Illuminating Faulting Complexity of the 2017 Yellowstone Maple Creek Earthquake Swarm

The 2017 Maple Creek earthquake swarm was one of the most prolific swarms to occur in the Yellowstone region in the past few decades, with nearly 2,500 routinely detected earthquakes up to M-w 4.4 between June and September 2017. To gain insight into the mechanics and underlying mechanisms of this swarm, we enhanced the seismic catalog, detecting and precisely locating nearly 16,000 earthquakes, while estimating magnitudes for more than 30,000 events. We further utilized the cross-correlation measurements derived from this processing to determine relative polarities for event pairs and to estimate focal mechanisms for the relocated population of events. The results reveal a complex network of faults activated progressively during the swarm, which may reflect diffusion of aqueous fluid pressure through these structures. Fluid-driven earthquake sequences may naturally generate more complex faulting geometries compared to earthquake sequences dominated by stress transfer. Plain Language Summary We examine an earthquake swarm that occurred in the Yellowstone region from June to September 2017. To gain insight into the swarm, we enhanced the routine seismic catalog to include many more very small earthquakes and to precisely locate all events. In total, we located nearly 16,000 earthquakes and estimated magnitudes for more than 30,000 events. We also used a recently developed technique to estimate the orientation of slip for these earthquakes. This analysis reveals a complex spatial and temporal evolution of faulting in the earthquake swarm, which we hypothesize is related to changes in fluid (water) pressure driving the swarm.