| Summary: | In 2000 and 2003, two massive hydraulic fracturing experiments were carried out at the European Geothermal Hot
Dry Rock site at Soultz-sous-Forêts, France. The objective was to create a dense network of enhanced permeability
fractures, which would form the heat exchanger. The injection of water in the fractured rock generated a high level
of microseismic activity: around 30,000 and 90,000 micro-earthquakes were triggered during the injection of 2000
and 2003 respectively. From this around 14,000 and 9,000 events were then located to characterize the extent of the
stimulated zones and hence of the fracture network. Then, the source parameters of each event, like seismic
moments and stress drops, were computed automatically to characterize the mechanics of the fracturing. We found
for example that the total seismic moment released is proportional to the injected fluid volume. This suggests that
the injection flow rate could be a means to control the earthquake strength released during the stimulation and
perhaps also control the effectiveness of the stimulation. Finally, we performed a multiplet analysis of a subset of
these data to identify microearthquakes having similar waveforms. Multiplets are considered to be microearthquakes
that occur on the same fracture plane and therefore may represent either seismically activated structures and/or
permeable fractures induced by hydraulic fracturing. We identified 350 multiplets among 1000 analyzed events. We
relocated them precisely by cross-spectrum analysis and found that they belong to sub-horizontal structures, likely
permeable fractures stimulated by the injection.
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