YELLOWSTONE NATIONAL PARK (Idaho capital sun) – Seismic activity in central Idaho has been high for the past 20 months. These earthquakes are aftershocks of a 6.5 magnitude earthquake that occurred on March 31, 2020 – a classic example of a main shock aftershock sequence.

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The 6.5 magnitude (Mw) earthquake near Stanley on March 31, 2020, was the second largest earthquake on record in Idaho. Fortunately, it caused very little damage, but its tremors were felt throughout the area. Since that initial large earthquake, known as the main shock, there have been more than 4,300 aftershocks in the same region. This increase in seismic activity has alarmed some people.

A common question after many earthquakes in the western United States is, “Will this earthquake cause an eruption in Yellowstone?” Or “is this seismicity related to Yellowstone?” The simple answer is “no,” but let’s take a closer look at what’s going on in central Idaho and if it has anything to do with volcanic activity in Yellowstone.

replicas are a normal and predictable phenomenon as a result of tectonic earthquakes caused when the earth’s crust moves on itself along fractures called faults. This movement usually occurs very quickly when the stored tectonic force is released. The energy produced by the movement of the fault is converted into seismic waves which radiate outward from the fault. The faults only rupture over a length of a few tens to hundreds of kilometers at a time. The Stanley earthquake rupture was approximately 12 miles in length. The fault system is part of the Western United States Basin and Range Province, which expands due to tectonic activity and has no direct relation to the magmatic system beneath Yellowstone.

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After an earthquake, the crust surrounding the ruptured fault will undergo significant stress changes. The earthquake will reduce stress in some areas of the crust, but increase stress in other areas. As the crust adjusts to its new state of stress, it produces numerous small earthquakes – aftershocks – in the area around the main shock. Seismologists have recognized several interesting models of aftershocks after observing them around the world for more than 100 years. First, the number of aftershocks can be anticipated depending on the magnitude of the main shock. Aftershocks are very abundant immediately after the main shock, and their number declines over time at a predictable rate of decline. That’s why scientists from Boise State University, Idaho Geological Survey, and the US Geological Survey rushed to deploy dozens of sensors in the Stanley earthquake area as soon as possible after. M6.5 – because aftershocks are numerous immediately after the main shock but diminish over time.

Second, the magnitude of aftershocks can be predicted. Typically, the largest aftershock is about 1.2 magnitude increments smaller than the main shake. For the Mw6.5 earthquake in Stanley, we would expect the largest aftershock to be no more than around Mw5.3. The most significant aftershock observed so far has been a 4.8 Mw which occurred less than an hour after the main shock. An important exception to these patterns is that sometimes earthquakes like the Mw6.5 Stanley event are the beginnings of an earthquake of similar or larger size. For example, two moderate earthquakes (around Mw6.1 and ~ Mw6.0) occurred in the Stanley area a few months apart in 1944 and 1945. Unfortunately, there is no way of knowing if a given earthquake is a main shock or a pre-shock before the fact.

Third, aftershocks can last for tens or even hundreds of years, depending on the size of the main shock. For example, in eastern Idaho we are still recording aftershocks from the 1983 Mw6.9 Borah Peak earthquake, almost 40 years later. Aftershocks, and even deformations, associated with the 1959 Mw7.3 Hebgen Lake earthquake, just west of Yellowstone, continue today! And current seismic activity in the New Madrid seismic zone in the east-central US is linked to a pair of huge ~ Mw8.0 earthquakes there in 1811 and 1812.

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Is all of this seismic activity in central Idaho affecting the Yellowstone volcanic field? Not significantly.

Large, distant earthquakes can cause changes to Yellowstone’s geysers and hot springs, as the tremors fracture the fragile hot-water underground plumbing systems. However, the important tectonic stresses are not transmitted very far. The post-seismic stress of the main shock and the resulting aftershocks only extend to about 1-2 fault lengths of the main shock (20-40 km, or 12-24 miles, in the case of the earthquake by Stanley). The part closest to the Yellowstone Caldera is 275 km (about 110 miles) from Stanley Main Shock. Additionally, there are at least five other active faults between the Stanley earthquake and Yellowstone that would be susceptible to rupture if a significant stress were transmitted this far.

Much bigger and closer earthquakes, like the 1959 Hebgen lake earthquake Mw7.3, occurred without triggering any known volcanic activity in Yellowstone (although this earthquake caused many changes to the geyser and hot spring systems).

Hopefully this shed some light on what’s going on in central Idaho regarding all the small earthquakes over the past year and a half – why this is a normal, natural process, and why distant earthquakes will not affect Yellowstone. The next time a large earthquake hits anywhere in the world, keep an eye on the aftershock sequence and see if you can recognize the common patterns of those sequences.

Yellowstone Caldera Chronicles is a weekly column written by scientists and contributors at the Yellowstone Volcano Observatory for the Idaho Capital Sun’s sister publication, Le Quotidien Montanais. This week’s contribution is from Zach Lifton, a geologist with the Idaho Geological Survey.