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

Abstract:

Increasing rates of seismicity in the Delaware Basin, Texas, 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 physical processes. In this study, we focus on earthquake focal depths to help constrain mechanisms for seismic genesis. We focus on a study area straddling the Reeves/Pecos county line along highway 258, where TexNet seismic stations PB04 and PB16 are atop the seismicity which includes several M 3+ events and numerous smaller ones. We analyze earthquakes from August 2019 onward, during which time no wells were fracked in the study area according to FracFocus. Short epicentral distance enables us to reliably estimate the hypocentral depth using seismic phase picks and standard location methods. We use deep-learning-based softwares, PhaseNet (Zhu and Beroza, 2018) and EqTransformer (Mousavi et al., 2020) to detect earthquakes and time the phase arrivals for both P- and S-waves. The resolved hypocentral locations reveal that the seismicity concentrates between 1.5 and 3.5 km below ground. Refraction of P-waves at distances of several kilometers along a shallow salt layer complicates both manual and automatic phase picking. Consequently, we also invert the waveforms for the moment tensor and focal depth to independently determine hypocentral depths for the M 3+ events. Results using gCAP (Zhu and Ben-Zion, 2013) confirm that these earthquakes are indeed shallow. We finally relocate the events using hypoDD, anchored to the larger earthquakes whose hypocenters are reliably determined. Our results reveal north-northwest trending, high-angle normal faults with the earthquakes located below the base of the Ochoa salt at 1.5 km to as deep as the production horizons of the Wolfcamp formation at 3+ km.