The 9.0-magnitude earthquake that struck off the eastern coast of Japan Friday happened exactly where seismologists expected it would — about 125 kilometres away from shore, in what is known as the subduction zone between the Pacific and North American tectonic plates.

The Pacific plate off Japan's coast is slowly but constantly sliding underneath the North American plate on which the country sits, says Stephen Halchuk, a Natural Resources Canada seismologist in Ottawa. Despite its name, the North American plate extends from North America to northwestern Russia and most of Japan.

"This movement [between the plates] is on the order of five to 10 centimetres per year," he said. "So, it's about as fast as your fingernails grow, or a little faster. But it's not a smooth movement. Two plates are grinding against each other and building up pressure. The plates stick against each other, and eventually, there's so much pressure built up that it's released in the form of earthquakes."

CBC meteorologist Johanna Wagstaffe says the quake was preceded by a series of foreshocks relieving the stress. The biggest foreshock happened on March 9, registering magnitude 7.2.

"A magnitude 8.9 quake is actually 8,000 times more powerful than a 7.2 quake," said Wagstaffe. "Because the earthquake occurred under the ocean floor, massive amounts of water were displaced above the shaking ocean ground, generating a tsunami."

Halchuk says his office has recorded more than 50 aftershocks reaching magnitudes of 5 or higher since the quake hit on Friday.

But initial reports suggest most of the damage was, in fact, not because of the quake.

"The Japanese have very stringent building codes," Halchuk said. "They design their buildings to withstand very large earthquakes, so the tsunami actually did most of the damage."

University of Ottawa tsunami expert Ioan Nistor says that when he lived in Japan, he regularly witnessed authorities sending ships into the safety of the open ocean during tsunami warnings. Out there, the vast depths of water mask the strength of the waves, he said.

"The sea surface would look flat," Nistor said. "You cannot see a tsunami wave in the middle of the ocean. But as it approaches the shoreline, it pushes the water up. The wave slows down as it approaches the shoreline but increases in height."