267 results found
Bommer J, Ake JP, Munson CG, 2023, Seismic source zones for site-specific probabilistic seismic hazard analysis: the very real questions raised by virtual fault ruptures, Seismological Research Letters, Vol: 94, Pages: 1900-1911, ISSN: 0895-0695
Site‐specific probabilistic seismic hazard analyses (PSHAs) very often include areal source zones to represent diffuse seismicity that cannot be associated with known geological faults. Most modern ground‐motion prediction models use distance metrics that are defined relative to the extended fault rupture rather than the epicenter or hypocenter. For these distances to be calculated correctly, virtual fault ruptures are generated, having dimensions consistent with the earthquake magnitude, within source zones when performing PSHA calculations. Although the generation of these virtual ruptures is necessary to achieve compatibility between the seismic source and ground‐motion models within the hazard calculations, the ruptures should, by definition, represent potentially realizable seismogenic structures within the crust. Frequently, algorithms for the generation of these virtual ruptures are embedded within the PSHA code as an interim calculation without generating any outputs to enable visualization of the location and extension of the resulting ruptures. Such visualizations can reveal features of these hypothetical ruptures that may challenge the assumptions underlying the definition of the source zone boundaries that separate and enclose distinct regions of diffuse seismicity, as well as raising questions regarding the recurrence parameters within each source, especially in terms of the assumed maximum magnitudes. Visualizing the virtual ruptures generated in PSHA calculations and ensuring their consistency with the criteria established, explicitly or otherwise, for the definition of seismic source zones, could lead to important improvements in the modeling of diffuse seismicity in PSHA. We propose that this visualization should become a standard step in any PSHA study that includes source zones of diffuse seismicity. In addition, the choice of strict or leaky source zone boundaries relative to these hypothetical ruptures should always be explained and justified ra
Ntinalexis M, Kruiver P, Bommer J, et al., 2022, A database of ground-motion recordings, site profiles, and amplification factors from the Groningen gas field in the Netherlands, Earthquake Spectra, Vol: 39, Pages: 687-701, ISSN: 8755-2930
A comprehensive database that has been used to develop ground motion models for induced earthquakes in the Groningen gas field is provided in a freely accessible online repository. The database includes more than 8500 processed ground motion recordings from 87 earthquakes of local magnitude ML between 1.8 and 3.6, obtained from a large network of surface accelerographs and borehole geophones placed at 50 m depth intervals to a depth of 200 m. The 5%-damped pseudo-acceleration spectra and Fourier amplitude spectra of the records are also provided. Measured shear-wave velocity (VS) profiles, obtained primarily from seismic Cone Penetration Tests (CPTs), are provided for 80 of the ∼100 recording stations. A model representing the regional dynamic soil properties is presented for the entire gas field plus a 5 km onshore buffer zone, specifying lithology, VS, and damping for all layers above the reference baserock horizon located at about 800 m depth. Transfer functions and frequency-dependent amplification factors from the reference rock horizon to the surface for the locations of the recording stations are also included. The database provides a valuable resource for further refinement of induced seismic hazard and risk modeling in Groningen as well as for generic research in site response of thick, soft soil deposits and the characteristics of ground motions from small-magnitude, shallow-focus induced earthquakes.
Bommer JJ, Stafford PJ, Ruigrok E, et al., 2022, Ground-motion prediction models for induced earthquakes in the Groningen gas field, the Netherlands, Journal of Seismology, Vol: 26, Pages: 1157-1184, ISSN: 1383-4649
Small-magnitude earthquakes induced by gas production in the Groningen field in the Netherlands have prompted the development of seismic risk models that serve both to estimate the impact of these events and to explore the efficacy of different risk mitigation strategies. A core element of the risk modelling is ground-motion prediction models (GMPM) derived from an extensive database of recordings obtained from a dense network of accelerographs installed in the field. For the verification of damage claims, an empirical GMPM for peak ground velocity (PGV) has been developed, which predicts horizontal PGV as a function of local magnitude, ML; hypocentral distance, Rhyp; and the time-averaged shear-wave velocity over the upper 30 m, VS30. For modelling the risk due to potential induced and triggered earthquakes of larger magnitude, a GMPM for response spectral accelerations has been developed from regressions on the outputs from finite-rupture simulations of motions at a deeply buried rock horizon. The GMPM for rock motions is coupled with a zonation map defining frequency-dependent non-linear amplification factors to obtain estimates of surface motions in the region of thick deposits of soft soils. The GMPM for spectral accelerations is formulated within a logic-tree framework to capture the epistemic uncertainty associated with extrapolation from recordings of events of ML ≤ 3.6 to much larger magnitudes.
