New research led by the University of Victoria (UVic) has illuminated a significant and previously unrecognized source of seismic hazard for the Yukon Territory of northwestern Canada.
The Tintina fault is a major geologic fault approximately 1,000 km long that trends northwestward across the entire territory. It has slipped laterally a total of 450 km in its lifetime but was previously believed to have been inactive for at least 40 million years. However, using new high-resolution topographic data collected from satellites, airplanes and drones, researchers have identified a 130-km-long segment of the fault near Dawson City where there is evidence of numerous large earthquakes in the much more recent geologic past (the Quaternary Period, 2.6 million years to present), indicating possible future earthquakes.
"Over the past couple of decades there have been a few small earthquakes of magnitude 3 to 4 detected along the Tintina fault, but nothing to suggest it is capable of large ruptures," says Theron Finley, recent UVic PhD graduate and lead author of the recent article in Geophysical Research Letters. "The expanding availability of high-resolution data prompted us to re-examine the fault, looking for evidence of prehistoric earthquakes in the landscape."
Currently, the understanding of earthquake rates and seismic hazard in much of Canada is based on a catalogue of earthquakes from oral Indigenous accounts, written historical records and modern seismic monitoring networks. Collectively, these records only cover the last couple hundred years. However, for many active faults, thousands of years can elapse between large ruptures.
When earthquakes are large and/or shallow, they often rupture the Earth's surface and produce a linear feature in the landscape known as a fault scarp. These features, which can persist in the landscape for thousands of years, are typically tens to hundreds of kilometres long, but only a few metres wide and tall. They are difficult to detect in heavily forested regions like Canada, and require extremely high-resolution topographic data to identify.
The team, consisting of researchers from UVic, the Geological Survey of Canada and University of Alberta, used high resolution topographic data from the ArcticDEM dataset from satellite images, as well as from light detection and ranging (lidar) surveys conducted with airplanes and drones. They identified a series of fault scarps passing within 20 km of Dawson City.
Crucially, they observed that glacial landforms 2.6 million years in age are laterally offset across the fault scarp by 1000 m. Others, 132,000 years old, are laterally offset by 75 m. These findings confirm that the fault has slipped in multiple earthquakes throughout the Quaternary period, likely slipping several meters in each event. What's more, landforms known to be 12,000 years old are not offset by the fault, indicating no large ruptures have occurred since that time. The fault continues to accumulate strain at an average rate of 0.2 to 0.8 millimetres per year, and therefore poses a future earthquake threat.
"We determined that future earthquakes on the Tintina fault could exceed magnitude 7.5," says Finley. "Based on the data, we think that the fault may be at a relatively late stage of a seismic cycle, having accrued a slip deficit, or build-up of strain, of six metres in the last 12,000 years. If this were to be released, it would cause a significant earthquake."
An earthquake of magnitude 7.5 or greater would cause severe shaking in Dawson City and could pose a threat to nearby highways and mining infrastructure. Compounding the hazard from seismic shaking, the region is prone to landslides, which could be seismically triggered. The Moosehide landslide immediately north of Dawson City and the newly discovered Sunnydale landslide directly across the Yukon River both show ongoing signs of instability.
Canada's National Seismic Hazard Model (NSHM) includes the potential for large earthquakes in central Yukon Territory, but the Tintina fault is not currently recognized as a discrete seismogenic fault source. The recent findings by this team will ultimately be integrated into the NSHM, which informs seismic building codes and other engineering standards that protect human lives and critical infrastructure. The findings will also be shared with local governments and emergency managers to improve earthquake readiness in their communities.
This research occurred on the territory of the Tr'ondëk Hwëch'in and Na-Cho Nyäk Dun First Nations
