Seismic Hazard Assessment for a Wind Farm Offshore England

Offshore wind has become a major contributor to reducing global carbon emissions. This paper presents a probabilistic seismic hazard analysis for the Sofia Offshore Wind Farm, which is located about 200 km north-east of England in the southern North Sea and will be one of the largest offshore wind farms in the world once completed. The seismic source characterization is composed of two areal seismic source models and four seismic source models derived using smoothed gridded seismicity with earthquake catalogue data processed by different techniques. The ground motion characterization contains eight ground motion models selected based on comparisons with regional data. The main findings are (1) the variation in seismic hazard across the site is negligible; (2) the main source controlling the hazard is the source that includes the 1931 Dogger Bank earthquake; (3) earthquake scenarios controlling the hazard are Mw = 5.0–6.3 and R = 110–210 km; and (4) the peak ground accelerations on rock are lower than for previous regional studies. These results could help guide future seismic hazard assessments in the North Sea.

Bayesian validation of the seismic source models using the Wylfa Newydd site in the UK as a case study

In probabilistic seismic hazard assessment, the development of the seismic source characterization, especially the geometry of the seismic source models (SSMs), is controversial because it often relies on expert judgment with different interpretations of the available data from seismology, tectonics, and geology. Based on the same input datasets, different teams of experts may derive different SSMs. In this context, the verification of the models through the comparison against a set of observations is a crucial step. We present a statistical tool to compare the SSMs with the observed seismicity and rank these SSMs based on their ability to replicate the past seismicity. We simulate many synthetic catalogues derived from candidate SSMs and compare them with the observed catalogue of mainshocks using the Metropolis-Hastings Algorithm to select those that fit the observed catalogue. The candidate SSMs are then expressed by a probability density function (pdf) using the set of synthetic catalogues accepted by the Metropolis-Hastings Algorithm and the Bayesian inference. To help practitioners in earthquake and civil engineering understand how this tool works in practice, the proposed approach is applied to a proposed new nuclear site in the United Kingdom, Wylfa Newydd.

Probabilistic Seismic Hazard Assessment for United Arab Emirates, Qatar and Bahrain

A probabilistic seismic hazard assessment in terms of peak ground acceleration (PGA) and spectral acceleration (SA) values, for both 10% and 5% probability of exceedance in 50 years, has been performed for the United Arab Emirates, Qatar, and Bahrain. To do that, an updated, unified, and Poissonian earthquake catalog (since 685 to 2019) was prepared for this work. Three alternative seismic source models were considered in a designed logic-tree framework. The discrimination between the shallow and intermediate depth seismicity along the Zagros and the Makran regions was also considered in this assessment. Three alternative ground-motion attenuation models for crustal earthquakes and one additional for intermediate-depth ones have been selected and applied in this study, considering the predominant stress regime computed previously for each defined source. This assessment highlights that the maximum obtained hazard values are observed in the northeastern part of the studied region, specifically at Ras Al-Khaimah, Umm Al-Quwain, and Fujaira, being characterized by mean PGA and SA (0.2 s) pair values equal to (0.13 g, 0.30 g), (0.12 g, 0.29 g), and (0.13 g, 0.28 g), respectively, for a 475-year return period and for B/C National Earthquake Hazards Reduction Program (NEHRP) boundary site conditions. Seismic hazard deaggregation in terms of magnitude and distance was also computed for a return period of 475 years, for ten emirates and cities, and for four different spectral periods.

Virtual Fault Ruptures in Area-Source Zones for PSHA: Are They Always Needed?

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.

Vertical force scaling in seismic source models of underground nuclear explosions

SUMMARY We have reanalysed observations of body waves and surface waves for 71 well-recorded underground nuclear explosions (UNEs) that were conducted between 1977 and 1989 at the Balapan subregion of the Semipalatinsk Test Site in Kazakhstan. To reconcile differences between body-wave and surface-wave amplitudes, we solve for a scaling factor between vertical and horizontal forces in the explosion model. We find that the estimated scaling factor is anticorrelated with the scaled depth of burial for the subset of UNEs at Balapan that have published depths. The observed anticorrelation and the inferred variations in force scaling suggest that recorded surface-wave amplitudes are significantly influenced by UNE burial depth as well as by previously recognized tectonic release. As part of our analysis, we revisit the relationship between teleseismic mb(P) and yield for UNEs at Balapan, and discuss the physical basis for effectiveness of the mb–MS discriminant.

Seismic Hazard Assessment in Australia: Can Structured Expert Elicitation Achieve Consensus in the “Land of the Fair Go”?

