Cascading multi-segment rupture in an injection-induced earthquake sequence with a Mw 5.3 mainshock

Rupture segmentation and propagating earthquake sequences are observed in major fault systems, however, the scalability of these processes to smaller fault systems is unclear. We investigated a 2011 earthquake sequence with a moderate magnitude mainshock (Mw 5.3) in the Trinidad zone, located in the Raton Basin coalbed methane field on the Colorado-New Mexico border, USA. The complexity of the 2011 earthquake sequence and broader evolution of seismicity from 2008-2022 was captured using machine-learning to build an initial earthquake catalog with an aftershock array followed by automated template detection. The 2011 Mw 5.3 mainshock was likely the culmination of cascading failure with slip across multiple well-separated segments. The majority (92%) of aftershock waveforms clustered into seven distinct groups, which are interpreted to result from slip asperities along multiple faults. The three southern clusters hosted foreshocks in the days to minutes prior to the mainshock, whereas the four northern clusters were inactive over the same period. The mainshock rupture initiated in the foreshock footprint and moved northeastward across the three southern clusters which are interpreted to occur on a NNE-SSW trending normal fault. The maximum slip patch from a previous geodetic inversion aligns with the third cluster along the rupture path from south to north. The ∼5.5-6 km rupture length across the three southern clusters is consistent with empirical scaling of rupture length (5-5.5 km) for a Mw 5.3 earthquake. Within an hour after the mainshock, seismicity was triggered on the four previously quiescent northern segments which appear to form a N-S oriented normal fault. The northern clusters filled a seismic gap between the two major earthquake sequences of the Trinidad zone, the 2011 Mw 5.3 and the 2001 MbLg 4.5 mainshock earthquake sequences. Seismicity in the Trinidad zone diminished from 2012-2016, and the earthquake rate from 2016-2020 is two orders of magnitude lower than that of the neighboring Tercio and Vermejo Park zones. Such complex earthquake sequences present a challenge for accurate hazard assessment in fluid-injection settings where fault geometry is often unresolved until seismic reactivation and the strength of the crust can be locally modified by fluid-injection.