The 28th February 1969 Earthquake and Tsunami in the Atlantic Iberian Margin

Abstract

On the 28th February1969, a massive earthquake stroke SW Iberia and NE Morocco triggering a tsunami recorded in more than 20 tide stations. The event occurred in the SW Iberian margin, the same seismogenic area of the 1st November 1755 mega event. Several studies were developed in the last 55 years to address its earthquake mechanism and the corresponding tsunami source. In some cases, the study of the 1969 event was also the base for inferences regarding the 1755 earthquake and indirectly to give some light on tsunamigenic processes related with the SW Iberian margin. In this study, we present a comprehensive review of the tsunami data and modelling, taking advantage from the great improvement that occurred on the quality of the bathymetric data, particularly on the shallow areas close to the tide stations. We used a larger set of tide-records than previous studies. All records were digitized from the original mareograms and processed according to modern standards. We address the possible landslide triggered at the NW coast of Morocco as the explanation of the tsunami observation at Casablanca. The new dataset combining both the earthquake and the landslide sources allows a better relocation of the tsunami source, enabling a quantitative comparison of the different source scenarios that have been developed for seismological research. The simulations presented here suggest that a thrust fault of 85 km × 20 km verging to the southeast is the best candidate to be responsible for the 1969 earthquake. The trace of this deep fault follows the one of the “Horseshoe Fault”, a northwest verging structure interpreted from the multichannel seismic data. Moreover, this deep structure may be accountable for both the 1969 event and the later 12th February 2007 M6 earthquake. Even more, the “Deep Horseshoe Fault” is a strong candidate to be the source of the 1st November 1755 event up to now elusive to multiple geological and geophysical studies.