The strongest event of the entire period was the magnitude ML 4.3 Elm/Steinibach earthquake, which struck the Glarus Alps in eastern Switzerland on October 25, 2020. Felt reports submitted by residents suggested shaking intensities of up to degree V on the European Macroseismic Scale, a level at which hanging objects swing and small objects fall. Yet when seismologists combined modeled ground motions with instrumental measurements from nearby stations, the picture near the epicenter was considerably more severe, hinting at intensities approaching degree VI–VII, where structural damage to buildings becomes possible. This discrepancy between felt reports and instrumentally constrained shaking is a classic challenge in seismic hazard assessment, particularly in sparsely populated mountain terrain where few observers are close to the source.

What makes the Elm/Steinibach sequence scientifically valuable is the forensic detail extracted from its aftershocks. By deriving focal mechanisms, which describe the orientation and sense of slip on the rupturing fault, and by applying relative hypocenter relocation techniques that sharpen the positions of fore- and aftershocks to sub-fault precision, researchers imaged a dextral strike-slip fault zone striking WSW–ENE to W–E with a total length of about 3.5 kilometers. Strike-slip faults move horizontally, with blocks sliding past one another, and the dextral designation means that, viewed from either side, the opposite block moves to the right.

Perhaps the most striking finding from the Glarus Alps sequence is its depth. Well-constrained focal depths of only 1 to 2 kilometers place the fault zone in the uppermost part of the crystalline basement of the eastern Aar Massif, a block of ancient rocks that forms part of the Alpine core. Shallow earthquakes of this kind are rare in intraplate settings and carry outsized hazard implications, because the same magnitude released at 1 to 2 kilometers depth produces far stronger shaking at the surface than a deeper event. The Elm event therefore offers a rare, well-documented example of active deformation in the very top of the Alpine basement.

A second exceptional sequence unfolded in November 2019 between Anzère and the Sanetsch Pass in the Rawil Depression of southwestern Switzerland. Within just ten days, more than 300 earthquakes rattled this cluster, and 16 of those events reached local magnitudes between 2.5 and 3.3, strong enough to be felt in nearby valleys. The swarm-like behavior, in which seismicity migrates through a small volume rather than producing one dominant mainshock, is characteristic of fluids or complex fault geometry modulating the stress field, although the detailed triggering mechanisms for this particular sequence remain a subject of ongoing analysis.

The structural interpretation of the Rawil sequence is where the report delivers a significant tectonic payoff. Focal mechanisms and relative relocations of the swarm events image the reactivation of a contractional stepover, a geometric configuration in which two overlapping fault segments are connected by a zone of compression between their tips. The imaged stepover confirms the previously proposed segmented nature of the Rawil Fault Zone north of the Rhône Valley, one of the major active structures in the Swiss Alps. Confirming fault segmentation matters for hazard assessment, because segmented faults can rupture in patches, producing clustered sequences like the 2019 swarm rather than single large events, and the size of individual segments helps bound the maximum credible earthquake.

Further west, the magnitude ML 4.2 Novel earthquake of May 28, 2019, in the Préalpes region south of Lake Geneva, added weight to a provocative recent hypothesis: the existence of a domain of NE–SW oriented extensional to transtensional deformation along the Alpine Front, in the transition zone between the Central and Western Alps. Extensional deformation means the crust is being pulled apart, and transtension combines that pulling with lateral strike-slip motion. In a mountain belt conventionally understood as being squeezed by the ongoing collision between the African and Eurasian plates, finding zones where the crust is instead stretching is a reminder that the modern Alpine stress field is far from uniform, with local structures responding to rotations of the stress direction and to the gravitational spreading of high topography.

Evidence for the same style of deformation appears at the southwestern edge of the Mont-Blanc Massif, where a remarkable earthquake cluster occurred near the Grandes Jorasses mountain in the border region between France and Italy. The focal mechanisms from this cluster indicate transtensional deformation, consistent with the pattern proposed along the Alpine Front. That two independent areas, separated by more than a hundred kilometers, display compatible kinematics strengthens the case that the extensional domain is a real, spatially coherent tectonic feature rather than a local curiosity, and it gives seismologists new targets for monitoring along one of Europe’s most closely watched plate boundaries.

North of the Alps, the report documents deformation in the foreland as well. A vigorous earthquake sequence on the Bodanrück Peninsula in southern Germany in 2019 revealed transtensional deformation of the Hegau-Bodensee Graben, a rift-like structure in the northern foreland of the Alps. Graben structures form where the crust subsides along normal faults, and their reactivation under the present-day stress field shows that the tectonic forces reshaping the Alpine region extend well beyond the high mountains themselves, reaching into the densely populated areas of southern Germany and the Lake Constance region.

Finally, the magnitude ML 3.5 Arolla earthquake provided evidence for unusually shallow seismicity in the domain of the Dent-Blanche nappe, one of the highest structural units of the Pennine Alps. Taken together, the 2019–2020 period stands out as exceptional in terms of both the absolute number of earthquakes and the number of events with ML ≥ 2.5, and the sequences it produced, from the shallow Glarus basement rupture to the Rawil stepover and the transtensional signals along the Alpine Front, offer seismologists an unusually rich natural laboratory for understanding how the Alps deform today, knowledge that feeds directly into the seismic hazard models protecting Switzerland and its neighbors.

Subject of Research: Seismicity and tectonic fault analysis in Switzerland and surrounding regions during 2019–2020

Article Title: Earthquakes in Switzerland and surrounding regions during 2019 and 2020

Article References: Diehl, T., Cauzzi, C., Clinton, J., Kraft, T., Kästli, P., Massin, F., Lanza, F., Simon, V., Grigoli, F., Hobiger, M., Roth, P., Haslinger, F., Fäh, D., & Wiemer, S. (2025). Earthquakes in Switzerland and surrounding regions during 2019 and 2020. Swiss Journal of Geosciences, 118(1), Article 15. https://doi.org/10.1186/s00015-025-00489-4

Image Credits: AI Generated

DOI: 10.1186/s00015-025-00489-4

Keywords: Switzerland, seismicity, earthquakes, Elm earthquake, Rawil Fault Zone, strike-slip fault, transtension, Alpine Front, Aar Massif, focal mechanisms, seismic hazard, Swiss Seismological Service

The Algeciras Fault System stretches roughly 540 kilometers in a northeast-southwest direction along the eastern flank of Colombia’s Central Cordillera, cutting through the departments of Meta, Huila, Cauca, Nariño, Caquetá, and Putumayo. It is a transpressional structure, meaning it combines sideways strike-slip motion with vertical compression, and it marks part of the boundary zone where the Nazca and South American plates grind against each other. The fault system passes beneath towns including Uribe, Algeciras, Neiva, Gigante, Pitalito, Sibundoy, Mocoa, and Ipiales, across terrain that rises from 600 meters to more than 3,000 meters above sea level. Historical records document destructive earthquakes in the Huila-Caquetá region, and instrumental monitoring confirms persistent moderate-to-strong seismicity along its trace.

