Earthquake swarm near Great Sand Dunes, Colorado, investigated with temporary seismic network and machine learning sesmic phase analysis

In November of 2018, residents living in the Zapata Subdivision south of Great Sand Dunes National Park and Preserve reported hearing and feeling multiple small earthquakes. Reports of additional earthquakes continued, escalating in late February of 2019, when the USGS recorded over 27 magnitude 0.9 and larger earthquakes over a two-day period. Subdivision residents became concerned that these could be foreshocks to a future, larger earthquake. To further study these earthquakes, we installed a temporary network of seismometers in the area during 2019 and used a convolution neural network seismic phase picker along with the GLASS3 associator to detect over 700 earthquakes in a 3.5-month period during the earthquake swarm. The earthquakes were located using a regional velocity model and a double-difference algorithm. The Northern Sangre de Cristo Fault (NSCF) cuts through the subdivision at the base of the Sangre de Cristo Mountains. Based on geologic evidence, it is one of the most active faults in Colorado but has been nearly aseismic historically. Initially, minor movement on the NSCF was suspected of being the geologic source of the earthquakes. However, nearly all recorded epicenters lie east of the trace of west-dipping fault and are not located on it. Instead, the earthquake epicenters define a narrow, linear, east-west-trending zone that projects eastward across the entire Northern Sangre de Cristo Range and into the headwaters of the Huerfano River Valley. We propose several possible geologic sources for the earthquakes including several mapped, but unnamed faults. Available evidence for any particular source in this geologically complex area is not conclusive. Additional geologic and geophysical investigations are needed to better understand the geology of the earthquake swarm and its implications for seismic hazards.