By David Jacobson, Temblor
Yesterday, at 6:48 p.m. local time, a M=4.4 earthquake struck the western edge of Yellowstone National Park in Wyoming. According to a press release from the University of Utah, the quake was felt throughout the national park as well as in the nearby towns of West Yellowstone and Gardiner. On the USGS website, only 97 people reported feeling the quake, though the number that actually felt it is likely much higher. So far, there are no reports of damage, which is not surprising given the quake’s magnitude, which only registered light shaking near the epicenter, based on the USGS ShakeMap.
According to the University of Utah, this M=4.4 earthquake is part of a swarm that began on June 12, that has included 30 M=2+ earthquakes, and four M=3+ earthquakes, including the one yesterday. They also point out that swarms like this are extremely common in the Yellowstone region, and comprise approximately 50% of the seismicity. While there are mapped faults around the location of this active swarm, based on the northwest trend in the seismicity, and a focal mechanism produced by Bob Herrmann of St. Louis University, this quake appears to have occurred on a left-lateral strike-slip fault which connects the east-west-trending extensional faults in the region. It should also be pointed out that this M=4.4 quake is the largest to strike the area since 2014, when there was a M=4.8.
Seismic activity around Yellowstone is extremely common and is caused by a variety of factors. Some of the smaller earthquakes are a result of rising and moving magma beneath the surface. However, Yellowstone is unlikely to experience a large earthquake because the hot magma beneath the surface causes the bedrock to behave more ductile, meaning it is less likely to rupture. While large earthquakes within the park are unlikely, the region is highly sensitive to remote earthquakes. What this means is that seismic waves from earthquakes hundreds of kilometers away can slightly destabilize the volcanic and hydrothermal systems, resulting in small earthquakes and hydrothermal eruptions.
The area around today’s small-moderate earthquake is also of interest as it is only 15-20 km from the epicenter of the 1959 M=7.3 Hebgen Lake earthquake. This earthquake killed 28 people and resulted in millions of dollars worth of damage. The majority of these fatalities were the result of a massive landslide triggered by the shaking. Therefore, while rare, large earthquakes can occur just outside of Yellowstone. Based on the Global Earthquake Activity Rate (GEAR) model, which is available in Temblor we can see that yesterday’s M=4.4 earthquake in Yellowstone should not be considered surprising, given the region is susceptible to M=5.25+ quakes. This model uses seismicity since 1977 and global strain rates to forecast what the likely earthquake magnitude is in your lifetime anywhere on earth. What we can also see from this model is that the M=7.2 Hebgen Lake earthquake just to the west of yesterday’s shock was an extremely rare and surprising event. Should the characteristics of this ongoing swarm in Yellowstone National Park change, we will update this post.
University of Utah Seismograph Stations
Bob Herrmann (St. Louis University)
Robert B. Smith (University of Utah)
Jamie Mark Farrell (University of Utah)