What makes the new study unusual is its data sources. The researchers drew on monthly gravity solutions from NASA’s GRACE mission and its successor GRACE Follow-On, which measure tiny changes in Earth’s gravitational field from orbit. Because those satellites also see the weight of water moving through rivers, soils, and aquifers, the team subtracted hydrological contributions using the Global Land Data Assimilation System, or GLDAS, a land-surface model driven by meteorological and satellite observations. Even after this correction, the authors caution that residual non-tectonic signals cannot be fully excluded, so the gravity signals they interpret must be treated carefully.

To check the quality of the satellite gravity data, the researchers compared it against airborne gravimetric surveys flown by Colombia’s National Hydrocarbons Agency in 2006 and 2009 over the Yari-Caguán and Putumayo basins. At coarse regional scales, the agreement was remarkably tight: when the airborne data were smoothed with a 500-kilometer upward-continuation filter, average differences from the satellite solutions dropped to between roughly 0.49 and 1.54 percent. This cross-check supported the team’s decision to use the GFZ-processed RL06 solutions at full 96-by-96 spherical harmonic resolution, which preserve small-amplitude spatial variations that stronger filtering would erase.

The seismic side of the analysis drew on 14,592 earthquakes recorded by the Colombian Geological Survey between May 2002 and July 2020. The team converted moment magnitudes to local magnitudes where necessary and estimated the energy released by each event. When they sorted earthquakes by whether they occurred in areas of gravity increase or gravity decrease, a clear asymmetry emerged: 52.1 percent of events fell in gravity-decrease zones, but those zones accounted for 93.1 percent of the total released seismic energy. Stronger, shallower earthquakes were especially concentrated where gravity was falling.

The most dramatic result came from combining radar interferometry with gravity. Using 85 processed Sentinel-1 satellite images and the Small Baseline Subset technique, the researchers mapped vertical ground motion across the fault corridor and compared it month by month with GRACE-derived gravity variations. They identified two kinds of domains: direct-correlation areas, where gravity and elevation move up or down together, and inverse-correlation areas, where one rises as the other falls. When they tallied the earthquakes falling within each domain type, they found that areas of simultaneous decrease in both gravity and elevation accounted for approximately 99 percent of the total seismic energy released in the analyzed subset. For earthquakes of magnitude 3.0 or larger at depths of 20 kilometers or less, 89 percent of events occurred in these gravity-and-elevation-decrease zones.

This pattern is geophysically meaningful. Simultaneous gravity loss and ground subsidence can indicate mass redistribution at depth, whether through compaction, fluid migration, or the elastic response of the crust to tectonic loading. The authors suggest that such zones may reflect a closer spatial association between mass redistribution and crustal deformation during seismically active periods. Comparable gravity-deformation relationships have been documented in other settings, including Mount Etna in Italy and regions of nonlinear land subsidence, suggesting the pattern is not unique to the Colombian Andes but may be a general feature of actively deforming crust.

The team also cross-correlated gravity variations with ground displacements measured at nine permanent GNSS stations from Colombia’s GeoRED network. Correlation coefficients ranged from nearly zero to 0.61, with time lags stretching up to 45 months. The vertical component showed the strongest correlations of any measured quantity, with the LAB1 station recording a maximum direct correlation of 0.6143 at a four-month lag and a maximum inverse correlation of -0.5593 at six months. Stations close to the main fault trace, like LAB1, tended to show rapid responses consistent with elastic deformation of the upper crust, while stations farther away, like QUIL and OVSC, showed longer-delayed correlations that may reflect deeper viscoelastic processes or rheological differences along the fault corridor.

As a case study, the researchers examined the magnitude 6.0 Mesetas earthquake of December 24, 2019, which struck at a depth of about 13 kilometers. In the months surrounding the event, the GRACE-derived gravity time series displayed a distinctive W-shaped fluctuation, bounded by the two most abrupt gravity variations in the entire 2002-2020 record. At the earthquake location, the team noted a temporal coincidence between a gravity decrease of 0.461 milligal and an elevation change of -0.062 meters. Spectral analysis using the Fast Fourier Transform showed that most of the signal’s power was concentrated at low frequencies, with a dominant peak at roughly 0.2 cycles per month, corresponding to a period of about five months. The authors interpret this sequence as potentially compatible with stress accumulation, rupture, and post-event crustal readjustment, though they emphasize that the coarse spatial resolution of GRACE and possible residual hydrological effects mean the observation is exploratory rather than conclusive.

The study’s authors, Edwin Mauricio Pinzon Monroy and Carlos Alberto Vargas Jiménez, are careful to frame their results as associations rather than causal proof. Satellite gravimetry resolves features hundreds of kilometers wide, far larger than the rupture zone of a magnitude 6 earthquake, and hydrological models cannot perfectly remove water-related signals. GNSS time series vary in length and continuity, and InSAR measurements can be affected by atmospheric conditions and topographic artifacts. Yet the convergence of independent data streams, satellites weighing the Earth from orbit, radar measuring the ground’s rise and fall, and seismometers tallying where energy is released, offers a template that can be applied to other active fault systems worldwide. As gravity missions improve in resolution and longer geodetic records accumulate, this kind of multi-source approach may become a standard tool for identifying which segments of a fault are quietly loading up, and which are already letting go.

Subject of Research: Integration of satellite gravimetry, GNSS, InSAR, and seismicity to analyze the Algeciras Fault System in Colombia

Article Title: Seismic – Gravimetric analysis of the Algeciras fault system (Colombia): spatio-temporal correlations from GRACE and GNSS-InSAR

Article References: Pinzon Monroy, E. M., & Vargas Jiménez, C. A. (2026). Seismic – Gravimetric analysis of the Algeciras fault system (Colombia): spatio-temporal correlations from GRACE and GNSS-InSAR. Frontiers in Earth Science, 14, Article 1938095. https://doi.org/10.3389/feart.2026.1938095

Image Credits: AI Generated

DOI: 10.3389/feart.2026.1938095

Keywords: Algeciras Fault System, GRACE, GRACE Follow-On, GNSS, InSAR, satellite gravimetry, seismicity, Colombia, tectonic deformation, Mesetas earthquake, GLDAS, seismic hazard

The research team, led by Thomas Ulrich of Ludwig-Maximilians-Universität München and Xiaoyu Zou of the Scripps Institution of Oceanography, combined satellite radar measurements, optical image correlations, seismic recordings, and even security camera footage with an unprecedented ensemble of physics-based computer simulations. Their central conclusion is that the Sagaing Fault, one of the most mature and geometrically simple strike-slip faults on Earth, behaves in ways that pre-event observations had already hinted at, provided scientists know how to read the signals.

