Discrimination between natural and induced seismicity
An ability of differentiation of natural and induced seismicity is rather difficult but extremely important.
Seismic events observed in the vicinity of oil and gas operations can be induced, triggered, or natural. The currently used criteria, for distinguishing the natural and triggered seismicity, developed in the last century, are based on a subjective comparison of injection activities and seismic rates. For example, a direct cross-correlation of an earthquake and injection rates is intrinsically positive-valued because both, injection and earthquake rates, are positive. We objectively measure a potential connection between the earthquake and the injection rates by a modified correlation approach. Applied to multiple case studies of clear-cut cases of induced seismicity, our method gives high cross-correlation values dominated by a positive maximum delayed typically by 12-24 hours. This delay corresponds to the seismicity following the injection. On the other hand, our method exhibits no correlation for natural seismicity cases or for a randomly simulated seismicity and injection. In such cases the result of this method provides correlation values close to zero or oscillates between positive and negative values, typically between +0.5 and -0.5.
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