Ruigrok E, Rodriguez-Marek A, Edwards B, et al., 2022, Derivation of a near-surface damping model for the Groningen gas field, Geophysical Journal International, Vol: 230, Pages: 776-795, ISSN: 0956-540X
Seismic damping of near-surface deposits is an important input to site-response analysis for seismic hazard assessment. In Groningen, the Netherlands, gas production from a reservoir at 3 km depth causes seismicity. Above the gas field, an 800 m thick layer of unconsolidated sediments exist, which consists of a mixture of sand, gravel, clay and peat strata. Shear waves induced at 3 km depth experience most of their anelastic attenuation in these loose sediments. A good estimate of damping is therefore crucial for modeling realistic ground-motion levels. In Groningen, we take advantage of a large network of 200 m deep vertical arrays to estimate damping from recordings of the induced events. As a first step, we apply seismic interferometry by deconvolution to estimate local transfer functions over these vertical arrays. Subsequently, two different methods are employed. The first is the ’up-going’ method, where the amplitude decay of the retrieved up-going wave is used. The second is the ’up-down’ method, where the amplitude difference between retrieved up- and down-going waves is utilized. For the up-going method, the amplitude of the up-going direct wave is affected by both elastic and anelastic effects. In order to estimate the anelastic attenuation it is necessary to remove the elastic amplification first. Despite the fact that elastic compensation could be determined quite accurately, non-physical damping values were estimated for a number of boreholes. Likely, the underlying cause was small differences in effective response functions of geophones at different depths. It was found that the up-down method is more robust. With this method, elastic propagation corrections are not needed. In addition, small differences in in situ geophone response are irrelevant because the up- and down-going waves retrieved at the same geophone, are used. For the 1D case we showed that for estimating the local transfer function, the complex reverberations nee
Boore DM, Youngs RR, Kottke AR, et al., 2022, Construction of a ground-motion logic tree through host-to-target region adjustments applied to an adaptable ground-motion prediction model, Bulletin of the Seismological Society of America, Vol: 112, Pages: 3063-3080, ISSN: 0037-1106
The purpose of a median ground‐motion logic tree is to capture the center, body, and range of possible ground‐motion amplitudes for each earthquake scenario considered in a seismic hazard analysis. For site‐specific hazard analyses, the traditional approach of populating the logic tree branches with ground‐motion prediction models (GMPMs) selected and weighted on the basis of vaguely defined applicability to the target region is rapidly being abandoned in favor of the backbone GMPM approach. In this approach, the selected backbone model is first adjusted to match the earthquake source and path characteristics of the target region, and then it is separately adjusted to account for the site‐specific geotechnical profile. For a GMPM to be amenable to such host‐to‐target adjustments, the magnitude scaling of response spectral ordinates should be consistent with the theoretical scaling of Fourier amplitude spectra. In addition, the influence of individual source and path parameters should be clearly distinguished in the model to allow the adjustments to be applied individually, and reliable estimates of the source and path parameters from the host region of the GMPM should be available, as should a reference rock profile for the model. The NGA‐West2 project GMPM of Chiou and Youngs (2014; hereafter, CY14) has been identified as a very suitable backbone model. Moreover, rather than adopting generic source and path parameters and a rock site profile from the host region for CY14, which is not easily defined because the data from which it was derived came from several geographical locations, recent studies have inverted the model to obtain a CY14‐consistent reference rock profile and CY14‐compatible source and path parameters. Using these host‐region characteristics, this study illustrates the process of building a ground‐motion logic tree through the sequential application of multiple host‐to‐target‐region adjustments, each represented by a node on the logic tree to achiev
Stafford P, Boore DM, Youngs RR, et al., 2022, Host-region parameters for an adjustable model for crustal earthquakes to facilitate the implementation of the backbone approach to building ground-motion logic trees in probabilistic seismic hazard analysis, Earthquake Spectra, Vol: 38, Pages: 917-949, ISSN: 8755-2930
