Submarine landslide origin of a tsunami at the Black Sea coast: Evidence based on swath bathymetry and 3D seismic reflection data

Abstract

Tsunami waves were observed along the Bulgarian Black Sea coastline on 7 May 2007. The maximum rise and fall of the sea level were 1.2 and 2.0 m, respectively, with wave oscillations between 4 and 8 min. At first, submarine landsliding and then later on atmospheric disturbance were suggested as the cause of the tsunami. Numerical modeling, assuming a landslide displacing 30–60 million m3 material on the slope with a thickness range of more than 20–40 m, could reproduce the main characteristics of the recorded tsunami. In this early model, the landslide initiated on the shelf at a water depth of 100 m with a runout of approximately 20 km into 1000 m water depth. Subsequent and recent numerical modeling suggested that the failure may have initiated on the slope, anywhere between 200 and 1500 m seafloor depth. The runout of the transported sediments in these latest model was at 1850 m water depth. Just a few years after the tsunami, OMV and its joint venture partners, TOTAL and Repsol, acquired modern deepwater data sets in the same area where the submarine landsliding was assumed to occur. These data sets included multibeam swath bathymetry area and 3D reflection seismic data. These data sets offer a possibility to establish the presence of speculative submarine landslide responsible for the tsunami, with its geometry and nature. Our results provide direct evidence for the occurrence of large nonseismic catastrophic sediment failures along the Bulgarian coast. In this study, we illustrate Quaternary submarine landslides on 3D seismic reflection data immediately below the one responsible for the 2007 event; we also briefly point out the potential interpretation pitfall related to sediment waves and mass transport complexes.