The earthquake probably originated along the Gorringe Ridge, a submarine mountain range lying off Cape St. Vincent in the Atlantic Ocean. This underwater geological formation sits along the Azores-Gibraltar Transform Fault, one of the most seismically active zones in the eastern Atlantic. The same tectonic forces that produced the 1356 earthquake would later generate the devastating 1755 Lisbon earthquake, the 1722 Algarve earthquake, and the 1761 Portugal earthquake. Medieval Lisbon, a growing port city with stone churches and tightly packed wooden houses, was poorly equipped to withstand such forces. The quarter-hour duration of sustained shaking was extraordinarily long, suggesting a powerful rupture along a major fault segment.

What little we know of the 1356 earthquake comes from scattered medieval sources. The Livro da Noa, a chronicle belonging to the library of the Monastery of the Holy Cross in Coimbra, recorded the event. But over the centuries, the earthquake slipped from collective awareness. Portugal experienced so many subsequent seismic disasters that this medieval tremor was simply forgotten. It was not until the 21st century that the earthquake regained scholarly attention, when an inscription was discovered on the wall of the Torre do Paco inside Sao Jorge Castle, perched on the highest hill in Lisbon. Someone, centuries ago, had carved a record of the disaster into stone -- a message that outlasted the memories of the people who witnessed the event.

The 1356 earthquake is part of a long pattern of seismic activity that has shaped Portugal's history and architecture. The Iberian Peninsula sits near the boundary where the African and Eurasian tectonic plates converge, making it one of Europe's most earthquake-prone regions. Lisbon alone has been struck by multiple major earthquakes across the centuries. Each disaster reshaped the city -- new building techniques replaced old ones, neighborhoods were redesigned, and the population learned and forgot and learned again the lessons of living on unstable ground. The 1356 earthquake, though largely lost to history, belongs to this recurring cycle of destruction and rebuilding that defines Lisbon's relationship with the earth beneath it.

In hindsight, the 1356 earthquake reads like a warning that went unheeded for four centuries. When the great 1755 earthquake and tsunami devastated Lisbon -- killing tens of thousands, leveling the city center, and sparking fires that burned for days -- it shocked Enlightenment Europe into philosophical crisis. Voltaire wrote about it. Kant studied it. The entire concept of optimism was called into question. Yet the 1356 event demonstrated that Lisbon had always been vulnerable. The inscription on the castle wall, waiting silently through the centuries, was a message from the medieval past to the modern world: this ground has moved before, and it will move again.