The 2003 Lefkada earthquake: Field observations and preliminary microzonation map based on liquefaction potential index for the town of Lefkada

摘要

A strong earthquake (Mw=6.2) occurred offshore the island of Lefkada (or Lefkas) on August 14, 2003. The maximum intensity has been evaluated Io=VIII (EMS) at Lefkada municipality, while VI to VII+ intensities were evaluated at many other villages of the island. The offshore NNE-SSW oriented strike-slip right-lateral fault was activated by the main shock. This fault is the northern termination of the Cephalonia transform fault. The most characteristic macroseismic effects were extensive typical ground failures like rock falls, soil liquefactions, subsidence, densification, ground cracks and landslides. These macroseismic effects are remarkably similar to those reported from some historical Lefkada shocks, e.g. 1704, 1914 (Ms=6.3) and 1948 (Ms=6.5). Sand boils and ground fissures with ejection of mud were observed at the seaside of the town of Lefkada, and in the villages of Nydri and Vassiliki. In situ soil profiles are obtained based mainly on borings after the earthquake. Boreholes records with SPT values (standard penetration test) are obtained and the "simplified procedure" originally developed by Seed and Idriss was employed to evaluate the liquefaction resistance of soils. The results indicated that the silty sandy layer, which lies beneath the artificial fill in the coastal zone in Lefkada town, had liquefied during the 14 August Earthquake. An attempt was also made to establish a preliminary microzonation map for Lefkada town using the data from Liquefaction Potential Index analyses. Our map was validated by the occurrence of liquefaction phenomena inside the town.