“Did You Feel It 50 Years Ago?” The 1969 Mw 7.8 Cape Saint Vincent Earthquake

Abstract

Abstract On 28 February 1969, an Mw 7.8 earthquake occurred 180 km southwest of Cape Saint Vincent, Portugal. The earthquake was widely felt in mainland Portugal, Madeira Island, Spain, Morocco, and even as far as Andorra and France, reaching a maximum intensity of VIII in the south of Portugal. In 2019, on the 50th anniversary of the earthquake, there was launched online an international “Did You Feel It 50 Years Ago?” (DYFI-50y) survey, published in several languages, for citizens to report the observed effects of the earthquake. This initiative had two main purposes: first, to collect and preserve community observations of the earthquake; and second, to test the online implementation of the DYFI questionnaire at Instituto Português do Mar e da Atmosfera for a damaging event. The DYFI-50y questionnaire was disseminated widely in the media, and a school project was launched for school-age children to collect reports from grandparents or other close elders. In total, more than 3000 reports were collected, a number that is close to the number of reports received for a recent M 6 earthquake in 2009. The collected community reports provide good coverage of mainland Portugal, particularly in Lisbon and Porto’s densely populated urban centers. In low-population areas, the limited results do not allow for robust statistics, and variability becomes significant. The distribution of intensities obtained shows a generally good agreement with other sources, indicating that the community reports are reliable and complement existing intensity maps for this earthquake, further clarifying some of the observed patterns. It was concluded that an online questionnaire is an efficient tool for gathering data several years after the earthquake.