he backbone approach to constructing a ground-motion logic-tree for probabilistic seismic hazard analysis (PSHA) can address shortcomings in the traditional approach of populating the branches with multiple existing, or potentially modified, ground-motion models (GMM) by rendering more transparent the relationship between branch weights and the resulting distribution of predicted accelerations. In order to capture epistemic uncertainty in a tractable manner, there are benefits in building the logic tree through the application of successive adjustments for differences in source, path and site characteristics between the host-region of the selected backbone GMM and the target region for which the PSHA is being conducted. The implementation of this approach is facilitated by selecting a backbone GMM that is amenable to such host-to-target adjustments for individual source, path, and site characteristics. The NGA-West2 GMM of Chiou and Youngs (2014, CY14) has been identified as a highly adaptable model for crustal seismicity that is well suited to such adjustments. Rather than using generic source, path and site characteristics assumed appropriate for the host region, the final suite of adjusted GMMs for the target region will be better constrained if the host-region parameters are defined specifically on the basis of their compatibility with the CY14 backbone GMM. To this end, making use of a recently developed crustal shear-wave velocity profile consistent with CY14, we present an inversion of the model to estimate the key source and path parameters, namely the stress parameter and the anelastic attenuation. With these outputs, the effort in constructing a ground-motion logic-tree for any PSHA dealing with crustal seismicity can be focused primarily on the estimation of the target-region characteristics and their associated uncertainties. The inversion procedure can also be adapted for any application in which different constraints might be relevant
Bommer JJ, 2022, Earthquake hazard and risk analysis for natural and induced seismicity: towards objective assessments in the face of uncertainty., Bulletin of Earthquake Engineering, Vol: 20, Pages: 2825-3069, ISSN: 1570-761X
The fundamental objective of earthquake engineering is to protect lives and livelihoods through the reduction of seismic risk. Directly or indirectly, this generally requires quantification of the risk, for which quantification of the seismic hazard is required as a basic input. Over the last several decades, the practice of seismic hazard analysis has evolved enormously, firstly with the introduction of a rational framework for handling the apparent randomness in earthquake processes, which also enabled risk assessments to consider both the severity and likelihood of earthquake effects. The next major evolutionary step was the identification of epistemic uncertainties related to incomplete knowledge, and the formulation of frameworks for both their quantification and their incorporation into hazard assessments. Despite these advances in the practice of seismic hazard analysis, it is not uncommon for the acceptance of seismic hazard estimates to be hindered by invalid comparisons, resistance to new information that challenges prevailing views, and attachment to previous estimates of the hazard. The challenge of achieving impartial acceptance of seismic hazard and risk estimates becomes even more acute in the case of earthquakes attributed to human activities. A more rational evaluation of seismic hazard and risk due to induced earthquakes may be facilitated by adopting, with appropriate adaptations, the advances in risk quantification and risk mitigation developed for natural seismicity. While such practices may provide an impartial starting point for decision making regarding risk mitigation measures, the most promising avenue to achieve broad societal acceptance of the risks associated with induced earthquakes is through effective regulation, which needs to be transparent, independent, and informed by risk considerations based on both sound seismological science and reliable earthquake engineering.
Kruiver PP, Pefkos M, Meijles E, et al., 2022, Incorporating dwelling mounds into induced seismic risk analysis for the Groningen gas field in the Netherlands, Bulletin of Earthquake Engineering, Vol: 20, Pages: 255-285, ISSN: 1570-761X
In order to inform decision-making regarding measures to mitigate the impact of induced seismicity in the Groningen gas field in the Netherlands, a comprehensive seismic risk model has been developed. Starting with gas production scenarios and the consequent reservoir compaction, the model generates synthetic earthquake catalogues which are deployed in Monte Carlo analyses, predicting ground motions at a buried reference rock horizon that are combined with nonlinear amplification factors to estimate response spectral accelerations at the surface. These motions are combined with fragility functions defined for the exposed buildings throughout the region to estimate damage levels, which in turn are transformed to risk in terms of injury through consequence functions. Several older and potentially vulnerable buildings are located on dwelling mounds that were constructed from soils and organic material as a flood defence. These anthropogenic structures are not included in the soil profile models used to develop the amplification factors and hence their influence has not been included in the risk analyses to date. To address this gap in the model, concerted studies have been identified to characterize the dwelling mounds. These include new shear-wave velocity measurements that have enabled dynamic site response analyses to determine the modification of ground shaking due to the presence of the mound. A scheme has then been developed to incorporate the dwelling mounds into the risk calculations, which included an assessment of whether the soil-structure interaction effects for buildings founded on the mounds required modification of the seismic fragility functions.
