ABSTRACT

Several recent clusters of earthquakes of moment magnitude M3 in Alberta have been identified as potentially induced seismicity, triggered by hydraulic fracturing, oil and gas production, and wastewater disposal. We characterize ground motions from these events, recorded within 100 km, focusing on the empirical definition of response spectral shapes and amplitudes. We compare amplitudes statistically with binned‐average response spectra (for the same magnitude–distance bin) from induced events in Oklahoma, as well as with three benchmark ground‐motion prediction equations (GMPEs) derived from natural events in central and eastern North America (CENA) and California. We observe that small‐magnitude events (M<3.8) in Alberta have notably lower amplitudes than Oklahoma events at high frequencies (f>35  Hz), whereas at larger magnitudes the amplitude differences are less distinct. Because the discrepancies appear to be magnitude dependent, we venture that they are indicative of a source effect. For events greater than M4, the ground motions appear equivalent in the two regions, despite different mechanisms for triggering the seismicity. There is no apparent trend with hypocentral distance in the residuals of Alberta amplitudes relative to those in Oklahoma, indicating that attenuation is similar in the two regions. We therefore conclude that, for M>3.6 in Alberta and Oklahoma, median ground‐motion response spectra are comparable in terms of their spectral shapes and amplitudes.

Three GMPEs are used as benchmarks for further comparisons; Yenier and Atkinson (2015; hereafter, YA15CENA), Atkinson (2015; hereafter, A15), and Abrahamson et al. (2014; hereafter, ASK14). To minimize significant site effects in the Alberta data, we compare vertical‐component response spectra with the GMPEs. Comparisons with YA15CENA suggest that high‐frequency motions from M<3.5 events in Alberta appear to be weak, relative to natural events in CENA, even those at shallow focal depths. For larger magnitudes, the residuals between the Alberta motions and the YA15CENA, A15, and ASK14 GMPEs suggest that Alberta ground motions are in reasonable accord with the models.

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