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Lisbon 1755; a case of triggered onshore rupture?

Susana P. Vilanova, Catarina F. Nunes and Joao F. B. D. Fonseca
Lisbon 1755; a case of triggered onshore rupture?
Bulletin of the Seismological Society of America (October 2003) 93 (5): 2056-2068


It has been widely recognized, both in classical and in modern studies, that the Lisbon earthquake of 1755 was a multiple event, composed of three shocks separated by a few minutes (see, e.g., Reid, 1914). Attempts to constrain the location of the source have led to a diversity of proposals, reflecting apparent contradictions in the data. The tsunami and damage along the south and southwest Iberian coast and in Morocco favor an offshore source, whereas the presence of an additional zone of strong shaking in the Lower Tagus Valley (LTV), near Lisbon, favors a more northerly location. By combining the contemporary accounts with intensity data from other earthquakes, we favor a compound source with a large distance between the faults. We propose that, although the mainshock was offshore, the resulting stress changes induced the rupture of the LTV fault, at a distance on the order of 350 km (but subject to large uncertainty in the offshore location), a few minutes after the mainshock. We favor this model, rather than site effects causing high intensities in the Lisbon area, because the highest intensities show a negative correlation to soft soil. Several other phenomena described in the eyewitness accounts can also be explained by the local rupture now proposed, such as a tsunamilike wave in the Tagus River, ground deformation affecting the course of the Tagus River, and the spatial pattern of damaging aftershocks. Recognition of this "missing" episode of rupture on the LTV fault significantly changes the hazard estimate for the Lisbon area.

ISSN: 0037-1106
EISSN: 1943-3573
Serial Title: Bulletin of the Seismological Society of America
Serial Volume: 93
Serial Issue: 5
Title: Lisbon 1755; a case of triggered onshore rupture?
Affiliation: Instituto Superior Tecnico, Division of Earthquake Engineering and Seismology, Lisbon, Portugal
Pages: 2056-2068
Published: 200310
Text Language: English
Publisher: Seismological Society of America, Berkeley, CA, United States
References: 55
Accession Number: 2004-016014
Categories: Seismology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sketch map
N37°00'00" - N42°10'00", W09°30'00" - W06°10'00"
N38°55'00" - N40°46'60", W09°00'00" - W01°43'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200405
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