, 1998, Cocco and Rice, 2002, and references therein), Coulomb failure stress (CFS) on a specific plane can be presented as follows:CFS=τ+μσn,where τ is the shear stress computed along the slip direction on the assumed fault plane (positive for the along slip direction), μ = μ(1  B) is the apparent friction coefficient that is relative to the friction coefficient μ and the Skempton coefficient B, and σn is the normal

The modeled stress tensors compared to the observations at each site

Thus far we have presented the OOP modeling based on the slip deficit model in the Nankai region (Fig. 6a) and the coseismic dislocations of the two 2004 earthquakes (Fig. 6b). In the following discussion, we compare in-situ observations of the stress state with the OOP model assuming slip deficit and the 2004 earthquakes.

Summary and discussion

In-situ observations from NanTroSEIZE drillings such as logging, coring, and imaging in the shallow portion provide an opportunity for understanding the stress field in Nankai. Despite the stress analysis being limited by the drilling depth and borehole conditions, also by the deviations of slip deficit due to GEONET station coverage waiting for future discussions, the modeled OOPs using possible sources could address the stress state at any certain depth and within a wider area corrected with

Acknowledgments

This research was supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Japan and the National Science Council in Taiwan. Authors gratefully thank IODP and CDEX for providing the logging data. We thank the editor and two anonymous reviewers for their constructive comments.

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