Developments in the Tomographic Processing of Eastern-Nankai Trough OBS Data

Multi-fold wide-angle seismic data were collected offshore the eastern Nankai trough (Tokai area) in July-august 2001 by JAMSTEC (Japan Marine Science and Technology Center). One hundred 3C-OBSs were deployed with a 1km-spacing along a 140km-long shot profile that was orientated perpendicularly to the Nankai through. The experiment aims to illuminate the intraplate level of the subduction system and to obtain tomographic models with the highest achievable resolution in order to propose and/or confirm hypothesis concerning the structural factors that control the nucleation of large earthquakes. The resulting data were processed with a linearized, first arrival traveltime tomography technique. The validity of the outcoming tomographic model is assessed by showing its weak dependence with respect to a wide range of randomly generated realistic starting models. An a posteriori resolution study is also performed through checkerboard tests in order to characterize the areas of the model that are reliably recovered. Some features can be identified in this model among which a slight velocity anomaly that could support the hypothesis of a possibly locking subducted structure, referred to as the ``Paleo-Zenisu'' ridge in the literature. The absence of crustal thickening below the Zenisu ridge, at the front of the subduction, is also a noticeable feature of our model. Further interpretations are made difficult by the lack of resolution offered by first-arrival traveltime tomography. To address this problem, a pre-stack depth migration of the reflected arrivals, based on this velocity model, is planned. A complementary non-linear traveltime approach is also the subject of ongoing works in order to independently obtain a second model for comparisons. Results obtained with these various techniques will be presented. Future developments should combine traveltime and frequency domain waveform tomography methods.