In the west and central segments, the Africa-Eurasia plate boundary is a well defined narrow fault zone. This plate movement has caused various types of deformation: extension along the Terceira Ridge around the Azores Islands in the west strike-slip along the Gloria Fault in the central segment and convergence and collision in the Gorringe Bank and the Strait of Gibraltar to the east. Nevertheless, several large magnitude earthquakes have occurred here, including the 1755 event, a M7.6 earthquake in 1816, and a M7.9 earthquake in 1969. That's relatively slow compared to other active plate boundaries. Here the Africa plate rotates counterclockwise relative to the Eurasia plate at a rate of about 4 mm/yr. The Africa-Eurasia plate boundary extends from the mid-Atlantic ocean ridge triple junction near the Azores eastward to the Strait of Gibraltar. Active tectonics and historic seismicity in the 1755 Lisbon earthquake source zone (GB = Gorringe Bank, AP = Abyssal Plain, LTV = Lower Tagus Valley the arrows indicate the rotation of the Africa Plate). As it turns out, that more likely scenario has the potential to produce losses as high, or even higher than the occurrence of a much larger earthquake offshore.įigure 1. It also addresses recent research that suggests that Lisbon's earthquake risk today is driven not by the prospect of a recurrence of a 1755-type event-which is estimated to have a return period of between 3,000 and 4,000 years-but rather by the possibility of an earthquake of more moderate magnitude, but one much closer to the capital. This article describes the tectonic processes that gave rise to the disaster that occurred some 250 years ago. 1 In total, Pereira estimates losses of 64,000 to 72,000 contos, about 32% to 48% of Portugal's GDP at the time. The effects of the tsunami were noted as far away as England to the north and in the Caribbean islands across the ocean.Īccording to a 2003 study by Alvaro Pereira, nearly 70% of Lisbon's dwellings-about 23,000 buildings-were destroyed or substantially damaged by the combined effects of shaking, fire and tsunami. Towns along Portugal's western and southern coasts, southwestern Spain and most of Morocco's Atlantic coast also suffered significant damage and loss of life. Approximately half an hour after the first shock, massive tsunami waves up to 12 meters in height began to arrive, wrecking ships and drowning thousands of people. Some residents sought refuge in the harbor at the mouth of the Tagus River, but their relief from the fires and falling debris was short-lived. Intense fires that were likely ignited by curtains and woodwork that fell into the flames spread within minutes throughout the city and burned out of control for days. Those who survived collapsing buildings fled into the streets, leaving behind lighted candles on church altars and cooking fires in kitchens. Beginning at around 9:30 am, three separate jolts spaced a few minutes apart violently shook southwestern Portugal. Much of the city's population, estimated to be around 200,000, was gathered for mass in Lisbon's opulent churches and cathedrals. The Great Lisbon Earthquake occurred in the morning hours of All Saints Day, a Catholic high holiday, on November 1, 1755.
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