Fluid‐induced earthquakes with variable stress drop
Résumé
The static stress drop of an earthquake, which quantifies the ratio of seismic slip to the size of the rupture, is almost constant over several orders of magnitudes. Although variations are often observed, it is difficult, however, to attribute these variations either to a well-defined phenomenon or simply to measurement uncertainty. In this study we analyze the static stress drop of earthquakes that occurred during a water circulation test in the Soultz-sous-Forêts, France, geothermal reservoir in 2010. During this circulation test, 411 earthquakes were recorded, the largest event having a magnitude MD2.3. We show that several earthquakes in the reservoir can be combined into groups of closely located similar repeating waveforms. We infer that the amplitudes, and hence magnitudes, vary between the repeaters although the waveforms and spectra are both similar in shape. We measure similar corner frequencies for these events despite their different magnitudes, suggesting a similar rupture size. Our results imply that events at the same location may exhibit stress drop variations by as much as a factor of 300. We interpret that this variation in stress drop is caused by fluid pressure at the interface reducing the normal stress. We also hypothesize that the observed variations reflect a transition from stable to unstable slip on the imaged asperities.
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