Quakes shake from Purcell Trench fault

SANDPOINT — Research shows area earthquakes may actually be coming from the Purcell Trench fault, not the Hope fault originally suspected.

According to Daisuke Kobayashi, a geology graduate student at the University of Idaho, the Purcell Trench fault runs north-south through the Sandpoint area and was awakened by crustal contraction. The earthquakes are a result of that awakening.

“Most past earthquakes in the area were too small to indicate on which fault they occurred with a certainty,” Kobayashi said. "The earthquakes in April were larger than most of the past events, which helped us determine on which fault they occurred.”

The fault may have been reactivated before the April earthquake and some of the past earthquakes may have been on the Purcell Trench fault. The Purcell Trench is part of the Lewis Clark Fault Zone, extending from eastern Washington to central Montana. Before the study, researchers believed the earthquakes were taking place along the Hope fault, the major fault line that runs northwest to southeast through the area.

Kobayashi said the area needs more data to determine earthquake patterns and predictions. However, there are currently no seismic stations in North Idaho.

“We need more seismic and GPS stations to answer important questions, such as how often on average the similar earthquakes can occur there, whether larger earthquakes can occur in the area, and which segments of which fault are ready to slip as earthquakes,” he said.

Research comes from surrounding states’ seismic networks. He said having one or more seismic stations in North Idaho would benefit research to analyze the earthquake hazard risk in the area. However, Kobayashi said federal funding for the area is not prioritized for earthquake studies compared to cities with larger risks.

The study led to two surprise findings, Kobayashi said. The Sandpoint events are a crustal contraction type, while most of the quakes in the Lewis Clark Fault Zone in Montana are from crustal extension, the other type of earthquake. The other major finding in the study was that the Purcell Trench fault is active, where there was no documentation of that before, Kobayashi said.

The Purcell Trench fault formed millions of years ago as a result of crustal extension. Research shows the Sandpoint earthquakes indicate contraction.

“The change of the style of faulting like this is called a reactivation,” Kobayashi said. "The Sandpoint earthquakes suggest the reactivation of the Purcell Trench fault, which may have been happening even before as small earthquakes.”

Kobayashi said there are different styles of earthquakes, including extension and contraction. Deformations found in Earth's tectonic plates shift, stretch, shorten, uplift, subside or make a combination of those. Stretching is extension, and shortening is contraction for styles of earthquakes.

“Earthquakes in the same style will probably keep occurring in the area,” Kobayashi said. "Now we know a style of some earthquakes in Northern Idaho, which is the first step of seismic hazard analysis.”

Earthquakes have shaken the Sandpoint area this year, starting with a 4.1-magnitude on Apr. 23. More recently, a 3.4-magnitude earthquake rumbled the Sandpoint area Nov. 23.

More than 98 people reported feeling the tremors in November from 14 ZIP codes, all in North Idaho.

Accoding to the U.S. Geological Service, the quake was felt all around Lake Pend Oreille.