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Tsunami hazard for Crete and Rhodes

Flouri Evaggelia

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Summary

Historical reports and several scientic studies attest to numerous tidal waves impacting the Aegean in the past. The most famous report of a tsunami was made by Marinatos (1939) who claimed that a tsunami triggered by the eruption of Thera volcano in 1620 BC caused the collapse of the Minoan civilization on Crete. In historical and modern times, possibly two of the most powerful earthquakes in the Mediterranean of 365 and 1303 AD, with sizes Mw > 8, have epicenters located close to Crete and Rhodes, and both triggered destructive tsunami waves. Such large events are relatively rare and extreme events with low probability, but the existence of the Hellenic arc subduction zone and the overall high seismicity of Greece, increase the tsunami hazard. Moreover, today the residential, commercial and tourism development of coastal areas is much larger than before, and therefore much smaller events could have a signicant impact.The present study aims to estimate the tsunami hazard in the largest islands ofSouth Aegean, Crete and Rhodes, using modern methodologies based on numerical simulation and inundation computation.For this purpose we used the tsunami simulation model MOST (Method Of Splitting Tsunami) of Titov and Synolakis (1998), which has been extensively tested with benchmark problems and is now used operationally by the National Oceanic and Atmospheric Administration (NOAA) to support the Tsunami Warning center of the U.S., on the Pacic Ocean. This numerical model is capable for simulating all three phases of the tsunami; that is the generation, the propagation in the open sea, and nally the inland penetration and inundation.The initial phase of the tsunami generation, requires as initial condition the detailed description of earthquake rupture. For this purpose we searched the literature for recent studies on the estimation of focal mechanisms (e.g. Floyd et al, 2010) for potential earthquake scenarios in the Aegean, that are expected to have signicant impact for the two island. We tested probabilistic scenarios and/or worst case scenarios.Then, based on the seismic data, full numerical simulation of all proposedscenarios was made with MOST. In the phase of the wave propagation in the open 7 sea, accurate bathymetry and topography data of various resolutions are needed which were collected from various sources and formulated into suitable telescopic gridded datasets focusing to the islands. Finally, for the nal phase of inundation a detailed digital terrain model (Digital Elevation Model) bathymetry and topography of high resolution is required. Suitable digital models were acquired or produced for specicific areas of interest, e.g. large urban centers with signicant coastal infrastructure.Simulation results were then collected and processed to produce detailed ood maps (inundation maps), and important tsunami damage metrics were calculated for the study of risk.The risk is usually dened as the probability of disasters as a result of the interaction of natural disasters and vulnerability conditions. Towards more resilientsocieties, Ewing and Synolakis (2010), the focus now begins to shift from the assessment of natural disasters in the assessment of vulnerability. For the vulnerability of coastal areas from tsunami we need to study the exposure and the vulnerability of various socioeconomic data and structures, among which are usually the population, the buildings, and many other infrastructures. For this reason, appropriate spatiotemporal data from available sources were collected and processed through geographic information systems. Finally, the results were combined with those from the numerical simulations to produce detailed hazard maps.The ultimate goal of the research is to provide estimates that will contribute,where possible, in decision making for appropriate prevention, optimized risk management, aiming to improve the resilience of the local communities against the danger of tsunami.

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