UK earthquake monitoring 2012/2013

The British Geological Survey (BGS) operates a network of seismometers throughout the UK in order to acquire seismic data on a long-term basis. The aims of the Seismic Monitoring and Information Service are to develop and maintain a national database of seismic activity in the UK for use in seismic hazard assessment, and to provide near-immediate responses to the occurrence, or reported occurrence, of significant events. The project is supported by a group of organisations under the chairmanship of the Office for Nuclear Regulation (ONR) with major financial input from the Natural Environment Research Council (NERC). In the 24th year of the project, two new broadband seismograph stations were established, giving a total of 40 broadband stations. Real-time data from all broadband stations and nearly all other short period stations are transferred directly to Edinburgh for near real-time detection and location of seismic events as well as archival and storage of continuous data. Data latency is generally low, less than one minute most of the time, and there is a high level of completeness within our archive of continuous data. All significant events were reported rapidly to the Customer Group through seismic alerts sent by e-mail. The alerts were also published on the Internet (http://www.earthquakes.bgs.ac.uk). Monthly seismic bulletins were issued six weeks in arrears and compiled in a finalised annual bulletin (Galloway, 2013). Four papers have been published in external journals. A chapter was also published in the New Manual of Seismological Observatory Practice. Three presentations were made at international conferences. Four BGS internal reports were prepared. We have continued to collaborate widely with academic partners across the UK and overseas on a number of research initiatives

The Moidart earthquakes of 4 August 2017

The Moidart earthquake of 4 August 2017 (4.0 ML) was the largest earthquake in Scotland for 18 years. The earthquake was felt widely across the west of Scotland. Only five other earthquakes of this size or greater have been observed in the period of instrumental recording from 1970 to present. Historical observations and instrumental recordings have been used to estimate that an earthquake of 4.0 ML or greater occurs somewhere in Scotland roughly every 8-9 years on average. The earthquake hypocentre was calculated using an iterative linearized method. The results suggest that the earthquake occurred in the mid-Crust at a depth of approximately 12 km. This is largely consistent with observed focal depths for other earthquakes in the region, which are distributed throughout the upper 20 km of the Crust. The strong similarity between the recorded ground motions from the mainshock and the four recorded aftershocks suggests that they all occurred within a small source volume, of the order of a few hundred metres in extent and had similar source mechanisms. The modelled source displacement spectra provide a good fit for the observed displacement spectra and suggest a moment magnitude (Mw) of 3.6 ± 0.1. This is slightly less than that expected for an earthquake with a local magnitude of 4.0 ML using commonly used empirical relationships relating local and moment magnitude, which gives an expected moment magnitude of 3.7. The calculated focal mechanism suggests that the earthquake resulted from strike-slip faulting on a fault plane that strikes either SW-NE or NW-SE and dips steeply, although the dip of both fault planes is rather poorly constrained. This is in good agreement with focal mechanisms calculated for other earthquakes across the region, which all show similar solutions. Seismicity in northwest Scotland is clustered around a number of large, steeply dipping major faults that strike either NE-SW or NW-SE suggesting that earthquake activity across the region is driven by reactivation of such fault systems by deformation associated with first-order plate motions rather than deformation associated with glacioisostatic recovery. Although there are no mapped major fault systems in the immediate vicinity of the Moidart earthquake, it seems likely that the earthquake also occurred on a steeply dipping fault that strikes either NE-SW or NW-SE but remains unmapped

A New Procedure for Evaluating Ground-Motion Models, with Application to Hydraulic-Fracture-Induced Seismicity in the United Kingdom

