An Explosive Collision
On December 6, 1917, at approximately 8:45 AM, two ships collided in the harbor narrows of Halifax, Nova Scotia. The Norwegian vessel SS Imo, heading out for New York to pick up war relief supplies, struck the French cargo ship SS Mont-Blanc. The Mont-Blanc was inbound, fully laden with a volatile cargo destined for the battlefields of Europe: 2,367 metric tons of picric acid, 250 metric tons of TNT, 62 metric tons of guncotton, and 246 metric tons of highly flammable benzol fuel in barrels on its deck.
The low-speed impact gashed the Mont-Blanc's hull and toppled barrels of benzol, which quickly ignited from sparks created by the grinding steel hulls. A fire rapidly consumed the ship. The crew, aware of the catastrophic potential, abandoned ship and tried to warn others. For nearly 20 minutes, the burning vessel drifted toward the Halifax shore, drawing crowds of spectators who were unaware of the danger. At 9:04:35 AM local time, the ship detonated. The resulting explosion was the largest human-made blast of the pre-nuclear era, releasing energy equivalent to 2.9 kilotons of TNT. The blast obliterated the ship and instantly vaporized the water around it, creating a pressure wave that radiated outwards at more than 1,000 meters per second and a fireball with a temperature of 5,000°C.
The Seismic Signature
Over 3 kilometers from the explosion, a primitive seismograph in the basement of the physics building at Dalhousie University recorded the event. In 1915, Mainka bifilar horizontal seismographs had been installed at the university. The ground shock from the detonation traveled through the bedrock at more than 6.5 kilometers per second, registering on the instrument. This physical record provided a precise time for the detonation—9:04:35 AM—that became important for historical accounts.
The explosion created both a powerful ground tremor and an atmospheric pressure wave. The ground shock was recorded locally, but the air blast was detected much farther away, shattering windows up to 60 miles distant and being felt as a tremor over 200 kilometers away. The event provided a rare and tragic opportunity for early 20th-century scientists to study the characteristics of a large, surface-level detonation. The data from the Dalhousie seismograph was one of the first instances where a precisely timed, human-caused event was captured with seismic instruments, contributing to the nascent field of seismology. The devastation was immense, destroying 1.6 square kilometers of the city, killing at least 1,782 people, and injuring 9,000 more. The blast also triggered an 18-meter-high tsunami that swept the shoreline, completely destroying the Mi'kmaq community of Turtle Grove on the Dartmouth side of the harbor.