Submarine and coastal sediment failure triggered by the 1995, M s = 6.1 R Aegion earthquake, Gulf of Corinth, Greece

Abstract

On June 15th, 1995 a locally destructive earthquake of magnitude M s 6.1 on the Richter scale occurred offshore, 7.5 km NNE of the town of Aegion, in the western Gulf of Corinth (Greece). An offshore survey using 3.5 kHz subbottom profiling system and a remote operated vehicle (ROV) has shown that the earthquake caused small sized subaerial to submarine sediment failure in at least four sites, in three fan delta deposits: the Rododafni; the Eliki and the Tolofonas. The fan deltas were located within a radius of about 9 km from the epicentre. The areal size of the four sediment failure sites ranged from 2 × 10 4 m 2 to 6 × 10 5 m 2, whilst the volume of the failed masses ranged from 4 × 10 3 to 3 × 10 6 m 3. The sediment deformation types identified at the failure sites consist of ground cracking, rotational slides, elongated slides, sediment gravity flows and extrusion of mixtures of water and sand (sand boils). The sediment failure in the four sites affected the upper 5–6 m of well layered Holocene (?) topset and foreset fan delta deposits. The failure occurred on slopes ranging from 0.2 ° to 23 ° and the slip planes were all bedding planes which have a gradient from 0.2 ° to 21 °. The dominant instability mechanism that caused the sediment failure in the Rododafni and Eliki fan deltas is considered to be liquefaction of a shallow sub-surface horizon. The liquefaction was caused by elevated pore pressure enhanced perhaps by the presence of gas, resulting from the cyclic loading induced by the earthquake. The liquefied layer is assumed to have temporarily provided a failure surface for sliding to take place on and caused movement of sand that was ejected onto the surface. In the Tolofonas fan delta the causative mechanism in the case of the multi-block rotational slide is considered to be deformation of the underlying sediments caused by remoulding and/or liquefaction while in the case of the elongated slide it is considered to be the result of a combination of shear stress increase and/or strength degradation of the unconsolidated sediment. Both of these conditions could have been generated during the cyclic loading resulting from the Aegion earthquake. The study of historical documents reveals that sediment failure like those described above have also occurred at least four times during the past 2500 years in the same locations. Therefore, it is suggested that they could be repeated in the future by any earthquake event with a magnitude greater than 6 R, depending upon the proximity of the site to the earthquake epicentre.