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December 10, 2018

In June of 2017, an earthquake swarm began beneath the western edge of Yellowstone National Park, just east of Hebgen Lake. This swarm proved to be one of the more persistent swarms observed in Yellowstone, with the main episode lasting more than 3 months and producing thousands of recorded earthquakes.

This is a map of earthquakes in the Yellowstone area in 2017.
Map of earthquakes in the Yellowstone area in 2017 that were individually located using traditional methods by University of Utah Seismograph Stations. The Maple Creek earthquake swarm, northwest of the caldera (red outline), is the second-longest-lasting ever recorded in the region. Black line shows Park boundary, and white lines are roads. Dashed lines are state boundaries.

Most of the earthquakes were very small, but a few were felt in the park, including the largest, a magnitude 4.4 earthquake on June 16, 2017.

Because of investments made in upgrading the seismic network over the past several years, this swarm was captured in more detail than any previous large swarm in Yellowstone. This means that scientists, including those involved in YVO, have more data than ever to detect and precisely locate earthquakes in the swarm, which can provide evidence of the causes of seismic swarms in the area.

Using a research technique that involves directly comparing the waveforms of the thousands of recorded earthquakes (instead of the routine processing that detects and locates earthquakes individually), scientists can greatly improve the precision of earthquake locations. Not only that, but we can also detect and locate thousands of earthquakes that were too small to be readily located as individual events (most of these earthquakes are less than M1.0. The patterns of seismicity from the swarm in time and space that have been revealed from this processing are striking—the swarm involved numerous fault structures over its course. Many of these faults are parallel and oriented along east-northeast trends, but some faults with orientations that are nearly perpendicular to that trend were also activated. Over the 3 months of the swarm, seismicity migrated outward from its initial activation area, both laterally and in depth. At times, this migration was rapid; at other times it was slow.

What do these patterns tell us about the physical processes driving the swarm? Although movement of magma can sometimes generate earthquake swarms at volcanoes, the patterns of this swarm (especially the rapid migration and lack of nearby surface deformation) instead suggest that water is diffusing through small cracks in the subsurface. The involvement of this water may in part explain why these swarms are sometimes long-lived, why they expand dramatically over time, and why the fault structures are so complex. This also may explain why swarms are common in volcanic areas, where water is a byproduct released from deeper magma as it cools. We often see chemical evidence for this type of water at surface springs and fumaroles.

This is a plot showing the evolution of the 2017 Maple Creek earthquake swarm.
Evolution of the 2017 Maple Creek earthquake swarm. Plots show earthquake locations colored by time. a) Map view. b) West-east cross-section. c) Three-dimensional view, looking from the east-southeast, along the axis for much of the swarm activity.

Because this water is under great pressure in the deep crust where it is released, it tends to migrate upward and sometimes laterally. When it interacts with cooler, more brittle rocks stressed by tectonic and volcanic processes, this water may trigger earthquakes. In fact, earthquakes themselves may allow the fluid to migrate more efficiently, through faults in the rock.

Without of other signs of volcanic unrest (like rapid surface deformation or changes in gas emissions), earthquake swarms probably do not indicate increased volcanic hazard. Although the 2017 Maple Creek swarm was a larger-than-average example, and therefore garnered significant public and scientific interest, earthquake swarms such as this likely reflect ongoing low-level tectonic and volcanic processes in Yellowstone. Even though this swarm might be "business as usual" for the caldera, the outstanding monitoring systems at Yellowstone provide an exceptional window into this "business," which will allow us to better understand future earthquake swarms not only at Yellowstone but also at other volcanoes around the world.

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