Shortly after 5 pm on 18 November 1929, an earthquake shook Canada’s eastern provinces. In Halifax, Nova Scotia, a seismometer needle jumped right off track. Across the Gulf of St Lawrence, on Newfoundland’s Burin peninsula, the tremors sent people running into the streets. But for them, worse was to come. Two hours later, 7-metre waves hit the shore, their momentum carrying them as far as 27 metres above the high-tide level. Boats were smashed and quaysides stripped bare. When the waves retreated, 28 were dead or dying and 10,000 were homeless. Then, to add to Burin’s woes, a blizzard hit. It would be three days before news of the disaster reached the outside world. Only now do scientists finally understand the cause of the tsunami: a giant landslide, deep beneath the waters of the Grand Banks.
FIVE-YEAR-OLD Pearl Brushett was snug in bed when the first wave struck the Burin peninsula, lifted her home off its foundations and carried it out to sea. She must have been a sound sleeper because the first thing she remembered was being woken by her mother and peering outside with her four sisters. By then, the house had grounded in shallow water next to an island.
Then the second wave struck, refloating the house. This time it wound up back on the beach not far from where it started. The family didn’t wait for the third wave. Unable to use the door, Pearl’s mother smashed a pane in the parlour window and with the help of a neighbour they all escaped.
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None of the family was seriously hurt, although Pearl’s mother gashed her arm when she smashed the window. “Years later Mom still had pieces of glass coming out of her wrist,” an ageing Pearl later recalled for a book of reminiscences.
The house also survived. Solidly built, it was washed back to sea yet again by the third and final wave, but was later recovered by a schooner and towed back to town, where it was converted to a storage shed for fish.
“The house was washed out to sea yet again by a third and final wave”
Pearl was one of the lucky ones. Thirteen-year-old Marion Kelly was doing homework when she saw the sea coming in, “like a mountain… but slowly”. She grabbed her 3-year-old brother Elroy and jumped a fence, just as the oncoming water flowed beneath it. “I don’t know how I did it,” she wrote, “because Elroy was biggish.” Safe on high ground, she saw the wave take her house, along with her mother and sister. Their bodies were never recovered.
Across the Gulf of St Lawrence, the people of Halifax, Nova Scotia had no idea of the disaster that had overwhelmed their neighbours in Newfoundland. In Halifax, the earthquake had done little more than topple a few chimneys and break some dishes, and the local newspaper’s main concern was to reassure its readers. There was so little to report that the Halifax Chronicle devoted much of its story to describing in excruciating detail how a seismometer worked and the “heroism” of local telephone operators who refused to leave their posts. Nor did the lack of communication with the Burin peninsula cause any alarm. The telegraph lines had been down for days, probably a casualty of the November weather.
The day after the quake, the Chronicle was able to state blithely that the most serious damage appeared to be breaks in “several cables of the Western Union Cable Company… somewhere off Newfoundland”.
The quake’s magnitude was later assessed at 7.2, with its epicentre beneath the Grand Banks some 250 kilometres south of Newfoundland. Today, the offshore location of the epicentre would ring immediate alarm bells, but earthquakes are relatively rare in the Atlantic – so unusual that no one understood the implications. And tsunamis are even rarer in the Atlantic than earthquakes: even the residents of the Burin peninsula were slow to comprehend what had hit them. “We didn’t know what a tidal wave was,” wrote survivor Louise Emberley, who was 23 at the time. “We thought the place was sinking or something.”
Scientifically, the Burin disaster posed two questions: why had there been such a big earthquake, so far from the usual tectonic centres? And why had the quake created a tsunami so strong that its ripples were felt in Portugal on the far side of the Atlantic?
The first question proved easiest to answer. Scientists now know that such quakes occur when far-away tectonic forces are transmitted to ancient, rarely active fractures. Similar quakes have been felt along the eastern seaboard of North America, from Florida to Baffin Island.
As for the tsunami, the snapped cables would prove a crucial clue. All told, a dozen cables broke. Oddly, they broke in sequence, with delays of up to 13 hours – and the further the cable was from the epicentre, the longer the delay. At the time, scientists thought the breaks were caused by aftershocks, spreading slowly from the epicentre. But that made no sense because the timing of the breaks had been recorded and no corresponding aftershocks had registered on seismometers in Halifax and Boston.
The loss of the submarine cables was more than just a nuisance. “This was shortly after the Wall Street crash, and it took six months to a year to get the cables relaid,” says David Piper of the Bedford Institute of Oceanography in Dartmouth, Nova Scotia. The breaks, he speculates, may even have contributed to the Great Depression by impeding communication between American and European banks at a critical time.
It was more than 20 years before scientists had any real inkling of what had happened. In the 1950s, geologists took core samples of the seabed and found layers of sand indicative of a large landslide. Sonar mapping then revealed that the edge of the continental shelf off the mouth of the Gulf of St Lawrence is riven by deep canyons. Gradually, oceanographers began to realise that the quake had triggered an enormous underwater landslide, shifting as much as 200 cubic kilometres of sediment. Mapping the sequence and timings of the cable breaks revealed that debris had roared downslope at speeds of up to 67 kilometres per hour.
An ordinary landslide wouldn’t travel that fast or that far underwater. Instead, the sediment must have mixed with water to form a muddy liquid dense enough to pour downwards under its own weight – a phenomenon known as a turbidity flow. These had been produced in laboratories, but sedimentologists now realised they could happen in nature.
With the help of high-resolution multi-beam sonar, scientists are now able to chart how slabs of sediment broke off high in the canyons, then avalanched down like submarine flash floods, building up power and snapping cables as they went.
Small turbidity flows are remarkably common. In 2001, scientists at the Monterey Bay Aquarium Research Institute in Santa Cruz, California, planted some equipment at the bottom of the Monterey Canyon, a deep submarine cleft running offshore. Three months later, the gear had vanished. They found it 550 metres down the canyon, partially buried in mud. They tried again, but this time their instruments were mangled by at least two turbidity flows in just five months.
The flows in Monterey Canyon were too small to do more than wreck expensive scientific instruments, but elsewhere there’s potential for Burin-style disasters. The prime candidates are places where you have a combination of three factors: a sloping seabed, an earthquake and a lot of loosely consolidated sediment, deposited by a river or washed down at the end of the last ice age. “These features aren’t remarkable,” says Piper’s colleague, David Mosher. “We see similar features all along the Canadian east coast.”
Many scientists are concerned that the risk might worsen if global warming triggers the melting of underwater gas hydrates, ice-like substances produced when natural gas percolating up from below interacts with cold, high-pressure water. This could turn once-solid slopes into landslide-prone ooze.
Glacial rivers in the far north may also be setting the stage for numerous local tsunamis. In Alaska many rivers have deposited large banks of sediment at the heads of deep fjords. The last time the region was struck by a giant earthquake, in 1964, 106 residents were killed by tsunamis. Only 25 of those died in a tsunami triggered directly by the earthquake – the rest were killed by tsunamis triggered by landslides.
The Atlantic, however, is considerably less active than the Pacific. “The risk is probably small in a single person’s lifetime,” says Mosher. “But we know it does happen, because it did.”
