Hydraulic fracturing (HF) at Preston New Road (PNR), Lancashire, United Kingdom, in August 2019, induced a number of felt earthquakes. The largest event (⁠ML 2.9) occurred on 26 August 2019, approximately three days after HF operations at the site had stopped. Following this, in November 2019, the United Kingdom Government announced a moratorium on HF for shale gas in England. Here we provide an analysis of the microseismic observations made during this case of HF‐induced fault activation. More than 55,000 microseismic events were detected during operations using a downhole array, the vast majority measuring less than Mw 0. Event locations revealed the growth of hydraulic fractures and their interaction with several preexisting structures. The spatiotemporal distribution of events suggests that a hydraulic pathway was created between the injection points and a nearby northwest–southeast‐striking fault, on which the largest events occurred. The aftershocks of the ML 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 Mw 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 that rely on extrapolating microseismicity observed during injection to forecast future behavior. 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 with 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 HF.

中文翻译：

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普雷斯顿新路PNR-2作业中的水力压裂诱发的英国兰开夏郡ML 2.9 2019年8月2.9级地震的高分辨率成像

2019年8月，英国兰开夏郡普雷斯顿新路（PNR）的水力压裂（HF）引发了许多毡状地震。最大的事件（ML 2.9）发生在2019年8月26日，大约是现场的HF操作停止后三天。此后，2019年11月，英国政府宣布暂停在英国使用页岩气的HF。在此，我们对在HF引起的断层激活过程中进行的微震观测进行了分析。在使用井下阵列进行的作业过程中，检测到超过55,000个微地震事件，绝大多数测量值小于Mw0。事件位置揭示了水力压裂的增长以及它们与几个先前存在的结构的相互作用。事件的时空分布表明，在注入点和附近的西北-东南冲动断层之间形成了一条水力通道，发生了最大的事件。ML 2.9事件的余震清楚地描绘了破裂平面，其空间分布形成了主震破裂区域周围活动的光晕。在整个事件簇中，幅度分布明显是双峰的，对于Mw 0以上的事件，其Gutenberg–Richter b值较低，这表明与水力压裂扩展有关的事件与与断层激活有关的事件之间的标度破裂。这对缓解策略提出了挑战，这些策略依赖于在注入过程中观察到的微地震的推论来预测未来的行为。激活的断层在区域应力场中的破裂方向很明确，比在PNR于2018年的先前作业中激活的断层要好得多。应力场内的不同方向可能解释了为什么该PNR-2断层与尽管接收到的注射液量较小，但仍按2018年顺序进行。这表明断层定向和原地应力条件在控制由HF引起的地震活动的严重性方面起着关键作用。