Dynamic process of turbidity generation triggered by the 2011 Tohoku‐Oki earthquake

Abstract

We analyzed the distribution of turbidity in seawater near the epicenter of the 2011 Tohoku earthquake ( M w 9.0) as well as the mineral composition and grain size distribution of suspended particles and of shallow sediments in cores from the same area. One month after the earthquake in an area where large changes in bathymetry accompanied the earthquake, we observed a strong turbidity anomaly above the seafloor (maximum 5 mg/L) that greatly exceeded turbidity measured several years before the earthquake (average 0.59 mg/L). The suspended particles were composed of material similar to the surface material of the sediment cores, and we inferred that they were generated locally by a disturbance due to slope failure. We estimated slope stability on the basis of the geophysical characteristics of the sediment cores as well as the acceleration of the earthquake. Our results showed that a submarine landslide could have been induced by the very large ground acceleration of the Tohoku earthquake, as high as 10–15 m/s 2 , even if the sediment layer on the sliding surface was thin. We interpreted the turbidity anomaly observed one month after the Tohoku earthquake as the result of shallow submarine landsliding and stirring up of unconsolidated sediment around the epicentral area.