Stanley Earthquake

Summary

At 5:52 pm local time (MDT) on March 31, 2020, a magnitude (M) 6.5 earthquake occurred approximately 19 miles northwest of Stanley, Idaho. Shaking lasted approximately 10 seconds and was felt over a very large area.

If you felt it, you can submit a Did You Feel It? report here. Over 39,000 felt reports were received by the USGS:

No injuries and only minor damage were reported:

• Lemhi County residents reported some damage to windows and pictures falling off walls. Critical infrastructure, such as the courthouse, post office, and some propane lines, was inspected for structural damage.
• ITD did not report any major issues. There were several rock slides on Highway 21, but crews cleared them within a few days of the earthquake. ITD crews examined bridges and roads in districts 3, 4, 5, and 6.
• Boise County did not report any major damage and all roads remained open. Initial assessments of historic brick and mortar buildings in Idaho City showed very minor damage. Initial assessment of other critical infrastructure showed no damage. The Sheriff's office sustained some minor damage. Some residents reported broken windows.
• No major damage reported in the town of Stanley or Custer County.

Interactive Webmap

This map shows basic geologic data related to the earthquake, such as bedrock mapping, faults, aerial reconnaissance information, and earthquake epicenters. A static layer of seismicity from 3/31/2020 (the day of the M6.5 mainshock earthquake) through 3/26/2021 is shown by default. This layer will be updated periodically. You can also turn on a live feed of seismicity from the USGS catalog. Note that the earthquake feed epicenters are updated automatically and this layer displays earthquakes <M4.5 from the past 7 days, and earthquakes <M3.0 from the past 3 days. Search the complete catalog here.

Open the side panel (box in upper left with arrows) to see legend and turn layers on/off.

View larger map

Whose Fault?

The earthquake mechanism is not as straightforward as one would expect. Simple extensional motion on the Sawtooth normal fault seems like the most obvious mechanism, however the focal mechanism is more complicated and suggests complex strike-slip motion. Below left is the focal mechanism from USGS, showing a partially non-double couple solution. Below right is a more recent focal mechanism from Jamie Farrell and Katherine Whidden at University of Utah Seismograph Stations that is nearly a pure double couple.

Geologists and seismologists are still collecting data to determine which fault caused this earthquake. The Sawtooth fault, a normal fault which runs along the east side of the Sawtooth Mountain range, is the largest and best known fault in the region. However, the Sawtooth fault is still not well understood. We only have a general idea of the long-term slip rate and the timing of past earthquakes (e.g., Thackray et al., 2013).

The M6.5 earthquake was located just north of the mapped trace of the Sawtooth fault, and the subsequent aftershocks outline an approximately north-south trend extending off the north end of the Sawtooth fault. Because aftershocks often occur along the same fault as the mainshock, they can provide information about the fault structure (e.g., location, dip, depth). In this case, the aftershocks have occurred west and south of the mainshock. Aftershocks have defined a relatively linear pattern suggesting a fault oriented north-northwest. This fits one of the fault planes described by the focal mechanisms, and suggests the fault motion is left-lateral strike-slip. The preferred fault plane, at least used by USGS in their finite fault modelling, is a N-S striking fault with left-lateral motion. Initial InSAR interferograms of the epicentral region show a distinct deformation pattern that seems to fit a N-S striking fault. The examples below were created by Sotiris Valkaniotis (@SotisValkan).

This is somewhat unusual because the Sawtooth fault is believed to be a typical Basin and Range-style normal fault. Left-lateral motion on a N-S striking fault is not easily explained by the regional stress field. Furthermore, there is no mapped fault or clear surface topographic expression of a fault along the trend of aftershocks. Earthquakes like this may illustrate the complexity of fault tips (e.g., Bruno et al., 2017), and scarcity of detailed mapping and paleoseismic data in Idaho.

The aftershocks have also defined a linear east-west oriented pattern of seismicity, extending east from and perpendicular to the primary north-northwest pattern. It is still not clear if these patterns are defining previously active faults, or how these structures are interacting with the Sawtooth fault.

Further analysis of aftershocks (see Seismic Monitoring below) will help resolve the orientation and geometry of the fault plane.

Field Reconnaissance

(See IGS Stanley Earthquake Reconnaissance Report, 4/6/20)

Conducting field reconnaissance as soon after an earthquake is important for collecting perishable data. According to Marie Peppler, USGS (email communication 3/31/20), “In the immediate time following a natural disaster, like an earthquake, access to damaged areas to collect perishable data is critical to assessing risk for first responders, defining the hazards clearly and providing information that informs models and future response and mitigation activities. The data will not last long (i.e. landslides are cleaned off of roads, buildings are repaired) so it needs to be collected immediately following the event.”