The first surprise came from the geometry of the fault itself. Although the Sagaing Fault appears as a smooth, nearly vertical line at the surface, the pattern of ground deformation recorded by satellites told a different story. The horizontal displacements measured by the Sentinel-1A and ALOS-2 radar satellites and the Sentinel-2 optical instrument were strikingly asymmetric across the fault trace, with larger movements on the eastern side in the north of the rupture and the opposite pattern toward the south. The researchers systematically tested competing explanations, including contrasts in rock stiffness across the fault and variations in fault strike, but neither could reproduce the observed asymmetry. Instead, the data pointed to a fault plane that is not vertical at all but tilted, dipping eastward at roughly 70 to 80 degrees in the north before rotating through vertical to a westward dip of about 80 degrees at the southern end. Assuming these transitions occur gradually, the fault follows a helix-shaped, corkscrew geometry as it descends through the crust, a configuration the authors note resembles independent evidence for the San Andreas Fault in California.

This helical geometry mattered for the earthquake’s behavior. When the team inverted the full suite of geodetic observations for the distribution of slip on their curved, three-dimensional fault model, the fit was excellent, achieving a 96 percent variance reduction and matching independent optical data not used in the inversion. The model revealed predominantly right-lateral strike-slip motion with a peak slip of about 5.5 meters near the hypocenter, and an equivalent moment magnitude of 7.81. Critically, inversions that assumed a vertical fault left systematic residuals of about half a meter near the rupture, further confirming the dipping geometry. The model also showed remarkably little shallow slip deficit, meaning slip at the surface was nearly as large as slip at depth, a hallmark of very large, mature strike-slip ruptures.

The second major finding concerns how much the fault moved compared with how much strain had accumulated since its last major ruptures. By comparing the average coseismic slip on each fault segment with the slip deficit, calculated as the geodetic slip rate of roughly 16 to 24 millimeters per year multiplied by the time since the penultimate earthquake, the researchers found a striking pattern. Segments south of the epicenter that had been locked since the 1839 earthquake, part of the so-called Meiktila seismic gap, released slip in good agreement with the accumulated deficit. But the two northern segments that had already ruptured in 1946 and 1956 produced more slip than had accumulated since those events, an overshoot. This mirrors the behavior of the 2023 magnitude 7.8 and 7.7 Kahramanmaraş earthquake doublet in Türkiye, which was described as loosely slip-predictable. The authors propose the term minimally slip-predictable: very large, multi-segment ruptures tend to release at least the accumulated slip deficit, and sometimes more, by tapping residual stresses left over from earlier events.

To understand the dynamics of the rupture itself, the team turned to high-performance computing. Using the open-source SeisSol software, they generated an ensemble of 180 three-dimensional dynamic rupture simulations, each starting from the geodetically derived fault geometry and slip distribution but varying three fault-wide parameters that control prestress heterogeneity, fracture energy, and the ratio of stress drop to frictional strength drop. The simulations were then ranked against independent observations the static model had never seen: the moment rate function from teleseismic data, a strong-motion record from the near-fault station NPW, slip rates and a curved slip trajectory extracted from CCTV footage that captured the rupture passing a roadside camera, and rupture speeds imaged by teleseismic back-projection using seismic arrays in Alaska, Australia, and Europe.

The preferred scenario is remarkable. The rupture begins bilaterally, propagating in both directions from the hypocenter, but after about 25 seconds it transitions to predominantly unilateral southward propagation that races along at supershear speeds, faster than the shear wave velocity of the surrounding rock, before the earthquake finally terminates roughly 90 seconds after nucleation. This transition to supershear rupture occurred almost immediately at depth, within a deep, high-slip, high-prestress band, while the shallow portion of the fault remained subshear over a 150-kilometer section south of the hypocenter. The result is a strongly curved rupture front, with slow, shallow slip persistently catching up as it is driven by stresses radiated from the faster deep pulse. This depth-dependent behavior elegantly reconciles three seemingly contradictory observations: the subshear, pulse-like phase recorded by the CCTV camera, the rapid average rupture speed of about 4.9 kilometers per second implied by the arrival of the main pulse at station NPW, and the high-frequency radiation patterns imaged by back-projection, which resolved about 20 seconds of northward rupture at 2.5 kilometers per second and southward speeds accelerating to roughly 6 kilometers per second.

The physics behind this behavior points to a structurally mature fault zone. The best-fitting model requires unusually low fracture energy, about 0.83 megajoules per square meter, and low effective normal stress, both consistent with the weak, fine-grained materials documented along the Sagaing system, including clay-rich gouge and phyllosilicate-bearing serpentinites containing talc and chlorite. Such minerals can promote dynamic weakening and strain localization, and low-permeability gouge can develop fluid overpressure that further reduces effective normal stress. The team also tested whether a bimaterial contrast, the fault juxtaposing the slower Central Myanmar Basin against the faster Shan-Thai Block, could explain the north-south asymmetry in rupture speed. A second ensemble of 180 simulations with elastic properties perturbed by 10 percent across the fault did qualitatively reproduce the observed contrast, with transient unclamping ahead of the southward rupture tip promoting supershear transition. However, no bimaterial model fit the CCTV slip-rate data as well as the reference ensemble, and the modeled ruptures arrived at station NPW 13 seconds too late, suggesting bimaterial effects contribute but are not the sole control.

The implications extend well beyond Myanmar. The authors argue that for other mature strike-slip faults believed to be late in their earthquake cycles, such as the Istanbul-Marmara section of the North Anatolian Fault and the Southern San Andreas Fault, asymmetric interseismic deformation patterns already recorded by geodesy may reveal dipping, helix-shaped fault geometries before the next big rupture. Because ground shaking is expected to be significantly stronger on the hanging wall side of a dipping strike-slip fault, constraining this geometry in advance is directly relevant to predicting ground motion. Moreover, if very long ruptures are indeed minimally slip-predictable, the interseismically accumulated slip deficit provides a reasonable lower bound on the slip a maximum-size earthquake could deliver, implying that hazard estimates based on limited historical and paleoseismic records may significantly underpredict the true danger. The study demonstrates that even for sparsely instrumented fault systems, combining interseismic deformation measurements with dynamic rupture modeling offers a practical pathway toward physics-based forecasts of how the next great earthquake will unfold.

