Recommendations for the shape of the design response spectrum in the New Zealand seismic loadings technical specification

The U.S. Geological Survey (USGS) National Seismic Hazard Model (NSHM) is the scientific foundation of seismic design regulations in the United States and is regularly updated to consider the best available science and data. The 2018 update of the conterminous U.S. NSHM includes significant changes to the underlying ground motion models (GMMs), most of which are necessary to enable the new multi-period response spectra (MPRS) requirements of seismic design regulations that use hazard results for 22 spectral periods and eight site classes. This article focuses on the GMMs used in the western United States (WUS) and is a companion to a recent article on the GMMs used in the central and eastern United States (CEUS). In the WUS, for crustal and subduction earthquakes, two models used in previous versions of the NSHM are excluded to provide consistency over all considered periods and site classes. To more accurately estimate ground motions at long periods in the vicinity of Los Angeles, San Francisco, Salt Lake City, and Seattle, the 2018 NSHM incorporates deep sedimentary basin depth from local seismic velocity models. The subduction GMMs considered lack basin depth terms and are modified to include an additional scale factor to account for this. This article documents the WUS GMMs used in the 2018 NSHM update and provides detail on the changes to GMM medians, aleatory variability, epistemic uncertainty, and site-effect models. It compares each of these components with those considered in prior NSHMs and discusses their total effect on hazard.

As part of the update of the 2018 National Seismic Hazard Model (NSHM) for the conterminous United States (CONUS), new ground motion and site effect models for the central and eastern United States were incorporated, as well as basin depths from local seismic velocity models in four western US (WUS) urban areas. These additions allow us, for the first time, to calculate probabilistic seismic hazard curves for an expanded set of spectral periods (0.01 to 10 s) and site classes (VS30 = 150 to 1500 m/s) for the CONUS, as well as account for amplification of long-period ground motions in deep sedimentary basins in the Los Angeles, San Francisco Bay, Seattle, and Salt Lake City areas. Two sets of 2018 NSHM hazard data (hazard curves and uniform-hazard ground motions) are available: (1) 0.05°-latitude-by-0.05°-longitude gridded data for the CONUS and (2) higher resolution 0.01°-latitude-by-0.01°-longitude gridded data for the four WUS basins. Both sets of data contain basin effects in the WUS deep sedimentary basins. Uniform-hazard ground motion data are interpolated for 2, 5, and 10% probability of exceedance in 50 years from the hazard curves. The gridded data for the hazard curves and uniform-hazard ground motions, for all periods and site classes, are available for download at the U.S. Geological Survey ScienceBase Catalog ( https://doi.org/10.5066/P9RQMREV ). The design ground motions derived from the hazard curves have been accepted by the Building Seismic Safety Council for adoption in the 2020 National Earthquake Hazard Reduction Program Recommended Seismic Provisions.

The United States Geological Survey (USGS) National Seismic Hazard Model (NSHM) is the scientific foundation of seismic design regulations in the United States and is regularly updated to consider the best available science and data. The 2018 update of the conterminous US NSHM includes major changes to the underlying ground motion models (GMMs). Most of the changes are motivated by the new multi-period response spectra requirements of seismic design regulations that use hazard results for 22 spectral periods and 8 site classes. In the central and eastern United States (CEUS), the 2018 NSHM incorporates 31 new GMMs for hard-rock site conditions [Formula: see text], including the Next Generation Attenuation (NGA)-East GMMs. New aleatory variability and site-effect models, both specific to the CEUS, are applied to all median hard-rock GMMs. This article documents the changes to the USGS GMM selection criteria and provides details on the new CEUS GMMs used in the 2018 NSHM update. The median GMMs, their weights, epistemic uncertainty, and aleatory variability are compared with those considered in prior NSHMs. This article further provides implementation details on the CEUS site-effect model, which allows conversion of hard-rock ground motions to other site conditions in the CEUS for the first time in NSHMs. Compared with the 2014 NSHM hard-rock ground motions, the weighted average of median GMMs increases for large magnitude events at middle to large distance range, epistemic uncertainty increases in almost all situations, but aleatory variability is not significantly different. Finally, the total effect on hazard is demonstrated for an assumed earthquake source model in the CEUS, which shows an increased ring of ground motions in the vicinity of the New Madrid seismic zone and decreased ground motions near the East Tennessee seismic zone.

