The development of most unconventional oil and gas resources relies upon subsurface injection of very large volumes of fluids, which can induce earthquakes by activating slip on a nearby fault. During the last 5 years, accelerated oilfield fluid injection has led to a sharp increase in the rate of earthquakes in some parts of North America. In the central United States, most induced seismicity is linked to deep disposal of coproduced wastewater from oil and gas extraction. In contrast, in western Canada most recent cases of induced seismicity are highly correlated in time and space with hydraulic fracturing, during which fluids are injected under high pressure during well completion to induce localized fracturing of rock. Furthermore, it appears that the maximum-observed magnitude of events associated with hydraulic fracturing may exceed the predictions of an often-cited relationship between the volume of injected fluid and the maximum expected magnitude. These findings have far-reaching implications for assessment of inducedseismicity hazards.

A key issue in the assessment of hazard due to induced seismicity from fluid injection activity is to determine the potential ground motions. Although wastewater disposal typically receives the most attention, hydraulic fracturing is increasingly recognized as a significant source of seismic hazard. We present an analysis of the ground motions from the three largest events of 2014 that occurred along the deformation front marking the western boundary of the stable Canadian craton: an M 4.0 and an M 4.2 near Fort St. John (FSJ), British Columbia, and an M 3.9 near Rocky Mountain House (RMH), Alberta. The two FSJ events were likely induced by hydraulic fracturing activities in the region. Although the cause of the RMH event remains unclear, it is of interest because it is of similar magnitude to the other events and had significant consequences to the public. The event triggered an automatic shutdown of a nearby gas plant and a subsequent precautionary flaring of gas, and several hundred people were without power for a prolonged period. We examine the ground motions and intensities for these events. We find that ground motions at frequencies up to about 2 Hz are in agreement with corresponding observations for similar-sized events in California and with the predictions of applicable empirical ground-motion prediction equations. However, high-frequency ground motions appear to be lower than those predicted, suggesting that these events may be associated with a low stress drop; we believe that this is likely a focal depth effect, which may be a mitigating factor that limits high-frequency ground motions from induced events. Our preliminary findings suggest that moderate-induced events (M 4-5) may be damaging to nearby infrastructure, because the shallow focal depth may result in localized strong ground motions to which some infrastructure may be vulnerable; this is a particular concern in low-to-moderate seismicity regions, because seismic design measures for structures in these regions may be minimal. Our results highlight the importance of seismic monitoring in the immediate vicinity of fluid injection sites (both wastewater disposal and hydraulic fracturing) to accurately characterize injection-induced seismicity and ultimately mitigate the associated risk.

The points raised by Dr. Schieber are most efficiently addressed through a joint Reply from both Plint and Cheadle, who are currently working on Cretaceous mudrocks in the Canadian portion of the Western Interior Seaway. Before making our reply, both Plint and Cheadle, as relative neophytes in the world of mudrock sedimentology, are pleased to acknowledge the guidance and inspiration provided by the numerous, groundbreaking, publications of Juergen Schieber. We hope that our research on Canadian mudrocks can also make some modest contribution to the exciting, tantalizing and sometimes frustrating field of mudrock sedimentology. We thank Dr Schieber for the interest he has taken in the paper by Plint (2014) concerning prodelta mudstone in the Cretaceous Dunvegan Formation. Below, we comment briefly on the main points raised in Schieber's Discussion.

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