Field survey and modelling of the Caspian Sea tsunami of 1990 June 20

The Rudbar-Tarom earthquake of 1990 June 20 (M0 = 1.4 × 1027 dyn cm), the largest one in Iran over the past 35 yr, was accompanied by a small tsunami in the Caspian Sea, which produced run-ups of up to 2 m, and inundations of 1 km, reaffirming the existence of tsunami hazard along the Caspian shoreline, as suggested by historical reports. We present the results of a field survey, documenting the concentration of the effects of the tsunami along a section of coastline not exceeding 30 km in length. A hydrodynamic simulation using the earthquake dislocation as the source of the tsunami fails to reproduce both the amplitude of the waves, and especially their concentration between the cities of Kiashahr and Jafrood. Rather, we show that the model of an underwater landslide, presumably triggered by the earthquake, and taking place on the steep slopes of the continental shelf approximately 10 km offshore, can fit the principal inundation characteristics identified during the survey. We suggest that the occurrence of such underwater landslides should become a primary ingredient to the assessment of tsunami risk along the Southern shores of the Caspian Sea.