Slow slip events and megathrust coupling changes contribute to the earthquake potential in Oaxaca, Mexico

Stress accumulation on the plate interface of subduction zones is a key parameter that controls the location, timing and rupture characteristics of earthquakes. The diversity of slip processes occurring in the megathrust indicates that stress is highly variable in space and time. Based on GNSS and InSAR data, we study the evolution of the interplate slip-rate along the Oaxaca subduction zone, Mexico, from October 2016 through October 2020, with particular emphasis on the pre-seismic, coseismic and post-seismic phases associated with the June 23, 2020 Mw 7.4 Huatulco earthquake (also known as La Crucecita earthquake), to understand how different slip regimes contribute to the stress accumulation in the region. Our results show that continuous changes in both the aseismic stress-releasing slip and the coupling produced a high stress concentration prior to the event on the region with the highest moment release of the Huatulco earthquake and a stress deficit zone in the adjacent updip region . This region under negative stress accumulation can be explained by possible recurrent shallow Slow Slip Events (SSE) offshore Huatulco as well as by the stress shadow from adjacent locked segments. Two months prior to the event, a Mw 6.6 long-term SSE also occurred about 80 km northwest from the hypocenter, between 25 and 55 km depth. Transient increments of the interplate coupling around the adjacent 1978 (Mw 7.8) Puerto Escondido rupture zone correlate with the occurrence of the last three SSEs in Oaxaca far downdip of this zone,  possibly associated with along-dip fluid diffusion at the subduction interface. Throughout the four-year period analyzed, the interface region of the 1978 event experienced a high CFS build up, primarily attributable to both the co-seismic and early post-seismic slip of the Huatulco rupture, that, considering the 55 year average return period of the region, indicates large earthquake potential near Puerto Escondido. Continuous monitoring of the interplate slip-rate thus provides a better estimation of the stress accumulation in seismogenic regions than those given by long-term, time-invariant coupling models, and improves our understanding of the megathrust mechanics where future earthquakes are likely to occur.