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Abstract

Properties of small earthquakes induced by fluid injection reflect the spatiotemporal evolution of important reservoir parameters, including the local stress field, pore space, rock permeability, fluid overpressure, and distributions of pre‐existing fractures. Resolving detailed earthquake mechanisms and their uncertainties is key to the physical interpretation of source processes. Here, we report moment tensor (MT) patterns in response to two enhanced geothermal system stimulations through ∼6‐km‐deep boreholes in the crystalline basement of the Fennoscandian Shield. A local monitoring network of 32 surface and borehole stations provided high‐signal‐to‐noise ratio seismograms of the body‐wave arrivals in the 5–10 Hz target range. Using a probabilistic waveform‐fitting method with carefully selected data and manually revised inversion parameters, we obtain robust centroid MT solutions for 301 induced earthquakes with moment magnitudes between 0.3 and 2.0. Most events exhibit reverse‐faulting double‐couple (DC) mechanisms that are not compatible with extrapolations from regional stress modeling. Our analysis resolves spatially variable distributions of the compensated linear vector dipole (CLVD) component around the injection well, which reflects the governing effect of the injection pressure. The spatial variation of the volumetric or ISO component suggests an ambient pore pressure or a geological control. Together, the CLVD and ISO components resolve a change in earthquake properties between the 2018 and 2020 injection experiments. We propose a source process that involves the interaction between a pressurized hydraulic fracture and surrounding shear fractures to explain coseismic compression and dilatation that are associated with the DC and CLVD components. Resolving the behavior of fluid‐induced earthquakes in crystalline rock at this level of detail demonstrates the effectiveness of the employed processing methods and helps improve our understanding of hydromechanical feedback mechanisms in geothermal systems.