Earthquake-induced coastal sediment instabilities in the western Gulf of Corinth, Greece

On June 15 1995, a locally 6.2-R destructive earthquake occurred in the western Gulf of Corinth and caused extensive onshore and offshore sediment instabilities. A detailed offshore geophysical survey along the Tolofonas/Eratini coastline, soon after the earthquake, revealed the existence of two major sediment failures affecting the surficial 8 and 2 m of the recent sedimentary cover. The sizes of the two sediment failures are approximately 200 000 and 50 000 m2. The geotechnical study has shown that (1) the coastal sediments are stable under gravitational stresses and cyclic loading stresses induced by the 6.2-R earthquake and (2) the sediment failures were initiated due to liquefaction of subsurface layers. The failed sediments very quickly disintegrated and transformed to mass flows. The deformation of the failed sediment was caused by loss of sediment strength due to development of high pore water pressure induced by the earthquake. The study suggests that the alluvial fan deposits along the coastal zone of the western Corinth Gulf are susceptible to liquefaction by any earthquake event with a magnitude greater than 6 R, depending upon the proximity of the site to the earthquake epicenter. Earthquakes of magnitude 6 R are expected to occur every 22.7 years and therefore the recurrence interval for sediment failure events is about the same.