Suboceanic Rayleigh waves in the 1755 Lisbon Earthquake

SRW are large-amplitude, long-duration surface waves generated by shallow seismic sources in the oceano These waves result from the coupling of crustal Rayleigh waves with acoustic water waves across the seafloor (Ewing et al., 1957). They propagate efficiently along oceanic paths and are well recorded at stations inland. SRW have been comm only observed for long distance paths (> 2000 km) and their effect has never been considered in earthquake engineering so far. However, recent observations in peninsula r Italy during moderate earthquakes in the southern Tyrrhenian Sea (Rovelli et al., 2004) demonstrate that SRW attain large amplitude and long du ration also at regional distances (300-2000 km). In this study, we investigate moderate-size oceanic earthquakes record ed by the broadband station PMST in Lisbon to find the imprint of SRW. To confirm our interpretation we compare the data recorded from Tyrrhenian Sea eartbquakes with PMST data by applying a frequency-time representation of tbe signals. Finally, we perform some simple numer ical simulations to evaluate (i) if wave propagation in l-D crustal and upper mantle models, including a water layer, can return s the amplitude and long coda of observed SRW, and (ii) if a possible occurrence of SRW during the Great 1755 Lisbon earthquake can explain the high level of damage and duration of sbaking felt in southern Portugal