The research, published in the Bulletin of Earthquake Engineering, was led by Snježana Markušić of the University of Zagreb together with colleagues from the University of Zagreb’s Faculty of Geotechnical Engineering and the Croatian Geological Survey. The team used the OpenQuake Engine, developed by the Global Earthquake Model Foundation and now the de facto international standard for hazard computation, running it in classical mode to integrate earthquake occurrence models, ground motion prediction equations, and site characterisation into a single probabilistic framework. The result is a set of hazard maps covering peak ground acceleration and spectral accelerations between 0.1 and 2.0 seconds, computed on a 0.1-degree grid for seven return periods ranging from 95 to 2475 years.

The study’s principal scientific contribution lies not in the computational workflow but in the construction, justification, and validation of a Croatia-specific seismic source model. The model comprises 32 area source zones grouped into nine seismotectonic super-zones, spanning the country’s three major tectonic domains: the comparatively rigid Adriatic Foreland, the NW-SE trending Dinarides fold-thrust belt dominated by reverse and thrust faulting, and the Pannonian Basin in the north, a Miocene back-arc extensional basin where normal and strike-slip faulting prevail. Earthquake recurrence parameters for each zone were estimated with the Weichert maximum likelihood method, which allows completeness to vary by magnitude, and the entire model was validated against independent seismicity observations before being combined with ground motion models.

The earthquake catalogue underpinning the analysis merged three sources: the Croatian Earthquake Catalogue, the International Seismological Centre Bulletin, and the SHARE European Earthquake Catalogue. After careful deduplication, a single declustering pass using the Gardner-Knopoff algorithm with Grünthal space-time windows yielded 90,003 mainshocks, with 74.2 percent of all events identified as aftershocks and 10.9 percent as foreshocks. Crucially, the catalogue incorporates the 2020 Petrinja sequence, something the pan-European ESHM20 model, with its 2020 data freeze, could not capture. Magnitude of completeness was determined independently for each super-zone through a temporal rate-stability analysis, revealing marked differences between the well-monitored Dinarides, where completeness stabilises from the 1980s onward, and parts of the Pannonian-Bosnian domain where detection is still improving.

One of the study’s most distinctive methodological innovations concerns focal mechanisms. Rather than assigning a single dominant faulting style to each zone, as even continental-scale models typically do, the team derived zone-specific nodal plane distributions from the CroFMS 2024 catalogue of 410 focal mechanism solutions. Using Kagan-angle clustering with silhouette-optimised cluster selection and moment-tensor averaging, they represented the observed diversity of faulting within each zone as a weighted, multi-cluster distribution. In the Zagreb zone, for example, 51 mechanisms resolved into three clusters of reverse, strike-slip, and oblique-reverse character, none exceeding 40 percent weight. Because ground motion models typically shift median peak ground acceleration by 10 to 20 percent between reverse and strike-slip ruptures, collapsing such a mixture into a single mechanism would systematically bias hazard estimates.

The ground motion characterisation was equally rigorous. From an initial candidate set of 19 models evaluated against a regional Croatian strong-motion dataset, five were selected and weighted using a combined ranking based on the Deviance Information Criterion and the Euclidean Distance-Based Ranking method, with meta-weights of 0.80 and 0.20 respectively. The top-ranked model, Kowsari et al. 2020 Model Y5, calibrated on a pan-European and Mediterranean dataset that closely matches Croatian strong-motion characteristics, was not available in OpenQuake’s library, so the team implemented it as a custom Python module validated to within 1 percent of published median predictions. Hazard was computed on a spatially varying, real ground-surface site model built from 733,761 site locations, capturing local soil amplification directly rather than defaulting to Eurocode 8’s uniform reference-rock condition.

The resulting hazard maps reveal a striking spatial pattern. Hazard peaks along the Adriatic coast, with the Dubrovnik-Neretva coast identified as the highest-hazard zone in the country at every intensity measure and return period. At Dubrovnik, 475-year peak ground acceleration reaches 0.40 g, with spectral acceleration at 0.2 seconds reaching 0.94 g, and 2475-year values climbing to 0.85 g and 2.00 g respectively. Split follows at 0.32 g and Rijeka at 0.27 g for the 475-year return period. Perhaps counterintuitively, the Sisak-Moslavina area, despite hosting the destructive 2020 Petrinja sequence, falls only in the moderate tier at approximately 0.22 g, while Osijek in the low-seismicity Pannonian domain records the country’s lowest values at just 0.08 g, roughly one-fifth of the coastal maximum.