An essential component of seismic hazard analysis is the prediction of ground shaking (and its uncertainty), using ground-motion models (GMMs). This article proposes a new method to evaluate (i.e., rank) the suitability of GMMs for modeling ground motions in a given region. The method leverages a statistical tool from sensitivity analysis to quantitatively compare predictions of a GMM with underlying observations. We demonstrate the performance of the proposed method relative to several other popular GMM ranking procedures and highlight its advantages, which include its intuitive scoring system and its ability to account for the hierarchical structure of GMMs. We use the proposed method to evaluate the applicability of several GMMs for modeling ground motions from induced earthquakes due to U.K. shale gas development. The data consist of 195 recordings at hypocentral distances (R) less than 10 km for 29 events with local magnitude (ML) greater than 0 that relate to 2018/2019 hydraulic-fracture operations at the Preston New Road shale gas site in Lancashire and 192 R<10  km recordings for 48 ML>0 events induced—within the same geologic formation—by coal mining near New Ollerton, North Nottinghamshire. We examine: (1) the Akkar, Sandikkaya, and Bommer (2014) models for European seismicity; (2) the Douglas et al. (2013) model for geothermal-induced seismicity; and (3) the Atkinson (2015) model for central and eastern North America induced seismicity. We find the Douglas et al. (2013) model to be the most suitable for almost all of the considered ground-motion intensity measures. We modify this model by recomputing its coefficients in line with the observed data, to further improve its accuracy for future analyses of the seismic hazard of interest. This study both advances the state of the art in GMM evaluation and enhances understanding of the seismic hazard related to U.K. shale gas development

Integrating Outcomes from Probabilistic and Deterministic Seismic Hazard Analysis in the Tien Shan

In this study, we have evaluated the probabilistic and deterministic seismic hazard for the city of Almaty, the largest city in Kazakhstan, which has a population of nearly two million people. Almaty is located in the Tien Shan belt, a low‐strain‐rate environment within the interior of the Eurasian plate that is characterized by large infrequent earthquakes. A robust assessment of seismic hazard for Almaty is challenging because current knowledge about the occurrence of large earthquakes is limited, due to the short duration of the earthquake catalog and only partial information about the geometry, rupture behavior, slip rate, and the maximum expected earthquake magnitude of the faults in the area. The impact that this incomplete knowledge has on assessing seismic hazard in this area can be overcome using both probabilistic and deterministic approaches and integrating the results. First, we simulate ground‐shaking scenarios for three destructive historical earthquakes that occurred in the northern Tien Shan in 1887, 1889, and 1911, using ground‐motion prediction equations (GMPEs) and realistic fault‐rupture models based on recent geomorphological studies. We show that the large variability in the GMPEs results in large uncertainty in the ground‐motion simulations. Then, we estimate the seismic hazard probabilistically using a Monte Carlo‐based probabilistic seismic hazard analysis and the earthquake catalog compiled from the databases of the International Seismological Centre and the British Geological Survey. The results show that earthquakes of M w Mw 7.0–7.5 at Joyner–Boore distances of less than 10 km from the city pose a significant hazard to Almaty due to their proximity. These potential future earthquakes are similar to the 1887 Verny earthquake in terms of their magnitude and distance from Almaty. Unfortunately, this is the least well understood of the destructive historical earthquakes that have occurred in the northern Tien Shan

Potential risks of induced seismicity from high volume hydraulic fracturing of shales in Northern Ireland

