High resolution imaging of the M​L​ 2.9 August 2019 earthquake in Lancashire, UK, induced by hydraulic fracturing during Preston New Road PNR-2 operations

Hydraulic fracturing at Preston New Road, Lancashire, UK, in August 2019 induced a number of felt earthquakes. The largest event (M​ 2.9) occurred on August 26t​h​, 2019, approximately 3 days after hydraulic fracturing operations at the site had stopped. Following this, in November 2019, the UK government announced a moratorium on hydraulic fracturing for shale gas in England. Here we provide an analysis of the microseismic observations made during this case of hydraulic fracturing-induced fault activation. More than 55,000 microseismic events were detected during operations using a downhole array, the vast majority measuring less than M​w​ 0. Event locations revealed the growth of hydraulic fractures and their interaction with several pre-existing structures. The spatiotemporal distribution of events suggests that a hydraulic pathway was created between the injection points and a nearby NW-SE striking fault, on which the largest events occurred. The aftershocks of the M​L​ 2.9 event clearly delineate the rupture plane, with their spatial distribution forming a halo of activity around the mainshock rupture area. Across clusters of events, the magnitude distributions are distinctly bimodal, with a lower Gutenberg-Richter b-value for events above M​w​ 0, suggesting a break in scaling between events associated with hydraulic fracture propagation, and events associated with activation of the fault. This poses a challenge for mitigation strategies which rely on extrapolating microseismicity observed during injection to forecast future behaviour. The activated fault was well oriented for failure in the regional stress field, significantly more so than the fault activated during previous operations at PNR in 2018. The differing orientations within the stress field likely explain why this PNR-2 fault produced larger events compared to the 2018 sequence, despite receiving a smaller volume of injected fluid. This indicates that fault orientation and in situ stress conditions play a key role in controlling the severity of seismicity induced by hydraulic fracturing.