Complex Shear-Wave Anisotropy from Induced Earthquakes in West Texas

Abstract

We have analyzed shear-wave splitting (SWS) data from local earthquakes in the Permian basin in west Texas to understand crustal stress change and induced seismicity. Two SWS parameters, the fast polarization direction and the delay time, are computed using a semiautomatic algorithm. Most measurements are determined in the Delaware basin and the Snyder area. In both regions, SWS fast directions are mostly consistent with local SHmax at stations that are relatively far from the earthquake clusters. Varying fast directions at one station are related to different ray paths and are probably caused by heterogeneity. In the Snyder area, most northeast–southwest fast directions are from the events in the northern part of the cluster, whereas the northwest–southeast fast directions are mostly from the southern part. The northeast–southwest and northwest–southeast fast directions could be attributed to the northeast-trending normal faults and the northwest-trending strike-slip faults, respectively. SWS results in the Delaware basin have two unique features. First, most shallow earthquakes less than 4 km deep produce relatively large delay times. This observation implies that the upper crust of the Delaware basin is highly fractured, as indicated by the increasing number of induced earthquakes. Second, diverse fast directions are observed at the stations in the high-seismicity region, likely caused by the presence of multiple sets of cracks with different orientations. This situation is possible in the crust with high pore pressure, which is expected in the Delaware basin due to extensive wastewater injection and hydraulic fracturing. We propose that the diversity of SWS fast directions could be a typical phenomenon in regions with a high rate of induced seismicity.

A Numerical Study of Elastic Wave Arrival Behavior in a Naturally Fractured Rock Based on a Combined Displacement Discontinuity-Discrete Fracture Network Model

“…Elastic waves in natural fracture systems involving systematic fracture sets and variable fracture lengths may exhibit much more complicated wavefield phenomena compared to those in idealized fracture networks. A typical example is the velocity anisotropy in the crust (Liu and Crampin 1990 ; Crampin and Lovell 1991 ; Liu et al 2004 ; Fontaine et al 2009 ; Al-Harrasi et al 2011 ; Frietsch et al 2015 ; Robinson et al 2020 ) intrinsically caused by the complicated spatial distribution of fractures (Kohler et al 2003 ; Matonti et al 2017 ; Zhang et al 2020 ), which often results in a highly variable arrival behaviors of seismic waves (Majer et al 1988 ; Spetzler and Snieder 2001 ; Santos et al 2015 ; Shao and Pyrak-Nolte 2016 ; Xu et al 2018 ; Najdahmadi et al 2018 ). Studying the linkage between the wave arrival behavior and the fracture network distribution can be useful for characterizing crustal heterogeneities in many rock engineering applications.…”

Multivariate Statistical Appraisal of Regional Susceptibility to Induced Seismicity: Application to the Permian Basin, SW United States

“…Recent emergent seismicity in the western Permian Basin, Texas (e.g., Frohlich et al., 2020; Robinson et al., 2020; Savvaidis et al., 2020; Skoumal & Trugman, 2021; Skoumal et al., 2020; Staniewicz et al., 2020; Trugman & Savvaidis, 2021) provides a new opportunity to understand the fundamental drivers of induced seismicity in this region, and whether these factors are similar to, or differ from, other regions of extensive induced seismicity, such as in Oklahoma. Seismicity is very spatially clustered in the western Permian Basin (Figure 1a).…”