Summary
Induced seismicity occurs both in conventional oil/gas fields due to production and water injection and in unconventional oil/gas fields due to hydraulic fracturing. Source mechanisms of these induced earthquakes are of great importance for understanding their causes and the physics of the seismic processes in reservoirs. Previous research on the analysis of induced seismic events assumed a double-couple (DC) source mechanism. However, recent studies have shown a non-negligible percentage of a nondouble- couple (non-DC) component of source moment tensor in hydraulic fracturing events (Rutledge et al. 2003, ŠÍlený et al., 2009; Warpinski and Du, 2010; Song and Toksöz, 2011). In this study, we determined the full moment tensor of a large number of earthquakes from an oil/gas field. The seismicity of the field and source mechanisms of events using DC assumption have been studied extensively (Sarkar, 2008; Li et al., 2011, 2012). Song and Toksöz (2011) developed a full waveform based complete moment tensor inversion method to investigate induced events. In this study, we inverted and compared the source mechanisms of induced events with and without DC constraints. We investigated the accuracy of moment tensors on number of stations, azimuthal distribution, and signal to noise ratio. We conducted tests with synthetic data to validate the method before the application to the real data. Our results show that the double-couple component of the MT is determined most accurately. The accuracy of isotropic and CLVD components depend on data quality and station coverage.
Introduction
Induced micro-earthquakes widely happen in conventional and unconventional oil/gas fields. Induced seismicity study is of great importance in monitoring and understanding the processes of hydraulic fracturing, fluid injection and oil/gas extraction.
Previous research on the analysis of induced seismic events in conventional oil/gas fields assumed a double-couple (DC) source mechanism. However, recent studies have shown interests in a non-double-couple (non-DC) component of source moment tensor in hydraulic fracturing events (ŠÍlený et al., 2009; Warpinski and Du, 2010; Song and Toksöz, 2011).