The rise of performance-based earthquake engineering, in combination with the complexity associated with selecting records for time-history analysis, demonstrates an expressed need for localized default suites of ground motion records for structural designers to use in the absence of site-specific studies. In the current research investigation, deaggregations of probabilistic seismic hazard models (National Seismic Hazard Model, Canterbury Seismic Hazard Model, and Kaikōura Seismic Hazard Model) and the location-specific seismological characteristics of expected ground motions were used to define eight seismic hazard zonations and accompanying suite profiles for the South Island of New Zealand to satisfy the requirements of the New Zealand structural design standard NZS1170.5 for response-history analyses. Specific records, including 21 from the recent Kaikōura, Darfield, and Christchurch earthquakes, were then selected from publicly-available databases and presented as default suites for use in time-history analyses in the absence of site-specific studies. This investigation encompasses seismic hazards corresponding to 500-year return periods, site classes C (shallow soils) and D (deep soils), and buildings with fundamental periods between 0.4 and 2.0 seconds.

The current New Zealand seismic design provisions are expected to be updated with a proposed Technical Specification (TS). The update is motivated primarily by the recent release of the 2022 National Seismic Hazard Model, with new seismic hazard estimates across the country. The updates are being carried out by the Seismic Risk Working Group (SRWG). One of the SRWG’s primary intentions for the proposed TS was to maintain the Building Code objective of safeguarding people from injury in light of these hazard changes. While previous code development in New Zealand has not explicitly assessed whether the life-safety risk was tolerably low, the SRWG sought to implement a new life-safety risk assessment methodology. The risk assessment takes an Ultimate Limit State (ULS) design spectrum as input and provides an expected distribution of the fatality risk, representing the variety of buildings that could be designed in accordance with the minimum requirements associated with ULS. The methodology is made up of four modules representing (A) the shaking hazard, (B) the building performance (collapse fragility), (C) the probability of fatality given collapse, and (D) the variability in performance among code-conforming buildings. The first three modules quantify fatality risk for a single building, while the fourth module iterates over many buildings to produce a full risk distribution. Using this methodology, the SRWG found that for Importance Level 2 (IL2) buildings, a ULS design spectrum with an annual probability of exceedance (APoE) of 1/500 typically corresponds to an annual individual fatality risk (AIFR) ranging between 10−6 and 10−5. Comparison with the ULS design spectra from the current NZS 1170.5:2004 provisions shows that the proposed spectra result in more uniform risk across the country, across different site classes, and across different periods. Additionally, the IL3 ULS design spectrum with an APoE of 1/1000 was considered, demonstrating that increasing the importance level to mitigate mass casualty events in high occupancy buildings is functionally equivalent to reducing the tolerable AIFR level. In summary, the risk assessment methodology can provide valuable information to the code development process by evaluating and comparing the life-safety risk associated with various options for the ULS design spectra.

We present the updated seismic source characterization (SSC) for the 2014 update of the National Seismic Hazard Model (NSHM) for the conterminous United States. Construction of the seismic source models employs the methodology that was developed for the 1996 NSHM but includes new and updated data, data types, source models, and source parameters that reflect the current state of knowledge of earthquake occurrence and state of practice for seismic hazard analyses. We review the SSC parameterization and describe the methods used to estimate earthquake rates, magnitudes, locations, and geometries for all seismic source models, with an emphasis on new source model components. We highlight the effects that two new model components—incorporation of slip rates from combined geodetic-geologic inversions and the incorporation of adaptively smoothed seismicity models—have on probabilistic ground motions, because these sources span multiple regions of the conterminous United States and provide important additional epistemic uncertainty for the 2014 NSHM.

As part of the U.S. Geological Survey’s 2023 50-State National Seismic Hazard Model (NSHM), we make modest revisions and additions to the central and eastern U.S. (CEUS) fault-based seismic source model that result in locally substantial hazard changes. The CEUS fault-based source model was last updated as part of the 2014 NSHM and considered new information from the Seismic Source Characterization for Nuclear Facilities (CEUS-SSCn) Project. Since then, new geologic investigations have led to revised fault and fault-zone inputs, and the release of databases of fault-based sources in the CEUS. We have reviewed these databases and made minor revisions to six of the current fault-based sources in the NSHM, as well as added five new fault-based sources. Implementation of these sources follows the current NSHM methodology for CEUS fault-based sources, as well as the incorporation of a new magnitude–area relationship and updated maximum magnitude and recurrence rate estimates following the methods used by the CEUS-SSCn Project. Seismic hazard sensitivity calculations show some substantial local changes in hazard (−0.4g to 1.1g) due to some of these revisions and additions, especially from the addition of the central Virginia, Joiner ridge, and Saline River sources and revisions made to the Meers and New Madrid sources.

