Microseismic monitoring during a massive hydraulic injection experiment was performed in the Yufutsu gas field, JAPAN in May 2005. The reservoir is naturally fractured granite and overlaying conglomerate located at around 4 km depth. The monitoring network consisted of four deep stations and three shallow stations. 35 mutiplet groups were extracted from 3700 of microseismic events observed by the deep stations. We relocated these multiplets using the precise phase picking and the pseudo 3D velocity model. Location uncertainties of less than 20 m were small enough to delineate the fracture systems and the two dominant strikes of N-S and NE-SW were interpreted from the principal directions and the locations of the multiplet clusters. Furthermore, the focal mechanisms of the representative events of the every multiplet groups were constrained by the grid search technique performed on the observed P-wave polarity and S-wave polarization. The N-S and ENE-WSW dominant fracture systems are also estimated by the mechanisms. These dominant fracture orientations are consistent with the most likely orientations of shear slippages in the stress condition around the injection well, which is a strike-slip stress regime whose maximum horizontal stress axis is in NNE-SSW direction.


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