Settling of earthquake-induced turbidity on the accretionary prism slope of the central nankai subduction zone

Abstract

Studied of turbidite deposits can provide valuable paleoseismological information, such as earthquake recurrence intervals. However, there are few studies based on in situ observations of gravity flows during or soon after earthquakes. We used a remotely operated vehicle in dive surveys near the epicenters of the 2004 off the Kii Peninsula earthquakes (maximum magnitude 7.4) soon after and 6 years after the events. Video observations 20 days after the main shock revealed two turbid layers exhibiting different degrees of turbidity: A dilute suspension layer (DSL) and a bottom turbid layer (BTL). Pervasive distributions of DSL on the prism slope suggest that clouds of suspended sediments were induced by sliding, slumping, seafloor shaking, or sediment gravity flow at multiple locations during the earthquakes. BTL is a highly turbid layer of around 2.4 m thickness, the upper surface of which is sharply defined. That BTL was observed only in a slope basin implies that it was formed by a sediment gravity flow dumping muddy deposits in a depression. Our dive observations revealed that seafloor disturbance by seismic shaking plays a significant role in sediment redistribution in subduction zones. © Springer Science+Business Media B.V. 2012.