Hydraulic fracturing (HF) has made it possible to economically produce hydrocarbons directly from low‐permeability reservoirs such as shales by injecting high pressure fluids to create fracture networks. However, over the last decade the number of observations of induced earthquakes caused by HF operations around the world has increased as the shale gas industry has developed. Data from the US and Canada suggest that on average around 1% of HF wells can be linked to earthquakes with magnitudes of 3 or greater. Earthquakes of this size are large enough to be felt by people. However, in some areas of the US and Canada the percentage of wells associated with induced earthquakes is much higher (>30%). This variability is often explained in terms of geological factors such as proximity to existing faults. In a small number of cases, HF operations have triggered earthquakes large enough to cause potentially damaging ground motions. Such earthquakes cannot be confidently predicted in advance of operations. These observations suggest that the risk from induced seismicity during HF operations is not negligible. Earthquakes with magnitudes greater than around 2 result from slip on existing faults that is triggered by stress changes caused by the injection of fluid during the HF process. The size of the earthquake will depend on both the area of the ruptured part of the fault and the amount of slip. Since such faults may extend outside the hydraulically fractured zone, the maximum magnitude will be controlled by local geology and tectonics, not operational parameters such as the amount of injected fluid. As a result, the maximum magnitude is highly uncertain. Induced earthquakes have been observed in wide variety of geological settings and in areas where there are relatively few tectonic earthquakes. In some areas, the resulting hazard from induced earthquakes due to HF operations is significantly greater than the hazard from tectonic earthquakes. As a result, the low hazard from tectonic earthquakes in Northern Ireland does not guarantee that the hazard from induced seismicity will also be low. Induced earthquakes are likely to be clustered in space and time around the locus of HF operations. Hazard is likely to increase with the number of wells and will be highest during or shortly after HF operations. Hazard may also be a function of total injected volume, with larger injected volumes leading to more earthquakes and increasing the probability of larger events. Operations that target shallow formations may pose a higher hazard, since for a given magnitude, the intensity of ground motions at the surface will be greater. The potential for actual damage depends on the intensity of motions and both the number and vulnerability of buildings exposed to ground shaking. As a result, the risk of damage to buildings will be higher in densely populated urban areas than in rural areas. Risk studies for the UK have shown that cosmetic and minor structural damage may occur for earthquakes with magnitudes as low as 3. Higher resolution geophysical data is needed to identify fault structures and depth to basement in sedimentary basins with hydrocarbon potential in Northern Ireland in order to help mitigate risk of induced seismicity from hydraulic fracturing. Improved regional seismic monitoring should also be considered. Similarly, the present-day stress regime and stress state of faults in both the Lough Allen and Rathlin basins is poorly known. Further work is needed to address this. Current risk-mitigation strategies have had limited success. There may be insufficient data to identify geological faults prior to operations and even where high resolution data are available, there may still be hidden faults. Similarly, traffic light systems based on specific earthquake magnitude thresholds have often failed. Statistical methods that relate the volume of injected fluid or the injection rate to induced earthquake activity may allow useful probabilistic forecasts in the future but may be associated with considerable uncertainties without calibration for local conditions

Water, oceanic fracture zones and the lubrication of subducting plate boundaries - insights from seismicity

We investigate the relationship between subduction processes and related seismicity for the Lesser Antilles Arc using the Gutenberg-Richter law. This power lawdescribes the earthquakemagnitude distribution, with the gradient of the cumulative magnitude distribution being commonly known as the b-value. The Lesser Antilles Arc was chosen because of its alongstrike variability in sediment subduction and the transition from subduction to strike-slip movement towards its northern and southern ends. The data are derived from the seismicity catalogues from the Seismic Research Centre of The University of the West Indies and the Observatoires Volcanologiques et Sismologiques of the Institut de Physique du Globe de Paris and consist of subcrustal events primarily from the slab interface. The b-value is found using a Kolmogorov-Smirnov test for a maximum-likelihood straight line-fitting routine. We investigate spatial variations in b-values using a grid-search with circular cells as well as an along-arc projection. Tests with different algorithms and the two independent earthquake cataloges provide confidence in the robustness of our results. We observe a strong spatial variability of the b-value that cannot be explained by the uncertainties. Rather than obtaining a simple north-south b-value distribution suggestive of the dominant control on earthquake triggering being water released from the sedimentary cover on the incoming American Plates, or a b-value distribution that correlates with on the obliquity of subduction, we obtain a series of discrete, high b-value 'bull's-eyes' along strike. These bull's-eyes, which indicate stress release through a higher fraction of small earthquakes, coincide with the locations of known incoming oceanic fracture zones on the American Plates. We interpret the results in terms of water being delivered to the Lesser Antilles subduction zone in the vicinity of fracture zones providing lubrication and thus changing the character of the related seismicity. Our results suggest serpentinization around mid-ocean ridge transform faults, which go on to become fracture zones on the incoming plate, plays a significant role in the delivery of water into the mantle at subduction zones

