What's the mudslide risk in your community?

The hard part about predicting mudslides isn't the "where" — it's the "when."

They're like earthquakes that way. Pointing to areas on a map where there's potential trouble — California, Japan, Iran — is a fairly simple job for any geologist worth his or her salt. But knowing whether the Big One will strike today, next year or sometime in the next 10,000 years is a question one might as well refer to a Tarot deck.

It's the same with landslides and mudslides, such as the 2.6-square-kilometre mass of earth that over the weekend devastated the community of Oso, Wash.

Often the best geologists, hydrologists and the like can do is read the terrain, rainfall patterns and other conditions around a community and say whether the ground is at risk of what they call "failure."

"The kind of failure we saw in Washington occurs when a number of factors come together," said Prof. Andrew Miall of the department of Earth sciences at the University of Toronto.

"Very soft sediment and a fairly steep slope facing a river," will set the stage, he said, though a slide itself is often triggered by heavy rainfall or other saturation of the soil.

"Heavy rain can do it. The springtime, when things start to thaw, makes those setups particularly prone to collapse," he said.

There are three regions in Canada that fit this general profile, say geologists: the Lower Mainland of British Columbia, the eastern side of the Rocky Mountains in Alberta and the St. Lawrence Valley up into the Eastern Townships in Quebec.

All three have the right combination of hilly or mountainous terrain, water and unreliable soil. Much of B.C. is covered with the same soil found across Washington state — loose silt and sediment left behind when the last local glacier melted.

In Quebec, the culprit is leda clay, another relic of the last ice age that sometimes lurks deep under seemingly more stable ground, and which contributed to the deadly landslide in 2010 that killed a family of four.

B.C. has seen at least a dozen dangerous or deadly slides over the past 10 years, though one geologist said there are probably many more that go unreported because they occur in remote areas.

"We have higher frequency [in B.C.] but we also benefit from lower population density," said Erik Eberhardt, a professor of geological engineering at the University of British Columbia. "Hundreds or thousands probably happen every year, but we just don't hear about them."

Construction or deforestation due to logging or fire can also make bad situations more dangerous. Older communities — established without the help of modern engineering and safety standards — might also be at more risk, though there's no clear date after which a site can be considered entirely safe. The more recent, the better, say experts.

Mud and landslides can also happen anywhere there's an unstable slope — even flat, featureless Toronto has had a few, thanks in part to a previous century's poor idea of using garbage to reinforce ravine walls.

But it is hard to predict when, if ever, any of these factors will add up to actual trouble.

"We can't say when the land will fail," said Eberhardt. "It could be one year, five years or 50 years."

Indeed, it came to light this week that the U.S. Army Corps of Engineers once warned that a "large catastrophic failure" was looming over Oso, though it took 15 years for that prediction to come true.

"Specialists can talk about risk and conditions that apply, but it's like talking about the risk of earthquakes. You hear it, but you don't necessarily change your behaviour," said Miall.

Even after a community has been hit by a slide (and areas are often struck more than once, though the slides might be decades or more apart), residents may give in to what experts call "hazard amnesia," eventually forgetting about the need for diligence and disaster preparedness as the most recent disaster recedes into the past.

"It's one thing to talk about risk, but if you're not talking about when something that's going to happen — something that's intangible like an earthquake or slide — it's very difficult to know how to respond," he said.

"You could tell people today to stock up on batteries and bottled water but [the disaster] could be 200 years from now or it could be tomorrow."