Hidden Reason Behind Portugal’s Deadly Earthquakes Finally Explained

The catastrophic 1755 Lisbon earthquake was one of the deadliest earthquakes in European history. The catastrophe spawned massive tsunamis, fires and shaking that killed tens of thousands of people and destroyed large swaths of Lisbon. A smaller quake on record struck in 1356. And the region was struck by another big quake, almost sizmic 8, in 1969. Unlike all those others, modern instruments were recording it, and they tracked it to that Horseshoe Plain, freshwater Aquatic, rimmed by water miles deep and very far from any known tectonic faults.

Mysterious Earthquakes Far from Fault Lines

According to the research published in Nature Geoscience, for years, scientists puzzled over how such powerful quakes could occur so far from fault lines. We now have have some of the answers. A high-velocity anomaly beneath the Horseshoe Abyssal Plain was revealed by seismic refraction imaging data as well as by ocean-bottom seismometer data. Their simulations revealed odd geological shenanigans far beneath the seabed.

Oceanic Plate Delamination as the Hidden Cause

The anomaly was a chunk of old oceanic lithosphere that had collapsed. Oceanic plates are typically rigid, but here, the lower part began to sink into the Earth. This process is called delamination. It was startling because delamination had previously been observed only in continental plates. According to Phys.org, the split was due to “serpentinization,” the process of seawater entering fractures in rocks, chemically altering them to serpentinite and making the crust weaker.

This weakening allowed a large thrust fault to form, the scientists said. According to their models, the lithosphere block that subsequently subducted from the north was enclosed by two fault zones. This movement is more likely than not the cause of the catastrophic earthquakes in Portugal, included the 1755 Lisbon earthquake and the 1969 faulting.

The findings also suggest that delamination could be the first step in a sequence of events that ultimately drives subduction — and, with it, large earthquakes in several parts of the world. The discovery could also help explain why Portugal still suffers powerful quakes to this day, and may help improve earthquake preparedness in the region in the future.