Hybrid focal mechanism determination: Constraining focal mechanisms of injection induced seismicity using in situ stress data

Determining the focal mechanisms of injection induced microseismicity provides beneficial information not only for understanding the features of induced seismicity but also for characterizing the fracture system in the reservoir. However, focal mechanisms ofmicroseismicity are often not well determined with first motion information alone due to limited station coverage. We propose a new method to better constrain first motion focal mechanisms of fluid-induced seismicity by introducing in situ stress information estimated from borehole analysis and wellhead pressure. Since injection induced seismicity is dominantly caused by increases of pore pressure, we can use in situ stress magnitude/orientation information and the injection wellhead pressure to constrain the range of focal mechanisms. Then, the range of possible focal mechanisms estimated from available first motion data is constrained by superposing the range of focal mechanisms that can slip from the in situ stress condition. The use of the combined sets of information places a better constraint on focal mechanisms than can be obtained from either of the individual data sets. We apply our method to the microseismic data recorded during the hydraulic stimulation in Basel, Switzerland where the stress state is known from borehole measurements. Our method successfully constrains the range of focal mechanisms, though the degree of constraint is dependent on the distribution of possible focal mechanisms that are consistent with the first motions and the distribution that are consistent with the stress condition. We compared the range of focal mechanisms constrained by our method using data from only nearby stations with well-constrained focal mechanisms determined from first motions that are recorded by more than 50 surface stations. In the best case, 80 per cent of the focal mechanisms that are consistent with the first motion data were removed because they were inconsistent with the stress data. The constrained range of focal mechanisms includes the true focal mechanisms with significantly high probability of 0.86 even considering the uncertainty in stress information that is used for constraint. This demonstrates the ability of our method to constrain the range of focal mechanisms in situations with limited station coverage, and the benefits of introducing other geophysical data into seismological analyses.