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

The researchers begin from a sober demographic observation. Low-elevation coastal zones cover only about two percent of the planet’s land surface but held ten percent of the world’s population in 2000, and in the Caribbean roughly seventy percent of people live in large coastal cities. Population density on the Anguilla Bank is striking: around 160 inhabitants per square kilometre on Anguilla, about 500 on Saint-Martin/Sint Maarten and some 530 on Saint-Barthélemy. On these flat forearc islands, there is often no high ground within a reasonable walking distance, so even a modest tsunami could trap residents and tourists on narrow coastal strips between the open sea and shallow salt ponds.

Although the Caribbean accounts for only about five percent of global tsunami records — roughly seventy of the 1,540 events catalogued by NOAA — the region’s history is far from quiet. The infamous Lisbon earthquake of 1 November 1755, with an estimated moment magnitude of 8.5 off the Iberian Peninsula, sent destructive waves across the Atlantic that reached the Lesser Antilles about ten hours later. Contemporary documents describe several-metre waves and inundations throughout the arc. On Saint Martin, historical compilations report a run-up of 4.5 metres and a dramatic sea withdrawal: a sloop anchored in roughly 4.6 metres of water was reportedly left lying dry on her broadside. A second, lesser-known transatlantic event followed on 18 November 1755, when the Cape Ann earthquake in Massachusetts apparently produced a sea withdrawal and a six-foot return wave at Saint Martin’s harbour, as recorded in a letter by Professor Winthrop.

The most destructive regional tsunami struck on 18 November 1867, after a magnitude 7.2 earthquake ruptured a 50-kilometre segment of a fault along the northern border of the Virgin Islands basin. Waves reached several metres — up to about ten metres — in the U.S. Virgin Islands within fifteen minutes and were reported from Puerto Rico to Grenada. In the Anguilla Bank, damage was documented in Saint Martin and Saint Barthélemy, where the 2.1-metre-deep Carenage was left dry, vessels were stranded and then swept adrift, and one schooner was carried by currents at a rate of ten knots before being secured hours later. Beyond these three documented events on the bank, geological evidence points to older, pre-colonial tsunamis: boulders transported up to six metres above sea level on Saint Martin, sediment cores from Etang Guichard recording five extreme-wave events over the past 3,500 years with a 300-to-400-year recurrence, and a regionally extensive inundation dated to roughly 1200–1500 years before present identified on Anegada, Saint Thomas and Scrubb Island.

To translate this history into forward-looking hazard estimates, the team built twelve rupture scenarios. Seven (S01 to S07) represent local upper-crustal faults mapped around the Anguilla Bank from high-resolution bathymetry and seismic reflection profiles, selected for their ability to produce magnitude greater than 6 earthquakes and for their near-complete azimuthal coverage of the archipelago. Fault dimensions were converted into moment magnitudes, rupture areas and coseismic slips using the empirical scaling relationships of Wells and Coppersmith, Leonard, and Thingbaijam, with fault dips and slip directions constrained by seismic profiles and seismotectonic data. Three further scenarios reproduce historical events: the 1755 Lisbon source (S08), the 1867 Virgin Islands rupture (S09) and the 1843 Guadeloupe earthquake (S10), the largest known event in the Lesser Antilles at an estimated magnitude 8.5. Two final scenarios (S11 and S12) relocate that 1843 megathrust rupture northward along the subduction interface — one beneath the Anguilla Bank region and one in the Puerto Rico subduction zone — to test how source position controls impact.

The simulations were run with COMCOT, the Cornell Multi-grid Coupled Tsunami model, which solves linear and non-linear shallow-water equations on a system of nested grids. Three grid levels were used: a 2-arcminute grid of the North Atlantic built from GEBCO bathymetry, an intermediate 15-arcsecond grid over the northeastern Caribbean, and a finely resolved 3.75-arcsecond grid — about 115 metres — covering the Anguilla Bank itself, assembled from the HOMONIM and Litto 3D datasets offshore and SRTM topography on land, all referenced to local mean sea level. Seafloor displacement for each scenario was computed with Okada’s elastic dislocation model, and inundation was activated on the two inner grids with Manning’s friction coefficients of 0.013 for the seabed and 0.0025 for land. Local and regional scenarios were propagated for ten hours; the Lisbon teletsunami was tracked for thirty-six.