Imaging the subsurface using induced seismicity and ambient noise: 3D Tomographic Monte Carlo joint inversion of earthquake body wave travel times and surface wave dispersion

Seismic body wave travel time tomography and surface wave dispersion tomography have been used widely to characterise earthquakes and to study the subsurface structure of the Earth. Since these types of problem are often significantly non-linear and have non-unique solutions, Markov chain Monte Carlo (McMC) methods have been used to find probabilistic solutions. Body and surface wave data are usually inverted separately to produce independent velocity models. However, body wave tomography is generally sensitive to structure around the sub-volume in which earthquakes occur and produces limited resolution in the shallower Earth, whereas surface wave tomography is often sensitive to shallower structure. To better estimate subsurface properties, we therefore jointly invert for the seismic velocity structure and earthquake locations using body and surface wave data simultaneously. We apply the new joint inversion method to a mining site in the U.K. at which induced seismicity occurred and was recorded on a small local network of stations, and where ambient noise recordings are available from the same stations. The ambient noise is processed to obtain inter-receiver surface wave dispersion measurements which are inverted jointly with body wave arrival times from local earthquakes. The results show that by using both types of data, the earthquake source parameters and the velocity structure can be better constrained than in independent inversions. To further understand and interpret the results, we conduct synthetic tests to compare the results from body wave inversion and joint inversion. The results show that trade-offs between source parameters and velocities appear to bias results if only body wave data are used, but this issue is largely resolved by using the joint inversion method. Thus the use of ambient seismic noise and our fully non-linear inversion provides a valuable, improved method to image the subsurface velocity and seismicity

Recent scientific advances in the understanding of induced seismicity from hydraulic fracturing of shales

The Secretary of State for Business, Energy & lndustrial Strategy has commissioned the British Geological Survey to write a short report about seismic activity associated with hydraulic fracturing (HF) of shales to extract hydrocarbons. The specific terms of reference are available at https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/fi le/1066525/BGS_Letter.pdf. These ask six questions related to recent scientific research on the hazard and risk from induced seismicity during hydraulic fracturing of shale rocks. Our report considers the scientific advances in this area since 2019 that have been published in peer reviewed scientific journals as well as other recent studies commissioned by regulatory authorities. The main conclusions of our report in relation to each of the questions in the terms of reference are as follows: Forecasting the occurrence of large earthquakes and their expected magnitude remains a scientific challenge for the geoscience community. This is the case for both tectonic and induced earthquakes. (Questions 1 and 2) Methods to estimate the maximum magnitudes of induced earthquakes based on operational parameters and observed seismicity have been tested using data from both Hydraulic Fracturing (HF) operations and data from other industries. These methods have shown some applicability to guide operational decisions using real-time data. However, they do not currently account for the possibility of events that occur after operations have stopped or earthquakes on faults that extend outside the stimulated volume whose magnitude is not controlled by operational parameters alone. (Questions 1 and 2) Probabilistic methods widely applied to model and forecast tectonic earthquake sequences show some promise when modified to incorporate information about HF operations and appear capable of providing informative forecasts of the observed earthquake patterns. Operators could make forecasts for operations in new wells using either generic parameters or ones calibrated for operations in adjacent wells. Further testing of these methods may allow them to be further developed for operational scenarios. (Questions 1 and 2) Enhanced seismicity monitoring and measurement based on machine learning (ML) has been shown to reveal previously undetected earthquakes and hidden faults, essential for both more reliable earthquake forecasts and characterisation of fault reactivation potential. This can compensate for both limited numbers of seismic stations and faults that remain unmapped even by 3D exploration seismic data. (Questions 1 and 2) Widely used probabilistic methods to assess hazards and risks for tectonic earthquakes can also be applied to induced seismicity. However, there are important differences between how tectonic and induced seismicity evolves in space and time. Recent studies have suggested possible solutions, but further work is needed to develop these models and incorporate them in risk assessments. (Questions 1 and 2) Traffic light systems remain a useful tool for the mitigation of risks from induced seismicity. New research shows how red-light thresholds can be chosen to reduce the probability of the scenario to be avoided to a required level. This research recommends that there should be sufficient space between the amber and red-light thresholds to ensure that operators have an opportunity to modify operations to mitigate risks. (Questions 1 and 2) Induced seismicity has been observed in other industries related to underground energy production both in the UK and elsewhere. In the absence of a seismic building code in the UK, consistent risk targets, i.e., scenarios to be avoided, could be considered for all energy related industries that present a risk of induced earthquakes. (Question 3) Recent research using high quality exploration data that is available for some parts of the UK reveals localised structural and stress heterogeneity that could influence fault reactivation. However, it is not possible to identify all faults that could host earthquakes with magnitudes of up to 3 prior to operations, even with the best available data. (Questions 4 and 5). Recent research from the USA demonstrates the importance of geomechanical modelling to identify faults that are most likely to rupture during operations. This information can be used to assess risks prior to and during operations. However, these models require accurate mapping of sub-surface faults, robust estimates of stress state, and knowledge of formation pore pressures and the mechanical properties of sub-surface rocks. While this information is available in areas with unconventional hydrocarbon potential such as the Bowland Basin, more data is needed from other basins to apply this more widely (Questions 4 and 5). Limited exploration data from other basins with unconventional hydrocarbon potential of the UK means that there are significant gaps in our knowledge of sub-surface structure of potential shale resources in these places. (Questions 4 and 5) The rates of HF-induced seismicity in other countries where shale gas production has been ongoing for many years are observed to vary widely. The limited number of HF operations in the UK means that it is difficult to make a valid comparison of the rates of occurrence of induced seismicity with elsewhere. This underlines the importance of knowledge exchange in monitoring and operational practices. (Question 6) Our review focusses on recently published geoscience related to induced seismicity caused by HF of shales. Ongoing and future research may bring new insights that may reduce uncertainties and improve mitigation of risks. We did not consider socio-economic research on perception of risks or the benefits of shale gas. Similarly, we do not consider technological advances in hydraulic fracturing

