Effects of Normal Stress Reduction on Seismic Triggering

The injection of water during hydraulic fracturing leads to effective normal stress reduction on faults and may trigger earthquakes. Different injection loading schemes may lead to various slip behaviors in fault zones. In addition to direct pressurization (i.e., direct reduction of effective normal stress), cyclic pressurization has also been introduced. In this study, to reveal the underlying seismic triggering mechanisms during hydraulic fracturing, we employ double direct-shear tests to investigate the frictional behaviors of fault gouges under sine-shaped normal stress oscillation (NSO) and direct reduction of normal stress, conducted at a reference background normal stress of 40 MPa and constant shear stress (CSS) conditions. In all experiments, during the slip process, the fault slip velocity initially increases (slip-acceleration stage) and then decreases (slip-deceleration stage). NSO can significantly reduce the peak slip velocity of a fault compared to the direct reduction of normal stress. Faults sliding at a higher acceleration rate during the slip-acceleration stage also show a higher deceleration rate during the slip-deceleration stage. Repeating the NSO on the same fault under identical conditions causes a gradual decrease in its peak slip velocity, indicating permanent changes in the fault during slips. Various factors, including the compaction effect on healing and the rate of normal stress reduction, control the different slip behaviors triggered by NSO. Quantifying controlling factors in the field and optimizing NSO parameters can effectively reduce the potential of seismic activity, which is critical for safe hydraulic fracturing operations.