The results reveal a strongly source-dependent hazard. The pure strike-slip scenario S01 produces little more than 30 centimetres of wave amplitude, consistent with the small tsunamis that followed recent large strike-slip earthquakes elsewhere in the region. In contrast, six of the seven local fault scenarios push coastal amplitudes above 30 centimetres, at least one location above 1 metre, and five of them above 2 metres somewhere along the coast. Scenario S05 emerges as the most impactful, with maximum amplitudes of 2 to 5 metres at some coastal sites, waves exceeding 2 metres on the northern coast of Saint Martin and more than 2 metres at Saint-Barthélemy’s low-lying, northeast-facing Baie de Saint-Jean. The bank’s numerous submarine canyons play a decisive role, focusing wave energy onto the flanks of canyon heads while sheltering the areas directly in front of them — a pattern well documented in other ocean basins.

The historical scenarios tell a subtler story. The 1755 Lisbon simulation produces generally modest impacts, with amplitudes above 30 centimetres in bays near the populous towns of Marigot and Sandy Ground and between 0.5 and just over 1 metre at Nettlé Bay and Great Bay, but the modelled wave fails to enter Simpson Bay Lagoon and falls far short of the reported 4.5-metre run-up. The authors suggest the historical observation may be erroneous, misattributed to another island, or that the current 115-metre grid resolution cannot fully capture coastal shoaling and refraction. The 1867 Virgin Islands scenario produces 0.3-to-1-metre offshore amplitudes, overtops the Sandy Ground spit and floods parts of the east coast, while the 1843 megathrust scenario drives amplitudes above 1.5 metres along most of Saint Martin and about 2 metres inside Simpson Bay Lagoon. When the same rupture is shifted northward (S11), it generates the largest amplitudes of all twelve cases — 2 to 5 metres along nearly every coast of the archipelago.

Currents prove as consequential as wave heights. Previous studies identify 1.5 metres per second as the threshold at which tsunami flows become dangerous to people and navigation, and experiments show an adult can begin sliding backward in only 26 centimetres of water at 1.8 metres per second. The Lisbon scenario stays mostly below this threshold around Saint Martin, with localized hotspots off Pointe du Bluff, Pinel Island and in Great Bay. The 1867 event pushes speeds to or beyond the threshold at those same sites plus Marigot, Baie de Grand Case and Baie de l’Embouchure, while the 1843 scenario exceeds it along nearly the entire coastline. Conversely, zones of low current speed — below 0.5 metres per second — could serve as sheltering anchorages during a tsunami alert. Overlaying the simulations on satellite-derived building footprints shows that Saint Martin’s largest urban centres sit either within modelled flood zones or directly facing high-amplitude, high-current areas.

The authors stress that their twelve scenarios are a preliminary, deliberately limited set: alternative rupture geometries, uncharacterized faults and non-seismic sources such as submarine landslides and volcanic eruptions could produce larger waves. The steep slopes flanking the Anguilla Bank on both its Caribbean and Atlantic sides are candidates for future collapse, and the nearby volcano Saba represents a direct threat, with at least 47 flank-collapse events identified along the Lesser Antilles Arc, 15 of them within the last 12,000 years. Tsunami management plans for Saint Martin and Saint Barthélemy are still under construction, and the study argues that horizontal evacuation along limited coastal roads should be supplemented with vertical shelters. It also calls for an expanded monitoring network: the northern Lesser Antilles currently hosts only a handful of working sea-level gauges, the Saint Martin gauge has delivered no usable signal since February 2020, the planned Gustavia station was never completed, and the nearest deep-ocean DART buoys lie roughly 630 to 750 kilometres away — leaving these idyllic islands, for now, largely blind to the next approaching wave.

Subject of Research: Earthquake-related tsunami hazard assessment for the Anguilla Bank Archipelago in the northern Lesser Antilles using historical data, fault characterization and numerical tsunami simulation.

Article Title: Tsunami hazard in the Anguilla Bank Archipelago, Lesser Antilles: historical data, fault characterization and numerical simulation

Article References: Roger, J. H. M., & Philippon, M. (2026). Tsunami hazard in the Anguilla Bank Archipelago, Lesser Antilles: historical data, fault characterization and numerical simulation. Natural Hazards, 122(19), Article 636. https://doi.org/10.1007/s11069-026-08367-4

Image Credits: AI Generated

DOI: 10.1007/s11069-026-08367-4

Keywords: tsunami hazard, Anguilla Bank, Lesser Antilles, Caribbean, numerical modelling, COMCOT, earthquake, subduction zone, Saint Martin, Saint Barthélemy, crustal faults, natural hazards

The Guerrero gap is a segment of the Mexican subduction margin where the Cocos Plate dives beneath the North American Plate. Unlike many portions of the boundary, this region has experienced a relative absence of large, recently recorded earthquakes, even though it is capable of storing enormous tectonic stress. That combination has made the area a focus of research into earthquake hazards, slow slip, and the processes that determine whether accumulated strain is released suddenly or gradually.

