Consideration of Turbidity Current occurrence caused by Typhoon No.24, 2018 at northern part of Suruga Bay Japan-Accidental phenomenon on the Sea Floor detected and traced by OBS Arrays-

In August 2018, seismic reflection and refraction structural exploration using Airgun-OBSs was started to carry out under the project name of "Integrated Research Project for the Fujikawa Fault Zone" in northern Suruga Bay. Fujikawa Fault Zone is located around the Suruga Bay at the central part of Japan. We had been deployed pop-up typed 18 OBSs beneath the survey line across the Bay from offshores Shimizu to Heda. We also deployed another 4 OBSs in other survey lines for seismic activity observations around the Suruga Bay.

During the exploration period from September 30 to October 1 in 2018, the Typhoon No.24 was closely passed in the vicinity of the Suruga Bay. Severe damages were given to various parts on OBSs. Following various OBS accidents had occurred by these damages after the Typhoon left: Four OBSs were released and drifted to the beach. Two OBSs were forced to move to southeast approx.max.800m apart from its original position. One OBS had been missing with no transponder response. Two OBSs were not released to pop-up with transponder responses.

By these above statuses, we have considered that OBSs had got several accidents due to the phenomenon occurred a by typhoon No. 24 at in northern part of Suruga Bay.

We have analyzed these phenomenon using seismic waves and submarine topography.

・According to the waveform records obtained by OBSs, the accidents were spread from Fujikawa estuary extension to the center of Suruga Bay.

・Topography from Fujikawa estuary to the survey lines are tilted to easy to occur turbidity flows.

From the above results, the cause of the accidents of OBSs in northern Suruga Bay were presumed that Fujikawa originated muds streamed into the Suruga bay, then huge sediments pushed OBSs away from its original position. From this point of view, the Turbidity Currents had occurred in northern Suruga Bay.