Developments in understanding seismicity triggered by hydraulic fracturing

As recently as 2015, it was common in the scientific literature to find assertions that the risk of triggering a damaging earthquake by hydraulic fracturing (HF) - an industrial process where pressurized fluids are used to create or open fractures within rock layers - could be treated as negligible. However, that viewpoint has changed dramatically. It is now clear that the hazard from induced seismicity (including HF) exceeds the natural hazard in low-to-moderate seismicity environments. As such, to mitigate risk to vulnerable and critical infrastructure, it is important to address the likelihood and triggering mechanisms of HF-induced earthquakes. Although it is sometimes claimed that HF-induced earthquakes can be accurately predicted, avoided or controlled, critical knowledge gaps still remain. In this Review, we discuss six fundamental issues surrounding induced seismicity, focusing specifically on HF-induced events, including: the triggering mechanisms of HF seismicity; the relationship between tectonic environment and HF seismicity; the similarities and differences between induced and natural events; the damage potential associated with HF-induced seismicity; whether HF-induced events can be predicted; and the relative hazards of HF-induced and natural seismic events. We finish by outlining future research directions that are required to minimize the uncertainty and hazard that surround induced seismicity. Hydraulic fracturing can trigger seismic events though a variety of direct and indirect mechanisms, and could result in injuries, fatalities and damage to critical infrastructure. This Review considers six fundamental issues to address how induced seismicity can be predicted or mitigated.