Background seismic noise level in DONET seafloor cabled observation network

One of the observation target of the seafloor cabled observation network called Dense Ocean Network for Earthquake and Tsunamis (DONET) in the Nankai Trough, south of Japan, is slow and weak seismic motion. The target necessitated installation of broadband seismometers in the seafloor in a very quiet background noise environment. We developed a technique to install seismometer in a buried caisson using a piston corer and a remotely operated vehicle. We installed 18 observatories in the DONET using the technique, and evaluation of the resultant seismic background noise level and its relationship with the quality of the installation, was performed.