In Hot Dry Rock (HDR) or Hot Fractured Rock (HFR) geothermal energy development, micro-earthquake events were observed to evaluate the location and size of fracture distribution of artificially created reservoirs during hydraulic stimulation operations. Many micro-earthquake events were observed during hydraulic stimulation operations from depths of around 1,000 m to 4,200 m at three different sites: Ogachi and Hijiori in Japan and Cooper Basin in Australia. Comparison of the water injection operations to observed micro-earthquake hypocenter locations and the rate of events led to the observation of some differences and similarities. Seismic activities at the beginning of the water injection phase were low in Ogachi and Hijiori until the injected water volume reached that which was injected in a previous test, but high activity was observed from the onset of injection at the Cooper Basin site. This difference is thought to be caused by the injected water recovery from the fractures.

It was seen in Cooper Basin that the previously activated area was quiet in the successive stimulation operations. This is thought to be caused by stress release from previous water injection.

The water volume injected and the seismically activated area showed linear correlation. However, the inclination of the relationship is different and depends on the site. The seismically activated region at Cooper Basin extended much further than other sites. These differences are thought to be strongly dependent on regional natural joint systems.

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