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The 1755 Nov.1st Great Lisbon Earthquake: a seismological, historical and cultural landmark

The November, 1st, 1755 Lisbon earthquake and related disasters (tsunami, fires) deeply influenced the french philosophers of the Enlightenment Age ("Le Siècle des Lumières"), and then the European cultural heritage. Voltaire questioned the philosophical notion that "all is for the best" ("tout est bien") (Poème sur le désastre de Lisbonne; Poem on the Lisbon disaster):O malheureux mortels! ô terre déplorable!
O de tous les mortels assemblage effroyable!
D'inutiles douleurs éternel entretien!
Philosophes trompés qui criez: "Tout est bien"
Accourez, contemplez ces ruines affreuses
Ces débris, ces lambeaux, ces cendres malheureuses,
Ces femmes, ces enfants l'un sur l'autre entassés,
Sous ces marbres rompus ces membres dispersés;
Cent mille infortunés que la terre dévore,
Qui, sanglants, déchirés, et palpitants encore,
Enterrés sous leurs toits, terminent sans secours
Dans l'horreur des tourments leurs lamentables jours!
Voltaire also strongly denounce those who appeal to the Divine Will to explain the disaster in this world closely controlled by religion:"Tout est bien, dites-vous, et tout est nécessaire."
Quoi! l'univers entier, sans ce gouffre infernal
Sans engloutir Lisbonne, eût-il été plus mal?
Etes-vous assurés que la cause éternelle
Qui fait tout, qui sait tout, qui créa tout pour elle,
Ne pouvait nous jeter dans ces tristes climats
Sans former des volcans allumés sous nos pas?
Borneriez-vous ainsi la suprême puissance?
Lui défendriez-vous d'exercer sa clémence?
L'éternel artisan n'a-t-il pas dans ses mains
Des moyens infinis tout prêts pour ses desseins?
Je désire humblement, sans offenser mon maître,
Que ce gouffre enflammé de soufre et de salpêtre
Eût allumé ses feux dans le fond des déserts.
Je respecte mon Dieu, mais j'aime l'univers.
The cultural "shock" was so strong that some writers even claim that the earthquake influenced the European (and especially french) atheism expansion.Even if opposed to the Voltaire's analysis (they had harsh exchanges on this topic), J.J. Rousseau, another french philosopher, used the earthquake to support his desire for a closer to nature life. According to him, the extent of the disaster was largelly due to the development of the city and the agglomeration of inhabitants.
J.J. Rousseau and Voltaire. Panthéon façade, Paris.Damages were huge. At around 9.30-9.40 AM, Lisbon was first damaged by shaking. The very pious population of the city was massed in the churches for this All Saints' Day, and many people died under the collapsed roofs and walls. The survivors, who rushed along the seashore and Tagus riverside, could have seen the strange water ebb, the sign of the following tsunami waves. They submerged the city around 30 minutes after shaking and the wave height could have exceeded 10 meters. A severe fire devastated the remnant buildings during five days. Many other cities, in Portugal and Spain, suffered the effects of the earthquake and its tsunami. The total death toll is estimated to 70,000.
A copper engraving depicting the city of Lisbon, hit by the tsunami and fire after the November 1st earthquake (1755)From the historical archives relating damages and other effects, the maximum recorded intensity has been assessed to 10 to 10.5. The magnitude has been estimated to 8.5 and the epicentre location, though very uncertain, is suspected to be 250 km south-south-west of Lisbon (see AHEAD, European Catalogue of Historical Earthquakes).
Map of effects and damages due to the 1755 earthquake, according to AHEAD catalogue.
What is the source of the earthquake? This is quite speculative to definitely state about an offshore historical earthquake. However, the size of the geological source (fault) has to be large enough to produce such a earthquake magnitude. A minimal estimation of the fault size can be inferred: if we consider a Mw>8.0 value, the minimal surface area should be 7000 km² (Matias et al., 2013). The earthquake epicenter is located in a complex seismotectonic setting, within the diffuse Nubia-Eurasia continental plate boundary, east of the oceanic transform fault (Gloria Fracture Zone). In the epicenter area, several large faults were proposed as individual or composite seismogenic sources for the 1755 event, including the Gorringe Fault, the Horseshoe thrust or the Marqués de Pombal fault.

Tectonic map of the Gulf of Cadiz region showing relief (bathymetry and topography) and major mapped faults. Map available on Wikipedia, Author: M.A. Gutscher. Red star is the 1755 Lisbon earthquake epicenter according to AHEAD catalogue.

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