Focal mechanism determination for induced seismicity using the neighbourhood algorithm

Abstract

Induced seismicity is widely detected during hydraulic fracture stimulation and oil/gas development. To better understand how seismic events are induced, a thorough knowledge of the source mechanism is required. In this study, we have developed a new method to determine the focal mechanism for induced seismicity. Three misfit functions are used in our method to measure the differences between observed and modelled data from different aspects, including the waveform, P-wave polarity and S/P amplitude ratio. We minimize these misfit functions simultaneously using the neighbourhood algorithm. Through synthetic data tests, we show the ability of our method to yield reliable focal mechanism solutions and study the effect of velocity inaccuracy and location error on the solutions. To mitigate the impact of the source location uncertainties, we develop a joint inversion method to find the optimal source depth and focal mechanism simultaneously. Using the proposed method, we have determined the focal mechanisms of 40 induced seismic events in an oil/gas field in Oman. By investigating the results, we find that the reactivation of pre-existing faults is the main cause of the induced seismicity in the monitored area. Other observations obtained from the focal mechanism solutions are also consistent with earlier studies in the same area.