CONSTRAINS ON THE SOURCE OF THE 1755 LISBON TSUNAMI INFERRED FROM NUMERICAL MODELING OF HISTORICAL DATA ON THE SOURCE OF THE 1755 LISBON TSUNAMI

The 1755 Lisbon tsunami was felt all over the North Atlantic, being on e of the first major events of this kind relatively well documented by historical sources. However, in spite of the extensive research work on the historical reports by a considerable number of authors, the epi centre location of this event is still uncertain and its focal mechani sm is still not well understood, implying a great uncertainty in the t sunami generating mechanism, The generally assumed epicentre, inferred from isoseismal maps, is located slightly north of the Gorringe Bank (SW Iberia) and the rupture mechanism has been assumed in the past. to be similar to the well studied 1969.02.28 event. While all previous s tudies have used a seismic-based approach, this paper uses all that is known about the tsunami parameters at the coast - presented in a comp anion paper - to define the location and geometry of the tsunami sourc e. For that purpose some backward ray-tracing techniques were develope d and their results were used to define the initial fields in a number of shallow water simulations of the water height at the coastal locat ions where the most reliable historical data are available. The source parameters also took into account the estimated seismic energy releas ed, The results obtained here suggest that the 1755 tsunami probably o riginated on the continental shelf, implying an epicentre area located between the Gorringe Bank and the Iberian coast, in a geodynamic cont ext quite different from the one implied in the 1969.02.28 event, The amplitude of the initial movement in the source region, required by th e shallow water simulations to account for the reported magnitudes, su ggests an elongated but shallow rupture area, extending along the shel f, It is suggested that this location of the rupture would have signif icant implications in the geology of the region. (C) 1997 Elsevier Sci ence Ltd.