In southwestern Colorado, USA, the USA Bureau of Reclamation high-pressure injects waste water at between ~800 and ~1,300 l/min, 4.3 to 4.8 km below the surface. The injection has induced more than 4,400, surface-recorded seismic events (i.e., magnitude ≥ M0). The event locations group into two distinct zones: a principle zone (>95% of the events) asymmetrically surrounding the injection well to a maximum distance of ~3+ km and a secondary, ellipsoidal zone, displaced ~8 km northwest of the injection well. Within the principal zone, the events align into groups distinctly showing at-depth stratigraphy and the local fracture and fault system. The fractures mapped by seismic locations align with the fault planes determined from the seismic waveforms. The major faults of the system are aseismic, running parallel to the principal stress direction (determine from seismic waveforms), which agrees with the direction between the secondary seismic zone and the injection well. This shows that the main faults act as long, fluid conduits from injection well to the displaced secondary seismic zone. Since 1996, when continuous injection began, Reclamation has altered injection 3 times to control the seismicity while maintaining acceptable brine injection volume.


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