Yellowstone – the year 2017 in review

The publication of the first issue of Yellowstone Caldera Chronicles on New Year's Day of 2018 provides an excellent opportunity to review the activity that occurred in Yellowstone over the past year. And what a year it was!

2017 began calmly enough. In the first several months of the year, only a few hundred earthquakes were located by the University of Utah Seismograph Stations, which has the lead in monitoring seismic activity in the Yellowstone region. This trend continued the low earthquake rates of 2015 and 2016, during which only about 1,000 earthquakes were located per year (typically, between 1,000 and 3,000 earthquakes are located in any given year).

All of this changed on June 12, when the Maple Creek earthquake swarm began on the west side of the Park, a few miles north of West Yellowstone, Montana. Over the ensuing three months, about 2,400 earthquakes were located by the University of Utah Seismograph Stations (many more earthquakes occurred but were too small to be located). The swarm lasted until early September, and small bursts of seismicity occurred in the same area in late September and late October. The largest event in the sequence was an M4.4 event on June 16.

Additional swarms of earthquakes occurred outside the Yellowstone area, near Lincoln, Montana, and Soda Springs, Idaho. These swarms were each kicked off by widely felt M5+ earthquakes, but the seismicity is not related to the Yellowstone magmatic system. Instead, the earthquakes are caused by faulting associated with tectonic extension of the western United States.

Overall for the year, over 3,300 earthquakes, including about 20 that were felt, were located in the Yellowstone region, making 2017 one of the most seismically active years ever recorded. Almost 80% of all the earthquakes that were located occurred as part of approximately 13 swarms, of which the Maple Creek swarm was by far the largest. But was that swarm the largest ever recorded at Yellowstone?

In fact, the 2017 Maple Creek swarm comes in a distant second to that of 1985. In that year, an earthquake swarm also near West Yellowstone, Montana, lasted for 3 months and included over 3,000 located earthquakes, with the largest reaching M4.9. If today's monitoring system had been in place in 1985, it is likely that many more earthquakes would have been located. The 2010 Madison swarm, just southwest of West Yellowstone, is now the third largest swarm ever recorded, with about 2,300 located events.

Interestingly, the 1985 and 2010 swarms were associated with a change in deformation style of the caldera. During both years, the caldera switched from uplift to subsidence, suggesting that the swarms were associated with a release of fluids from the caldera region.

In 2017, however, there was no significant change in deformation at the time of the swarm. Throughout most of the year, the caldera subsided and the area around Norris Geyser Basin continued to uplift, as indicated by numerous continuous Global Positioning System stations in the region. Both caldera subsidence and Norris uplift have been ongoing since 2015. In early-December, however, that pattern began to change, with subsidence beginning at Norris. Whether this pattern will continue remains to be seen.

Instead of being related to fluid migration, it is also possible that the Maple Creek earthquake swarm is a lingering effect of the 1959 M7.3 Hebgen Lake earthquake—the largest earthquake ever recorded in the Yellowstone region. Future research will help to unravel this story.

We will be sure to update you on the results of our research in this column, so stay tuned for more information in the months to come. In the meantime, we hope you enjoy this recurring column, and we look forward to sharing interesting stories and new information every week.

The many members of the YVO consortium wish you and your loved ones a wonderful start to 2018. Happy New Year!