Detailed seismic reflection profiles in the Nankai subduction zone with multiple splay faults, southwestern offshore of the Kii Peninsula, Japan

Abstract:

To explore characteristics of the subducting oceanic plate and overlying accretionary sediments in the Nankai subduction zone, we conducted dense two-dimensional seismic surveys in a southwestern offshore area off the Kii Peninsula, Japan. The multichannel seismic data were collected along a total of approximately 2,500 km survey lines during the cruise KM18-10 from November to December in 2018 using R/V Kaimei using a 4.5 – 5.7 km hydrophone streamer and a totaling 10,600 cubic-inch airgun array firing at 50 m intervals. To obtain subsurface reflection profiles every 4 km intervals, we processed the data along 23 lines in the NW-SE direction and 4 lines in the EN-WS direction. After preprocessing the data including noise attenuation, deghosting, designature, and demultiples, we carried out prestack depth migration. Interpreting structural features from the vertical sections, we observed topographic variations on the subducted plate interface and multiple major splay faults dividing the accretionary prism to several domains. We also identified a deep depression of the oceanic crust by normal faults in the eastern part and past seamounts in the western part outside of the trench, which were buried with thick sediments on the incoming plate in the Shikoku Basin. These buried structures on the incoming plate imply an analogy of possible origin of the topographic variation of the subducted plate interface. In the accretionary prism with imbricated thrust faults, a thick slope basin that contains several unconformities with dipping strata and past landslides was clearly imaged. The structural features may imply the intermittent deposition in the slope basin and structural change by thrust faults with strike-slip motion in the underlying accretionary sediments. The geometrical features on the plate interface and detailed structures in the overlying sediments are important factors to better understand the spatial variation of seismic activities in this region.