Thanks to a recent earthquake swarm, the Yellowstone supervolcano has seen upwards of 200 quakes since February 8, along with countless smaller tremors. The largest earthquake was an unremarkable magnitude 2.9, and all of them have hit about five miles beneath the surface. Larger earthquakes have rocked the region in the past, some as destructive as the Hebgen Lake quake and others causing minimal damage.
With this most recent swarm, scientists say there's no reason to worry. "Supervolcano" and "earthquake swarm" might seem like daunting terms on the surface, but in Yellowstone National Park, these geologic features are relatively nonthreatening.
Here, we take a closer look at what's going on beneath the ground.
So what is an earthquake swarm?
Earthquake swarms occur when a single area experiences an increase in quakes over a short period of time without the trigger of a single, larger "mainshock." Swarms can result from changes in stress along fault lines, which can be caused by either large-scale tectonic forces or pressure buildup due to changes in magma, water, or gas underneath Earth's surface. (Related: "Why Alaska is Uniquely Prone to Earthquakes and Tsunamis")
The area where this current swarm is happening—about 8 miles northeast of West Yellowstone, Montana—is under pressure from both these forces, since Yellowstone is a hotbed for seismicity. But earthquake swarms are frequent in the region, accounting for more than half of the parks' seismic activity. And they haven't triggered any volcanic eruptions yet.
Last year, a swarm ten times larger than the current one rocked the same region, generating about 2,400 earthquakes between June and September 2017. This year's swarm could actually be just a continuation of last year's, since seismic activity in the area can be sporadic but ongoing.
What is a supervolcano?
A supervolcano is exactly what it sounds like—a massive volcano that is capable of erupting with devastating global consequences. A huge magma pocket lurks below the Yellowstone supervolcano, which violently erupted more than 630,000 years ago. That burst rocked the region, spewing forth ash and rock and carving out a volcanic depression that cradles most of the national park today. (Peek inside Yellowstone's supervolcano with our magazine interactive.)
"It is not an imminent hazard," volcanologist Guillaume Girard told National Geographic in 2012. "Every study has concluded that there is no magma that is ready to erupt within any foreseeable future."
Still, there's no way to tell when the supervolcano actually is going to blow. The U.S. Geological Survey puts the odds of another massive blast at about 1 in 730,000, which is similar to the chance of a disastrous asteroid collision. (Read about Italy's Campi Flegrei, one of the world's most dangerous supervolcanoes.)
When it does erupt, Yellowstone's supervolcano will most likely burst in one of three parallel fault zones that run north-northwest across the park. Knowing this helps scientists know which parts of the park should be monitored for future eruptions.
Still, earthquake swarms and supervolcanoes… Should we be worried?
"While it may seem worrisome, the current seismicity is relatively weak and actually represents an opportunity to learn more about Yellowstone," researchers Michael Poland and Farrell write in a column for the Yellowstone Volcano Observatory. "It is during periods of change when scientists can develop, test, and refine their models of how the Yellowstone volcanic system works."
Earthquake swarms are common in Yellowstone, and there's nothing to be concerned about. Of the 1,000 to 3,000 quakes that happen each year in Yellowstone, most are not felt. The largest swarm on record happened in 1985, when more than 3,000 earthquakes hit the area over the course of three months.
It's worth noting that Yellowstone is one of the best monitored volcanoes in the world, closely studied by diligent researchers armed with a slew of data from sensors and satellites. Their task is to look for geologic changes in the area.
As for the current earthquake swarm hitting the Yellowstone supervolcano?
"It's slowly petering out, although these things wax and wane, so it's a bit difficult to say that it's ending," Poland tells Newsweek. "Yellowstone is just a very swarmy place."
Mount Semeru, seen with an ash plume, is the highest volcano on the Indonesian island of Java and has been in a constant eruption since 1967. It lies at the southern end of the Tengger caldera, which contains smaller volcanoes Mount Bromo and Mount Batok (both seen in the foreground), and several others.