On September 11, 1999, at the same time that runners were pounding the pavement in the final leg of the Klondike Road Relay, a different sort of movement was taking place deep within the earth. Grinding of the rock along the Denali Fault set off a magnitude 5.0 earthquake at 2:23 PM, the biggest earthquake to shake this immediate area in more than 50 years.

This quake was fairly typical of the tremors regularly experienced in this part of the world. In Haines Junction, 60 kilometres north of the centre of the quake, one person reported falling out of bed. In Whitehorse, only people who were lying down or sitting quietly reported feeling any shaking.

More then 220 earthquakes of magnitude 2.0 or greater have been recorded in the last year in the Yukon or adjacent areas, but people reported feeling only six of them. Yukoners do not typically lose much sleep worrying about the next "big one," but it is worth recognizing that we do live in one of the most seismically active areas of Canada.

Both the southwest and the northeast corners of the territory are considered to be high hazard areas for earthquakes. The Yukon's largest recorded quake -- magnitude 7.9 -- occurred in the extreme southwest corner of the territory in 1959 in the St. Elias Mountains.

These peaks tower above the boundary between the giant Pacific and North American plates. The dozen or so large plates that make up the earth's crust are constantly moving. When the plates rub up against one another, the strain that builds up in the crust can produce an earthquake.

"The Yukon is an interesting area," says Taimi Mulder. A seismologist with the Geological Survey of Canada, she is responsible for locating earthquakes in western Canada. "The St. Elias Mountains are in a transition region from a strike-slip boundary to the south to a subducting boundary to the northwest."

The San Andreas fault in California is one famous example of a strike-slip fault where the plates slide horizontally past one another, occasionally sticking and setting off quakes as they break free. In a subduction zone, one large plate dives beneath its neighbour, and it is this process that kicks off the really big earthquakes.

On Good Friday in 1964, the Pacific Plate plunged beneath the North American plate and set off the second largest earthquake ever recorded. The magnitude 9.2 quake devastated the city of Anchorage.

The combined forces of subduction and strike-slip movement are pushing up the St. Elias Mountains at the rate of about 30 mm/year. The movement also causes a steady stream of small earthquakes in the area, most of them along the Denali Fault Zone.

This fault cuts a diagonal line across the southwest portion of the territory, and continues on into Alaska. Movement along this fault line has thrust up Denali, the highest peak on the continent. In the Yukon the fault roughly parallels the Alaska Highway south to Haines Junction, continuing south to the Haines area.

The northern Rocky Mountain region is another seismically active area, and earthquakes regularly shake both the Mackenzie and Richardson Mountains. Several magnitude six-plus earthquakes have occurred in the Richardson Mountains over the last 60 years.

A major quake occurred in 1985 when a magnitude 6.9 earthquake rocked the Nahanni area just east of the Yukon border. The seismic waves set off by this quake had the strongest ground accelerations ever recorded at that time.

To put these quakes into perspective, the recent quake in Turkey was a magnitude 7.4, while the one in Taiwan was magnitude 7.6. A 7.0 quake sets off seismic waves with an amplitude 10 times larger than a 6.0 quake, and it releases 32 times as much energy as the smaller quake.

Mulder works at the Geological Survey of Canada's Pacific Geoscience Centre in Sidney, BC. In the Yukon, seismograph stations in Whitehorse, Haines Junction and Dawson City send a steady stream of data via satellite to the centre. Seismographs in Haines, Alaska, and Inuvik, NWT pick up tremors in both the Yukon and neighbouring areas.

Mulder says that after bigger quakes, they usually get a phone call right away from someone who has felt the tremor. Then they can use a computerized system to locate the centre of the quake. Every morning they check reporting stations and locate earthquakes from the previous 24 hours.

But in a sparsely settled area like the Yukon, there often are not many people around to feel the earth shaking. The Geoscience Centre's records show that on November 25, 1997, a magnitude 5.1 tremor was "felt strongly by a trapper at Square Lake, about 60 kilometres SSW of the epicentre."

In true Yukon style, the trapper quickly scribbled the details of the quake on paper torn from a dog food bag, and sent it to geologists in Whitehorse.

For more information on earthquakes in the Yukon and western Canada, contact the Pacific Geoscience Centre at (250) 363-6500. You can find their web page at www.pgc.nrcan.gc.ca.