Focal Mechanism Solutions for Earthquakes in Eastern Canada and Alberta Using the Generalized Cut and Paste (gCAP) MethodÂ
Alex Carey, McGill University, Montreal, QC, Canada, Hongyu Yu, ITAG Institute of Theoretical and Applied Geophysics, Peking University, Beijing, China, Yajing Liu, McGill University, Department of Earth and Planetary Sciences, Montreal, QC, Canada and Rebecca M Harrington, Ruhr University Bochum, Institute of Geology, Mineralogy and Geophysics, Bochum, Germany
Contact First Author: Alex Carey; alexander.carey@mail.mcgill.ca
English Abstract:
Earthquake focal mechanism solutions (FMSs) provide information on the fracture orientation and the ambient stress field under which seismic ruptures take place. In this study, we first applied the generalized Cut and Paste (gCAP) method (Zhu and Helmberger, 1996) to several recent earthquakes in the Charlevoix Seismic Zone (CSZ) and the Western Quebec Seismic Zone (WQSZ). One attribute of the gCAP method is that it can invert the five phases (two P and three S phases) independently, increasing the weight of P waveforms for the overall inversion and decreasing the influence of earthquake location. Using waveform recordings of high signal-to-noise ratio from a combined 11 CNSN and Earthscope Transportable Array stations in the gCAP, we found the May 17
th, 2013, Ladysmith earthquake in the WQSZ was a reverse-faulting event on a high-angle fault, with Mw=4.5 and depth at 12 km (variance reduction of 85.6). Our result is consistent with the FMS determined by another study using the inversion of long-period Rayleigh waves from more distant stations (Ma and Audet, 2014).
Next, we will apply gCAP in the source characterization of induced seismicity from hydraulic fracturing operations in Alberta. We will focus specifically on the Rocky Mountain House (RMH) cluster in August 2014, which includes a magnitude 4.3 event on August 9th that was possibly induced by hydrofracturing. We will also use the USGS Coulomb software to calculate the local stress field perturbations due to injection. By 1) comparing the RMH cluster focal mechanisms to earthquakes during periods without injection in the same region and 2) quantitatively evaluating the spatial correlation in RMH seismicity and Coulomb stress change, we will work toward clarifying the relationship between the RMH seismicity sequence and hydraulic fracturing activity.
