Group , Poster #141, Fault and Rupture Mechanics (FARM)

Role of background stress state in fluid induced aseismic slip and dynamic rupture on a 3 meter laboratory fault

Sara B. Cebry, Chun-Yu Ke, & Gregory C. McLaskey
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Poster Presentation

2021 SCEC Annual Meeting, Poster #141, SCEC Contribution #11313 VIEW PDF
Fluid injection for oil and gas operations, CO2 sequestration, or enhanced geothermal systems (EGS) stimulates seismicity far from active tectonic regions however, the details of how propagating fluid causes stress changes and potentially triggers earthquakes remain poorly understood. We conducted laboratory experiments using a biaxial loading apparatus to measure the induced slip and stress changes along a 3 m saw-cut granite fault during direct fluid injection. Slip and local strain were measured using 16 eddy current displacement sensors and 16 semiconductor strain gauge pairs spaced along the entire fault. Pressure sensors were used to measure sample-average shear and normal stress and i...njection well fluid pressure. We compared events induced with different initial shear stress levels while fluid injection rate and normal stress were held constant at 10 mL/min and 4 MPa, respectively. In all experiments, aseismic slip initiated near the location of fluid injection, and dynamic fault rupture eventually initiated from within the aseismic slipping patch. When the fault was near critically stressed, seismic slip initiated only seconds after MPa-level injection pressures were reached. When the initial fault stress was at a lower level, dynamic rupture initiated hundreds of seconds later, after aseismic slip had significantly redistributed stress from within the pressurized region to neighboring locked patches. We find that slow fault slip initiates based on local stresses exceeding Coulomb failure criteria, but dynamic rupture propagation is based not just on local stress levels, but also on the along-fault distribution of overstress (initial stress above the residual fault strength levels). Fluid-induced aseismic slip is an effective means of redistributing the stress, increasing the stress on locked portions of the fault, and preparing the fault for dynamic rupture.
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