Crustal deformation and faults configuration along the eastern Nankai Trough obtained by seismic reflection survey

Abstract:

The large disaster earthquakes had often occurred in the Nankai Trough with a great Tsunami event. In order to reduce a great deal of damage to coastal area from both strong ground motion and tsunami generation, it is necessary to understand rupture synchronization and segmentation of the Nankai megathrust earthquake. In the eastern Nankai Trough, it is known to be formed complicated structure by subducting Zenisu ridge. However, there are only old seismic profiles by Nakanishi et al. (1998) and Takahashi et al. (2002). It is difficult to evaluate the record of seismoturbidite in the sediments from old seismic data. Therefore we planned new survey around the Zenisu ridge in order to obtain the high resolution structure.

Japan Agency for Marine-Earth Science and Technology carried out new multi-channel seismic reflection (MCS) surveys over 1,000 km of line length from Kii Peninsula to Izu Peninsula in 2012 using tuned airgun array of 7,800 cu.in. and 6 km long streamer with 444 ch hydrophones.

We obtained five MCS lines along NW-SE direction and two MCS lines along NE-SW direction. Every profiles show the clear reflection from sediments, the top of oceanic crust and crustal structure. The reflectors from Moho discontinuity are identified in some survey lines. The sediments on the Shikoku Basin between the Nankai Trough and the Zenisu ridge have no deformation by subduction. On the other hand, the sediments of the southeastern edge of the Zenisu ridge indicate the compression structure by subduction. These results show the effect of the crustal deformation is stopped by the front of the Zenisu and/or paleo-Zenisu ridge. The lateral variations of sediment and basement along the Nankai Trough are also appeared in NE-SW survey line before subduction. Some convex structure of the basement in these variations are corresponds to the magnetic anomaly by Le Pichon et al. (1996). It is important to understand the incoming structure for the evaluation of rupture boundary.