The Amorgos, Greece earthquake and tsunami of 09 July 1956: Focal mechanism and field survey

E.A. Okal, C.E. Synolakis, and A.C. Yalçiner

The earthquake of 09 July 1956 near the Greek island of Amorgos (MPAS = 7.8) is the largest event of the past 75 years in the Aegean. It created the most damaging tsunami to hit Greece in the past century, with reported run-up of 25 m on the Eastern coast of Amorgos. This prompted Ambraseys [1960] to propose that underwater landslides may have occurred. We use the PDFM method introduced by Reymond and Okal [2000] to invert a moment tensor from a limited set of spectral amplitudes of mantle waves. Our solution features a normal faulting mechanism (phi = 245 deg; delta = 67 deg; lambda = 281 deg.) and a moment of 3.9 * 1027 dyn-cm. In parallel, we have started a systematic survey of tsunami run-up heights in the Aegean Islands and the Asia Minor coast of Turkey, through the interview of elderly witnesses of the tsunami. Our growing dataset presently includes 29 data points on seven islands and at eight villages on the Turkish Coast. We confirm a single run-up value of 20 m on Eastern Amorgos, with measured run-up limited to 8 m on Astypalea and at most 3 m at other locations (1 m on the Turkish coast). The uniqueness of the large run-up value, in the proximity of locales with milder values, does suggest that it could be due to the influence of a localized underwater landslide.