A Numerical Investigation of the 2020 M4.2 Stanton, Texas Seismicity Sequence Using 3D Poroelastic Modeling
Authors: Jin L. , Lippoldt R. , Curry W. J. , Hussenoeder S. , Bhargava P. 
Published: October 2022

ABSTRACT:
This paper investigates a seismicity sequence that occurred near Stanton, TX between December 2020 and February 2021 with event magnitudes peaking at 4.2. The historically aseismic area has been undergoing shallow salt water disposal (SWD) since the early 1980s, and deep SWD in the last decade. We perform a 3D fully coupled hydro-geomechanical investigation into potential links between the M4.2 event sequence and the surrounding decades-long, multi-zone SWD activities involving 183 SWD wells in a 900 mile2 area. We include faults interpreted from reflection seismic data into the model and survey six fault scenarios with various fault-zone structures and fault upper extents. We compare the modeled Coulomb stress changes against earthquake triggering thresholds obtained from a detailed in-situ stress model, and identify implausible and plausible scenarios. The plausible scenarios yield an excellent match between the critical Coulomb failure function (CFF) propagation and the onset of seismicity in both space and time, and are further supported by their revealing of possible earlier events that were later detected through template matching. Together, these findings strongly suggest a SWD origin of the M4.2 seismicity sequence. The modeling also suggests that near-hypocenter Coulomb stresses are sourced primarily from several deep disposal wells and secondarily from the remaining deep wells and shallower wells given the likely fault configurations. We also analyze roles of fault-zone structures, non-seismogenic faults, and mechanically time-dependent formations. Our work illustrates the importance of physics-based modeling that accounts for faults, formations, wells, and poroelasticity in understanding causes of seismicity.

1. INTRODUCTION
Fluid injection, including salt water disposal (SWD) into the subsurface, has long been recognized as capable of inducing earthquakes (Wesso & Nicholson, 1987; Ellsworth, 2013; National Research Council, 2013). There has been an increase in seismicity associated with SWD activities within the United States in the last decade (e.g., Frohlich et al., 2011; Kim 2013; Walsh & Zoback, 2015), and most recently in the Permian basin (e.g., Skoumal & Trugman, 2021). An earlier study proposed empirical criteria for determining whether earthquakes are induced based on their spatial-temporal correlations with injection activities and deviation from regional background seismicity (Davis & Frohlich, 1993). This correlative approach has remained popular in studying origins of induced seismicity, and has been complemented with additional steps, such as earthquake relocation and focal mechanism determination (Kim 2013), analysis of seismicity lineations relative to locally mapped faults (Frohlich et al., 2016), examination of data such as Vp-to-Vs ratio time series (Improta et al., 2015), and space-time clustering analysis (Savvaidis et al., 2020). Some studies employ pore pressure diffusion modeling to further support correlative analysis (e.g., Keranen et al., 2013; Peterie et al., 2018; Chen et al., 2018).