Taking a Closer Look at a Yellowstone Earthquake Swarm.
When earthquake swarms happen at Yellowstone, we receive a lot of questions about whether the volcano is headed toward eruption. The swarm in June 2017 garnered the same reaction, so let's take a look back at what we learned from the previous swarm in 2010, about 20 km SSE of the June activity.
On January 17, 2010, a 3-week-long swarm took place on the Madison Plateau, just northwest of the Yellowstone Caldera. About 2500 earthquakes were located by the University of Utah, the group that runs the Yellowstone Seismic Network through a cooperative agreement with the USGS. Seismic networks can locate earthquakes by comparing the arrival times of seismic waves emanating from the earthquake location, or hypocenter. Very small earthquakes (< M1) cannot be detected on distant seismometers, which means they frequently cannot be located by traditional techniques. Even well-located earthquakes may have horizontal and depth uncertainties of more than 0.5 km.
Nevertheless, scientists can examine a specific timeframe of seismic data and use mathematical algorithms to detect and relocate the earthquakes – even ones that were not large enough to have been given an official location when the swarm was first analyzed. USGS seismologist David Shelly used such techniques to study the 2010 swarm. He and his colleagues were able to recognize 8710 events, including many small events with magnitude as low as -1. For reference, the energy released by a M -1 earthquake is similar to the kinetic energy of the impact when a large man (100 kg) jumps off a 2-m-high fence. That is, this allows us to look at really tiny events and to understand more about the relative location of all the earthquakes.
The research was published in 2013 in the Journal of Geophysical Research, co–authored with colleagues from the University of Utah. One important finding was that most of the earthquakes took place within a single inclined fault zone, beginning at ~10 km depth then migrated up, down and sideways over the three-week swarm. The authors concluded that pressurized fluids (probably hot and gassy water) triggered the rupture of a pre-existing crustal fault system. Quite possibly, release of fluid from deep magma causes pressurization, which can trigger slip on faults that are under stress.
With their 2013 paper, Shelly and his colleagues provided a time-lapse video animation that depicts how earthquakes migrated along the crustal fault during the 2010 swarm. Because the Yellowstone Caldera started to subside in 2010, after about six years of uplift, most scientists now agree that earthquake swarms can be a key contributor to uplift/subsidence cycles in the caldera (Figure 8 of Shelly et al., above).
The 2017 swarm is farther north, and would more likely be linked to pressurization around the Norris Geyser Basin, or even the Hebgen Lake Fault Zone north of West Yellowstone. Future studies will allow us to learn more about the details of the swarm, and its effect on the surrounding crust.
As with the other well-studied earthquake swarms near Yellowstone in the past 50 years, damage to facilities and infrastructure has been minimal, as is potential for related volcanic eruptions. Fortunately, these events do provide an incredible learning opportunity for earth scientists around the globe.