North America’s Deadliest Landslide
Still Poses Questions – 100 Years Later

Researchers set up monitoring equipment in an effort to unlock the mysteries of Turtle Mountain, responsible for the famous and tragic Frank Slide.

Native lore calls it ‘the mountain that moves.’ Researchers want to know how often and how much – and whether Turtle Mountain is preparing a sequel to the legendary and deadly Frank Slide of 1903.

BY BILL CORBETT
Freelance Writer

In the early morning of April 29, 1903, more than 90 million tonnes of limestone slid from the east face of Turtle Mountain and swept through a sleeping town in Alberta’s Crowsnest Pass, killing an estimated 70 people. The Frank Slide, named for the town it buried, quickly found purchase in the public imagination as the most celebrated and fatal landslide in North American history.

Although the mountain has been extensively studied and sporadically monitored over the years, uncertainty remains over exactly what happened – and if, or when, it might slide again.

To mark the 100th anniversary of the Frank Slide, the Alberta Government announced last spring a $1.1-million program to monitor every creak, groan and shift in the potentially unstable rock mass on South Peak, one of the two prominent peaks left on Turtle Mountain by the 1903 slide. The primary goal is public safety – to better predict a future slide that could threaten people and valley infrastructure.

But researchers are also improving their understanding of landslides in general and the mountains that spawn them, testing new technologies and educating a public still fascinated by the story.

Shifting Technology

The program provides a perfect opportunity to deploy state-of-the-art geotechnical and geophysical monitoring technology, some of which was originally developed for oilfield monitoring, notes one project researcher.

“Most of our research here is developing methods for exploring hydrocarbons,” says Dr. Robert Stewart, P.Geoph., a geophysics professor at the University of Calgary who also teaches a popular course in natural disasters. “One of our technologies involves installing seismic monitors over oilfields and monitoring the popping, cracking and fracturing of petroleum reservoirs as they’re being produced.

“The same technology can be used to monitor fracturing or micro-seismicity in Turtle Mountain.”

Last summer and fall, Dr. Stewart’s team began installing a series of seismic monitoring stations on the mountain to measure microseismic activity associated with such things as vibrations, fracturing, and deformation of the rock mass. Over a dozen other teams will be installing surface and subsurface geotechnical and geophysical monitoring systems to measure displacement, pore pressure, temperature, water outflow, and climatic data. They’ll also conduct supporting geological, satellite and photogrammetric studies.

The results from all the installed systems (including deformation monitoring by an independent Iowa State University researcher) are being wirelessly transmitted to the Frank Slide Interpretive Centre, where they can be watched and analyzed in near real-time.

“The whole monitoring effort is based on the idea that these events (landslides) have precursors. We suspect Turtle Mountain will give us some warning if something is going to happen – according to aboriginal legend, it was known as the ‘mountain that moves’,” says Dr. Stewart, an APEGGA councillor.

“To be successful, the monitoring has to be sustained and continuous. We don’t know if something might happen next week or in the next millennium. But we do know this is a particularly unstable mountain.”

The Turtle’s Geology

But why is it unstable? As one would assume, part of the answer lies in Turtle Mountain’s very make-up.

Limestone layers have been geologically squeezed into an arch, or anticline, which is broken at the mountain’s top into large fissures. Water percolates down through these cracks, dissolving the limestone and increasing rock pore pressure by freezing and then expanding.

In the process, the rock layers on the downward slope of this anticline become like shingles loosely attached to a roof – just waiting for something to tip the balance.
Scientists believe the excavation of coal at the turn of the 20th century from the Frank Mine, which traverses the toe of the mountain, might have helped trigger the Frank Slide. Near the coal seam, the Turtle Mountain Thrust Fault and a small, secondary fault were further destabilizing factors.

But scientists still speculate about what caused the Turtle’s east flank to fail on such a massive scale or to slide so far.

The Theories
An early theory was that the massive layers of rock slid on a compressed cushion of air. Yet old run-out slides on Mars have been observed, and there’s no atmosphere there.

A more recent theory is acoustic fluidization. Large volumes of material generate their own seismic or vibrational energy, reducing friction and allowing for long run-outs, behaving more like a fluid than a solid mass.

Whatever the cause, such slides are not peculiar to Turtle Mountain. Historically, they’ve been quite common in the Canadian Rockies; indeed, there is evidence of such a slide in an overgrown area on nearby Bluff Mountain.

What makes the Frank Slide significant is its size and length, and that it occurred in a valley crowded with people, coal mines and a railway. Although the adjacent and intact South Peak of Turtle Mountain is similarly unstable, the good news is it poses less risk to humans and their interests.

“While it (South Peak) contains five million cubic metres of rock of the same general structure, it’s only one-sixth of the original slide’s volume,” says Dr. Rodney Read, P.Eng., P.Geol. “In an extreme case, the maximum probable run-out would reach Highway 3.”

Dr. Read is the Okotoks-based project engineer for monitoring Turtle Mountain. Apparently, interest in the slide runs in his family – Dr. Read’s great-great-uncle did the first post-Frank Slide measurements in 1903. Interestingly, the same uncle – David Alexander Stewart – may be related to Dr. Robert Stewart, too.

More to Learn
And 100 years later, there’s still plenty for scientists to learn, Dr. Stewart believes, about what makes Turtle Mountain tick – or fall apart. “Monitoring landslides is still new enough around the world that it is not strictly an engineering solution,” he says.

“ A lot of the technology we’re testing here could be used for monitoring everything from ice thickness on roads to snow avalanches in the mountains. They all have similar components.”

The effort to satisfy the public’s appetite for Frank Slide information continues, too. Because data and images from the Turtle Mountain monitoring equipment, weather stations and video cameras will be accessible, visitors to the Frank Slide Interpretive Centre and perhaps Internet users will be able to follow and learn from the research.

Dr. Stewart says some of the program monitoring has already been incorporated into U of C geophysics and geology courses.

The $1.1-million monitoring project covers some 20 months and is being spread over 18 work packages. Much of the work will take place in the summer of this year, including the drilling of boreholes. Into those holes will go measuring equipment such as micro-seismic sensors, inclinometers and extensometers.

Longer-term funding is still undecided, as is the government agency that will oversee the ongoing operation of the monitoring system. Whatever happens, it seems that there’s little doubt the Frank Slide will continue to intrigue scientists, students and lay people alike for many years to come.

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