Verdon J, Bommer J, 2021, Comment on "Activation Rate of Seismicity for Hydraulic Fracture Wells in the Western Canadian Sedimentary Basin" by Ghofrani and Atkinson (2020), Bulletin of the Seismological Society of America, Vol: 111, Pages: 3459-3474, ISSN: 0037-1106
Baker JW, Bradley BA, Stafford P, 2021, Seismic Hazard and Risk Analysis, Seismological Research Letters, Vol: 92(5), Pages: 3248-3250, ISSN: 0895-0695
The incorporation of local amplification factors determined through site response analyses has become standard practice in site-specific probabilistic seismic hazard analysis (PSHA). Another indispensable feature of the current state-of-practice in site-specific PSHA is the identification and quantification of all epistemic uncertainties that influence the final hazard estimates. Consequently, logic trees are constructed not only for seismic source characteristics and ground-motion models (GMMs) but also for the site amplification factors, the latter generally characterized by branches for alternative shear-wave velocity (VS) profiles. However, in the same way that branch weights on alternative GMMs can give rise to unintentionally narrow distributions of predicted ground-motion amplitudes, the distribution of amplification factors obtained from a small number of weighted VS profiles will often be quite narrow at some oscillator frequencies. We propose an alternative approach to capturing epistemic uncertainty in site response in order to avoid such unintentionally constricted distributions of amplification factors using more complete logic-trees for site response analyses. Nodes are included for all the factors that influence the calculated amplification factors, which may include shallow VS profiles, deeper VS profiles, depth of impedance contrasts, low-strain soil damping, and choice of modulus reduction and damping curves. Site response analyses are then executed for all branch combinations to generate a large number 2 of frequency-dependent amplification factors. Finally, these are re-sampled as a discrete distribution with enough branches to capture the underlying distribution of amplification factors (AFs). While this approach improves the representation of epistemic uncertainty in the dynamic site response characteristics, modeling uncertainty in the AFs is not automatically captured in this way, for which reason it is also proposed that a minimum level of e
Edwards B, Crowley H, Pinho R, et al., 2021, Seismic hazard and risk due to induced earthquakes at a shale gas site, Bulletin of the Seismological Society of America, Vol: 111, Pages: 875-897, ISSN: 0037-1106
Hydraulic fracturing of the first shale gas well at Preston New Road (PNR), Blackpool, United Kingdom, in late 2018, marked the end of a 7 yr United Kingdom‐wide moratorium on fracking. Despite a strict traffic‐light system being in place, seismic events up to ML 2.9 were induced. The ML 2.9 event was accompanied by reports of damage and was assigned European Macroseismic Scale 1998 (EMS‐98) intensity VI by the British Geological Survey. The moratorium was subsequently reinstated in late 2019. The study here presents a pseudo‐probabilistic seismic risk analysis and is applied to the larger of the induced events at PNR, in addition to hypothetical larger events. Initially, site characterization analysis is undertaken using direct and indirect methods. These analyses show low‐velocity deposits dominate the region (VS30¯¯¯¯¯¯=227 m/s). We test existing ground‐motion prediction equations using spatially dependent VS30 to determine applicability to the recorded waveform data and produce a referenced empirical model. Predicting median and 84th percentile peak ground velocity fields, we subsequently determine macroseismic intensities. Epicentral intensities of IV, IV–V, and VI–VII are predicted for the observed ML 2.9, and hypothetical ML 3.5 and 4.5 scenarios, respectively. A probabilistic analysis of damage is performed for 3500 ground‐motion realizations (2.1≤ML≤4.5) using the OpenQuake‐engine, with nonlinear dynamic analysis undertaken to define building fragility. Based on these analyses, the onset of cosmetic damage (DS1) in terms of median risk is observed for the ML 2.9 event. Mean modeled occurrences of DS1 and DS2 (minor structural damage), 75 and 10 instances, respectively, are consistent with reported damage (DS1:97, DS2:50). Significant occurrences (median≥30 buildings) of DS2, DS3, and DS4 (minor to major structural damage) are likely for ML 3.5, 4.0, and 4.5 events, respectively. However
Bommer J, Verdon JP, 2021, Green, yellow, red, or out of the blue? An assessment of Traffic Light Schemes to mitigate the impact of hydraulic fracturing-induced seismicity, Journal of Seismology, Vol: 25, Pages: 301-326, ISSN: 1383-4649
Mitigating hydraulic fracturing-induced seismicity (HF-IS) poses a challenge for shale gas companies and regulators alike. The use of Traffic Light Schemes (TLSs) is the most common way by which the hazards associated with HF-IS are mitigated. In this study, we discuss the implicit risk mitigation objectives of TLSs and explain the advantages of magnitude as the fundamental parameter to characterise induced seismic hazard. We go on to investigate some of the key assumptions on which TLSs are based, namely that magnitudes evolve relatively gradually from green to yellow to red thresholds (as opposed to larger events occurring “out-of-the-blue”), and that trailing event magnitudes do not increase substantially after injection stops. We compile HF-IS datasets from around the world, including the USA, Canada, the UK, and China, and track the temporal evolution of magnitudes in order to evaluate the extent to which magnitude jumps (i.e. sharp increases in magnitude from preceding events within a sequence) and trailing events occur. We find in the majority of cases magnitude jumps are less than 2 units. One quarter of cases experienced a post-injection magnitude increase, with the largest being 1.6. Trailing event increases generally occurred soon after injection, with most cases showing no increase in magnitude more than a few days after then end of injection. Hence, the effective operation of TLSs may require red-light thresholds to be set as much as two magnitude units below the threshold that the scheme is intended to avoid.
