Slowly migrating fracture swarms in an actively serpentinizing borehole

Peridotite rocks are primary targets for engineered geological carbon sequestration efforts because they accommodate transfer carbon from aqueous fluids to rock during alteration reactions. Sequestration efforts must necessarily open fractures in the rocks surrounding a pumped borehole, but the current understanding of fracture growth during serpentinization of peridotite is limited to theoretical models and laboratory experiments on small samples. We deployed hydrophone arrays in peridotite boreholes established by the Oman Drilling Program and detected downward migrating earthquake swarms that represent the first field observations of active fracture growth in a serpentinizing rock. More than two years after the boreholes were established, we detected four fracture swarms during an interval of elevated pore pressure following large rain events. All of the swarms occurred within a partially-confined section of the local aquifer, beginning at a depth of ~170 m and migrating to the bottom of the 400 m-deep hole at average rates of~6-20 cm.s^-1. Pore fluid processes can explain both triggering of the fracture swarms and their slow migration rates, which are characteristic of slow earthquakes, and water-rock reactions likely play a role in maintaining near-critical stresses at the crack tips as fractures grow away from the borehole. Our results indicate that fractures propagating away from actively serpentinizing boreholes maintain near-critical crack tip stresses such that relatively small increases in fluid pressure can trigger tensile fracturing episodes, and that pore fluid processes can limit the propagation speed of these tensile fractures in much the same way as they do for shear fractures.