Sandpoint quakes reveal seismic activity on ancient fault

A swarm of earthquakes near Sandpoint last April indicates that an ancient fault is active again, a University of Idaho researcher said.

The quakes, widely felt in Bonner County, struck April 23 and 24 with magnitudes in the 4.1 to 4.2 range. They were larger than other recent quakes, which allowed scientists to study them in greater detail, said Daisuke Kobayashi, a doctoral student in UI’s geological sciences program.

At first, researchers thought the earthquakes occurred along the Hope Fault, a major fault credited for past quakes in the area. But closer scrutiny revealed that the Purcell Trench Fault was responsible, Kobayashi said.

“Before the quakes in April, we didn’t know if that fault was active,” he said.

The Purcell Trench Fault formed between 34 million and 56 million years ago. It runs about 185 miles through North Idaho and into British Columbia.

Kobayashi noticed something else interesting about the April quakes: They were caused by the Earth’s crust pressing together. Other quakes in nearby fault zones have been caused by the Earth’s crust pulling apart.

For reasons that aren’t entirely clear, much of the landmass in the Northwest is slowly rotating clockwise, Kobayashi said. His research suggests that the Sandpoint quakes, located at 12 on a theoretical clock face, indicate where the rotation stops and where the Earth’s crust is slowly squeezing together. Compression from the clockwise rotation may have reactivated the Purcell Trench Fault, which was originally formed by the Earth’s crust pulling apart, he said.

Kobayashi’s research will be published in a forthcoming edition of Idaho Geological Survey. The findings are “the first step to analyzing seismic risk” along the fault, which runs through Sandpoint and is also close to Coeur d’Alene, he said.

North Idaho needs more seismic and GPS stations to monitor what is happening along the fault, Kobayashi said. The closest seismic stations are in Newport, Washington, and Montana. Better GPS monitoring also could reveal subtle changes in the Earth’s crust.

In most locations, earthquakes occur in cycles, he said. In 1942, a magnitude-5 earthquake struck Sandpoint, though it’s not clear which fault was responsible.

A damaging quake of that size is possible again in the area, Kobayashi said.