The most consequential finding, however, emerges from a systematic comparison with the Eurocode 8 design spectra in Croatia’s National Annex. The comparison revealed a previously undocumented, period-dependent discrepancy: probabilistic hazard spectra exceed code values at short periods but fall below them at longer periods. At the peak ground acceleration level, the new hazard estimates exceed the National Annex anchor values by 31 to 46 percent at four of the six representative cities, Petrinja, Rijeka, Split, and Dubrovnik, consistent with real-surface site amplification exceeding the implicit reference-rock basis of the current zonation. Yet at a spectral period of 1.0 second, the ratio drops below unity at every city without exception, meaning the current Eurocode 8 Type 1 spectrum decays more slowly beyond its 0.4-second corner period than the hazard analysis suggests is warranted, making the code comparatively more conservative for longer-period structures such as tall buildings and bridges.

The authors are careful to position the work as a scientific foundation rather than a statutory document. It does not set design values or follow the formal drafting procedure a National Annex requires, but it offers exactly the kind of transparent, independently reproducible basis that national standardisation bodies can draw upon, much as earlier studies informed the current first-generation Annex. The findings arrive at a pivotal moment, as Croatia prepares to implement the second generation of Eurocode 8, published in 2024, which introduces revised spectral shape parameterisation allowing country-specific corner periods, a unified European reference hazard model, and new ground-type classifications. The study’s results on the period-dependent scaling of hazard between the 475-year and 2475-year return periods, which ranges from roughly 2.0 at short periods to 2.87 at 2.0 seconds, also suggest that a single uniform importance factor for essential structures may not capture the full site and period dependence of rare-event hazard.

Limitations remain, and the authors flag them candidly. The source model relies solely on area sources, with explicit fault sources for major structures such as the Petrinja and Pokupsko faults deferred to the next model generation, since a defensible hybrid model would require slip-rate data that do not yet exist for the faults responsible for Croatia’s largest earthquakes. The site model depends partly on inferred values, and the strong-motion database used for ground motion ranking is still relatively small. A sensitivity check on maximum magnitude assignment revealed that this single parameter can matter more than the ground motion or b-value branches that typically receive the most logic-tree attention, particularly at the 2475-year hazard level used for essential-facility design. With its open, reproducible input package and validated source model, the study provides both a warning and a toolkit: Croatia’s design spectra need period-dependent recalibration, and the scientific machinery to accomplish it now exists.

Subject of Research: Probabilistic seismic hazard analysis and earthquake hazard mapping for Croatia with implications for Eurocode 8 seismic design

Article Title: Probabilistic seismic hazard analysis for Croatia: hazard mapping and Eurocode 8 implications

Article References: Markušić, S., Stanko, D., Žilić, I., Fiket, T., Majurec, A., & Korbar, T. (2026). Probabilistic seismic hazard analysis for Croatia: hazard mapping and Eurocode 8 implications. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02691-7

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02691-7

Keywords: probabilistic seismic hazard analysis, Croatia, Eurocode 8, OpenQuake Engine, seismic hazard maps, Dinarides, Adriatic coast, Petrinja earthquake, ground motion models, uniform hazard spectra, seismic source model, site amplification

For decades, seismic hazard assessment in most parts of the world has concentrated on horizontal ground motion, driven by the observation that lateral forces are usually the primary cause of structural collapse. Yet engineers have increasingly recognized that vertical shaking can impose severe axial demands on columns, beams, and connections, particularly in reinforced concrete frames, bridges, and base-isolated structures. Laboratory testing and post-earthquake investigations have repeatedly shown that reinforced concrete columns subjected to combined horizontal and vertical accelerations lose load-carrying capacity faster than those facing lateral motion alone, and that vertical ground motion has been implicated in damage to historical structures and modern buildings alike. Until now, El Salvador’s building codes and hazard maps have lacked a rigorous, nationally calibrated basis for quantifying that vertical threat.

The new work presents time-independent probabilistic seismic hazard maps expressed in terms of vertical peak ground acceleration and spectral ordinates at periods of 0.2 and 1 seconds, computed for 5 percent of critical damping. These values were produced for rock site conditions and flat topography across five return periods: 50, 95, 475, 975, and 2475 years. The return-period framework mirrors the conventions used in modern design standards, allowing engineers to select ground-motion intensities appropriate to the importance and intended lifespan of a structure. A 475-year return period, corresponding to roughly a 10 percent probability of exceedance in 50 years, is the traditional benchmark for ordinary buildings, while the rarer 2475-year event informs the design of critical facilities such as hospitals and emergency centers.

To construct the hazard models, Salazar employed two complementary approaches: the classical area source method and the smoothed seismicity method. The area source approach divides a region into discrete zones assumed to share uniform seismicity characteristics, a technique rooted in Cornell’s foundational 1968 formulation of engineering seismic risk analysis. The smoothed seismicity method, pioneered by Gordon Woo in the 1990s, dispenses with rigid zonation boundaries and instead spreads earthquake occurrence spatially using kernel functions, letting the historical record itself shape the hazard landscape. Using both methods within a logic tree framework allows the analysis to capture epistemic uncertainty, the uncertainty arising from incomplete scientific knowledge about where and how frequently earthquakes occur.

