Ground Motions from Three Recent Earthquakes in Western Alberta and Northeastern British Columbia and their Implications for Induced-Seismicity Hazard in Eastern Regions

English Abstract:

A key issue in the assessment of hazard due to induced seismicity from fluid injection activity is to determine the potential ground motions. Though waste water 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 recent events that occurred along the deformation front marking the western boundary of the stable Canadian craton: a M4.0 and a M4.2 near Fort St. John, British Columbia, and a M3.9 near Rocky Mountain House, Alberta. The two Fort St. John events were likely induced by hydraulic fracturing activities in the region. Though the cause of the Rocky Mountain House 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 shut-down of a nearby gas plant, and a subsequent precautionary flaring of gas; moreover 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 events in California and with the predictions of applicable empirical GMPEs. However, high frequency ground motions appear to be consistently 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 (M4 to 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 waste water disposal and hydraulic fracturing) in order to accurately characterize injection induced seismicity and ultimately mitigate the associated risk.