Modern seismologists estimate the magnitude at around 6.0, with a maximum felt intensity of VII to VIII on the Mercalli scale. The epicentre was somewhere beneath the Strait of Dover, the narrow stretch of sea between Calais and the white cliffs where the English Channel meets the North Sea. The tectonics responsible are buried deep: a boundary called the Northern Variscan Thrust Front runs roughly west-northwest to east-southeast under the eastern Channel, separating older rocks deformed during the Variscan mountain-building of 300 million years ago from the relatively stable London-Brabant Massif to the north. Strain still accumulates and occasionally releases. In 1580 another major Dover Strait earthquake would crack stones in London. In 1382 it was Canterbury's turn.

Canterbury Cathedral was the spiritual heart of medieval England, the seat of Thomas Becket's shrine, the destination of the pilgrims Chaucer would describe just a few years later. On 21 May 1382 it took serious damage. The campanile, the bell-tower beside the south transept, partially collapsed. Pinnacles fell from the upper walls. Glass shattered. The wooden cloister roof split. Repairs were still being recorded years afterward. The cathedral was not the only victim. Damage was reported across south-east England, and across the water in Ypres, Bruges, Liege, and Ghent. In London itself, the steeple of an unnamed parish church reportedly fell, and the spire of one of the friaries was bent. People in the streets thought the world might be ending.

Courtenay had called the Blackfriars synod to examine twenty-four propositions drawn from the writings of John Wycliffe, the Oxford theologian who argued the Bible should be available in English and questioned papal authority. Wycliffe had powerful protectors at court, but his ideas were spreading and the church wanted them stopped. The earthquake interrupted the proceedings mid-debate. The bishops paused. The earl was helped back to his feet. Then they kept going. Courtenay used the tremor in his closing argument. The earthquake, he said, was the Lord purging the air of the noxious vapours of heresy, just as the planet had purged itself of bad humours through a great convulsion. Ten of the twenty-four propositions were condemned as heretical; the remaining fourteen as erroneous. Wycliffe escaped formal condemnation himself but his followers, called Lollards, would be hunted for the next century and a half.

Medieval chroniclers treated the earthquake as a portent and competed to interpret it. The monk Henry Knighton wrote that the shaking was a sign of God's anger at Wycliffe's heresies. The Lollards turned the same evidence the other way, arguing the earthquake was the earth itself protesting the synod's verdict. Both sides agreed the tremor was meaningful. Nobody yet thought of it as a tectonic accident. The notion of plates and faults was five centuries away, although educated men of the time did know that earthquakes were natural events, attributed in the prevailing Aristotelian model to wind trapped in caverns under the earth. The theological reading and the natural reading coexisted comfortably. What was striking about 1382 was simply that an earthquake had arrived exactly when both factions wanted a sign and given each a reason to claim it.

England is not a seismically active country by world standards, but the Dover Strait is one of its noisier corners. The London-Brabant Massif has produced earthquakes for thousands of years; the 1382 event sits among a small group of significant historical shocks felt across the Channel. None has come close to causing the kind of damage that would shock a modern city, but the strait is one of the busiest stretches of water on the planet, and engineering for cross-Channel infrastructure, including the rail tunnel that now runs beneath these same waters, has to take the seismicity seriously. Standing on the chalk cliffs above Dover on a clear day, looking out across the strait toward Cap Gris Nez, the sea looks like nothing more dangerous than blue water and shipping. Six centuries ago it shook the country.