Ground-motion analysis of hydraulic-fracturing induced seismicity at close epicentral distance

The application of seismic hazard analysis methods developed for natural earthquakes can provide an effective framework for managing risks of induced seismicity (IS) sequences, particularly for assessment of potential risk to nearby infrastructure. Among various factors, the reliability of these methods depends on the accuracy of the ground-motion prediction equation (GMPE)-especially at close epicentral distances where ground motions are expected to be highest. In addition, potential impacts on local infrastructure can be assessed based on ground-motion-derived intensity values, which provide a basis for some traffic-light protocols that are used for managing fluid IS. GMPEs in many areas of the world, however, are poorly calibrated at close epicentral distances, because the availability of near-source seismograph stations is generally very sparse. This study investigates ground motions generated by an IS sequence, up to local magnitude (ML) 3.77 that occurred during a hydraulic-fracturing stimulation in the Duvernay shale play in central Alberta, western Canada. The sequence was monitored using a near-surface array that was located directly above the hydraulically fractured horizontal wells, providing accurate ground-motion measurements at hypocentral distances <10 km. The local array consisted of a combination of geophones cemented in shallow wellbores to depth of approximately 27 m, shallow buried broadband seismometers, and a strong-motion accelerometer. This configuration enabled direct measurement of near-surface seismic velocities in the top 27 m, which provided robust VS30 data used to correct observed ground motions for local site-amplification effects. Our data set extends previous analyses in this region by providing new measurements very close to the epicenters of moderate earthquakes and shows that a recently developed GMPE provides accurate near-source ground-motion estimates.