By Sid Perkins
From San Francisco, at a meeting of the American Geophysical Union
On the morning of Nov. 1, 1755, the thriving port city of Lisbon, Portugal, was devastated by three earthquakes, the tsunamis they triggered, and an ensuing fire. Tens of thousands of residents lost their lives. Now, tsunami simulations suggest that a newly discovered fault zone beneath the Atlantic Ocean could be the source of most of the seismic energy released that day.
Lisbon experienced the three temblors within 10 minutes, says Maria Ana Baptista, a geophysicist at the University of Lisbon. Scientists have long debated where those earthquakes originated because the two known fault zones beneath the ocean southwest of Portugal–the Guadalquivir Ridge and the Gorringe Bank faults–aren’t long enough to have released the total seismic energy of that day’s quakes.
Also, Baptista notes, the estimated arrival times for tsunamis generated by quakes along those faults don’t match historical accounts of when the killer waves reached the port. Finally, researchers haven’t found any evidence of large undersea landslides that could have triggered the tsunamis, so most scientists suspect that all three waves were generated by the earthquakes themselves.
Recent seismic surveys of the Atlantic seafloor east of Gibraltar suggest that there’s a subduction zone where the African tectonic plate dips below the Eurasian plate. An earthquake there in which fault surfaces slipped about 20 meters–the sort of earthquake that might occur every 1,500 years or so–could have generated a magnitude 8.8 earthquake. That temblor, with smaller ones along the Guadalquivir Ridge and Gorringe Bank faults, could account for the size of the Lisbon quakes on the disastrous day in 1755, as well as the heights and arrival times of the tsunamis the quakes triggered, says Baptista.
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