Earth Planets Space, Vol. 57 (No. 4), pp. 327-332, 2005LETTER
Earthquake Research Institute, University of Tokyo, Tokyo 113-0031, Japan
(Received November 30, 2004; Revised March 23, 2005; Accepted April 11, 2005)
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.
Key words: Trench-outer rise, bending, interplate locking, lithosphere, neutral plane, Izu Peninsula, collision, Zenisu Ridge.