Subject of Research: Rupture dynamics and slip predictability of the 2025 M7.8 Mandalay earthquake on the Sagaing Fault

Article Title: On predictability of slip, rupture geometry, and speed of the 2025 M7.8 Mandalay, Myanmar, earthquake

Article References: Ulrich, T., Zou, X., Marchandon, M., Schliwa, N., Tan, F., Gabriel, A.-A., Fan, W., Shearer, P., Thant, M., Tin, T. Z. H., Lindsey, E. O., & Fialko, Y. (2026). On predictability of slip, rupture geometry, and speed of the 2025 M7.8 Mandalay, Myanmar, earthquake. Nature Communications, 17(1), Article 10173. https://doi.org/10.1038/s41467-026-77203-x

Image Credits: AI Generated

DOI: 10.1038/s41467-026-77203-x

Keywords: Mandalay earthquake, Sagaing Fault, supershear rupture, slip predictability, fault geometry, dynamic rupture modeling, space geodesy, seismic hazard, Myanmar, strike-slip fault, fracture energy, back-projection

In a study published in the Bulletin of Earthquake Engineering, Anabella Cotovanu, Elisei Cojan and Radu Vacareanu of the Technical University of Civil Engineering Bucharest and the Military Technical Academy Ferdinand I present a unified framework that decomposes the variability of Vrancea ground motions into source, path and site contributions. Their conclusion is striking: the apparent anomalies that have caused Vrancea to be treated as an exception and excluded from global comparative studies do not reflect fundamentally different physics. They arise from pronounced structural heterogeneity and the strong interdependence of source, path and site effects, which previous approaches failed to separate coherently.

The scale of the problem becomes clear when you consider how divergent the published estimates of key parameters have been. Stress drop, a measure of how much energy an earthquake releases per unit of fault area, has been variously reported for major Vrancea events at around 50 bar, 100 to 200 bar, as high as 850 bar statically and 950 to 1400 bar dynamically, and in the range of 300 to 1200 bar by researchers using empirical Green’s function methods. One influential series of analyses by Oth and colleagues employed stress-drop values roughly twenty times higher than commonly adopted reference values simply to make their models consistent with observations. Stress drop, in other words, had effectively become a correction dial rather than a physically constrained quantity.

The situation was hardly better for wave propagation. The literature contains six different attenuation functions describing how ground-motion amplitudes decay with distance outside the Carpathian Arc, split into two camps based on contradictory assumptions about geometrical spreading, with one group assuming amplitude decay proportional to the inverse of distance and the other to the inverse square root. Seismic refraction profiles from the VRANCEA99 and VRANCEA2001 experiments and the CALIXTO tomography project revealed why: the crust and upper mantle beneath the region are dramatically heterogeneous, with the asthenosphere lying at very different depths on either side of the arc, and sedimentary strata in the Focsani Basin bent to depths exceeding 30 kilometers.

Then there is the data problem, which is arguably the most severe constraint of all. Since the beginning of the twentieth century, Vrancea has produced fourteen earthquakes with moment magnitudes of at least 6.3, yet only four have been instrumentally recorded: the events of 1977, 1986, and the two shocks of May 1990. The devastating 1977 magnitude 7.4 earthquake, which killed roughly 1,500 people in Bucharest, was recorded within Romania at a single station. Attempting statistically robust source and path analyses with such a sparse database of major events is a formidable challenge, and one reason Vrancea has been left out of global ground-motion studies for decades.

The new framework tackles this problem head-on with a series of methodological innovations. First, rather than letting the recordings themselves define the grouping of data, the researchers built a zonation of the affected region from independent geological, tectonic, geophysical and geomorphological evidence, together with maps of the fundamental resonance frequency of near-surface sediments. This produced eight zones, from the Dobrogea Platform with its shallow ancient bedrock, through the deeply sedimented Dacian Basin accumulation zone, to the Focsani Basin with its exceptional sediment thickness and strong local topographic effects. Because the zoning is defined without reference to strong-motion data, it avoids circular reasoning and serves as an external pivot for organizing the region’s heterogeneity.

A preliminary spectral comparison across these zones immediately revealed systematic differences. The Dobrogea region showed a narrow amplification band peaking near 0.1 seconds, matching its independently mapped resonance frequency, while for larger earthquakes the Focsani Basin and the southern Moesia accumulation zone displayed pronounced amplification in the 0.5 to 2.5 second range. Critically, the two accumulation zones behaved similarly for moderate earthquakes but diverged sharply for large ones, suggesting that bigger ruptures activate deeper sedimentary packages. This supports what the authors call a dynamic boundary between path-controlled and site-controlled behavior, rather than a fixed depth dividing local site conditions from the deeper propagation medium.

The framework then performs what the team calls a uniformization of site effects. Because moderate-magnitude and large-magnitude datasets exhibit different site responses, an average ratio between their amplification patterns was computed and applied to the large-event data, bringing both datasets to a common reference site condition. This allowed the sparse large-event recordings and the more numerous moderate events to be combined into a single database for estimating path parameters. Stress drop was deliberately fixed at a physically plausible 100 bar and never used as a fitting parameter, a choice designed to avoid the source-site trade-offs that have plagued earlier work. Strikingly, when the analysis was repeated with stress-drop values ranging from 50 to 300 bar, the derived attenuation parameters did not change at all; the variations were absorbed entirely by site amplification, revealing the hidden coupling that had made earlier stress-drop estimates so unstable.

The results are internally consistent and, crucially, unremarkable. Geometrical spreading follows the physically expected inverse-distance decay for body waves, which dominate intermediate-depth ground motions. Frequency-dependent anelastic attenuation was determined independently for each zone, with coefficients of determination exceeding 85 percent for most zones and reaching about 98 percent in the best-constrained one. The zone-to-zone differences in attenuation, lowest over the North Moesia ridge and strongest around the Focsani Basin and Transylvanian side, directly explain why previous non-zonated studies produced such wildly different attenuation functions: they were averaging over irreducibly heterogeneous propagation paths. Site amplification envelopes derived for each zone are physically plausible and match independent stratigraphic evidence, with the deep basins amplifying long-period motion and the shallow-bedrock zones concentrating their response at short periods.

The implications extend well beyond Romania. Vrancea belongs to a very small global class of highly active intermediate-depth seismic nests, alongside Bucaramanga in Colombia and the Hindu Kush in Afghanistan, and its earthquakes shape seismic design requirements across more than two-thirds of Romanian territory. The long-period amplification characteristic of Vrancea ground motions is already embedded in the Romanian design code through spectral corner periods of 0.7, 1.0 and 1.6 seconds, far longer than the 0.3 to 0.5 second bands typical elsewhere, pushing the most severe seismic demand from low-rise buildings toward mid- and high-rise structures. The new framework, the authors argue, is transferable to any tectonic setting characterized by sparse strong-motion data and non-standard ground motions, provided sufficient independent geological and geophysical information exists to structure the regional heterogeneity. The broader lesson is perhaps the most satisfying one in science: what looks like an exception to the rules may simply be a complicated case in which the rules have not yet been applied coherently. Vrancea, it turns out, was never breaking the laws of seismology. It was just hiding them behind layers of complexity that no one had thought to peel apart.