The seismicity evaluation draws on a homogenized earthquake catalog spanning nearly five centuries, from 1528 to 2023, with moment magnitudes ranging from 5.0 to 8.1. Bringing nearly 500 years of historical and instrumental records into a consistent magnitude scale is one of the most labor-intensive aspects of hazard analysis in developing countries, where early accounts may consist only of damage descriptions in colonial archives. From this catalog the study derived classical Gutenberg-Richter frequency-magnitude relationships, which describe the predictable inverse relationship between earthquake size and occurrence rate, as well as average kernel bandwidth distances, a measure of the typical spacing among epicenters of earthquakes of the same size that parameterizes the smoothed seismicity calculations.

A distinctive strength of the study lies in its empirical testing of vertical ground-motion prediction equations against actual accelerometer recordings from El Salvador itself. The candidate equations were evaluated against data from the two destructive earthquakes of 2001: the January 13 event of magnitude 7.7, which ruptured within the Cocos plate subduction zone offshore, and the February 13 event of magnitude 6.5, which struck in the volcanic chain zone where shallow crustal faults cut across the densely populated interior. Both earthquakes caused catastrophic losses, and their contrasting tectonic settings make them ideal calibration points for distinguishing how subduction interface and upper-crustal earthquakes transmit vertical energy to the surface. Salazar also accounted for hanging-wall effects, the amplification of shaking that occurs at sites located above the up-dip edge of a dipping fault rupture, which can substantially elevate ground motions at near-fault locations.

The weighting of competing ground-motion models in the logic tree was informed directly by how well each vertical prediction equation reproduced the observed Salvadoran recordings, rather than being assigned arbitrarily or borrowed wholesale from other regions. This data-driven calibration matters because vertical-to-horizontal spectral ratios vary widely across tectonic regimes, and models developed for Japan, Italy, Taiwan, or the Mediterranean do not necessarily transfer cleanly to Central America’s unique combination of a rapidly subducting oceanic plate and an active volcanic arc. By anchoring the model selection to local data, the study reduces one of the largest sources of uncertainty in vertical hazard estimates.

Among the study’s most practical outputs are proposed relations between horizontal and vertical map accelerations, expressed across all the return periods considered. These vertical-to-horizontal ratios provide an efficient bridge for practicing engineers: instead of requiring new vertical hazard computations for every site, designers can derive vertical design forces directly from the horizontal hazard values already in use, scaled by the locally calibrated ratios. Given that international design provisions, including the American Society of Civil Engineers’ ASCE 7-22 standard, increasingly demand explicit treatment of vertical seismic effects, such locally derived conversion factors are precisely what national code committees need to modernize requirements without embarking on a parallel hazard analysis from scratch.

The implications extend beyond structural engineering practice. El Salvador’s seismic vulnerability was starkly demonstrated in 1986, when the San Salvador earthquake destroyed thousands of buildings, and again in 2001, when the two major quakes within a month devastated communities already struggling with recovery. Recent geodetic work using GNSS and InSAR has continued to map the accumulating tectonic deformation across the country, underscoring that the forces driving the hazard remain fully active. Regional hazard models developed for sovereign parametric insurance also depend on the kind of robust, probabilistic ground-motion characterization this study provides, meaning the new vertical maps could ultimately inform not only building design but financial instruments that transfer catastrophe risk at the national scale.

For a country that sits atop one of the planet’s most active seismic engines, the message of this research is clear: the ground does not only move sideways, and the engineering community must plan for the full three-dimensional character of earthquake shaking. By combining a five-century earthquake catalog, dual seismicity modeling methods, empirically tested vertical ground-motion equations validated against Salvadoran strong-motion records, and practical hazard maps spanning return periods from 50 to 2475 years, the study gives El Salvador a technical foundation that few nations of its size possess. The work was supported by research grants from the Catholic University of El Salvador and made use of Woo’s KERFRACT Fortran code for smoothed seismicity, and it stands as a template for how data-scarce, high-hazard countries can leverage both historical archives and modern instrumental networks to quantify the risks beneath their feet.

Subject of Research: Probabilistic seismic hazard mapping of vertical earthquake ground motion components in El Salvador

Article Title: Seismic hazard maps for El Salvador: the vertical component of motion

Article References: Salazar, W. (2026). Seismic hazard maps for El Salvador: the vertical component of motion. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02665-9

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02665-9

Keywords: seismic hazard, El Salvador, vertical ground motion, probabilistic seismic hazard analysis, ground-motion prediction equations, smoothed seismicity, subduction zone, earthquake catalog, logic tree, return period, vertical-to-horizontal ratio, Bulletin of Earthquake Engineering