A Systematic Study of Earthquake Source Mechanism and Regional Stress Field in the Southern Montney Unconventional Play of Northeast British Columbia, Canada

摘要

We provide a close look at the source mechanism of hydraulically fractured induced earthquakes and the in situ stress field within the southern Montney unconventional play in the northeast British Columbia, Canada. P-wave first-motion focal mechanisms were obtained for 66 earthquakes with magnitudes between 1.5 and 4.6. Results show that strike-slip movement is the prevailing source mechanism for the events in this area, although reverse faulting is also observed for a few earthquakes. The best-fitting nodal plane mostly strikes at similar to N60 degrees E, with most events having dip angles of >60 degrees. Using the Martinez-Garzon et al. stress inversion module, we obtained the orientation of the three principal compressive stress (S-1>S-2>S-3) and the relative intermediate principal stress magnitude (R) in five clusters. Assuming the best-fitting nodal plane to be the causative fault, R values are mostly between 0.8 and 0.9 suggesting that the magnitude of S-2 and S-3 are similar, which is consistent with strike-slip or reverse-faulting regimes. The plunge of S-1 varies between 1 degrees and 3 degrees, with its trend varying between N21 degrees E and N34 degrees E. On the other hand, the plunge of S-3 varies between 22 degrees and 50 degrees, with its trend varies between N68 degrees W and N58 degrees W. Following Lund and Townend (2007), we calculated the trend of maximum horizontal stress to vary from N22 degrees E to N33 degrees E, in comparison with the average trend of N41 degrees E from the World Stress Map (Heidbach et al., 2016). Through analysis of the Coulomb failure criterion and Mohr diagrams, we estimated the amount of pore-pressure increase necessary to initiate shear slip to range between 4 and 29 MPa (average of 14 +/- 8 MPa) in the study area.