EARTHQUAKE-TRIGGERED TURBIDITY CURRENT OBSERVED AT THE LONG TERM DEEP SEA FLOOR OBSERVATORY OFF KUSHIRO, NORTHEASTERN JAPAN

The seafloor observatory equipped with current meters detected a flow activity after the earthquake, which was M8 in Richter scale. The study area is located at a landward slope in the Kuril Trench where the Pacific plate subsides under Eurasia plate, and is a potential site of disastrous and cyclic earthquake activity. The earthquake occurred on Septempber 16th, 2003 and the estimated epicenter was about 25 km away from the observatory. A weak flow that started 22 minutes after the main shock was followed by a sudden downslope flow 130 minutes later. The maximum velocity reached 1.4 m/s, and the flow lasted more than 24 hours. The measurement by the Acoustic Doppler Current Profiler shows the velocity profile of the flow that was up to 80 m thick. While there were minor problems due to the earthquake shock, the fact that the observatory system had little damage by the flow indicates that the flow carried suspended sediments with little or no bedload. During the flow passage, a gradual increase in water temperature was observed. The temperature difference indicates that the source area was shallower than 1700 m and, considering the local bathymetry, at least 20 km away from the observatory. While the observed flow velocity was no more than 1.5 m/s, the estimated speed of flow propagation is 2.6 m/s or larger. Detailed bathymetric map of the area shows that there is a network of small submarine canyons. It is likely, therefore, that the main flow traveled in the canyon and the observatory detected an overspill flow spreading from the canyon. Although its size was small to moderate, this is a rare example of direct observation of seismically triggered turbidity current, which is unpredictable and potentially catastrophic. The observed flow properties and the sea floor survey suggest that the flow was not generated by a large-scale slope failure, but by resuspension or mobilization of surface sediments.