Bommer J, Stafford PJ, 2020, Selecting ground-motion models for site-specific PSHA: adaptability vs applicability, Bulletin of the Seismological Society of America, Vol: 110, Pages: 2801-2815, ISSN: 0037-1106
Capturing the center, the body and the range of ground-motion predictions is an indispensableelement of site-specific probabilistic seismic hazard analyses (PSHA), for which the logic tree isthe ubiquitous tool in current practice. The criteria for selecting the ground-motion models (GMMs)used in such studies have generally been focused on their potential applicability to the region andsite for which the PSHA is being conducted. However, except for applications within the fewregions with abundant ground-motion databases, it will rarely be the case that GMMs can beidentified which are perfectly calibrated to the characteristics of the target study region in termsof source and path properties. A good match between the generic site amplification model withinthe GMM and the site-specific dynamic response characteristics is equally, if not more, unlikely.Consequently, adjustments are likely to be made to the selected GMMs to render them moreapplicable to the target region and site. Empirical adjustments for host-to-target region sourcedifferences using local recordings are unlikely to be robust unless these have been generated byearthquakes from a wide range of magnitudes. Empirical adjustments for site characteristics areimpossible unless there are recordings from the target site. Therefore, the preferred approachmakes parametric adjustments to empirical GMMs, isolating each host-to-target difference to mapthe individual contributions to the epistemic uncertainty. For such an approach to be applied, theemphasis moves from selecting GMMs on the basis of their applicability to focusing on theiramenability to being adjusted to the target region and site. An adaptable equation is characterizedby well constrained host-region source, path and site characteristics and a functional form inwhich response spectral accelerations scale with source, path and site characteristics in a mannersimilar to the scaling implicit in stochastic simulations based on Fourier amplitude spectra.
Bommer J, Green RA, Stafford PJ, et al., 2020, Liquefaction hazard of the Groningen region of the Netherlands due to induced seismicity, Journal of Geotechnical and Geoenvironmental Engineering, Vol: 146, Pages: 04020068-1-04020068-15, ISSN: 0733-9410
The operator of the Groningen gas field is leading an effort to quantify the seismic hazard and riskof the region due to induced earthquakes, includingoverseeing one of the most comprehensive liquefaction hazard studies performedgloballyto date. Due tothe unique characteristics of the seismic hazard and the geologic deposits in Groningen, efforts first focused on developing relationships for a Groningen-specific liquefaction triggering model. The liquefaction hazard was then assessedusing a Monte Carlo method, wherein a range of credibleevent scenarios were considered in computingliquefaction damage-potentialhazard curves. Thiseffort entailed the use of a regional stochastic seismic source model,ground motion prediction equation,site response model,and geologic model that were developed as part of the broader regional seismic hazardassessment.“No-to-Minor Surficial Liquefaction Manifestations”arepredicted for mostsites across the study areafor a 75-year return period. The only sites where “Moderate Surficial Liquefaction Manifestations” are predicted are in the town of Zandeweer, with only some of the sites in the townbeing predicted to experience this severityof liquefactionfor thisreturn period.
Bommer J, Montaldo-Falero V, 2020, Virtual fault ruptures in area source zones for PSHA: are they always needed?, Seismological Research Letters, Vol: 91, Pages: 2310-2319, ISSN: 0895-0695
Seismic source models for probabilistic seismic hazard analysis (PSHA), except when using zoneless approaches based directly on the earthquake catalog, invariably include area‐source zones, even if active fault sources are modeled explicitly. Because most modern ground‐motion prediction equations (GMPEs) employ source‐to‐site distance metrics defined relative to extended fault ruptures rather than to the epicenter or hypocenter, it becomes necessary to generate virtual fault ruptures within the area‐source zones to enable calculation of the correct distance of each earthquake scenario from the site of interest. For a site‐specific PSHA, the work of defining the virtual rupture characteristics such as strike, dip, and style of faulting, for more distant source zones, and the computational effort of simulating these ruptures for each earthquake scenario in the hazard calculations, may be unnecessary. Beyond a certain distance from the site, it can be demonstrated that the loss of accuracy introduced by modeling the individual earthquake scenarios as point sources rather than as extended ruptures is usually sufficiently small to allow the distance metric in the GMPEs to be treated as epicentral or hypocentral distance. Such simplifications can significantly increase the efficiency of the hazard calculations and also relieve the seismic source modelers of considerable effort to characterize virtual ruptures far beyond the host zone of the site. Treating earthquake scenarios in the more remote source zones as points also brings the additional benefit of avoiding problems that can arise with the largest magnitude scenarios leading to ruptures that approach the site in cases for which the ruptures are not constrained to remain within the source boundaries.
