ABSTRACT

On 30 November 2018, three felt earthquakes occurred in the Septimus region of northeast British Columbia in an area where hydraulic fracturing was in progress. The proximity of oil and gas activities to populated areas and to critical infrastructure including major dams raises significant concern regarding the seismic hazard posed by moderate induced events and motivates study of their ground motions. Here, we analyze the ground‐motion amplitudes from these events recorded between 3 and 400 km. We use three‐component waveforms from 45 seismometer and accelerometer sensors to analyze the observed ground motions. The moment magnitude (Mw) of the first event is estimated as 4.6 using the vertical pseudoresponse spectral acceleration (PSA) based on the relations provided by Novakovic et al. (2018). The Mw for the two smaller earthquakes are 3.5 and 4.0. The intensity of shaking from the Mw 4.6 and 4.0 events generally exceeded modified Mercalli intensity (MMI) VI at distances <6  km. The maximum duration above the MMI VI threshold at the closest station (3.5 km distance) from the mainshock is 1.6 s. The observed ground motions agree with the ground‐motion prediction equation (GMPE) of Novakovic et al. (2018) for induced events in Oklahoma, with attenuation modified to match that for the study region, assuming typical regional site amplification. The inferred value of stress drop for the mainshock and the largest aftershock is approximately 50 bars based on the agreement of observed PSA values with the Novakovic et al. (2018) GMPE.

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