Earthquake Swarm S20200219.1 in the Yellowstone Region

Seismic swarm S20200219.1 was recorded between 06:53 on 18 February 2020 and 02:58 on 21 February 2020, approximately 26 km south-southwest of Mammoth, Wyoming. Over 68 hours and 4 minutes, the sequence comprised 62 earthquakes, with magnitudes ranging from -0.3 to 2.0 and focal depths between 0 and 9 km. The majority of events clustered between 2 and 5 km depth, consistent with shallow crustal processes in the area.

The swarm initiated with a magnitude 0.6 event at 9 km depth and featured several small clusters of activity, including a notable sequence on the evening of 18 February with multiple events near 4 km depth. Peak magnitudes occurred on 19 and 20 February, including events of 1.9, 2.0, and 1.7. Activity tapered off by the early hours of 21 February.

This swarm occurred within the Yellowstone volcanic system, a large caldera formed by massive eruptions over the past 2.1 million years. The most recent caldera-forming event took place approximately 631,000 years ago. Ongoing magmatic and hydrothermal processes drive frequent seismicity, with earthquake swarms representing a common expression of fluid migration and stress changes in the brittle crust above the magma reservoir.

Since 1 January 2000, 77 swarms have been documented in the region. Annual counts show variability, with elevated activity in 2000 (10 swarms), 2008 (7), 2013 (7), and 2014 (7). Lower rates occurred in several years, including single swarms in 2004, 2005, 2010, 2011, and 2016. This long-term pattern underscores the persistent, episodic nature of seismicity tied to the underlying volcanic and hydrothermal framework.

Swarm sequences in Yellowstone typically involve low-magnitude events and lack a dominant mainshock-aftershock pattern. Depths recorded in S20200219.1 align with the shallow seismogenic zone influenced by hydrothermal circulation and minor crustal adjustments. Such activity contributes to monitoring efforts that track potential changes in the magmatic system.

References

• U.S. Geological Survey, Yellowstone Volcano Observatory reports on regional seismicity.
• Yellowstone National Park geological summaries on caldera history and hydrothermal systems.