Experiment to detect temporal change of seismic velocity in the subducting plate boundary in the Nankai Trough using the DONET submarine cabled observation network and airgun controlled source

An experiment was performed to shoot airgun and receive the seismic signal with long-term seafloor seismic observatories (DONET seafloor and IODP borehole observatory) above the seismogenic zone in the Nankai Trough. The purpose of the experiment is to assess possibility to detect temporal change in seismic velocity structure which might change due to the stress accumulation in and around the subducting seismogenic plate interface, by repeated airgun survey, as well as to evaluate present stress state in the area through seismic anisotropy. The initial analysis of the experiment suggests that airgun shooting in accurate position especially in terms of the distance from the receiving observatory is necessary to treat the repeated shot record as the shooting in the same condition. P-wave first motion data from a circle shooting around the borehole observatory showed clear azimuthal dependency when correction was made for seafloor topography variation, which was consistent with known other indicators for seismic anisotropy and stress state.