Probabilistic seismic hazard analyses such as the U.S. National Seismic Hazard Model (NSHM) typically rely on declustering and spatially smoothing an earthquake catalog to estimate a long-term time-independent (background) seismicity rate to forecast future seismicity. In support of the U.S. Geological Survey’s (USGS) 2023 update to the NSHM, we update the methods used to develop this background or gridded seismicity model component of the NSHM. As in 2018, we use a combination of fixed and adaptive kernel Gaussian smoothing. However, we implement two additional declustering methods to account for the fact that declustering is a nonunique process. These new declustering methods result in different forecasts for the locations of future seismicity, as represented by spatial probability density functions that are later combined with a rate model to produce a full gridded earthquake rate forecast. The method updates, particularly in the separation of the spatial and rate models as well as revised regional boundaries, in some places cause substantive changes to the seismic hazard forecast compared to the previous 2018 NSHM. Additional updates to catalog processing and induced seismicity zones also contribute to changes in the gridded seismicity hazard since 2018. However, these changes are well understood and reflect improvements in our modeling of gridded seismicity hazard.

We present new maps and slip rate estimates for the Blue Mountain and Spylaw Faults of west Otago, and a re‐evaluation of the probabilistic seismic hazard for the area. Our mapping has resulted in an extension of the active section of the Blue Mountain Fault by about a factor of two, and reduction of the slip rates for the two faults by about a factor of two to five from the previous estimates. The incorporation of these revised fault data into the national seismic hazard model has resulted in the elimination of a zone of anomalously high seismic hazard in the national hazard maps, leaving behind a smooth gradient of increasing hazard from east to west across Otago towards the plate boundary. The study serves to show the magnitude of changes to seismic hazard estimates that result from significant changes to the recurrence parameters of individual fault sources. Future changes to hazard estimates may also be observed elsewhere in the country (e.g., Taupo Volcanic Zone) when significant updates to fault source data are made in the seismic hazard model.

Since 2014, research groups from Japan, Taiwan, and New Zealand have been collaborating to share research ideas and expertise about topics relevant to the development of national seismic hazard models (NSHMs) in each region. The trilateral collaboration is centered on a series of workshops designed to bring the researchers together. So far, three such meetings have been held: a kick‐off workshop in Taiwan in May 2014, a second workshop in Japan in August 2015, and a third workshop in New Zealand in November 2015. A fourth workshop is planned for late 2016. In this focus section on the joint Japan–Taiwan–New Zealand NSHM, we have collected eight papers that highlight some of the ongoing research that specifically targets improvements in the respective NSHMs.

In recent years, new approaches for developing earthquake rupture forecasts (ERFs) have been proposed to be used as an input for probabilistic seismic hazard assessment (PSHA). Zone- based approaches with seismicity rates derived from earthquake catalogs are commonly used in many countries as the standard for national seismic hazard models. In Italy, a single zone- based ERF is currently the basis for the official seismic hazard model. In this contribution, we present eleven new ERFs, including five zone-based, two smoothed seismicity-based, two fault- based, and two geodetic-based, used for a new PSH model in Italy. The ERFs were tested against observed seismicity and were subject to an elicitation procedure by a panel of PSHA experts to verify the scientific robustness and consistency of the forecasts with respect to the observations. Tests and elicitation were finalized to weight the ERFs. The results show a good response to the new inputs to observed seismicity in the last few centuries. The entire approach was a first attempt to build a community-based set of ERFs for an Italian PSHA model. The project involved a large number of seismic hazard practitioners, with their knowledge and experience, and the development of different models to capture and explore a large range of epistemic uncertainties in building ERFs, and represents an important step forward for the new national seismic hazard model.

