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When the hatch was bolted shut in preparation for a descent to the bottom of Lake Ontario, geologist Arsalan Mohajer wondered exactly what he was about to see. But as the orange mini-sub passed the 220-metre mark below the surface, his worst fears were confirmed.

In front of the thick portholes, illuminated by the submarine's powerful lights, was a long ridge of rock, about the height of a small apartment building. Prof. Mohajer and his colleagues, both veteran geologists, saw a disaster in the making: In their view, the ridge is part of a huge seismic fault that will one day unleash a magnitude 7 earthquake on the city of Toronto. According to his calculations, there is a 50-50 chance that Toronto will experience a magnitude 5 earthquake within 50 years. A magnitude 7, he calculates, will occur some time within the next 300 years.

"The evidence is there," says Prof. Mohajer, a geology professor at the University of Toronto who made the submarine descent in the summer of 1997. "If you look at it, this is the conclusion you come to."

Prof. Mohajer's dive, about 35 kilometres northeast of Rochester, was one in a series made to explore the underwater topography of Lake Ontario.

More dives were made in the waters south of Pickering, and off the shores of Toronto itself. Prof. Mohajer and his colleagues found a series of ominous geological warning signs: gas bubbling from underwater fissures; knobs of rock pushed up from below by massive forces, and evidence that plates of stone weighing millions of tonnes hang in precarious balance.

Their findings fly in the face of the geology community's generally accepted view that Toronto is built on relatively stable ground, with virtually no risk of a massive quake like the magnitude 7.6 tremor that is thought to have killed more than 35,000 people in Pakistan last week.

"Toronto is not a high-risk location," says Stephen Halchuk, a seismologist with Natural Resources Canada. "We don't think the evidence points to an active fault."

But Prof. Mohajer remains convinced his pessimistic view of the city's geological stability is the accurate one: "We've stated our conclusions. Now we'll see what happens."

If Prof. Mohajer and his colleagues are correct, it means that many of Toronto's buildings are structurally inadequate, since they face seismic forces that were never contemplated. "You design for the worst case," says Murat Saatcioglu, a professor of earthquake engineering at the University of Ottawa who also serves as a national building-code consultant. "But Toronto is never expected to have a major earthquake."

If Toronto does get hit by a magnitude 7 or greater quake, experts such as Prof. Saatcioglu say there would be widespread devastation, but the death toll would be a small fraction of that in Pakistan, since many of our buildings were built to meet national codes that ensure earthquake resistance.

The safest buildings, Prof. Saatcioglu says, are those built after the mid-1970s, since they had to meet sharply increased standards. But many older buildings, he says, will simply collapse.

Some of the most dangerous buildings, he says, are those made of solid masonry. (Although lay people who have read The Three Little Pigs would assume that brick buildings are especially safe, they have a fatal weakness that is exposed by earthquakes: they're heavy, and they're brittle.)

Many of Toronto's most beautiful and storied buildings, including churches and arenas, would collapse almost instantly in a powerful earthquake, experts say, since they were never designed to withstand the unique forces imposed by a major shift in the earth.

"You think of these as very strong buildings," Prof. Saatcioglu says. "But in an earthquake, they're not."

Modern high-rise buildings such as Toronto's downtown bank towers, the experts say, would do very well in an earthquake since they're designed to absorb energy by flexing -- an engineering quality known as ductility. The CN Tower would also fare well, engineers say, since it was designed with particular attention to earthquake risks, and built with special, highly reinforced concrete.

"I wouldn't worry about the CN Tower," Prof. Saatcioglu says. "But there are a lot of other buildings that wouldn't do well at all."

Shamim Sheikh, a professor of structural engineering at the University of Toronto, says the Pakistan earthquake has provided a sobering reminder of how dangerous it is to ignore earthquake risks.

Prof. Sheikh, who was born and raised in Pakistan, spoke about the need for improved structural standards when he visited the country last year, only to see his grim predictions come true. The scale of death in Pakistan, he says, is due to the high concentration of buildings made from unreinforced masonry. "You could see the risks," he said. "Any engineer could."

If a magnitude 7 quake did hit Toronto, Prof. Sheikh says, the death toll would probably be 20 to 30 times lower than in Pakistan. "We have better buildings," he says. "It makes a major, major difference."

Although Canada's stringent building codes mean that most buildings are safely constructed, there are no guarantees. Prof. Sheikh was recently called in to consult on a Burlington-area project that involved the construction of five commercial buildings. After examining them, Prof. Sheikh learned that none of them met the minimum earthquake-resistance standards required by the national building code.

(Two of the buildings had already been built, and required extensive modifications. The engineer who drew up the plans was investigated, and lost his licence.)

Prof. Sheikh says few Toronto buildings are designed to withstand an earthquake above magnitude 4 -- the strongest most experts expect the city to ever experience.

"Our buildings are not designed for the worst case," he says. "Because it's not supposed to happen here."

The dire seismic predictions made by Prof. Mohajer and his colleagues have been the subject of intense scientific and political debate, since they raise countless questions about public safety -- including the fate of the Pickering nuclear plant, just 30 kilometres east of Toronto.

Most scientists believe the faults discovered by Prof. Mohajer are benign. "There are faults everywhere," says Gail Atkinson, a Carleton University seismologist. "In Ontario, you can throw a rock and hit a fault. But that doesn't mean they're active."

Prof. Mohajer sees things differently.

In his view, the faults he and others discovered beneath Lake Ontario are part of an active fault that extends hundreds of kilometres the St. Lawrence River.

"A lot of people don't want to believe it," he says. "But the evidence so far points in that direction. As far as we're concerned, it's up to the non-believers to produce evidence to the contrary."

CN TOWER

Advanced engineering and

construction puts the world's tallest freestanding structure among the safest places to be

in an earthquake. Prestressed concrete and steel reinforcement make it extremely strong, yet flexible enough to absorb seismic energy.

OFFICE TOWERS

Modern high-rise office towers perform well in earthquakes thanks to strong steel framing that flexes, then returns to its original shape -- a quality

engineers refer to as ductility.

NEWER BRICK HOMES

Although a brick house is

extremely good at resisting loads such as gravity and wind (including the huffing and

puffing of the Big Bad Wolf), brick walls collapse easily when

shaken. Interior wood framing, which is lighter and more flexible, may protect the occupants.

OLDER BRICK BUILDINGS

Although it was built before the building code was revised to include earthquake protection, Maple Leaf Gardens has a core of reinforced concrete and extensive steel interior framing that would probably resist seismic forces relatively well.

OLDER BRICK HOMES

With thick walls built of clay brick and lime-based mortar, big, old brick homes such as

Casa Loma would be hard to knock down with a battering ram, but would probably

collapse quickly in a powerful earthquake due to the weight and inflexibility of their

structure.

Nov. 25, 1988

The largest earthquake in eastern North America in 53 years struck just south of Chicoutimi, Que., at magnitude 6.5 magnitude and rattled homes 800 kilometres away in Toronto.

Nov. 11, 1944

Although this earthquake was only of magnitude 5.6, it caused considerable damage in both Cornwall, Ont., and Massena, N.Y. and was felt more than 300 kilometres away in Toronto.

Nov. 1, 1935

Striking with a magnitude of 6.2 about 10 kilometres east of Témiscaming, Que., this earthquake caused vibrations 350 kilometres away in Toronto and triggered small landslides closer to its epicentre.

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