At a subduction zone, the incoming oceanic plate does not descend as a perfectly smooth slab. Its surface may carry ridges, volcanic seamounts, fractures, sediment, and variations in crustal thickness. As these features enter the trench, they can alter the geometry and frictional behavior of the plate interface. Scientists have therefore often expected a direct relationship between the structure arriving at the trench and the distribution of slip along the megathrust, the broad fault where the two plates meet.

Slip describes the relative movement between the plates during an earthquake or a slower tectonic event. In some sections of a subduction boundary, the fault may remain locked for years or centuries before rupturing in a powerful earthquake. Elsewhere, the same interface can creep continuously or release energy through slow-slip events that last weeks or months and produce little or no shaking. Mapping where and how this movement occurs is central to estimating seismic risk, but the physical controls remain difficult to isolate.

The study by T. Acquisto, A. Bécel, V. M. Cruz-Atienza and colleagues examined whether changes in slip behavior at the Guerrero gap could be directly explained by the structure of the plate entering the subduction zone. Its central conclusion, reflected in the paper’s title, is that the relationship is limited. In other words, incoming plate architecture may influence the fault system, but it does not provide a simple one-to-one explanation for why some parts of the interface slip differently from others.

That finding matters because it shifts attention away from single-factor explanations. If a seamount, ridge, fracture zone, or change in crustal properties does not consistently correspond to a particular style or amount of slip, then researchers must consider a wider combination of influences. These may include the composition and thickness of sediments in the trench, the pressure of fluids within the fault, the roughness and orientation of the plate boundary, temperature, mineral transformations, and the history of previous earthquakes and slow-slip episodes.

Fluids are especially important in this setting. Water carried downward by the oceanic plate can be released as minerals change under increasing pressure and temperature. The resulting fluids may raise pore pressure within the fault zone, reducing the effective force pressing the two plates together. In principle, that can make the interface more likely to slide. Yet fluid pathways are highly variable and may be controlled by fractures and permeability rather than by the large-scale shape of the incoming plate alone, helping explain why structural correlations can be weak.

The Guerrero gap is also a natural laboratory because its offshore environment allows scientists to compare processes beneath the seafloor with signals measured on land and at sea. Seismic waves, seafloor mapping, geodetic measurements, and models of fault motion can reveal different aspects of the plate boundary. Each method has limitations: seismic imaging can blur small structures, geodetic observations may average behavior over broad areas, and models depend on assumptions about the fault’s geometry and friction. Combining these approaches is therefore essential for separating robust patterns from apparent connections.

The study’s message is not that incoming plate structure is irrelevant. Rather, it suggests that its effect is indirect, filtered through the complex physical conditions of the subduction interface. This distinction could influence how scientists interpret earthquake forecasts and hazard maps. A visible feature on the oceanic plate should not automatically be treated as a reliable marker of a locked patch, a slow-slip zone, or a future rupture boundary. More realistic assessments will need to integrate geology, fault mechanics, fluids, temperature, and the timing of past deformation.

For the public, the research offers a reminder that earthquake science is increasingly precise but not yet simple. The absence of a straightforward structural fingerprint does not remove the seismic threat at the Guerrero gap, nor does it predict when a major earthquake might occur. Instead, it highlights why the most dangerous faults require detailed, multidisciplinary monitoring. By showing that slip variability cannot be explained directly by incoming plate structure alone, the researchers provide a more nuanced framework for understanding how Mexico’s subduction margin stores and releases tectonic energy.

Subject of Research: Slip variability and incoming plate structure at the Guerrero gap offshore Mexico

Article Title: Limited direct links between slip variability and incoming plate structure at the Guerrero gap offshore Mexico

Article References: Acquisto, T., Bécel, A., Cruz-Atienza, V.M. et al. Limited direct links between slip variability and incoming plate structure at the Guerrero gap offshore Mexico. Commun Earth Environ (2026). https://doi.org/10.1038/s43247-026-03796-2

Image Credits: AI Generated

DOI: 10.1038/s43247-026-03796-2

Keywords: Guerrero gap, Mexico, subduction zone, tectonic slip, earthquake science, incoming plate structure, megathrust, slow-slip events, seismic hazard