Seismic reflection and bathymetric evidences for the Nankai earthquake rupture across a stable segment-boundary

Jin-Oh Park,Shuichi Kodaira

doi:10.5047/eps.2011.10.006

Abstract

Seismic reflection profiles reveal steeply landward-dipping splay faults in segment B (the 1946 Nankai earthquake rupture, M 8.3) as well as segment C (the 1944 Tonankai earthquake rupture, M 8.1) of the Nankai subduction zone. The splay faults, branching upward from the plate-boundary interface, almost reach the seafloor, producing seafloor fault scarps. The swath-bathymetry map exhibits a ∼200-km-long, remarkable seafloor lineament with which the seafloor fault scarps align in the segments B and C. The seafloor lineament, which we believe is produced by repeating slips on the splay faults, is almost laterally continuous across a stable boundary off Kii Peninsula inbetween the two segments. These seafloor and subsurface features could be due to multiple, simultaneous coseismic slips across the B–C boundary, when subduction thrust earthquakes accompany the co-seismic slips on the splay fault. The splay faults are associated with (1) fluid expulsion, (2) dynamic deformation, and (3) tsunami generation.

Highlights

It is well known that a subduction zone is divided into several discrete segments marked by a megathrust earthquake rupture
We show multi-channel seismic (MCS) reflection profiles crossing the Nankai Trough, and Seabeam swath-bathymetry data off the Kii Peninsula, to resolve the controversy of whether, or not, coseismic rupture includes a previously-defined segment-boundary
The swath-bathymetry map exhibits a ∼200-km-long, remarkable sea oor lineament with which the sea oor fault scarps align in the segments B and C

Summary

Introduction

It is well known that a subduction zone is divided into several discrete segments marked by a megathrust earthquake rupture. The Nankai subduction zone off Kii Peninsula (Fig. 1) is characterized by steeply landward-dipping megasplay faults (Park et al, 2002; Moore et al, 2007) in the rupture area of the 1944 Tonankai (M 8.1) earthquake. The 1707 Hoei (M 8.7) earthquake is presumed to have extended across the B–C boundary and ruptured the segments A, B, C, and D at a single event, based on historical documents and numerical simulation (Kodaira et al, 2006). It has been controversial whether, or not, there has been an earthquake rupture. We present the potential implications of the splay faults in the Nankai subduction zone

Swath-Bathymetry Data and Their Interpretation

Conclusions