The 2018 National Seismic Hazard Assessment of Australia incorporated 19 alternative seismic-source models developed by members of the Australian seismological community. The diversity of these models demonstrates the deep epistemic uncertainty that exists with regards to how best to characterize seismicity in stable continental regions. In the face of similarly high uncertainty, a diverse range of ground-motion models was proposed for use. A complex logic tree was developed to incorporate the alternative component models into a single hazard model. Expert opinion was drawn upon to weight the alternative logic-tree branches through a structured expert elicitation process. Expert elicitation aims to transparently and reproducibly characterize the community distribution of expert estimates for uncertain quantities and thereby quantify the epistemic uncertainty around estimates of seismic hazard in Australia. We achieve a multimodel rational consensus in which each model, and each expert, is, in accordance with the Australian cultural myth of egalitarianism, given a “fair go”—that is, judged on their merits rather than their status. Yet despite this process, we find that the results are not universally accepted. A key issue is a contested boundary between what is scientifically reducible and what remains epistemologically uncertain, with a particular focus on the earthquake catalog. Furthermore, a reduction, on average, of 72% for the 10% in 50 yr probability of exceedance peak ground acceleration levels compared with those underpinning existing building design standards, challenges the choice of metrics upon which design codes are based. Both quantification of the bounds of epistemic uncertainties through expert elicitation and reduction of epistemic uncertainties through scientific advances have changed our understanding of how the hazard behaves. Dialog between scientists, engineers, and policy makers is required to ensure that as our understanding of the hazard evolves, the hazard metrics used to underpin risk management decisions are re-evaluated to ensure societal aims are achieved.

An updated and unified earthquake catalog from 1787 to 2018 for seismic hazard assessment studies in Mexico

Here we present a new updated and unified Poissonian earthquake catalog for Mexico. The details about the catalog compilation, the removal of duplicate events, unifying the magnitude scales, removal of dependent events through the declustering process and its completeness analysis are presented. Earthquake and focal mechanism data have been compiled from various local, regional and international sources. Large earthquake events (MW ≥ 6.5) have been carefully revised for their epicentral locations and magnitudes from trusted publications. Different magnitude-conversion relationships, compatible with available local and regional ones, has been established to obtain unified moment magnitude estimates for the whole catalog. Completeness periods for the declustered catalog were estimated for the definition of appropriate seismic source models for the whole territory. The final unified Poissonian earthquake catalog spans from 1787 to 2018, covering a spatial extent of 13° to 33°N and 91° to 117°W. This catalog is compatible with other published catalogs providing basis for new analysis related to seismicity, seismotectonics and seismic hazard assessment in Mexico.

Paleoliquefaction Studies and the Evaluation of Seismic Hazard

Recent and historical studies of earthquake-induced liquefaction, as well as paleoliquefaction studies, demonstrate the potential usefulness of liquefaction data in the assessment of the earthquake potential of seismic sources. Paleoliquefaction studies, along with other paleoseismology studies, supplement historical and instrumental seismicity and provide information about the long-term behavior of earthquake sources. Paleoliquefaction studies focus on soft-sediment deformation features, including sand blows and sand dikes, which result from strong ground shaking. Most paleoliquefaction studies have been conducted in intraplate geologic settings, but a few such studies have been carried out in interplate settings. Paleoliquefaction studies provide information about timing, location, magnitude, and recurrence of large paleoearthquakes, particularly those with moment magnitude, M, greater than 6 during the past 50,000 years. This review paper presents background information on earthquake-induced liquefaction and resulting soft-sediment deformation features that may be preserved in the geologic record, best practices used in paleoliquefaction studies, and application of paleoliquefaction data in earthquake source characterization. The paper concludes with two examples of regional paleoliquefaction studies—in the Charleston seismic zone and the New Madrid seismic zone in the southeastern and central United States, respectively—which contributed to seismic source models used in earthquake hazard assessment.

Peruvian Subduction Surface Model for Seismic Hazard Assessments

Throughout the years seismic hazard calculations in Peru have been developed using area sources models, having to date a great variety of models, however, since they are discretized planar models, they cannot adequately represent the continuity and subduction characteristics of the Nazca Plate. The main objective of this work is the developing of a surface subduction model (SSM), useful for seismic hazard assessments as well as the revision and control of previous models used in this sort of assessments. In this study a spatial interpolation was performed employing the Local Polynomial Interpolation method to capture short-range variation in addition to long-range trends. The data base is based on the compilation of seismic catalogs from Peruvian and international institutions such as the IGP, the USGS, the ISC and others, subsequently, in order to have independent events the elimination of duplicate events, aftershocks and foreshocks was carried out. Then, by interpolation of the focal depths of the independent events, a subduction surface model (SSM) was generated as well as a Standard Error Surface which supports a good correlation of the model. Furthermore, 14 transversal sections of the SSM was employed to compare with the hypocenter’s distributions, evidencing a good correlation with the spatial distribution of the events, in addition to adequately capturing the subduction characteristics of the Nazca Plate. Finally, a comparison was made between 2 Peruvian area models for seismic hazard and SSM developed in the present research, evidencing that seismic source models of the area type have deficiencies mainly in the depths they consider, thus is recommended the use of the present model for future seismic hazard assessments.