by Vivian Belik
When a 7.5 magnitude earthquake shook the ocean floor near Craig, Alaska, last week, it was strong enough to be felt by people 500 kilometres away in Whitehorse. The earthquake struck at midnight, and homes in Craig and Sitka swayed. But it was far enough offshore that it didn’t cause any serious damage to coastal towns.
That earthquake was the strongest to hit the southwest corner of Alaska since 1979, when a 7.5-magnitude earthquake ripped across the southern Alaska and Yukon border. The area is one of the most seismically active in Canada.
The Alaska Panhandle and the southwest corner of the Yukon sit on top of the North American tectonic plate. Immediately to the west is the Pacific Plate. West of the Panhandle, and off Haida Gwaii, these two plates are grinding past one another at a rate of about six centimetres per year. To the north, these two plates are colliding into each other.
“It’s a complicated corner,” says Natural Resources seismologist John Cassidy, explaining that what happens in that region is a combination of collision and horizontal slippage of two tectonic plates. “It’s essentially a transition zone.”
And seismologists have no way of predicting when the next big earthquake might hit. This writer spoke with Cassidy less than 12 hours before the earthquake rocked residents in Sitka. “They could occur at any time,” he said last Friday. “We can’t rule out that an earthquake might happen today, or it might be in 10 years or in a year.”
Even with the sophisticated machinery that exists to record earthquakes, scientists still have no way of knowing whether one will happen in the next hour. In some parts of the world, “early warning systems” are being tested. They require a large number of seismic stations to detect shaking, and automated systems to analyze, transmit, and use this information.
These systems take advantage of the fact that it takes time for seismic shaking to travel through the earth. For example, if an earthquake were to occur 100 kilometres from a city, it would take about 20 to 30 seconds for the waves that cause damage to arrive. “They’re not earthquake predictions, but they do take advantage of the fact that waves take time to travel,” says Cassidy.
When Japan experienced a magnitude 9 earthquake in 2011, thanks to its early warning system the government was able to halt its high-speed trains, shut down some power plants, and prevent traffic from driving onto bridges and tunnels that might have been damaged.
There are two different types of waves that follow an earthquake. The initial up-and-down shaking you feel is called a compressional wave and generally doesn’t cause any damage. It’s the slower sideways wave, or shear wave, that causes damage to buildings and bridges. “If you feel vertical shaking, it’s an indication that there could be stronger shaking on the way,” Cassidy warns.
The further away the earthquake, the more time you have between the compressional and shear waves reaching you. But it’s not a lot of time. “If the earthquake is 100 kilometres away you only have 10 seconds between those two waves,” says Cassidy. “If you feel shaking, you may have a few seconds to move away from any big windows that might break or from a large bookcase that might fall over onto you.”
Cassidy and his colleagues at Natural Resources study earthquakes to determine which areas of Canada are more likely to experience them and how large they’ll be. Using seismographs and GPS systems to track how the plates are moving, they’re able to produce earthquake hazard maps (used in the National Building Code of Canada, as well as in bridge and dam codes) that help engineers to design and construct buildings in a particular area. “It’s really important information if you’re deciding to build a pipeline, mine, or dam in a certain area,” says Cassidy.
In the Yukon it’s not only the southwest corner of the territory that’s susceptible to earthquakes. The northeast corner, where the tail end of the Rockies leads into the Mackenzie and Richardson mountains, also experiences fairly big quakes, some as large as magnitude 6.5.
“People forget about earthquakes because the big ones don’t happen very often,” says Cassidy. “Just looking over the past month, the Yukon has experienced three fairly substantial earthquakes.” In December there was a magnitude 4.2 earthquake near Beaver Creek, a magnitude 5.1 earthquake in northern Yukon, and a 5.1 earthquake northeast of Keno. Then there were the tremors felt in the Yukon from last week’s earthquake in Alaska.
Natural Resources has several seismic stations spread throughout the territory, including Whitehorse, Dawson, Beaver Creek, Haines Junction, and a temporary network around Kluane Lake. They’re usually installed on mountaintops and in bedrock, away from any sources of noise.
The first earthquake ever recorded in the Yukon was in 1850 by a Hudson’s Bay employee in Fort Selkirk. A series of magnitude 8 earthquakes on the BC/Alaska/Yukon border occurred in 1899. These remain the largest known quakes that have happened in the territory. But researchers are looking for evidence of other large earthquakes that took place thousands of years ago. Cassidy works alongside Simon Fraser University professor Jim Clague, who digs deep into layers of soil to find certain patterns that prove an area has been shaken by an earthquake.
This historical research may help seismologists move forward in predicting future earthquakes. “There may be things that occur before an earthquake, things that take place that we haven’t discovered yet. It’s something that we continue to work on,” says Cassidy. “We’ve made a lot of progress in improving our assessments of earthquake hazards over the last 30 years and it continues to be a really exciting area to work in.”
This column is co-ordinated by the Yukon Research Centre at Yukon College with major financial support from Environment Yukon and Yukon College. The articles are archived at www.taiga.net/yourYukon.