Seismologists and structural engineers say the city of Ottawa is at risk for a big earthquake.

The Val-des-Bois quake that shook the capital last year reminded people that Ottawa sits in an earthquake zone. And there is a chance it could one day produce a destructive quake far worse than February's deadly magnitude-6.3 tremor in Christchurch, New Zealand.

Claude Blais was at the pharmacy in Gracefield, Que., when last year's quake hit.

An expert examines a land collapse that occurred in Notre Dame de la Salette, Que., one day after the Val-des-Bois quake that shook the Ottawa area. (CBC)

"All the lights broke down, and all the shelves emptied themselves. It was just horrible," Blais said. "You kind of think it's the end of the world."

It wasn't like that in Ottawa. That's the difference between being close to the epicentre of a 5.2 earthquake, and being 65 kilometres away.

Even a small quake can be damaging, but seismologists say the Ottawa region is capable of something much bigger.

"We can certainly imagine something stronger could happen," seismologist John Adams of Natural Resources Canada said. "And we think there's a possibility of a magnitude seven in the Ottawa or St. Lawrence Valley."

That kind of earthquake close to the capital would be a doomsday scenario. But something like this year's quake in Christchurch is much more likely.

"What we would worry about is the magnitude 6.2 or 6.3, 30 or 40 kilometres away," Adams said. "That would be bad enough in Ottawa without going into the worst, worst case.

"We would probably see building collapses among older brick buildings in particular, and a lot of infrastructure would be damaged."

Frustrating efforts to prepare for such a disaster, the movements of tectonic plates near Ottawa are a mystery and can't be tracked. The mid-continental faults that underlie the region give few clues as to when they might shift again.

That means it could be in the next few minutes, or it could be millennia.