Seismic reflection images of shallow subduction zone in the Suruga Trough, Central Japan

Suruga Trough, Central Japan, is located at the eastern end of the Nankai Trough where the Izu-Bonin arc on the Philippine Sea Plate has been subducting beneath Eurasian plate. To clarify the subduction history and process of this megathrust, it is necessary to understand the shallow structures together with the help of intensive high-quality geological and geophysical observations. In this study, we have shown the results of shallow subduction zone images in the Suruga Trough by means of 2D seismic reflection surveys.

Portable 2-D seismic reflection system of Tokyo University of Marine Science and Technology consists of three 10-ft containers for Bolt air-gun source system, an air compressor system and Hydroscience 600-m 96-ch digital streamer-cable systems (Tsuruga et al., 2016, 2017).

We conducted the 2-D seismic reflection and refraction surveys in 2016, 2017 and 2018 by using our portable system and a research vessel 'Shinyomaru-IV' (986 tons,) (e.g., Tsuruga et al., 2017, 2018, 2019). All 9 survey lines were located in the northern to middle part of Suruga bay with a total length of 201 km.

In 2018, we used Bolt 'Tri-Gun' sources (a total volume of 1950 cu.in) with an shot interval of 25 m for the particular 60-km onshore-offshore seismic refraction survey (Sato et al., Baba et al., in this meeting). And we also conducted seismic reflection survey in the oceanic area with the survey line length of 20 km (this study). The seismic reflection profiles across Suruga trough axis show some important features for subduction images in Suruga Trough.

It indicates that some continuous west-tilting reflection phases may be appeared around an upper boundary of oceanic plate down-going toward west. Such reflection images similar to the seismic profiles observed at other survey line across the trough axis in the middle-southern area. Sato et al. (this meeting) suggests that such reflection phases can be continued to the other reflection phases observed in the deep structure along the onshore-offshore refraction survey.

The onshore-offshore seismic survey in 2018 was conducted under the Integrated Research project on the Fujikawa-kako Fault zone funded by the Ministry of Education, Culture, Sports, Science and Technology, Japan.