The 2017-2018 Maple Creek Earthquake Sequence in Yellowstone National Park, USA

We explore the detailed spatiotemporal evolution of 3,345 earthquakes that occurred near Maple Creek, Yellowstone, for the time period of 12 June 2017 to 13 March 2018. We generate high-accuracy relocations and near source V-p/V-s ratios using 4.4 million P wave and S wave differential travel times derived from waveform cross correlation. The hypocenters can be subdivided geographically into two major subpopulations: a northern cluster with planar structures striking mainly NW-SE and a southern cluster with planar structures striking mainly E-W. We observe V-p/V-s ratios of 1.39-1.66 in the northern cluster and a steady ratio of 1.50 in the southern cluster, suggesting the presence of CO2-filled cracks. We interpret the northern earthquake cluster primarily as long-lived aftershocks of the 1959 M-w 7.2 Hebgen Lake earthquake but with some influence of magmatic fluids. We interpret the southern earthquake cluster as a more classic, swarm-like sequence induced primarily by the migration of magmatic fluids. Plain Language Summary Seismicity in the Yellowstone volcanic region dominantly consists of bursts of earthquakes, often referred to as swarms. The three largest swarms in modern times-in 1985, 2008-2009, and 2010-were triggered by magmatic fluid flow. A fourth major burst of Yellowstone earthquakes occurred in 2017-2018 near Maple Creek and consisted of over 3,000 events. We determined accurate locations for these earthquakes and measured the ratio of P to S velocity in the source region. We find that the 2017-2018 Maple Creek sequence differs from the three previous large swarms in that many of the earthquakes can best be categorized as late aftershocks of the 1959 M-w 7.2 Hebgen Lake earthquake.