Fan delta development and processes offshore a seasonal river in a seismically active region, NW Gulf of Corinth

High-resolution geophysical surveys (seismic, side-scan sonar) offshore of the Eratini River, a seasonally flowing river in the NW Gulf of Corinth, Greece, revealed a small fan delta with a variety of bottom features (blocky deposits, chutes and sediment instabilities). Considering the relatively small size of this river, however, these features could not be explained as being produced solely by river flow processes. Based on morphological features, the fan delta can be subdivided into a high- and a low-energy area. Sedimentation processes in the fan delta are associated with flood-derived sediment input, hyperpycnal flows which erode the fan surface, mud settling from suspension plumes, shelf sedimentation and sediment failures. The observed blocky deposits are considered to be the result of earthquake-induced mass flows in 1965 and 1995, whereas the chutes would be produced both by erosive mass flows and by hyperpycnal currents. The bulk block sediment volume has probably resulted from the 1965 earthquake. The 1965 evacuation zone and the related chutes were buried by the prograding fan delta. The main causative factor triggering the observed sediment instabilities is considered to be liquefaction, which is caused by (1) frequent earthquake-induced cyclic loading and (2) low sediment shear strengths created by rapid deposition during floods, in both cases associated with high pore-water pressures.