Long-lasting acoustic resonant signal associated with the turbidity current triggered by the 2003 Tokachi-oki earthquake

Turbidity current triggered by the 2003 Tokachi-oki earthquake of M8 is observed at a multidisciplinary submarine cabled observatory deployed in southern Kuril subduction zone offshore of Hokkaido in Japan. The cabled observatory consists of a cable-end station, three ocean bottom seismometers (OBSs) and two pressure gauges (PGs). At each cable-end station and OBS, one hydrophone, whose sampling rate is 100 Hz, is also installed. The epicenter is located at 25 km west-northwest of the cable-end station. The arrival of the turbidity current is detected with both the electro-magnetic current meter and the acoustic Doppler current profiler (ADCP) at the cable-end station about 2 h after the earthquake occurred. In the waveform of the hydrophone at the cable-end station, relatively broadband signal is recognized to continue since the arrival of the turbidity current. About eight hours later, the peak frequency changes to 1 Hz and this resonant signal continues about four hours. Meanwhile, similar signals are not observed at OBS1 which is located 5 km northeast of the cable-end station. Although the source mechanism of the resonant signal is unknown to date, this indicates that the main body of the turbidity current passes the western side of the cable-end station.