Impacts of a large seamount subduction in the Nankai Trough off Cape Muroto revealed bya dense-OBS seismic survey

Abstract:

Seafloor roughness of the incoming oceanic plate, such as large seamounts, is one of the first-order controls on the subduction process, including the interplate earthquakes and the formation and the evolution of the forearc structure. In the Nankai subduction zone off Cape Muroto, there is a remarkable topographic high, called Tosa-bae. Based on the active-source seismic data, Kodaira et al. (2000) found a large seamount is subducting beneath the Tosa-bae and discussed the relationship between megathrust earthquakes and the seamount subduction there. However, the structural impacts caused by a large seamount subduction were not well constrained due to the limitation of the data quality obtained by the old seismic survey system of the previous century. Recently, small-scale roughness of the plate interface is considered to be key controlling on the distribution of the slow earthquakes such as shallow very low frequency earthquakes (sVLFE). From this point of view as well, detailed seismic structure regarding the seamount subduction is important.

In 2019, for revealing the detailed seismic structure around the Tosa-bae, we conducted an active-source seismic survey using a large tuned air-gun array of the R/V Kaimei and Ocean Bottom Seismometers (OBSs). First, we deployed 99 OBSs at a spacing of 2-km along two survey lines 2-km separated from each other. Next, we shot the guns along both lines as well as lines in between them. Then, we recovered all the OBSs. As a result, we obtained two 2-D OBS seismic data and quasi-3-D OBS seismic data. In this presentation, we show P-wave velocity (Vp) models along 2-D survey lines derived by the traveltime inversion. The preliminary Vp models showed complicated structural features within the overlying plate around the Tosa-bae which have not been revealed before, suggesting the seamount subduction causes significant impacts on the evolution and lateral variations of the overlying plate structure.