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.