Did Suboceanic Rayleigh Waves Play a Role on the Heavy Destruction Caused by the 1755 Lisbon Earthquake?

Moderate magnitude earthquakes in the south Tyrrhenian Sea generate efficient suboceanic Rayleigh waves that are well recorded in peninsular Italy. This observation, extrapolated to magnitudes larger than 8, suggests that the effect of large earthquakes in the ocean could be devastating at regional distance in terms of ground motion amplitude and duration. Through 1D models, we test the hypothesis that the high level of destruction and the long duration of shaking felt during the Great Lisbon earthquake of 1755 were caused by suboceanic Rayleigh waves. In this preliminary study, we check the role of critical model parameters. We find that durations and amplitudes are largest when the average thickness of the water layer is 2 km and shear-wave velocity of the ocean floor is close to the speed of sound in the water. Both conditions are realistic for a source in the Atlantic Ocean, few hundreds of kilometres southwest of Lisbon. Moreover, the propagation of suboceanic Rayleigh waves at regional distances accounts for durations of a ten minutes as the effect of a single large earthquake.