Green RA, Bommer JJ, 2020, RESPONSE TO Discussion of "What is the smallest earthquake magnitude that needs to be considered in assessing liquefaction hazard?" by Roger MW Musson, Earthquake Spectra, Vol: 36, Pages: 455-457, ISSN: 8755-2930
Nievas CI, Bommer J, Crowley H, et al., 2020, A database of damaging small-to-medium magnitude earthquakes, Journal of Seismology, Vol: 24, Pages: 263-292, ISSN: 1383-4649
Interest in small-to-medium magnitude earthquakes and their potential consequences has increased significantly in recent years, mostly due to the occurrence of some unusually damaging small events, the development of seismic risk assessment methodologies for existing building stock, and the recognition of the potential risk of induced seismicity. As part of a clear ongoing effort of the earthquake engineering community to develop knowledge on the risk posed by smaller events, aglobal database of earthquakes withmomentmagnitudes in the range from 4.0 to 5.5 for which damage and/or casualties have been reportedhas been compiledand is made publicly available. Thetwo main purposeswereto facilitate studies onthe potential for earthquakes in thismagnitude range to cause material damage and to carry out a statistical study to characterise the frequency with which earthquakes of this size cause damage and/or casualties(published separately). The present paper describes the data sources and processfollowedfor the compilation of the database, whileproviding critical discussions on the challenges encounteredand decisions made, which are of relevance for itsinterpretation and use.The geographic, temporal and magnitude distributions of the 1,958 earthquakesthat make up the databaseare presented alongside thegeneral statistics on damage and casualties, noting that these stem from a variety of sources of differing reliability.Despite its inherent limitations, we believe it is an important contributionto the understanding of the extent of the consequences that may arise from earthquakes in the magnitude range of study.
Bommer J, NIevas CI, Helen C, et al., 2020, Global occurrence and impact of small-to-medium magnitude earthquakes: A statistical analysis, Bulletin of Earthquake Engineering, Vol: 18, Pages: 1-35, ISSN: 1570-761X
Despite their much smaller individual contribution to the global counts of casualties and damage than theirlarger counterparts, earthquakes with moment magnitudes Mw in the range 4.0-5.5 may dominate seismichazard and risk in areas of low overall seismicity, a statement that is particularly true for regions whereanthropogenically-induced earthquakes are predominant. With the risk posed by these earthquakescausing increasing alarm in certain areas of the globe, it is of interest to determine what proportion ofearthquakes in this magnitude range that occur sufficiently close to population or the built environment doactually result in damage and/or casualties. For this purpose, a global catalogue of potentially damagingevents—that is, earthquakes deemed as potentially capable of causing damage or casualties based on aseries of pre-defined criteria—has been generated and contrasted against a database of reportedlydamaging small-to-medium earthquakes compiled in parallel to this work. This paper discusses the criteriaand methodology followed to define such a set of potentially damaging events, from the issues inherent toearthquake catalogue compilation to the definition of criteria to establish how much potential exposure issufficient to consider each earthquake a threat. The resulting statistics show that, on average, around 2% ofall potentially-damaging shocks were actually reported as damaging, though the proportion variessignificantly in time as a consequence of the impact of accessibility to data on damage and seismicity ingeneral. Inspection of the years believed to be more complete suggests that a value of around 4 to 5%might be a more realistic figure.
Green RA, Bommer JJ, 2019, Discussion of what is the smallest earthquake magnitude that needs to be considered in assessing liquefaction hazard? by Roger M.W. Musson, Earthquake Spectra, Pages: 071019EQS162A-071019EQS162A, ISSN: 8755-2930
Bahrampouri M, Rodriguez-Marek A, Bommer JJ, 2019, Mapping the uncertainty in modulus reduction and damping curves onto the uncertainty of site amplification functions, Soil Dynamics and Earthquake Engineering, Vol: 126, ISSN: 0267-7261
Probabilistic seismic hazard analysis often requires the use of site response analyses to calculate site amplification factors (AFs) that capture the effects of near-surface layers. The site response analyses must also be conducted in a probabilistic framework to quantify the variability in AFs resulting from variation in the rock input motions and uncertainty in the dynamic properties of the soil profile, including uncertainty in the shear-wave velocity and in the nonlinear behavior of the soil. In equivalent linear analyses, the latter is captured through modulus reduction and damping curves (MRD). The joint randomization of shear-wave velocity and MRD curves can result in large computational costs. The objective of this paper is to propose a procedure to separately calculate the contribution of uncertainty in the MRD curves to the total uncertainty in the AFs for a site. This procedure is illustrated in an application for a seismic hazard and risk assessment of the Groningen gas field in the Netherlands. The results show that the effect of uncertainty in MRD curves on the uncertainty in the AFs is highly dependent on the soil profile. This effect can be significant for soft soils and strong input motions. Results also indicate that the effects of MRD uncertainty can be correlated to elastic site properties.
Ntinalexis M, Bommer JJ, Ruigrok E, et al., 2019, Ground-motion networks in the Groningen field: usability and consistency of surface recordings, JOURNAL OF SEISMOLOGY, Vol: 23, Pages: 1233-1253, ISSN: 1383-4649
Several strong-motion networks have been installed in the Groningen gas field in the Netherlands to record ground motions associated with induced earthquakes. There are now more than 450 permanent surface accelerographs plus a mobile array of 450 instruments, which, in addition to many instrumented boreholes, yield a wealth of data. The database of recordings has been of fundamental importance to the development of ground-motion models that form a key element of the seismic hazard and risk estimations for the field. In order to maximise the benefit that can be derived from these recordings, this study evaluates the usability of the recordings from the different networks, in general terms and specifically with regards to the frequency ranges with acceptable signal-to-noise ratios. The study also explores the consistency among the recordings from the different networks, highlighting in particular how a configuration error was identified and resolved. The largest accelerograph network consists of instruments housed in buildings around the field, frequently installed on the lower parts of walls rather than on the floor. A series of experiments were conducted, using additional instruments installed adjacent to these buildings and replicating the installation configuration in full-scale shake table tests, to identify the degree to which structural response contaminated the recordings. The general finding of these efforts was that for PGV and oscillator periods above 0.1 s, the response spectral ordinates from these recordings can be used with confidence.
