Geometrical dependence on the stress and slip tendency acting on the subduction megathrust of the Nankai seismogenic zone off Kumano

Masataka Kinoshita,Kazuya Shiraishi,Weiren Lin,Evi Demetriou,Yoshitaka Hashimoto

doi:10.1186/s40645-018-0253-y

Abstract

We assess the dependence of megathrust geometry near the updip edge of the Nankai seismogenic zone according to the slip tendency of the megathrust based on a reprocessed 3D PSDM seismic volume image and International Ocean Discovery Program (IODP) NanTroSEIZE drilling data off the coast of the Kii Peninsula, central Japan. The plate boundary fault surface is manually picked from the 3D seismic volume and is divided into three groups: low dip (10°–15°) trending NW to N in the NE region, intermediate dip (~ 25°) trending north in the western region, and high dip (30°–40°) trending NW in the SE corner. We then calculate the overburden (Sv) by converting the modeled 3D velocity to bulk density. Here, Sv ranges from 100 MPa near the SW edge to 160 MPa on the NE corner. In order to derive horizontal principal stresses (SH and Sh) and the pore fluid pressure (Pp), we assign the ratio of horizontal-to-vertical principal stresses, r (= SH/Sv), and the ratio of pore fluid pressure-to-vertical stress, λ (= Pp/Sv) based on IODP drilling data. The directions of SH and the slip on the fault are set parallel to the plate convergence vector (N55W). Assuming a triaxial condition (SH > Sh = Sv), the slip tendency (Ts) can be calculated from the dip angle and dip azimuth of the fault surface, r, and λ. We found that Ts is sensitive to the variation in fault geometry. For r = 1.2 and λ = 0.7 (normalized pore pressure ratio λ* = 0.25), Ts is low (~ 0.1) in the low-angle dip region and higher (> 0.2) in the high-angle dip region. This suggests that the high-angle fault is optimally oriented under this condition. Low Ts in the low-angle dip region would correspond to a weaker region due to a low frictional strength, assuming that the fault surface should slip simultaneously. Using the high pore pressure ratio (λ* ~ 0.85) beneath the fault zone, the fault beneath IODP Site C0002 is very close to failure if r > 1.2.

Highlights

According to the Coulomb failure theory, an earthquake or a slip along a fault can occur if the ratio of shear stress to effective normal stress exceeds the friction coefficient on the fault surface
The slip direction of the 2016 Mie-ken Nanto-Oki event was obtained from the Japan Meteorological Agency (JMA) and U.S Geological Survey (USGS). b The critical pore-pressure ratio λ*cr is the value at which the slip likelihood (SL) becomes 1, assuming that the intrinsic friction coefficient is 0.6 and Phyd/Sv = 0.6 a steep dip region, the uncertainty is ± 5°, resulting in Slip tendency (Ts) = 0.23 ± 0.05
The geometry of the fault surface is divided into three groups: low dip (10°–15°) trending NW to N in the NE region, intermediate dip trending north in the western region, and high dip (30°–40°) trending NW in the SE corner

Summary

Introduction

According to the Coulomb failure theory, an earthquake or a slip along a fault can occur if the ratio of shear stress to effective normal stress exceeds the friction coefficient on the fault surface. This ratio is called the slip tendency (Ts). From the reprocessed 3D PSDM volume, we obtained the fine-scale geometry (dip angle and azimuth) of the updip region of plate boundary fault Integrating this fault geometry with deep drilling data obtained within the 3D survey region, we calculate the slip tendency of the fault and assess the degree to which the fault

Results

Discussion

Conclusion