The official adoption of Eurocode standards in North Macedonia, compared to developed EU countries, was only in recent years, leaving a significant gap that needs to be tackled. The new generation of Eurocode standards is posing demands to seismic hazard assessment’s outputs to use the most updated seismic hazard models, which will be crucial for the development of national annexes. To improve and refine the national seismic hazard model, it is essential to update and curate the existing datasets (i.e., earthquake catalogues, tectonics information and active faults), particularly in a country that has shown significant tectonic activity in the past decades with the occurrence of moderate to strong earthquakes. In just the last century, 12 earthquakes with magnitudes greater than 6.0 have occurred within the territory of North Macedonia, confirming the constant threat of earthquake to society. The main task in developing a refined national seismogenic source model is to improve and update the national database by increasing the quality and thoroughness of all available national and regional seismotectonic databases. This activity involves compilation and statistical analyses of the updated national earthquake catalogue that chronologically ranges from 1000 to 2020. In its compiling, priority was given to the national catalogue from the National Seismological Observatory at the Faculty of Natural Sciences and Mathematics (SORM), which was then supplemented by events from the International Seismological Centre (ISC). Additionally, an overview of the latest defined active faults in North Macedonia will be presented, serving as an input for the new national seismic hazard model. With recently collected and compiled data, along with updated information on seismically active faults in the region, we are now capable of elevating and upgrading the probabilistic seismic hazard assessment of the country, which previously was built primarily upon seismological data, i.e., data from past earthquakes.

The 2023 update to the National Seismic Hazard (NSHM) model is informed by several deformation models that furnish geodetically estimated fault slip rates. Here I describe a fault-based model that permits estimation of long-term slip rates on discrete faults and the distribution of off-fault moment release. It is based on quantification of the earthquake cycle on a viscoelastic model of the seismogenic upper crust and ductile lower crust and mantle. I apply it to a large dataset of horizontal and vertical Global Positioning System (GPS) interseismic velocities in the western United States, resulting in long-term slip rates on more than 1000 active faults defined for the NSHM. A reasonable fit to the GPS dataset is achieved with a set of slip rates designed to lie strictly within a priori geologic slip rate bounds. Time-dependent effects implemented via a “ghost transient” have a profound effect on slip rate estimation and tend to raise calculated slip rates along the northern and southern San Andreas fault by up to several mm/yr.

A team of earthquake geologists, seismologists, and engineering seis- mologists has collectively produced an update of the national probabilistic seismic hazard (PSH) model for New Zealand (National Seismic Hazard Model, or NSHM). The new NSHM supersedes the earlier NSHM published in 2002 and used as the hazard basis for the New Zealand Loadings Standard and numerous other end-user applica- tions. The new NSHM incorporates a fault source model that has been updated with over 200 new onshore and offshore fault sources and utilizes new New Zealand-based and international scaling relationships for the parameterization of the faults. The dis- tributed seismicity model has also been updated to include post-1997 seismicity data, a new seismicity regionalization, and improved methodology for calculation of the seismicity parameters. Probabilistic seismic hazard maps produced from the new NSHM show a similar pattern of hazard to the earlier model at the national scale, but there are some significant reductions and increases in hazard at the regional scale. The national-scale differences between the new and earlier NSHM appear less than those seen between much earlier national models, indicating that some degree of consis- tency has been achieved in the national-scale pattern of hazard estimates, at least for return periods of 475 years and greater. Online Material: Table of fault source parameters for the 2010 national seismic- hazard model.

Widespread surface creep is observed across a number of active faults included in the United States (US) National Seismic Hazard Model (NSHM). In northern California, creep occurs on the central section of the San Andreas fault, along the Hayward and Calaveras faults through the San Francisco Bay Area, and to the north coast region along the Maacama and Bartlett Springs faults. In southern California, creep is observed across the Coachella segment of the San Andreas fault, through the Brawley Seismic Zone, and along the Imperial and Superstition Hills faults. Seismic hazard assessments for California have accounted for creep using various data and methods, including the most recent Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3) in 2013. The purpose of this study is to expand and update the UCERF3 creep rate data set for the 2023 release of the US NSHM and to invert geodetic data and the surface creep rate data for the spatial distribution of interseismic fault creep on California faults using an elastic model with physical creep constraints. The updated surface creep rate compilation consists of a variety of data types including alignment arrays, offset cultural markers, creepmeters, Interferometric Synthetic Aperture Radar, and Global Positioning System data. We compile a total of 497 surface creep rate measurements, 400 of which are new and 97 of which appear in the UCERF3 compilation. We compute creep rate distributions for each of the five 2023 NSHM geodetic-based and geologic-based deformation models. Computed creep rates are used to reduce the total fault moment rate available for earthquake sequences in the NSHM model. We find that, despite relatively large variability in model long-term slip rates across all five deformation models, the variability in depth-averaged creep rate across all models is relatively small, typically 5–10 mm/yr along the creeping San Andreas fault section and only 2–4 mm/yr along the Maacama and Rodgers Creek-Hayward faults.