On the Depth of Earthquakes in the Delaware Basin: A Case Study along the Reeves–Pecos County Line

Abstract

Increased rates of seismicity in the Delaware basin, Texas, accompanying unconventional petroleum development have created intensive interest in determining their cause. Detailed and accurate spatial distribution of seismicity and focal mechanisms are critical components for understanding the underlying industrial processes responsible for inducing seismicity. We focus on a highly seismically active area straddling the Reeves–Pecos County line where two TexNet stations sit atop the seismicity, which includes 21 ML 3+ events from 2017 to 2020 (Advanced National Seismic System Comprehensive Earthquake Catalog). Short epicentral distance enables us to reliably estimate the hypocentral depth using seismic phase picks and standard location methods. We use a deep-learning-based method to detect earthquakes and time the phase arrivals. Hypocentral locations computed in a velocity model constrained by local well data reveal that the seismicity concentrates between 1.5 and 2.5 km below ground in the Delaware Mountain Group, the primary wastewater disposal zone at this location. Waveform inversions for the moment tensor and focal depth independently confirm the shallow depths. The moment tensor solutions define critically stressed high-angle normal faults, suggesting a causal connection between injection and seismicity.

Shallow Aseismic Slip in the Delaware Basin Determined by Sentinel‐1 InSAR

“…To date, the role of aseismic slip in induced seismicity has been largely limited to indirect inference and associated with hydraulic fracturing (Cornet et al, 1997;Eyre et al, 2019Eyre et al, , 2020Guglielmi et al, 2015;Zhu et al, 2020), so the implications of its occurrence in the Delaware Basin are challenging to know. Though Sheng et al (2022a) and our work suggest that wastewater disposal is likely inducing seismic and aseismic slip on normal faults in the DMG, it is unclear whether both are a direct consequence of the fluid injection, or whether aseismic slip triggers seismic events and/or vice versa. Based solely on our static 3D model, it is clear that the largest earthquakes along F1 and F3 do not coincide with the patches hosting the largest cumulative displacements (up to 25 cm), but rather are located around the periphery in patches with slip <12 cm.…”

“…In that work, they used a moment tensor analysis to determine focal mechanisms and depths for nine moderate events (M W > 2.7), and phase arrival times to determine the depths of smaller earthquakes. When considered together, our study and Sheng et al (2022a) suggest high-angle normal faults in the Delaware Mountain Group are activated by wastewater injection. We conclude with a discussion of the implications for the nature of induced seismicity in the greater Delaware Basin.…”

“…All of the moment tensor solutions are consistent with normal faulting on high-angle planes striking northwest-southeast, with the dip direction split almost evenly between northeast and southwest dips (Table 1). Sheng et al (2022a) also found no spatiotemporal correlation between fracked wells and the earthquakes, suggesting that they were not induced by hydraulic fracking; rather, they need to be explained by another driving mechanism, such as wastewater disposal, oil and gas production, or perhaps a combination of the two.…”

“…After selecting the number of faults (N) to include in the modeling, we manually select the endpoints of the top edge of each, thus defining their locations in the 3D space. For simplicity, we then select and assign identical d t , d b , and 𝐴𝐴 𝐴𝐴 to each fault plane, but permit the strikes (as determined by the endpoints) and dip direction to vary on each, noting that we do not allow significant deviations ( 𝐴𝐴 ± 10 trends created by the moment tensor solutions from Sheng et al (2022a). We then solve for the magnitude of uniform slip on each fault plane using the relationship in Equation 4:…”

“…After motivating the consideration of fault slip, we determine the geometry and slip of potential faults using analytic modeling of both vertical and east-west horizontal displacements, focusing on a small study area along the border of Reeves and Pecos counties (see Figure 1b). We compare our results to an improved seismic analysis in the same study area, which is presented in a companion paper by Sheng et al (2022a). In that work, they used a moment tensor analysis to determine focal mechanisms and depths for nine moderate events (M W > 2.7), and phase arrival times to determine the depths of smaller earthquakes.…”