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Turbidites and the Grand Banks Earthquake

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The Grand Banks Earthquake

On a mid-November afternoon in 1929, the year of the great stock market crash, the earth shook in Atlantic Canada. According to one report from Canso, NS, "It seemed as though a large steamer had bumped into the wharf and the bumping continued as though the engine and propeller were still going". In Ingonish, NS, cups were upset, windows were broken and a concrete door sill was jarred out of alignment. In Halifax and Lunenburg, NS, houses vibrated. Chimneys toppled in Sydney, NS. A fire bell was set off in Bridgewater, NS, and several parked cars were set in motion. Even as far away as Yarmouth, NS, small items were dislodged, but people walking on the street felt nothing. These disturbing events resulted from an earthquake of magnitude 7.2 on the Richter Scale that had its epicentre (marked in the image above by a red star)on the Grand Banks.

The Discovery of Turbidity Currents

A third effect of the Grand Banks earthquake (after the shaking of the ground and the tsunami) was the most puzzling. It involved the snapping of numerous trans-Atlantic telephone cables that had been laid under the ocean south of Newfoundland. Those near the epicentre of the earthquake broke at the same time as the quake itself (marked with yellow dots in the above image), which is not surprising. However, what was surprising was the progressively longer time delay of cable breaks further from the epicentre (marked with orange dots in the above image). The last cable, 600 km south, broke 13 hours after the earthquake. Scientists at the time were baffled by this progressive sequence of breaks.

Twenty-five years later, south of the earthquake site, a layer of sand was discovered on the sea floor, covering an area the size of the province of Quebec (shown above in light blue). And evidence of vast underwater landslides was found near the epicentre. These observations led to a theory that explained not just the cable breaks, but a whole class of sedimentary rocks around the world. This theory involved catastrophic underwater currents, called turbidity currents, that carry large amounts of sediment from the continental slope to the deep sea floor. The turbidity current triggered by the 1929 Grand Banks Earthquake travelled at speeds of up to 65 km per hour down an undersea valley 25 km wide and 300 m deep. The volume of sand and mud that it transported would be sufficient to completely fill Nova Scotia's Annapolis Valley. The geologic record preserves many ancient examples of such activity. For example, sedimentary rocks derived from such "turbidites" have been found in the last 30 years to be quite common. They are common, for example, in the strata of the Meguma Terrane around Halifax and Nova Scotia's south and eastern shores, as well as in the Miramichi Terrane of New Brunswick.

Other Earthquakes that Affected the Maritimes

The Grand Banks Earthquake is perhaps the most famous earthquake that has been felt in the Maritime Provinces, but it is not the earliest recorded nor the largest known. In 1764, the "Halifax Gazette" gave the first written account of an earthquake in the region, felt in the Saint John, NB, area. Since that shake, over a hundred relatively mild earthquakes have been reported in the Maritimes, mostly in New Brunswick, clustered in the Passamoquoddy Bay area and in the Miramichi Highlands.

The largest earthquake ever recorded in the Maritimes occurred on the 9th January, 1982 in the Miramichi Highlands, about 80 km west of Miramichi City. The main shock, of magnitude 5.7 on the Richter Scale, was felt just before nine o'clock in the morning, followed by aftershocks of magnitudes 5.1 and 5.4 later the same day and two days later, respectively. Although ground motion was felt over most of the province, damage was limited to hairline fractures in the walls of a few buildings in Miramichi City, Bathurst and Perth-Andover. No actual fault trace was found that could account for the earthquake, but this is not surprising since the area is heavily wooded and has thick soil.

Most earthquakes happen along tectonic plate boundaries, but the Maritime Provinces are presently well within the large North American Plate. Why, then, are there earthquakes, even though rare and relatively weak, in the Maritimes? The reason is that, although most tectonic action is indeed at plate boundaries, the plates themselves are in motion. Stress builds up within the North American Plate as it drifts slowly westward over the Pacific plate, and it is this stress that is released occasionally by mid plate earthquakes in the Maritimes and elsewhere.


    Last Modified: 2004-12-10