Subject of Research: Seismic complexity and ground-motion characterization of the Vrancea intermediate-depth seismic source in Romania

Article Title: Defining and resolving seismic complexity through a unified source–path–site framework: the Vrancea intermediate-depth ground motions

Article References: Cotovanu, A., Cojan, E., & Vacareanu, R. (2026). Defining and resolving seismic complexity through a unified source–path–site framework: the Vrancea intermediate-depth ground motions. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02713-4

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02713-4

Keywords: Vrancea, intermediate-depth earthquakes, seismic hazard, stress drop, anelastic attenuation, geometrical spreading, site amplification, ground motion prediction, Romania, Carpathians, sedimentary basins, seismology

The North Anatolian Fault Zone is a natural laboratory for this kind of work. It slices across northern Turkey for roughly 1,200 kilometers, accommodating the westward extrusion of the Anatolian plate as it grinds between the Eurasian and African-Arabian plate systems. The fault has a well-documented history of sequential ruptures, with large earthquakes migrating along its length over the past century, including the devastating 1999 Izmit and Duzce events. For the new study, the fault was divided into nine predefined segments, each treated as an analytical unit with its own seismic history and hazard profile. This fault-specific approach departs from broad regional hazard maps, which average risk over large areas and can obscure the concentrated danger posed by a particular locked segment.

At the heart of the framework is a dataset of 539 seismic events drawn from Turkish earthquake catalogs, including records maintained by the Turkish Accelerometric Database and Analysis System operated by AFAD, the country’s disaster and emergency management authority. From this catalog, Ozkan engineered a set of predictive features designed to capture what he describes as short-to-medium-term seismic memory indicators. These are statistical fingerprints of recent earthquake activity on a segment, such as patterns in the timing and magnitude of preceding events, that encode how the fault remembers its own recent behavior. The idea builds on decades of seismological thinking, from the classic Gutenberg-Richter frequency-magnitude relationship to the seismic gap hypothesis of McCann and colleagues in 1979, which holds that segments that have not ruptured recently accumulate the greatest pending hazard.

The machine learning core of the study is the Extreme Gradient Boosting algorithm, widely known as XGBoost. This ensemble technique builds a hierarchy of decision trees in sequence, with each new tree trained to correct the residual errors of its predecessors. Gradient boosting has become a workhorse of tabular data science because it handles heterogeneous features well, resists overfitting through regularization, and often outperforms deep neural networks on structured datasets of modest size. In head-to-head comparison within this study, XGBoost demonstrated superior predictive capability over alternative approaches, ultimately achieving a final test Area Under the Receiver Operating Characteristic Curve score of 0.8930. An AUC of that magnitude indicates that the model ranks a randomly chosen future major earthquake above a randomly chosen quiet period nearly nine times out of ten, a level of discrimination that is considered strong for a problem as notoriously difficult as earthquake forecasting.

Perhaps the most consequential part of the work is not the classifier itself but the calibration layer wrapped around it. Raw machine learning outputs are probabilities in name only; without calibration, a stated 40 percent chance may not correspond to any meaningful real-world frequency. Ozkan performed a high-resolution probability calibration and imposed what he calls a strict Zero Miss operational strategy. The logic is blunt: for disaster management, failing to warn of a major earthquake is far more costly than issuing occasional false alarms. The optimization therefore searched for the decision boundary that would capture every major event in the test set, even at the price of reduced precision. That optimal boundary was formally established at a 35 percent probability threshold.

The results at that threshold are striking. When the calibrated framework was evaluated, it successfully captured 100 percent of the major earthquakes in the test data, meaning no event of magnitude 5.6 or greater slipped through unflagged. At the same time, the model achieved a balanced accuracy of 87.3 percent, a metric that averages the detection rate for hazardous events and the correct rejection rate for quiet periods, thereby preventing a large class of negatives from inflating the score. In practical terms, the framework behaved like a smoke detector tuned to be extremely sensitive: it rarely stays silent when danger is present, while still avoiding the constant crying wolf that would erode institutional trust in any operational warning system.

With the classifier calibrated, the final step was deployment. Ozkan applied the framework to evaluate future hazard probabilities across each of the nine predefined fault segments for their upcoming statistical cycles. The output is a quantitative, segment-by-segment ranking of hazard potential, expressed as probabilities rather than vague qualitative categories. This kind of dynamic, data-driven assessment could give disaster management authorities a defensible basis for deciding where to position search-and-rescue capacity, pre-stage emergency supplies, prioritize retrofitting programs, and focus public preparedness campaigns. Static hazard maps, updated on decadal timescales, cannot offer that level of temporal responsiveness.

The study sits within a rapidly expanding field. Machine learning approaches to seismology have progressed from early artificial neural networks trained on magnitude time series and seismic electric signals, through radon monitoring models, to modern deep learning architectures such as LSTM networks applied to seismicity analysis and satellite data streams. Recent work has extended these methods to ground motion modeling for probabilistic seismic hazard analysis in subduction zones and to real-time prediction of spectral acceleration for earthquake early warning. What distinguishes the new framework is its explicit marriage of classification performance with operational threshold calibration, an element that much of the academic literature leaves as an afterthought but that determines whether a model can actually be used in an emergency management context.

Important caveats remain. The framework was developed and tested on a single fault system, and its 539-event dataset, while substantial for a fault-specific analysis, reflects the catalog completeness and magnitude homogeneity of the Turkish record. Earthquake prediction in the strict sense, meaning the deterministic specification of the time, place, and magnitude of a future event, remains beyond any existing method, and this study does not claim otherwise. What the framework offers is a probabilistic hazard potential, a statement about relative likelihood over statistical cycles, which is a fundamentally more modest and more defensible objective. The author also reports that no external datasets were generated or analyzed beyond the study itself, and the work was conducted without dedicated funding, underscoring the individual, exploratory nature of the research.

Even with those qualifications, the implications are considerable. Turkey’s population is concentrated along corridors that the North Anatolian Fault threads like a fuse, and the Marmara region in particular is regarded by many seismologists as overdue for a major rupture. A tool that can translate decades of cataloged seismicity into calibrated, segment-specific probabilities offers exactly the kind of quantitative guidance that logistical planners need, converting abstract seismological knowledge into actionable resource allocation. If the framework’s performance holds under independent validation and in other tectonic settings, threshold-calibrated machine learning could become a standard component of the seismic risk toolkit, sitting alongside probabilistic seismic hazard analysis and operational forecasting systems rather than replacing them. The study demonstrates that the bottleneck in earthquake forecasting may no longer be the raw predictive power of the algorithms, but the careful, operationally minded engineering that turns a good classifier into a trustworthy decision aid.