Green RA, Bommer JJ, Rodriguez-Marek A, et al., 2019, Addressing limitations in existing ‘simplified’ liquefaction triggering evaluation procedures: application to induced seismicity in the Groningen gas field, Bulletin of Earthquake Engineering, Vol: 17, Pages: 4539-4557, ISSN: 1570-761X
The Groningen gas field is one of the largest in the world and has produced over 2000 billion m3 of natural gas since the start of production in 1963. The first earthquakes linked to gas production in the Groningen field occurred in 1991, with the largest event to date being a local magnitude (ML) 3.6. As a result, the field operator is leading an effort to quantify the seismic hazard and risk resulting from the gas production operations, including the assessment of liquefaction hazard. However, due to the unique characteristics of both the seismic hazard and the geological subsurface, particularly the unconsolidated sediments, direct application of existing liquefaction evaluation procedures is deemed inappropriate in Groningen. Specifically, the depth-stress reduction factor (rd) and the magnitude scaling factor relationships inherent to existing variants of the simplified liquefaction evaluation procedure are considered unsuitable for use. Accordingly, efforts have first focused on developing a framework for evaluating the liquefaction potential of the region for moment magnitudes (M) ranging from 3.5 to 7.0. The limitations of existing liquefaction procedures for use in Groningen and the path being followed to overcome these shortcomings are presented in detail herein.
Green R, Bommer J, 2019, What is the smallest earthquake magnitude that needs to be considered in assessing liquefaction hazard?, Earthquake Spectra, Vol: 35, Pages: 1441-1464, ISSN: 8755-2930
Probabilistic assessments of the potential impact of earthquakes on the infrastructure entails the consideration of smaller magnitude events than those generally considered in deterministic hazard and risk assessments. In this context, it is useful to establish if there is a magnitude threshold below which the possibility of triggering liquefaction can be discounted because such a lower bound is required for probabilistic liquefaction hazard analyses. Based on field observations and a simple parametric study, we conclude that earthquakes as small as moment magnitude 4.5 can trigger liquefaction in extremely susceptible soil deposits. However, for soil profiles that are suitable for building structures, the minimum earthquake magnitude for the triggering of liquefaction is about 5. We therefore propose that in liquefaction hazard assessments of building sites magnitude 5.0 be adopted as the minimum earthquake size considered, while magnitudes as low as 4.5 may be appropriate for some other types of infrastructure.
Stafford PJ, Zurek BD, Ntinalexis M, et al., 2019, Extensions to the Groningen ground-motion model for seismic risk calculations: component-to-component variability and spatial correlation, Bulletin of Earthquake Engineering, Vol: 17, Pages: 4417-4439, ISSN: 1573-1456
A bespoke ground-motion model has been developed for the prediction of response spectral accelerations, peak ground velocity and significant duration due to induced earthquakes in the Groningen gas field in the Netherlands. For applications to the calculation of risk to the exposed building stock, extensions to the model are required. The use of the geometric mean horizontal component in the ground-motion predictions and the arbitrary horizontal component for the building fragility functions requires the addition of component-to-component variability. A model for this variability has been developed that both reflects the strong horizontal polarisation of motions observed in many Groningen records obtained at short distances and the fact that the strong polarisation is unlikely to persist at larger magnitudes. The other extension of the model is the spatial correlation of ground motions for the calculation of aggregated risk, which can be approximated through simple rules for sampling the variance within site response zones. Making use of ground-motion recordings from several networks in the field and the results of finite difference waveform simulations, a Groningen-specific spatial correlation model has been developed. The new model also combines results from traditional variogram fitting approaches with a new method to infer spatial correlation lengths from observed variance reduction. The development of the new spatial correlation model relaxes the need to approximate spatial correlation through the sampling of site response, although the results obtained herein suggest that similar results could be obtained using either approach. The preliminary consideration of the numerical waveform modelling results in this study paves the way for significant extensions to be made for the modelling of spatial correlations and the decomposition of apparent spatial variability into systematic and random components within a fully non-ergodic framework .
