Mapping Stress and Fluids on Faults by Nonshear Earthquakes

Fluids on faults facilitate triggering of earthquakes and affect seismic energy release. Their detection in the focal zone is, however, difficult. Here, we present mapping of stress changes and fluid flow based on determining seismic moment tensors (MTs), which describe focal mechanisms and shear/tensile/compressive fracturing modes of earthquake sources. We calculate highly accurate MTs of 4,500 microearthquakes that occurred in the West Bohemia swarm region in the period of 2008–2018. The extent and quality of the moment tensor data are unique and allow us to study detailed behavior of the nondouble‐couple (nonDC) components of MTs along the fault. The nonDC components were mostly zero or negative and indicated shear/compressive fracturing. However, also a patch with positive nonDC components, indicating tensile fracturing, was revealed. The inversion for stress and modeling of the Coulomb stress change confirmed that this patch is under anomalous stress conditions. Tensile fractures in the patch were opened due to interaction of tensile fault steps and filled by overpressurized fluids. Hence, evaluating nonDC components of earthquakes might help for tracing stress variations and fault interactions, identifying pathways of fluids and locating areas with overpressure prone to being activated. Key Points Nondouble‐couple moment tensors (MTs) show patches of compressive/tensile fracturing along the fault Tensile fracturing detected by MTs is produced by interaction of tensile fault steps Tensile patches on the fault serve as pathways for fluid escapes from the focal zone