Subject of Research: Machine learning-based fault-specific seismic hazard assessment along the North Anatolian Fault Zone

Article Title: A threshold-calibrated machine learning framework for evaluating fault-specific seismic hazard potentials

Article References: Ozkan, E. (2026). A threshold-calibrated machine learning framework for evaluating fault-specific seismic hazard potentials. Earth Science Informatics, 19(11), Article 210. https://doi.org/10.1007/s12145-026-02265-w

Image Credits: AI Generated

DOI: 10.1007/s12145-026-02265-w

Keywords: seismic hazard, machine learning, North Anatolian Fault, XGBoost, earthquake forecasting, threshold calibration, probability calibration, fault segments, Turkey, disaster management, balanced accuracy, seismicity indicators

The research, led by Arpita Biswas of the Indian Institute of Technology Kharagpur together with Sankar Kumar Nath of the University of Burdwan, drew on an extraordinary geotechnical and geophysical database of approximately 6,000 investigation sites across the basin. These included Standard Penetration Test boreholes, downhole seismic surveys, multichannel analysis of surface waves, ambient noise measurements and resistivity surveys, supplemented by laboratory tests on soil samples for grain size, moisture content, plasticity and density. The sheer scale of the dataset allowed the team to move beyond the city-level case studies — focused previously on Kolkata, Dhaka and Chittagong — that had dominated liquefaction research in the region.

The geological setting could hardly be more favourable for liquefaction. The basin contains the world’s deepest Neogene-to-recent sedimentary accumulation, exceeding 18 kilometres in thickness, and about 77 percent of the study area is blanketed in Quaternary alluvium — loose silt, sand, clay and gravel deposited by the Ganges, Brahmaputra and Meghna rivers. Statistical analysis of the borehole data revealed that the upper 15 metres of basin sediment is very loose to medium dense, with corrected penetration values averaging between 6 and 35 depending on depth, and a mean fines content of just 36.8 percent, indicating predominantly coarse-grained soils. Grain-size curves for sites in Dhaka, Rajshahi, Kalyani and Bhubaneswar showed that most soil layers fall within the internationally recognised boundaries of ‘most liquefiable’ and ‘potentially liquefiable’ soils.

Groundwater compounds the danger. Post-monsoon water tables across the basin range from just 0.27 to 4.71 metres below the surface, and shallow groundwater dramatically lowers the effective confining stress on sediments, creating ideal conditions for pore-pressure buildup during shaking. Historical precedent is sobering: the 1885 Bengal earthquake of magnitude 6.8, the 1897 Shillong earthquake of magnitude 8.1, the 1918 Srimangal earthquake of magnitude 7.6 and the 1934 Bihar-Nepal earthquake of magnitude 8.1 all triggered documented liquefaction, leaving ground fissures, sand boils, lateral spreading and soil slumping in their wake. Even moderate events such as the 2003 Kolabunia earthquake of magnitude 5.6 produced ground rupture along riverbanks in Rangamati.

Methodologically, the study is a tour de force of integrated hazard modelling. Because instrumental recordings are absent for the great historical earthquakes, the team used the EXSIM finite-fault stochastic simulation technique, which discretises a fault plane into numerous subsources and sums their contributions with realistic rupture propagation and travel-time delays. The resulting bedrock motions were then fed into two-dimensional nonlinear finite element site response analyses in PLAXIS 2D, capturing soil behaviour that simpler one-dimensional equivalent-linear methods miss. The simulated ground motions were validated against Modified Mercalli intensities derived from historical isoseismal maps, yielding a root-mean-square error of 0.91 — a respectable agreement for regional-scale work.

The probabilistic component is equally ambitious. Combining a comprehensive probabilistic seismic hazard model with fourteen ground-motion prediction equations, the researchers computed bedrock peak ground acceleration for a 475-year return period — the conventional design-basis earthquake — ranging from 0.08 to 0.58 g. Multiplying these values by spatially interpolated site amplification factors, which ranged from 1.30 to as much as 4.60 in the soft coastal deposits of the Sundarbans, produced surface-consistent hazard values reaching a remarkable 1.40 g near Sylhet in the northeast, close to the Dauki fault and the Indo-Burman subduction zone. Cities such as Mymensingh and Sylhet fell into the ‘severe’ category, while Dhaka, Kolkata and Khulna were rated ‘moderate’ and Bhubaneswar and Dhanbad ‘low’.

With these ground motions in hand, the team evaluated liquefaction using the stress-based framework of Idriss and Boulanger, computing factors of safety for every soil layer, then aggregating them into the Liquefaction Potential Index, the Liquefaction Risk Index, the probability of liquefaction and the probability of ground failure. The results are stark: under the probabilistic scenario, roughly 42 percent of the basin’s area falls into the ‘severe’ liquefaction potential class, with a further 35 percent rated ‘high’. Dhaka, Sylhet, Mymensingh, Chittagong, Barisal, Khulna and Rangpur all exceed the severe threshold of LPI greater than 15, while Rajshahi and Kolkata sit in the high class. The risk index map places nearly half the region — about 48 percent — in the ‘extremely high’ risk zone. At representative sites including Dhaka, Chittagong, Haldia and Kalna, probabilities of liquefaction exceeded 65 percent between depths of 5 and 20 metres, and cities from Dhaka to Barasat showed probabilities of ground failure above 0.9 — ‘extremely high to absolutely certain’.

Crucially, the modelled hazard aligns with reality. The predicted severity zones show an increasing trend in the number of documented liquefaction occurrences, providing qualitative validation of the approach. The spatial pattern also makes geological sense: the northeast around Sylhet and Mymensingh combines high seismic loading from nearby active faults with young floodplain deposits and shallow groundwater, while the southwest, underlain by older, stiffer sediments near the Indian Shield, fares considerably better. Notably, the study shows that strong shaking alone does not guarantee severe liquefaction — the Chittagong Fold Belt’s uplifted, folded sediments partially offset high seismic demand, whereas the unconsolidated Holocene deltaic plains of Khulna and Barisal remain highly susceptible despite more moderate loading.

Perhaps the most practically valuable contribution is the back-analysis of target penetration resistance. By reversing the standard liquefaction triggering equations, the researchers calculated the improved SPT-N values — denoted N-improved — needed at each site to guarantee no liquefaction under the probabilistic scenario, assuming a target factor of safety of 1.5. Mapped at depth intervals from 5 to 20 metres, these values range from as low as 1 in the stable west to 50 at 20-metre depth in the most demanding northeastern locations. Such maps give engineers, contractors and planners a first-order estimate of the densification or reinforcement required at specific sites, bridging the persistent gap between regional hazard assessment and ground-level engineering decisions.

The authors are candid about limitations: the empirical triggering relationships were calibrated on global case histories rather than Bengal Basin-specific data, uncertainties were not formally propagated through the entire framework, and the maps represent regional trends rather than site-specific design criteria. Ground improvement techniques — vibro-compaction, stone columns, gravel drains, grouting, dewatering and reinforcement — each have their place depending on geology, land use and economics, and the study suggests densification suits the young deltaic south while targeted foundation measures may be more realistic in congested megacities like Dhaka. Still, as a unified, basin-scale portrait of liquefaction threat across one of the world’s most populous regions, the work offers an essential foundation for risk-informed urban planning — and a sobering reminder that beneath the streets of Dhaka and the fields of Sylhet, the ground itself may be the region’s most insidious seismic hazard.

