Separating Primary and Secondary Mechanisms for Seismicity Induced by Hydraulic Fracturing

Abstract

ABSTRACT Hydraulic fracturing operations are generally associated with induced seismic events with magnitudes that are typically lower than those of tectonic-based earthquakes. Although fluid migration and the resulting pore-pressure increase appear to be the primary driving mechanisms, one must also understand fault activation and the secondary interevent triggering process(es) leading to the generation of aftershock sequences. This requires distinguishing between seismic events that are a direct consequence of fluid injections and those that predominantly arise because of secondary mechanisms such as static or dynamic stress triggering caused by preceding earthquakes. Focusing on a case study in the Montney area in western Alberta, Canada, where both surface array and borehole geophone data are available, we (1) establish the importance of secondary mechanisms and aftershock sequences in this area and (2) show that the statistical properties of the aftershock sequences can allow one to establish whether significant fault activation is at play and whether the activated fault zone was critically prestressed. Specifically, we find evidence for aftershock sequences using different statistical methodologies, although the presence of aftershocks in the studied hydraulic-fracturing-induced context is significantly less dominant compared with tectonic seismicity. The identified aftershock sequences are characterized by long-range spatial aftershock zones, setting them apart from other cases in fluid-driven settings but comparable to tectonic aftershock sequences and indicative of fault activation. Despite this observation, we find overall a strong dominance of smaller triggers for aftershock sequences and an aftershock production associated with large-magnitude events that is significantly lower than what has been observed in tectonic settings. This suggests that the activated fault zone as a whole was not critically prestressed, and the associated initial seismic hazard was significantly lower compared with tectonic cases.