How Did a 1929 Earthquake Snap 12 Transatlantic Telegraph Cables?

On November 18, 1929, a magnitude 7.2 earthquake off Newfoundland’s Grand Banks triggered a massive submarine landslide that snapped twelve transatlantic telegraph cables — and in doing so, accidentally recorded its own speed across the ocean floor.

The Earthquake That Set the Ocean Floor in Motion

The Grand Banks earthquake struck at 5:02 PM local time on November 18, 1929, with its epicenter roughly 250 miles south of Newfoundland. The shaking dislodged approximately 48 cubic miles of sediment from the edge of the continental shelf — a volume almost incomprehensible in scale. That sediment didn’t simply slide and settle. It became a turbidity current: a dense, fast-moving underwater avalanche of sediment-laden water that surged down the continental slope and across the abyssal plain.

The human toll was also significant. A tsunami triggered by the earthquake struck the Burin Peninsula of Newfoundland, killing 28 people and destroying dozens of coastal communities. But beneath the surface, something else was unfolding — something that scientists wouldn’t fully understand for more than two decades.

Six Cables Snapped Instantly — Then Six More

At the moment of the earthquake, six telegraph cables crossing the Atlantic seafloor broke simultaneously. The slope had collapsed directly on top of them. But then, over the next thirteen hours and seventeen minutes, six additional cables broke — one by one, in sequence, each snapping progressively farther from the epicenter.

Telegraph companies meticulously logged the exact time each cable went silent. At the time, these records were purely operational — a maintenance headache, nothing more. No one yet understood that these timestamps were something far more valuable: a stopwatch buried two miles beneath the Atlantic Ocean.

The Discovery That Rewrote Ocean Science

In 1952, oceanographers Bruce Heezen and Maurice Ewing published a landmark paper that changed everything. By cross-referencing the precise break times recorded by the telegraph companies with the known positions of each severed cable, they were able to calculate how fast the sediment cloud had traveled between each one.

The numbers were startling. The turbidity current had raced across the ocean floor at between 37 and 62 miles per hour — faster than most people drive on a highway — covering more than 620 miles before finally losing momentum in the deep abyssal plain. This was the first scientifically documented turbidity current in history, and the evidence had been hiding in telegraph company maintenance logs for twenty-three years.

Why Turbidity Currents Matter

Before Heezen and Ewing’s 1952 paper, geologists largely assumed that the deep ocean was a quiet, undisturbed environment where sediment settled slowly and nothing moved very fast. The Grand Banks event overturned that assumption entirely.

Turbidity currents are now understood to be one of the primary mechanisms by which sediment is transported from continental shelves to the deep ocean floor. They carve enormous underwater canyons, build vast sediment fans thousands of feet below the surface, and shape the geology of ocean basins over millions of years. The Grand Banks turbidity current deposited sediment across an area of roughly 100,000 square miles — larger than the entire United Kingdom.

The Cables as an Accidental Scientific Instrument

What makes the 1929 event uniquely powerful in the history of science is that the measurement tool — twelve telegraph cables — was already in place before anyone knew there was anything to measure. The sequence of cable breaks gave Heezen and Ewing a precise, time-stamped record of an event that no human had witnessed and no instrument had been designed to capture.

The ocean floor had, in effect, timed itself. A single earthquake transformed the Atlantic seafloor into the world’s first speed trap, and the data it left behind reshaped our understanding of how the deep ocean works — permanently.