Subject of Research: Assessment of earthquake-induced soil liquefaction potential and mitigation targets across the Bengal Basin using probabilistic seismic hazard analysis and historical earthquake scenarios

Article Title: Probabilistic seismic hazard and historical scenario-based assessment of liquefaction potential in the Bengal Basin for liquefaction mitigation strategies

Article References: Biswas, A., & Nath, S. K. (2026). Probabilistic seismic hazard and historical scenario-based assessment of liquefaction potential in the Bengal Basin for liquefaction mitigation strategies. Discover Geoscience, 4(1), Article 293. https://doi.org/10.1007/s44288-026-00672-6

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00672-6

Keywords: Bengal Basin, soil liquefaction, seismic hazard, liquefaction potential index, probabilistic seismic hazard analysis, ground motion simulation, site amplification, Standard Penetration Test, ground improvement, earthquake engineering, Ganges-Brahmaputra delta, geotechnical investigation

The region in question belongs to the Outer Thrust System of Italy, a first-order arcuate fold-and-thrust belt that stretches roughly 2,500 kilometers from northern Italy to Sicily. It formed during the Late Pliocene and Quaternary as the Apennine-Maghrebian belt propagated eastward into the Adriatic foreland. Along its length, the system features two great outward-convex arcs: the Padan-Adriatic arc in the north and the Ionian-Sicilian arc in the south. The northern arc can be subdivided into smaller third-order arcs, including Monferrato, Emilia, Ferrara, Adriatic, and Abruzzi. While the northern arcs have produced damaging earthquakes in recent decades, the Abruzzi segment has remained strikingly silent, and its deep geometry has been far less understood than that of its neighbors.

That silence is precisely what makes the region scientifically troublesome. Slowly deforming zones present a fundamental dilemma for seismologists: faults that load at low rates may either be active, releasing earthquakes only at long recurrence intervals, or genuinely inactive. GPS measurements indicate that convergence across the Italian Outer Thrust System proceeds at just 1 to 3 millimeters per year, and historic and instrumental earthquakes in the contractional province rarely exceed magnitude 6.0. Yet the record shows that silent thrusts can surprise. The 2012 Emilia earthquake, the 1968 Belice earthquake in Sicily, and the 2023 magnitude 6.8 event in Morocco’s High Atlas all struck areas previously considered to have low seismicity, rupturing faults that had shown no instrumental activity.

To peer into the crust, the team inverted an enormous dataset of seismic travel times: 42,176 P-wave and 29,045 S-wave arrival times from 5,712 earthquakes with magnitudes between 0.2 and 5.5, recorded by 37 stations of the Italian National Seismic Network between January 2009 and December 2020. The earthquakes were carefully filtered and relocated using three one-dimensional velocity models proposed for the area, with the best-constrained catalogue selected as the starting point. The tomography itself was performed with the Fast-Marching Tomography algorithm FMTOMO, which solves the eikonal equation on a regular grid to compute first-arrival travel times through heterogeneous media, then reconstructs ray paths and iteratively minimizes travel-time residuals to update the velocity model.

Resolution was a central concern. The researchers optimized the grid spacing, using about 5 kilometers horizontally and 2 to 3 kilometers vertically, and determined optimal damping and smoothing parameters from classical trade-off curves. They then verified the reliability of the resulting models with synthetic checkerboard and spike tests, which showed that the smallest resolvable anomaly is roughly 15 kilometers across, with locally finer resolution of about 10 kilometers at shallow depths. The final models achieved reductions in root-mean-square misfit and covariance of roughly 73 and 93 percent for P-wave velocity, and 65 and 88 percent for S-wave velocity, providing confidence in the features interpreted down to about 20 and locally 24 kilometers depth.

The velocity models reveal a layered picture of the crust. In the west, within the Apennine extensional domain, scattered low-velocity anomalies in the upper 8 kilometers correlate well with Quaternary intra-mountain basins such as the Fucino and Sulmona basins. East of the extensional domain, at the hanging wall of the buried thrust, five low-velocity zones between 2 and 8 kilometers depth correspond to coastal and fluvial deposits and to Miocene-to-Pleistocene sandstone and clay units. Two of these anomalies coincide with a large positive magnetic anomaly whose origin is debated; because sedimentary rocks are slower than magmatic ones, the team favors a deeper magnetic source within the basement rather than volcanic rocks trapped in the sedimentary cover.

The most significant discovery, however, lies deeper. Between roughly 14 and 24 kilometers depth, in an area bounded by latitudes 41.3 to 41.8 degrees north and longitudes 14.3 to 15.0 degrees east, the tomography images a broad velocity inversion: a lower-velocity layer of 6.0 to 6.6 kilometers per second beneath a higher-velocity layer of 6.6 to 7.0 kilometers per second. This configuration is consistent with a mid-crustal overthrust in which a stack of crystalline and Mesozoic units, including dolomitic lithologies, overrides a lower-velocity footwall likely composed of Triassic evaporites and Verrucano formations. A comparable high-velocity body had been documented slightly further north by earlier work, but without the depth resolution to resolve the underlying low-velocity structure that the new study now images.

Integrating the tomographic results with geological maps, deep wells, seismic reflection profiles, and newly computed focal mechanisms, the team built a three-dimensional conceptual model of the Abruzzi Arc basal thrust. The reconstructed fault surface extends approximately 170 kilometers along strike, forming a broad eastward-convex arc with an average dip of about 22 degrees and reaching depths of up to 24 kilometers. Along strike it is segmented into three fourth-order arcs named Abruzzo Citeriore, Frentani, and Daunia. The deep tomography-derived fault patch extends about 100 kilometers and dips southwest at roughly 15 degrees. The geometry closely matches that of the Ferrara and Emilia arcs to the north, supporting the idea that the Abruzzi thrust may be the southernmost element of a larger crust-scale frontal thrust, and confirming a thick-skinned deformation style in which the crystalline basement itself is involved.

The model also clarifies how three tectonic provinces coexist in the region. To the west, east- and west-dipping normal faults of the Apennine extensional province host most of the region’s seismicity at shallow depths. To the east, right-lateral strike-slip faults of the Adriatic foreland, including the sources of the 2002 San Giuliano and 2018 Montecilfone earthquakes, remain confined to depths of 10 to 25 kilometers. Their upward propagation appears to be inhibited by the low-angle basal thrust, which acts as a mechanical barrier, a configuration recognized in several other sectors of the Italian thrust belt. New focal mechanisms computed from a temporary seismic network reveal modest compressional activity in 2009 and 2018 at depths of 8 to 18 kilometers, associated with a back-thrust splaying from the main structure.

