Re-evaluation of Seismic Intensities and Relocation of 1969 Saint Vincent Cape Seismic Sequence: A Comparison with the 1755 Lisbon Earthquake

Seismic intensity for the February 28, 1969 (Mw = 7.8) earthquake have been re-evaluated using original documents in local archives, such as, contemporary newspapers, council minutes, monographic studies, among other sources for Spain, Portugal and Morocco and answers to macroseismic questionnaires for Morocco. This information is used to plot a new intensity map for the whole region affected by the earthquake: Portugal, Spain and Morocco. The intensity values vary from VIII to IX in the E-W coast of Algarve, southern Portugal, to II–III. Furthermore, we have relocated the hypocentres for main shock and 24 aftershocks using a new 3D crustal velocity model for the Gulf of Cadiz region and a non-linear probabilistic location methodology, most of them previously lacking a depth estimate. The new locations show an E-W distribution of epicenters, with focus located in the uppermost mantle, most of them with depths between 30 and 50 km. No earthquakes have been located at depths shallower than 30 km. A comparison between peak ground accelerations (PGAs) estimated from the observed intensities for the 1969 and the Lisbon 1755 earthquakes, and synthetic PGA values, generated assuming two different scenarios (using the 1969 and 2009 earthquakes) for the 1755 Lisbon event, shows that the observed damage produced by the 1755 earthquake may be better explained assuming a reverse dip-slip mechanism oriented in NE-SW direction, similar to that of the 2009 earthquake, rather than assuming focal mechanism similar to that of the 1969 earthquake.