Monitoring of induced seismicity during the first geothermal reservoir stimulation at Paralana, Australia

In order to understand the development of a fracture network generated during the first large-scale hydraulic stimulation at Paralana, South Australia, we analysed more than 7000 induced microearthquakes. In July 2011, about 3 million litres of water were injected in the Paralana 2 well to create a geothermal reservoir. A 3-D velocity model was built from seismic reflection data and used for absolute location of the events, which cluster at the base of the injection well. Hypocentre relocations were determined by inverting travel-time differences, improved by waveform cross-correlation. The geometry of the seismic cloud and the associated seismic moment vary during the injection experiment. Relocated microearthquakes outline NNE-SSW and ENE-WSW preexisting structures. The main part of the seismic moment is released during stimulation and is dominated by three Mw 2.4 events and one Mw 2.5 event. The largest event was associated with right-lateral reverse faulting on a plane striking N82°E and dipping 39°N.