What have we learned since the 2020 Stanley, Idaho, earthquake? • Idaho Capital Sun

For many years, the Sawtooth fault in central Idaho was recognized as an active fault, but remained unstudied. Then the Stanley earthquake shook much of the northwestern USA in 2020, providing major confirmation of tectonic activity in the region. The earthquake sparked a wave of research that shed light on the Sawtooth Fault, while also raising new questions about fault behavior and the history of earthquakes in central Idaho.

The M6.5 Stanley earthquake that occurred on March 31, 2020 was the second largest earthquake ever recorded in Idaho (after the 1983 M6.9 Borah Peak earthquake). Although not directly related to Yellowstone, the earthquake and its numerous aftershocks occurred in the Centennial Tectonic Belt, which was affected by the region's passage over the Yellowstone hotspot. This zone of seismic activity wraps around the hotspot like a bow wave around a boat.

This is a map for the “Did you feel it?” Issued by the US Geological Survey. Reports on the March 31, 2020, M6.5 Stanley, Idaho earthquake. (Courtesy of USGS)

After a strong earthquake, geologists and seismologists usually work quickly to collect data that helps better understand the event.

However, the Stanley earthquake occurred early in the COVID-19 pandemic and immediately after a major snowstorm. This made it difficult for scientists to conduct field work. Despite a slow start, we have learned a lot in the past four years.

The Stanley earthquake was more complex than initially expected. The location of the earthquake indicates that it was created on the Sawtooth Normal Fault which lies to the east. However, the focal mechanism suggested a complex, multi-fault rupture that was neither normal nor eastward dipping.

At least three different models have been proposed for the fault geometry responsible for the Stanley earthquake, including two parallel faults, two faults crossing at a sharp angle, and two perpendicular faults. Regardless of which is true, all models describe complex geometry.

Aftershocks are a natural consequence of all strong tectonic earthquakes. They typically occur along the fault plane of the main shock as the crust adapts to new stresses after energy is released.

Scientists from the Idaho Geological Survey, Boise State University, and the USGS were able to deploy temporary seismometers to measure aftershocks around the Stanley area — aftershocks that still occur today (the stronger the earthquake, the longer the aftershocks last ).

Mapping aftershocks in three dimensions paints a picture of the fault plane. The Stanley aftershocks show a north-trending fault that dips steeply to the west, parallel to the Sawtooth fault but dipping in the opposite direction, further evidence of the complexity of this earthquake.

The quake did not rupture the surface, which often happens with earthquakes of magnitude 6.5 or larger. Ground deformation data and modeling indicate that the fault movement that generated the earthquake stopped at a depth of about 10.6 miles.

Although no surface fault rupture occurred, the Stanley earthquake triggered numerous avalanches, debris slides, and rock falls.

One of the most dramatic effects of the earthquake was the liquefaction of Lake Stanley. Ground shaking can weaken loose, water-saturated sediments, making them behave almost like a liquid.

At Lake Stanley, the sandy area near the inlet stream has sunk significantly due to liquefaction. The famous beach at the inlet delta has completely disappeared, and large cracks and sand dimples have formed along the shore.

More information about this liquefaction event can be found on the Idaho Geological Survey website: https://www.idahogeology.org/geologic-hazards/earthquake-hazards/stanley-lake-liquefaction.

This is a ground fissure along the shore of Lake Stanley, Idaho, caused by liquefaction during shaking from the Stanley Earthquake on March 31, 2020. (Zach Lifton/Idaho Geological Survey)

New lidar data collected shortly after the earthquake allowed geologists to map the Sawtooth Fault in detail. The resulting map shows that instead of a single continuous line, the Sawtooth Fault is an intermittent fault zone with several branches.

Detailed maps of the fault allowed researchers to pinpoint locations of ancient trenches. These investigations involve digging a trench across the fault to record and date sediments displaced by past earthquakes. USGS geologists who dug a trench across the North Sawtooth Fault found evidence of at least one earthquake there, about 9,000 years ago.

Additional ancient trenches are planned and will help clarify the history of earthquakes along other parts of the fault.

This is a map of the Sawtooth Fault in Idaho based on new lidar topographic maps. (Zach Lifton/Idaho Geological Survey)

Researchers also used lake records to examine the history of the Sawtooth Fault. Sediment cores from Redfish Lake have preserved evidence of two earthquakes (4,300 and 7,600 years ago) along the central portion of the Sawtooth Fault.

Lake sediment cores from Petit Lake, 15 kilometers to the south, preserved evidence of a single earthquake about 5,100 years ago, which was different from the earthquake events at Redfish Lake.

Geologists examine an ancient trench across the Sawtooth Fault in Idaho in September 2022. Such trenches provide geologists with an opportunity to date sediments displaced by past earthquakes, thus determining when the fault ruptured. (Zach Lifton/Idaho Geological Survey)

Despite all these new ideas about the Stanley earthquake and the Sawtooth fault, many questions remain.

For example, did the entire Saw Fault explode in one earthquake? If not, what is the history of smaller earthquakes in shorter segments? Do earthquakes that occur in the northern part of the fault rupture several branches simultaneously or at different times?

Field work and modeling continue as geologists work to better understand this unique and important fault system in central Idaho.

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