Fairweather Fault and earthquakes in Yukon

The Fairweather Fault is one of the principal strike-slip faults along which the Pacific Plate is moving northwesterly (at 5.8 cm annually, relative to North America). Sections of the fault become ‘stuck’, creating small earthquakes for a time, followed by a powerful earthquake that releases much of the accumulated strain. On the segment nearest Mount Logan the most recent powerful earthquake was in 1958. Three very large earthquakes occurred there in less than a week in 1899. Between these major events are many smaller tremors that each contribute to the northward shift of the Pacific Ocean floor.

The geological record of past events is sometimes the only predictor of future geologic hazards. However, the natural world imperfectly holds the record of ancient earthquakes, and we have only about 130 years of historical data in northwestern North America – too small a window to establish the pattern of large but infrequent earthquakes. The location of many smaller events and a few large ones, indicate five seismically active regions in Yukon Territory.
Faults and relative motion superimposed on earthquake epicentres from 1899 to 1996 in northwestern Canada.
Faults and relative motion superimposed on earthquake epicentres from 1899 to 1996 in northwestern Canada
Credit: Canadian Seismological Service and R. Hyndman, Geological Survey of Canada

The Fairweather Fault in southeastern Alaska comes ashore 130 km west of Haines, Alaska. It has several splays heading northwest along glacier-filled valleys,. West-side-northward and upward movement on these faults accommodate the inexorable northwestward movement (about 6 cm/year) of the Pacific oceanic plate to where it subducts obliquely beneath the St. Elias Mountains and southern Alaska. In 1958, a M 7.9 earthquake tore the Fairweather Fault from Yakutat to Sitka, and moved the west side 6.5 m northward. Were such an event to occur today, non-earthquake-resistant structures in Alaska coastal towns, as well as transportation corridors would be severely damaged. The zone of low seismicity on the continuation of the Fairweather Fault southwest of Mount St. Elias could indicate a ‘locked’ segment, A portion of the over-riding continental crust, the Yakutat microplate, is being squeezed, pushing Yukon northward.

The Denali Fault lies inland, extending from Dezadeash Lake south of Haines Junction, Yukon, northwest toward Fairbanks, Alaska (the trace looks curved on a small-scale flat map, but is a straight line on the curved surface of the earth). Some sections, such as that beneath Kluane Lake, have no recorded seismicity, but on November 3, 2002, a magnitude 7.9 earthquake south of Fairbanks ruptured the fault almost to the Alaska-Yukon border (http://www.aeic.alaska.edu/Denali_Fault_2002). In Yukon, the most likely active trace follows the Totschunda and Duke River faults. The latter crosses the Slims River 6 km west of the Alaska Highway and has regular small earthquakes that are felt in Haines Junction. Larger earthquakes could trigger rock or mud-slides which could block the transportation links along Kluane Lake or the Alsek river valley to the south.

Two other well-defined zones of seismic activity occur at the northeastern margins of Yukon. In the Richardson Mountains, there are faults with periodic earthquakes (magnitude 6.4 and 6.2 in 1940). Secondly, along the mountain front north and east of the Ogilvie and Mackenzie Mountains respectively are earthquakes (magnitude 6.5 south of Peel River in 1955; M 6.9 east of Nahanni Park in 1985). Both types are related to northward and eastward push by the Yakutat microplate against the St. Elias Mountains

The seismic zoning map of the National Building Code of Canada (www.EarthquakesCanada.ca) shows a greater risk of earthquake damage in Yukon (where the population and building density is comparatively light) than in most of central Canada. However, the most affected areas are in the southwest because of its closeness to faults in southeast Alaska, as well as in the Peel River region. Neither faces the looming disaster predicted for southwestern British Columbia, where geological evidence suggests a large earthquake every 300 to 500 years (the last one may have occurred in 1701).