The September 5, 2004 off the Kii Peninsula earthquakes as a composition of bending and collision

Tetsuzo Seno

doi:10.1186/bf03352571

Abstract

Abstract The 2004 off the Kii Peninsula earthquakes ruptured the subducting Philippine Sea plate near the axis of the Nankai Trough. A series of major events (the fore- and mainshocks with Mw>7) showed very peculiar rupture characteristics. The depth extent of the foreshock is around 12–23 km below the sea surface; in contrast, that of the mainshock is around 7–17 km, shallower and closer to the upper surface of the plate than the foreshock. The focal mechanism of the foreshock is almost pure reverse faulting, with a P-axis directing roughly N-S, that is rotated clockwise by 20–25° from the trough normal. In contrast, the mainshock involved a significant component of strike-slip faulting, with a similar P-axis. The OBS survey revealed a double layer of the aftershock activity and the upper NW trending layer contains aftershocks with strike-slip focal mechanisms. These features indicate that the compressional and tensional stress domains due to bending are significantly modified by tectonic stress disturbance and produced the fore- and mainshock ruptures deviated from simple reverse and normal faulting. I show that a supposed N-S compressional stress, which might be produced by the collision south of the Izu Peninsula, can rotate the principal stress axes and change the stress type, which are consistent with the P-axes of these events and the strike-slip component for the mainshock. The complex rupture mode of the mainshock might have been produced by superposition of the depth-dependent tensional bending stress in the upper half of the lithosphere and the collisional stress. This collisional stress should be comparable to the bending stresses in magnitude, which is much larger than the stress perturbation caused by nearby interplate locking.

Highlights

IntroductionEarthquakes often occur in the trench-outer rise region prior to subduction
Within an oceanic plate, earthquakes often occur in the trench-outer rise region prior to subduction
The aftershock distribution revealed by the OBS survey conducted after these events (Sakai et al, 2004) and their nodal planes with dip angles amounting to ∼40◦ (Centroid moment tensor (CMT) solutions determined by National Research Institute for Earth Science and Disaster Prevention (NIED), JMA and Harvard University, and individual studies by Yamanaka (2004), Yagi (2004) and Hara (2005)) clearly indicate that they have occurred within the subducting Philippine Sea plate (PHS) beneath the trench-outer rise region

Summary

Introduction

Earthquakes often occur in the trench-outer rise region prior to subduction. The aftershock distribution revealed by the OBS survey conducted after these events (Sakai et al, 2004) and their nodal planes with dip angles amounting to ∼40◦ (Centroid moment tensor (CMT) solutions determined by National Research Institute for Earth Science and Disaster Prevention (NIED), JMA and Harvard University, and individual studies by Yamanaka (2004), Yagi (2004) and Hara (2005)) clearly indicate that they have occurred within the subducting Philippine Sea plate (PHS) beneath the trench-outer rise region. Both of the T - and P-axes are oblique to the trough normal

Findings

Are they bending events?

Conclusions