5-19. Semi-controlled experiments in South Africa Gold mines

 

In 2001 and 2002, ERI and JAMSTEC carried out onshore-offshore seismic refraction expeditions. Objectives of these expeditions are to elucidate the plate subduction structure and its dynamics associated with the Philippine Sea Plate. The profile line in the 2001 experiment was 480 km long, and laid out from Off Tokai to northern end of the Chubu District, crossing the plausible rupture area of the next Tokai earthquake. On the southern part of the onshore line, we also performed seismic reflection profiling under the cooperation of the University of Texas, USA, to map the deep structure of accretionary complexes and the Median Tectonic Line developing beneath this area. The onshore profile recorded strong reflection from the subducted Philippine Sea Plate, from which a detailed structure and physical properties of the subducted plate are being clarified. On the northern part of the onshore line, we can recognize midcrustal reflections, which will provide some constraints on the evolutionary process of the island arc crust.

The seismic line in the 2002 experiment was extended from Shikoku Island to Off Tottori. On its offshore part, 230 km in length, 35 OBS were deployed to record air-gun shooting. The onshore line was 240 km long, on which 2,234 receivers were set to record 10 explosive shots. The southern part of this onshore line was designed to almost coincide a land part of another profile undertaken 1999 from the Nankai trough to the middle part of the Chugoku District. This experimental design enables us to do combined analysis of the 2002 and 1999 lines for obtaining the whole structural image of subduction system from the Nankai Trough to the Sea of Japan.

Fig. 1. Seismic profile of the 2002 line in SW Japan.