Raton Basin Induced Seismicity Is Hosted by Networks of Short Basement Faults and Mimics Tectonic Earthquake Statistics.

The Raton Basin has been an area of injection induced seismicity for the past two decades. Previously, the reactivated fault zone structures and spatiotemporal response of seismicity to evolving injection have been poorly constrained due to sparse publicly available seismic monitoring. The application of a machine‐learning phase picker to 4 years of continuous seismic data from a local array enables the detection and location of ∼38,000 earthquakes. The events from 2016 to 2020 are ∼2.5–6 km below sea level and range from ML < −1 to 4.2. Most earthquakes occur within previously identified ∼N‐S zones of seismicity, however our new catalog illuminates that these zones are composed of many short faults with variable orientations. The two most active zones, the Vermejo Park and Tercio zones, are potentially linked by small intermediate faults. In total, we find ∼60 short (<3 km long) basement faults with strikes from WNW to NNE. Faulting mechanisms are predominantly normal but some variability, including reverse dip‐slip and oblique‐slip, is observed. The Trinidad fault zone, which previously hosted a Mw 5.3 earthquake in 2011, is quiescent during 2016–2020, likely in response to both slow accumulation of tectonic strain after the 2011 sequence, and the significant decrease (80% reduction) in nearby wastewater injection from 2012 to 2016. Unlike some other regions, where induced seismicity was triggered in response to higher injection rates, the Raton Basin's frequency‐magnitude and spatiotemporal statistics are not distinguishable from tectonic seismicity. The similarity suggests that seismicity in the Raton Basin is predominantly releasing tectonic stress. Plain Language Summary: The Raton Basin, located on the Colorado‐New Mexico border, has a low level of historical seismicity but experienced a significant increase of earthquakes in the past two decades, coinciding with increased wastewater injection and coal‐bed methane production. We use four years of continuous local seismic data to detect ∼38,000 earthquakes from 2016 to 2020. We find that the previously identified 10–20 km long zones of seismicity are composed of many shorter faults, <3 km long, with variable orientations and slip mechanisms. We observed very little seismicity in the zone that previously hosted the largest earthquake (Mw 5.3 in 2011) in the basin and attribute this finding to both the slow accumulation of tectonic strain following the 2011 sequence and the significant decrease (80% reduction) of wastewater injection to this zone from 2012 to 2016. Overall, the frequency‐magnitude and spatial‐temporal earthquake behavior of the Raton Basin is not distinguishable from tectonic seismicity. This statistical similarity suggests Raton Basin seismicity predominantly releases tectonic stress accumulated over geologic time. Key Points: Seismicity is hosted by short, <3 km long, reactivated basement faults with variable strikes and faulting stylesThe fault zone that hosted a Mw 5.3 event in 2011 exhibits low seismicity following an 80% reduction of the wastewater‐injection rateSimilarity of spatiotemporal‐magnitude basin statistics to tectonic sequences suggests earthquakes mainly release stored tectonic stress [ABSTRACT FROM AUTHOR]