Due to the deep socioeconomic implications, induced seismicity is a timely and increasingly relevant topic of interest for the general public. Cases of induced seismicity have a global distribution and involve a large number of industrial operations, with many documented cases from as far back to the beginning of the twentieth century. However, the sparse and fragmented documentation available makes it difficult to have a clear picture on our understanding of the physical phenomenon and consequently in our ability to mitigate the risk associated with induced seismicity. This review presents a unified and concise summary of the still open questions related to monitoring, discrimination, and management of induced seismicity in the European context and, when possible, provides potential answers. We further discuss selected critical European cases of induced seismicity, which led to the suspension or reduction of the related industrial activities.

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Current challenges in monitoring, discrimination, and management of induced seismicity related to underground industrial activities: A European perspective

Hydraulic Fracturing-Induced Seismicity

A major task in mines and even more in underground repositories for nuclear waste is to investigate crack formation for evaluation of rock mass integrity of the host rock Therefore, in situ acoustic emission (AE) monitoring are carried out in mines as part of geomechanical investigations regarding the stability of underground cavities and the integrity of the rock mass In this work, the capability of in situ AE monitoring in the context of structural health monitoring (SHM) in mines and in various geological settings will be reported SHM pointed out, that the AE network is able to monitoring AE activity in rock with a volume up to 10 6 cubicmeter and distances up to 200 m (eg, 100 m × 100 m × 100 m) in the frequency range of 1 kHz to 150 kHz Very small AE events with source size in approximately centimeter to millimeter scale are detected The results show that AE activity monitors rock deformation in geological boundaries due to convergence of the rock In addition, high AE activity occurs in zones of dilatancy stress in homogenous rock In conclusion in situ AE monitoring is a useful tool to observe instabilities in rock long before any damage becomes visible

Review on In Situ Acoustic Emission Monitoring in the Context of Structural Health Monitoring in Mines

Earthquakes induced by subsurface fluid injection pose a significant issue across a range of industries. Debate continues as to the most effective methods to mitigate the resulting seismic hazard. Observations of induced seismicity indicate that the rate of seismicity scales with the injection volume and that events follow the Gutenberg–Richter distribution. These two inferences permit us to populate statistical models of the seismicity and extrapolate them to make forecasts of the expected event magnitudes as injection continues. Here, we describe a shale gas site where this approach was used in real time to make operational decisions during hydraulic fracturing operations.Microseismic observations revealed the intersection between hydraulic fracturing and a pre‐existing fault or fracture network that became seismically active. Although “red light” events, requiring a pause to the injection program, occurred on several occasions, the observed event magnitudes fell within expected levels based on the extrapolated statistical models, and the levels of seismicity remained within acceptable limits as defined by the regulator. To date, induced seismicity has typically been regulated using retroactive traffic light schemes. This study shows that the use of high‐quality microseismic observations to populate statistical models that forecast expected event magnitudes can provide a more effective approach.

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Real time imaging, forecasting and management of human-induced seismicity at Preston New Road, Lancashire, England

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The Relationship between M and ML: A Review and Application to Induced Seismicity in the Groningen Gas Field, The Netherlands

Icequakes, microseismic earthquakes at glaciers, offer critical insights into the dynamics of ice sheets. For the first time in the Antarctic, we explore the use of fibre optic cables as Distribute...

Distributed Acoustic Sensing (DAS) for Natural Microseismicity Studies: A Case Study From Antarctica

Local seismic magnitudes provide a practical and efficient scale for the implementation of regulation designed to manage the risk of induced seismicity, such as Traffic‐Light Schemes (TLS). We demonstrate that significant magnitude discrepancies (up to a unit higher) occur between seismic events recorded on nearby stations (<5  km) compared with those at greater distances. This is due to the influence of sedimentary layers, which are generally lower in velocity and more attenuating than the underlying crystalline basement rocks, and requires a change in the attenuation term of the M L scale. This has a significant impact on the United Kingdom’s (U.K.) hydraulic fracturing TLS, whose red light is set at M L 0.5. Because the nominal detectability of the U.K. network is M L 2, this scheme will require the deployment of monitoring stations in close proximity to well sites. Using data collected from mining events near New Ollerton, Nottinghamshire, we illustrate the effects that proximity has on travel path velocities and attenuation, then perform a damped least‐squares inversion to determine appropriate constants within the M L scale. We show that the attenuation term needs to increase from 0.00183 to 0.0514 and demonstrate that this higher value is representative of a ray path within a slower more attenuating sedimentary layer compared with the continental crust. We therefore recommend that the magnitude scale M L=log( A )+1.17log( r )+0.0514 r −3.0 should be used when local monitoring networks are within 5 km of the event epicenters.

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Local Magnitude Discrepancies for Near‐Event Receivers: Implications for the U.K. Traffic‐Light Scheme

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Extending local magnitude ML to short distances

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.

High‐Resolution Imaging of the ML 2.9 August 2019 Earthquake in Lancashire, United Kingdom, Induced by Hydraulic Fracturing during Preston New Road PNR‐2 Operations

Earthquake magnitudes are surprisingly difficult to estimate accurately. This is especially true when evaluating small earthquakes: for example, those caused by human activities such as shale gas stimulation, CO2 and waste water storage, and enhanced geothermal systems. Uncertainties are created by a range of issues including: which ‘magnitude’ scale is used; what type of instrument records the earthquake; how the instruments are deployed; and the heterogeneity of the Earth between the source and the receivers. Errors can be larger than an order of magnitude in scale. For very small earthquakes this is not usually of much concern. However, occasionally, larger earthquakes induced by human activity are felt at the surface. This has led to regulatory frameworks that require accurate assessment of earthquake magnitudes before they reach the point of being felt. Hence, to monitor and mitigate felt seismicity there is a need to calculate accurate earthquake magnitudes in real time. Regulatory monitoring of induced seismicity is becoming a key issue in the successful development of reservoir projects that involve stimulation or storage. Here, we discuss the challenges with implementing such reservoir monitoring, and provide a suggested monitoring strategy.

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Antony Butcher

Papers

Distributed Acoustic Sensing (DAS) for Natural Microseismicity Studies: A Case Study From Antarctica

Local Magnitude Discrepancies for Near‐Event Receivers: Implications for the U.K. Traffic‐Light Scheme

Extending local magnitude ML to short distances

High‐Resolution Imaging of the ML 2.9 August 2019 Earthquake in Lancashire, United Kingdom, Induced by Hydraulic Fracturing during Preston New Road PNR‐2 Operations

How big is a small earthquake? Challenges in determining microseismic magnitudes