Locating microearthquakes induced by hydraulic fracturing in crystalline rock



Microearthquakes induced by hydraulic fracturing in crystalline rock at a depth of 3.5 km were located with a precision of better than 30 m to obtain information about the geometry and dimensions of the fracture system produced. The induced microseismicity was monitored by a network of five borehole seismic stations; a total of about 800 induced events were reliably located from arrival times. Event locations show a tabular distribution that strikes 350° and dips 65° east, subparallel to the injection well. The injection was intended to produce a fracture system that would hydraulically connect two subparallel wells. A lack of fluid communication between them is consistent with a lack of induced microearthquakes near the target wellbore. The 150 m thickness of the zone of seismicity far exceeds the relative locational uncertainties. The injected fluid appears to have stimulated a zone of rock, rather than simply a single fracture. The distribution of microseismic events presumably envelops the zone of fluid paths created by the fracture experiment.