Edwards B, Zurek B, van Dedem E, et al., 2019, Simulations for the development of a ground motion model for induced seismicity in the Groningen gas field, the Netherlands, Bulletin of Earthquake Engineering, Vol: 17, Pages: 4441-4456, ISSN: 1570-761X
We present simulations performed for the development of a ground motion model for induced earthquakes in the Groningen gas field. The largest recorded event, with M3.5, occurred in 2012 and, more recently, a M3.4 event in 2018 led to recorded ground accelerations exceeding 0.1 g. As part of an extensive hazard and risk study, it has been necessary to predict ground motions for scenario earthquakes up to M7. In order to achieve this, while accounting for the unique local geology, a range of simulations have been performed using both stochastic and full-waveform finite-difference simulations. Due to frequency limitations and lack of empirical calibration of the latter approach, input simulations for the ground motion model used in the hazard and risk analyses have been performed with a finite-fault stochastic method. However, in parallel, extensive studies using the finite-difference simulations have guided inputs and modelling considerations for these simulations. Three approaches are used: (1) the finite-fault stochastic method, (2) elastic point- and (3) finite-source 3D finite-difference simulations. We present a summary of the methods and their synthesis, including both amplitudes and durations within the context of the hazard and risk model. A unique form of wave-propagation with strong lateral focusing and defocusing is evident in both peak amplitudes and durations. The results clearly demonstrate the need for a locally derived ground motion model and the potential for reduction in aleatory variability in moving toward a path-specific fully non-ergodic model.
van Elk J, Bourne SJ, Oates SJ, et al., 2019, A probabilistic model to evaluate options for mitigating induced seismic risk, Earthquake Spectra, Vol: 35, Pages: 537-564, ISSN: 8755-2930
Common responses to induced seismicity are based on control of the anthropogenic activity causing the earthquakes, like fluid injection or withdrawal, in order to limit either the magnitudes of the events or the level of ground motion to within established thresholds. An alternative risk mitigation option is seismic retrofitting of the more vulnerable buildings potentially exposed to the ground shaking in order to reduce the risk to acceptable levels. Optimal mitigation strategies may combine both production control and structural strengthening, for which a probabilistic risk model is required that can estimate the change in hazard due to production or injection variations and the changes in fragility resulting from structural interventions. Such a risk model has been developed for the Groningen gas field in the Netherlands. The framework for this risk model to inform decision-making regarding mitigation strategies can be adapted to other cases of anthropogenically-induced seismicity.
Dost B, Edwards B, Bommer JJ, 2019, Erratum The Relationship between M and ML: A Review and Application to Induced Seismicity in the Groningen Gas Field, The Netherlands, Seismological Research Letters, ISSN: 0895-0695
Verdon JP, Baptie BJ, Bommer J, 2019, An improved framework for discriminating seismicity induced by industrial activities from natural earthquakes, Seismological Research Letters, Vol: 90, Pages: 1592-1611, ISSN: 0895-0695
Heightened concerns regarding induced seismicity necessitate robust methods to assess whether detected earthquakes near industrial sites are natural or induced by the industrial activity. These assessments are required rapidly, which often precludes detailed modeling of fluid pressures and the geomechanical response of the reservoir and nearby faults. Simple question‐based assessment schemes in current use are a useful tool but suffer from several shortcomings: they do not specifically address questions regarding whether available evidence supports the case for natural seismicity; they give all questions equal weighting regardless of the relative influence of different factors; they are not formulated to account for ambiguous or uncertain evidence; and the final outcomes can be difficult to interpret. We propose a new framework that addresses these shortcomings by assigning numerical scores to each question, with positive values for answers that support induced seismicity and negative values for responses favoring natural seismicity. The score values available for each question reflect the relative importance of the different questions, and for each question the absolute value of the score is modulated according to the degree of uncertainty. The final outcome is a score, the induced assessment ratio, either positive or negative (or zero), that reflects whether events were induced or natural. A second score, the evidence strength ratio, is assigned that characterizes the strength of the available evidence, expressed as the ratio of the maximum score possible with the available evidence relative to the maximum score that could be obtained if all desired data were available at a site. We demonstrate this approach by application to two case studies in the United Kingdom, one widely regarded as a case of induced seismicity, and the other more likely to be a series of tectonic earthquakes.
de Almeida AAD, Assumpção M, Bommer JJ, et al., 2019, Probabilistic seismic hazard analysis for a nuclear power plant site in southeast Brazil, Journal of Seismology, Vol: 23, Pages: 1-23, ISSN: 1383-4649
A site-specific probabilistic seismic hazard analysis (PSHA) has been performed for the only nuclear power plant site in Brazil, located 130 km southwest of Rio de Janeiro at Angra dos Reis. Logic trees were developed for both the seismic source characterisation and ground-motion characterisation models, in both cases seeking to capture the appreciable ranges of epistemic uncertainty with relatively few branches. This logic-tree structure allowed the hazard calculations to be performed efficiently while obtaining results that reflect the inevitable uncertainty in long-term seismic hazard assessment in this tectonically stable region. An innovative feature of the study is an additional seismic source zone added to capture the potential contributions of characteristics earthquake associated with geological faults in the region surrounding the coastal site.
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