The Lisbon 1755 earthquake is poorly understood due to its offshore location and complex macroseismic intensity pattern. Attempts to estimate its magnitude from isoseismal areas led to results in the range from 8.5 to 8.7. I use the available macroseismic data to reassess the moment magnitude and obtain a value of 7.7±0.5, significantly lower than previous results. Based on the pattern of intensity data outliers, I suggest that the source was complex and spatially distributed, with part of the rupture taking place onshore or inshore.
One of the major challenges to this reassessment is the evidence in support of a large transoceanic tsunami. This earthquake was assigned a tsunami magnitude of 8.5, albeit with large uncertainty. If a moment magnitude of 7.7 is adopted as proposed here, it would put the Lisbon 1755 earthquake in the cathegory of a tsunami earthquake.
I discuss the apparent incompatibility between several published tsunami source models and the tectonic grain and stress field of the epicentral region and speculate on possible alternative sources for the tsunami such as submarine landslides on the continental slope of rupture at the basis of the Cadiz gulf accretionary prism.