GEOPHYSICS: The Great Lisbon Earthquake

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Science  21 Nov 2003:
Vol. 302, Issue 5649, pp. 1295c
DOI: 10.1126/science.302.5649.1295c

On 1 November 1755, an estimated 8.5 magnitude earthquake occurred along the Africa-Eurasia plate boundary in the Atlantic Ocean southwest of Lisbon. This literally earthshaking event devastated one of the most beautiful cities in Europe, producing ground liquefaction, seiches, a tsunami, and subsequent fires. Shaking was felt throughout the rest of Portugal and in Spain, France, Italy, Switzerland, and northern Africa; the tsunami flooded much of the Atlantic and western Mediterranean coastlines; and seiches were noted as far away as Finland.

Afterward, the Portuguese government requested that priests provide information about the time and duration of shaking as well as the damage in their parishes. In addition, eyewitness accounts from inhabitants and sailors were recorded. Based on these descriptions, Vilanova et al. suggest that the initial offshore earthquake triggered within minutes another earthquake about 300 km away on the Lower Tagus Valley fault. A dynamic effect at this distance would be unusual but possible if the fault were already close to failure. The second event would account for reports of two periods of intense shaking, the sudden subsidence of the riverbed and impulsive rise of the river water, and the high intensity of the shaking within the valley region. The Lower Tagus Valley is an important component of the plate boundary and suffered ruptures in 1344, 1531, and 1909. Adding another event in 1755 would reduce the recurrence interval and likely increase the estimated seismic hazard. — LR

Bull. Seismol. Soc. Am. 93, 2056 (2003).

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