Development of a Combined Detection and Relocation Workflow (TM2DD) to Characterize Escalating Hydraulic Fracturing Induced Seismicity in the Karnes Trough Fault Zone of the Eagle Ford Shale Play, Texas

James Kirchenwitz , Michael Brudzinski , Mehrnaz Khalkhali, Kieana Mazzio

On January 30, 2025, an M4.7 earthquake shook the Eagle Ford Shale, Texas, the region's second M4 event in under eighteen months. Unlike much of the central United States, where induced seismicity stems from wastewater disposal, most of the earthquakes in the Eagle Ford are linked to hydraulic fracturing (HF) (Fasola et al., 2019; Fasola & Brudzinski, 2023). If HF-induced, the 2025 event would be the largest such earthquake in the United States, amid seismicity increasing in both magnitude and frequency. The seismicity appears to be associated with mapped normal faults in the Karnes Trough Fault Zone (KTFZ), but its detailed pattern and relationship to HF operations remain poorly characterized. This study leverages ~5 years of continuous TexNet recordings (~25 km station spacing) to better detect and locate this seismicity. We have developed a workflow (TM2DD) that integrates multistation template matching with automated per-template correlation thresholds, refines phase arrivals via template-detection cross correlation, and applies double-difference relocation using a local 1-D velocity model. This expands the detected catalog by 17.5-fold, with relocations revealing that the 2023 M4.0 and 2025 M4.7 sequences ruptured adjacent fault segments with 15-20 degree differences in dip angle. In addition, the larger event occurred at the deeper extent of the seismically active zone. All detected sequences correlate tightly in space and time with active HF wells within 3.5 km, with the deeper 2025 event showcasing mainshock - aftershock behaviour, whereas the shallower 2023 sequence is swarm-like. These results indicate that HF-induced seismicity in the KTFZ can propagate along adjacent, connected fault systems as nearby operations continue, with deeper segments potentially hosting larger, mainshock-dominated ruptures.