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On June 15th, 1995 a highly destructive earthquake of Ms 6,1R occurred in the Gulf of Corinth, 7,5km NNE of the town of Egio. (Fig. 5.1) An offshore survey using 3,5KHz subbottom profiling system, side scan sonar and a remote operated vehicle (ROV) has shown that the earthquake caused small-sized, subaerial to submarine sediment failures in at least four sites, in three fan delta deposits: the Rododofni, the Helike and Tolofona. (Fig. 5.1) (Papatheodorou and Ferentinos, 1997, Hasiotis et al 2002)
The fan deltas were located within a radius of about 9km from the epicenter. The areal size of the four sediment failure sites ranged from 2x104m2 to 6x105m2 whilst the volume of the failed masses ranged from 4x103 to 3x106.
The sediment deformation types identified at the failure sites were: (i) ground cracking and extrusion of mixtures of water and sand (sand boils), (ii) rotational and translational slides, (iii) elongated slides.
The sediment failure in the four sites affected the upper 5-6m of well layered Holocene topset and foreset fan delta deposits. The failure occurred on slopes ranging from 0,2o to 21o .
The geotechnical study has shown that: (i) the coastal sediments are stable under gravitational and cycling loading stresses induced by the 6,1R earthquake and (ii) the sediment failure was initiated due to liquefaction of subsurface layers.
The alluvial fan deposits along the coastal zone are susceptible to liquefaction by any earthquake of a magnitude greater than 6R, depending on the proximity of the site to the earthquake epicenter. Earthquakes of magnitude 6R are expected to occur every 22.7 years and therefore the recurrence interval for sediment failure events is about the same.
Fig. 5.1: Side scan sonar moseic along the Tolofonas river sediment failure.
Papatheodorou, G. & G. Ferentinos 1997. Submarine and coastal sediment failure triggered by the 1995, Ms-6,1 Aegion earthquake, Gulf of Corinth, Greece. Marine Geology 137: 287-304
Hasiotis T., Papatheodorou G., Bouckovalas G., Corbau C, Ferentinos G. 2002. Earthquake-induced coastal sediment instabilities in the western Gulf of Corinth, Greece. Marine Geology 186: 319-335.