“Major ancient faults like that can remain as weak zones in the Earth’s crust and then focus ongoing tectonic strain,” noted Theron Finley, whose recent research has turned conventional thinking about seismic hazard in northwestern Canada on its head. For decades, the Tintina fault a giant, 1,000-kilometer formation running through British Columbia into Yukon and on into Alaska was thought to be dormant, its most recent major activity having ceased before the end of the last ice age. But recent studies that utilize sophisticated geospatial tools now indicate that this “sleeping giant” has the potential to unleash a magnitude 7.5 earthquake in the lifetime of a human, a discovery that has far-reaching implications for seismic hazard analysis and engineering in the region.
The discovery follows a careful examination of Tintina fault strain accumulation over the last 2.6 million years. By making use of high-resolution satellite and lidar imagery, scientists picked out subtle fault scarps linear fractures of the landscape, just a few meters high buried under dense Yukon forests. These invisible to the naked eye and standard aerial photo features are telltale signs of ancient surface-rupturing earthquakes. “Those features can be hundreds of kilometers long in some cases, but they’re only on the order of a couple meters high or wide, so we need the really high-resolution topographic data,” Finley said.
By fusing ArcticDEM satellite imagery with airborne and drone-borne lidar surveys, the researchers were able to map the fault’s subtle distortions with unprecedented accuracy. Their examination indicated that 2.6 million-year-old glacial landforms dating back 2.6 million years are offset by 1,000 meters along the fault, whereas 132,000-year-old features are displaced by 75 meters. Most importantly, landforms created 12,000 years ago exhibit no such displacement, suggesting the lack of significant ruptures since then. This suggests a quiescent period of over 12,000 years, when the fault has been building up an estimated 6 meters of strain in silence a “slip deficit” ready to be unleashed.
The Tintina’s geological setting is complicated. It is one of a larger system of strike-slip and reverse faults stretching across Yukon, British Columbia, and Alaska, deformed by the collision and accretion of terranes along North America’s western margin. The local stress field, driven by the continued northward motion of the Pacific Plate and thickening of the Yakutat microplate, has reorganized patterns of deformation over millions of years. Although the Denali and Duke River faults in the vicinity have been more recently active, the seeming inactivity of Tintina covered up its possibilities as a seismic hazard. As Finley pointed out, “There has always been a question of whether it’s still a little bit active or still accumulating strain at a slower rate.”
Paleoseismic techniques provide a critical window into the fault’s behavior at timescales far longer than instrumental or even historical records. Trenching across fault scarps and radiometric dating of displaced sediments can provide information on when and with what strength prehistoric earthquakes occurred. But in the far-off New World bush country of the Yukon, such ground truthing is logistically difficult. The introduction of lidar has changed all that, allowing scientists to spot trench sites with precision and to differentiate between tectonic scarps and those produced by glacial or landslide processes. As delineated in recent paleoseismic research, coupling lidar with radiometric and stratigraphic information is now routine to quantify earthquake recurrence periods and slip rates in similar difficult environments.
Implications for Dawson City and the surrounding Klondike district are sobering. With a population of approximately 1,600 and widespread mining infrastructure, the region is exposed not only to immediate shaking but also to seismically triggered landslides. The Moosehide and Sunnydale landslides, both with persistent instability, had the potential to be triggered by intense ground motion. Canada’s National Seismic Hazard Model now acknowledges the existence of large earthquakes in central Yukon, but until recently, the Tintina fault was not designated as a distinct seismogenic source. The new findings are set to inform updates to seismic building codes and emergency preparedness plans, as well as guide engineering standards for critical infrastructure.
Even with these breakthroughs, great unknowns persist. “Right now, we just know that many [earthquakes] have occurred, but we don’t have a sense of how frequently,” Finley cautioned. The exact recurrence interval whether 6 meters of strain is normal between ruptures, or if much more can pile up before failure remains unsolved. More progress will demand focused paleoseismic trenching and ongoing tightening of geodetic monitoring, based on the foundation established by high-resolution topographic mapping.
As the character of the Tintina fault begins to emerge, the task for geoscientists and hazard experts is evident: to use these developing insights to create actionable risk assessments for communities and infrastructure sitting on Canada’s unstable northern crust.
