Re-evaluation of magnitude of 1755 Lisbon Earthquake

The magnitude of the Lisbon Earthquake may not have been as high as has been estimated so far. The devastating earthquake and tsunami that struck Portugal on All Saints’ Day in 1755 would have been in Richter’s 7.7 degrees, below the nearly consensual 8.5 to 8.9%.

This conclusion is evident in a study by João Duarte Fonseca of the Instituto Superior Técnico of the New University of Lisbon, which has now been published in the newsletter of the Seismological Society of America.

For the Portuguese researcher, who for years has been paying attention to the Lisbon Earthquake and has published many on the subject, the location of the epicenter was on the southwest coast of the Iberian Peninsula and earthquake faults may also have occurred.

To reach this conclusion, he used macro seismic data from Portugal, Spain and Morocco. This re-evaluation may have implications for the seismic hazard map, João Duarte Fonseca   said in the publication of the American society that is dedicated to seismology.

“While the current official map assigns the highest level of risk to southern Portugal, gradually decreasing to the north, the interpretation presented now concentrates the risk in the Greater Lisbon area ,” said João Duarte Fonseca.

The 1755 earthquake, along with the fires that spread throughout the city, is considered one of the most important events in seismic history, leading to the deaths of about 20,000 people, although there are higher estimates.

The 1755 earthquake is seen as unusual as it has caused extreme damage hundreds of miles from its epicentre. “The explanations given for extreme damage in Lisbon tend to invoke an unusually low attenuation of seismic energy as the waves move away from the epicenter, something that should not be observed anywhere else in the world,” the seismologist explained. “Current attempts to harmonize seismic risk assessment across Europe face major discrepancies in this region that need to be investigated for better risk mitigation and management through building codes and land use planning.”

The new magnitude estimate for the 1755 earthquake is similar to that of another major regional earthquake, the magnitude 7.8 recorded in 1969 at the Gorringe Bank in the Atlantic. However, the damage caused by this earthquake was much less severe, in part because ground faults did not accumulate enough stress to make them “ready for rupture,” says João Duarte Fonseca.


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