The 2020 national seismic hazard model for the United Kingdom

We present updated seismic hazard maps for the United Kingdom (UK) intended for use with the National Annex for the revised edition of Eurocode 8. The last national maps for the UK were produced by Musson and Sargeant (Eurocode 8 seismic hazard zoning maps for the UK. British Geological Survey Report CR/07/125, United Kingdom, 2007). The updated model uses an up-to-date earthquake catalogue for the British Isles, for which the completeness periods have been reassessed, and a modified source model. The hazard model also incorporates some advances in ground motion modelling since 2007, including host-to-target adjustments for the ground motion models selected in the logic tree. For the first time, the new maps are provided for not only peak ground acceleration (PGA) but also spectral acceleration at 0.2 s (SA0.2s) and 1.0 s for 5% damping on rock (time-averaged shear wave velocity for the top 30 m Vs30 ≥ 800 m/s) and four return periods, including 475 and 2475 years. The hazard in most of the UK is generally low and increases slightly in North Wales, the England–Wales border region, and western Scotland. A similar spatial variation is observed for PGA and SA0.2s but the effects are more pronounced for SA0.2s. Hazard curves, uniform hazard spectra, and disaggregation analysis are calculated for selected sites. The new hazard maps are compared with the previous 2007 national maps and the 2013 European hazard maps (Woessner et al. in Bull Earthq Eng 13:3553–3596, 2015). There is a slight increase in PGA from the 2007 maps to this work; whereas the hazard in the updated maps is lower than indicated by the European maps

National seismic hazard maps for the UK: 2020 update

This report is the published product of a study by the British Geological Survey (BGS) to update the national seismic hazard maps for the UK. This is to take account of advances in seismic hazard methodology since the last seismic hazard maps were developed by Musson and Sargeant (2007) and present the results in a format that will be compatible with the future Eurocode 8 revisions