However, several factors made immediate field reconnaissance difficult following the Stanley earthquake:

1. The earthquake occurred in a remote location
2. There was significant late season snow in the area (~27 inches near Cape Horn in the days before the earthquake).
3. The main highway in the area, Highway 21, was closed just west of the epicenter. ITD closed the road a few hours before the earthquake occurred because the avalanche danger was high. Several snow avalanches and rockfall blocked the highway (see photos below).
4. Idaho was under a statewide stay-at-home order because of the coronavirus pandemic. The route to Stanley passes through Ketchum. Both communities were hard hit by COVID-19 cases and/or inundation with travelers seeking shelter in remote locations.

On Thursday, April 2, IGS Director and State Geologist Claudio Berti and IGS Hazards Geologist Zach Lifton conducted an overflight of the earthquake epicenter region to make observations of possible earthquake effects. A second overflight took place on April 9, 2020. During both flights, there was significant snow cover across the area. We observed many small snow avalanches, including some that blocked Highway 21 and some small streams. Minor rockfall occurred on steep slopes, but was not very extensive. One large boulder rolled down onto Highway 21. We didn't observe any obvious ground rupture of the fault. It may have been obscured by snow, but the ground looked undisturbed and intact. Perhaps subtle ground rupture or deformation will be visible when the snow melts.

Geologists from IGS, Idaho State University, and the U.S. Geological Survey conducted ground reconnaissance of the epicentral area and the Sawtooth fault in September 2020 to make observations of possible earthquake effects. We did not observe any surface fault rupture or ground deformation related to the Stanley earthquake.

Seismic Monitoring

Aftershocks continue to occur one year after the M6.5 mainshock. At least 19 aftershocks greater than M4 have been recorded, including a M4.8. More than 3,200 aftershocks related to this event have been recorded in the USGS earthquake catalog.

USGS has created an aftershock forecast. Note that these are NOT predictions. Aftershock forecasts are based on statistics of other observed earthquakes.

Seismic monitoring is a critical component of earthquake response. With the sparse distribution of existing permanent seismometers in central Idaho, it is important to deploy additional temporary instruments to record the aftershock sequence. And since the number of aftershocks decays over time, instruments need to be deployed immediately. Instrument deployment and monitoring is being led by a team from Boise State University, including Professors Lee Liberty, Dylan Mikesell, and Jeffrey Johnson. They have installed 14 seismometers near the epicenter. The map below by Lee Liberty shows the location of the BSU and IGS/PASSCAL instruments, as well as earthquake epicenters, focal mechanisms, and historical seismicity. In addition, 24 nodal sensors (wireless geophones), and more seismometers borrowed from IRIS are being deployed.

IGS Director Claudio Berti, IGS Hazards Geologist Zach Lifton, and BSU Research Assistant Thomas Otheim installed two temporary seismometers at Diamond D Ranch and Middle Fork Lodge, both along the Middle Fork Salmon River. Data from these and other non-telemetered stations will be downloaded regularly.

Changes at the Stanley Lake Inlet Delta

On May 7th, colleagues from the US Forest Service reported that the inlet delta of Stanley Lake, a popular beach and recreational site, was now under many feet of water. Pictures from the field and reports of field observation suggest that the "disappearance" of the delta is a combination of liquefaction/compaction of saturated sediments and lateral spreading of the delta into the deeper part of the lake. IGS geologists visited Stanley Lake in May 2020 to record observations of the liquefaction event.

Visit the Stanley Lake Liquefaction page for more details.

Connection to Yellowstone?

We have received questions from the public about whether this earthquake could be related to the Yellowstone volcanic field. The answer is “No”. The Stanley earthquake is over 200 miles away from Yellowstone and located in a very different tectonic environment. Basin and Range extension is driving tectonic activity in central Idaho, there is no current volcanic activity in the area. Mike Poland, USGS Yellowstone Volcano Observatory scientist-in-charge, said:

“It’s been an eventful month for seismic activity in the western USA, with a M5.7 earthquake near Salt Lake City, Utah, on March 18, and a M6.5 in central Idaho on March 31. These earthquakes are caused by tectonic extension of the region and are not related to Yellowstone, nor will they have a significant impact on the Yellowstone system. Some strong earthquakes in the region, like the 1983 M6.9 Borah Peak, ID, and 1959 M7.3 Hebgen Lake, MT, earthquakes, have impacted geyser behavior, but that is due to the response of the shallow and fragile geyser conduits to shaking. It is not yet clear if the M6.5 in central Idaho will have a similar impact; observations of geyser activity over the coming days to weeks will answer that question.”

More from Yellowstone Volcano Observatory on the recent seismicity in the western U.S.

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