We relocate 303 microseismic events that were recorded in the North Sea Valhall oilfield in 1998. A semiautomated re-picking of P-wave and S-wave arrival times and array based P-wave polarisation analysis improves the accuracy of the locations. Our multiplet analysis identifies 40 multiplet groups that include 208 events. Using cross correlation we then further increase the relative location accuracy between multiplet members. The P-waveform similarities between the different multiplets indicate that they are generated by a small number of faults. We fit two planes through the sources from two clusters and find a near-perfect match with fault planes that were interpreted on 3D surface seismic. Differences between the multiplets are primarily attributed to differences in S-phases and P-to-S amplitude ratios. We also find that activity within certain multiplet groups correlates with measurement of different 'regimes' of percent anisotropy and fast S-wave polarisations as obtained from S-wave splitting results. This can be explained in the context of stress build-up as a result of oil production, followed by dissipation of those stresses through microseismic activity. The cyclic nature of this mechanism drives the short term variations in seismic anisotropy and the reactivation of microseismic source mechanisms.
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