Science of Tsunami Hazards (Feb 2021)
THE GREAT LISBON EARTHQUAKE AND TSUNAMI OF 1 NOVEMBER 1755 Evaluation of the Compression Convergence Mechanism
Abstract
The Great Lisbon earthquake of 1 November 1755 was an unprecedented, extremely destructive seismic event with a moment magnitude (Mw) estimated to range from 8.5 to 9.0. Its epicenter was off the southwest coast of Portugal in the abyssal plain close to the Gorringe Bank along the Azores-Gibraltar fracture zone (AGFZ), which has been created by complex tectonic interactions between the Eurasian, African and Iberian plates. Shortly after the earthquake, huge tsunami waves engulfed and caused extreme destruction and loss of life in the lower part of Lisbon, in many smaller towns along the Tagus river estuary, as well as along Portugal’s southern region of Algrave, in Morocco, in Gibraltar, and elsewhere. The far- field tsunami impact affected both North and South America, the Caribbean region and Northern Africa. The present study examines the source parameters of the earthquake and its unusual mechanism of tsunami generation, based on bathymetry, gravity anomalies and overall geomorphological features of the eastern segment of the AGFZ which indicate the existence of a large area of subsidence, characterized by a grabben with an approximate orientation of N45E, adjacent to a thrust fault, apparently both caused by continent to continent convergence/collision. Additionally, the present study evaluates: a) the dimensions and orientation of the 1755 tsunami generating area; b) the net ocean floor crustal displacements; c) the total energy release; e) the extensional, transcurrent and compressional forces of this tectonic regime; and f) the statistical approximation of earthquake and tsunami recurrence frequencies in the region. Additional objectives are the documentation of the historic aspects of this disaster, and of the complex interactions of on-going active continent to continent convergence near the AGFZ fault zone which, in addition to this great 1755 earthquake, has resulted in numerous lesser magnitude historical tsunamigenic events in the region. The present study speculates the 1755 earthquake resulted, not from the rupture and subsidence of one short segment, but from a combination of at least two successive ruptures along the eastern segments of AGFZ, closer to the Iberian plate where a seismic gap existed and successive stress had accumulated.