Whether the Abruzzi Arc basal thrust is capable of generating large earthquakes remains the crucial open question. Some destructive historical events, including the 1706 and 1933 Maiella earthquakes and the catastrophic 1456 sequence, have epicenters that could align with the thrust or with adjacent domains, and interpretations differ. Morphotectonic studies document ongoing uplift and shortening along parts of the coastal sector since at least the Middle Pleistocene. The authors argue that slow deformation rates and long recurrence intervals do not rule out future activity, and that if the thrust behaves like its northern counterparts, it could release moderate to strong earthquakes. The new three-dimensional model offers a foundation for a next generation of seismic hazard models for the central-southern Apennine transition zone, where a seemingly quiet fault may be quietly loading toward its next rupture.

Subject of Research: Seismic tomographic imaging of mid-crustal doubling and the buried Abruzzi Arc basal thrust in central-southern Italy

Article Title: First tomographic imaging of mid-crustal doubling at the Abruzzi outer thrust front, central-southern Italy

Article References: de Nardis, R., Talone, D., De Siena, L., Romano, M. A., Brozzetti, F., & Lavecchia, G. (2026). First tomographic imaging of mid-crustal doubling at the Abruzzi outer thrust front, central-southern Italy. Solid Earth, 17(4), 665-687. https://doi.org/10.5194/se-17-665-2026

Image Credits: AI Generated

DOI: 10.5194/se-17-665-2026

Keywords: seismic tomography, Abruzzi Arc basal thrust, Apennines, crustal doubling, thrust tectonics, seismic hazard, Italy, FMTOMO, focal mechanisms, thick-skinned tectonics, slowly deforming zones, Adriatic foreland

The Lerma Valley is an oddity within the Andean orogen. Unlike the high, actively shortening ranges on either side, the basin sits in what researchers call a passive orogenic setting, where deformation appears to be governed less by present-day tectonic forces than by inherited structures left over from hundreds of millions of years of geological history. Cretaceous extensional faults were later squeezed back together during Andean mountain building, and Quaternary faults continue to reactivate ancient weaknesses. Despite hosting a provincial capital, intensive agriculture, and mining operations, the valley had never been the subject of a systematic geophysical survey, leaving a conspicuous gap in scientists’ understanding of how the central Andes transfer stress from the active Pacific margin into the continental interior.

To fill that gap, the team deployed a temporary seismic network known as LEVARIS, the Lerma Valley Ring Installation of Seismometers, in August 2017. Thirteen stations, each pairing a Data-Cube3 digitizer with a Lennartz 3D/5s sensor buried roughly 60 centimeters underground and powered by solar panels, recorded continuously for thirteen months across an area spanning about 80 kilometers north to south and 30 kilometers east to west. Before this deployment, the entire valley relied on a single permanent short-period station operated by Argentina’s National Institute of Seismic Prevention. The new network captured both distant teleseismic earthquakes arriving from 30 to 90 degrees away and, crucially, waves from deep local earthquakes concentrated in the Jujuy seismic cluster roughly 200 kilometers beneath the region.

The methodological core of the study rests on two complementary seismic tools. The first is receiver function analysis, which isolates P-to-S wave conversions that occur when seismic waves from distant earthquakes strike sharp interfaces such as the crust-mantle boundary, or Moho. By deconvolving the vertical component of a seismogram from the horizontal components, the researchers stripped away the effects of the earthquake source and distant travel path, leaving a signal dominated by the structure directly beneath each station. The second tool is ambient noise tomography, which exploits the Earth’s ceaseless background vibration. By cross-correlating continuous noise recordings between pairs of stations, the team extracted empirical Green’s functions and measured Rayleigh wave phase velocities, effectively turning the hum of the planet into an imaging source that requires no earthquakes at all.

Combining these datasets required sophisticated inversion. The researchers jointly inverted stacked receiver functions and phase velocity dispersion curves using a Hamiltonian Monte Carlo approach, a Bayesian sampling technique well suited to exploring the high-dimensional, strongly correlated parameter spaces typical of seismic velocity models. They also applied an evolutionary algorithm, inspired by natural selection, to invert the dispersion curves alone, sharpening resolution in the upper five kilometers where conventional methods lose sensitivity. A key technical innovation was a layer-dependent correction for the vp/vs ratio, the ratio of compressional to shear wave speed. Because standard H-k stacking yields only a weighted average above each discontinuity, the team developed a bottom-up recursive scheme that reconstructs the true ratio in each layer, anchored at the Moho where measurements are most reliable.

The resulting image of the crust is strikingly layered. Four major discontinuities emerge at depths of roughly 53 to 43 kilometers, 35 to 30 kilometers, 10 to 8 kilometers, and 1.5 to 1.2 kilometers, corresponding respectively to the Moho, the boundary between the lower and middle crust, the top of a possible mid-crustal detachment zone, and the base of the sedimentary basin. The Moho sits at 48 plus or minus 5 kilometers, consistent with earlier regional estimates, and common conversion point stacking reveals that it dips southward, deepening beyond 50 kilometers beneath the southern part of the study area. An azimuthal shift in the transverse components of the receiver functions, centered near 200 degrees, independently supports this southward-dipping geometry, a pattern reminiscent of observations in New Zealand where Moho dips track subducting plate geometry.

Perhaps the most consequential finding is the pronounced north-south contrast in the shallow crust. Ambient noise tomography maps at periods of 3 to 5 seconds delineate a low-velocity southern sector, where shear wave velocities of only 1 to 2.5 kilometers per second indicate thick, poorly consolidated sediments, and a northern sector with velocities up to 3.5 kilometers per second, signaling more competent crustal material. The final five-layer velocity model quantifies this stratification: a soft upper sediment layer 0.8 kilometers thick with shear velocities of 1.25 kilometers per second, a medium-consolidated sediment layer 3.7 kilometers thick, a lower consolidated sediment layer 2 kilometers thick, a 32-kilometer-thick middle crustal layer, and a 10-kilometer-thick lower crustal layer. The corrected vp/vs ratios rise from about 1.65 at the Moho, suggesting a dry felsic lower crust, toward 2.0 in the upper layers.

One of the most intriguing discoveries appears only in the local receiver functions: a discontinuity at approximately 15 kilometers depth that is invisible to the longer-period teleseismic signals. The high-frequency content of waves from the deep Jujuy cluster gives local events the resolving power to detect fractured, damaged zones that smear out in lower-frequency data. The researchers interpret this interface as a mid-crustal detachment horizon, a zone of mechanical decoupling where strain is partitioned between the upper and lower crust. Detachment structures of this kind have long been invoked to explain the style of deformation in the Eastern Cordillera, where thick-skinned tectonics gives way to distributed strain at depth, but this is among the first direct seismic images of such a horizon beneath the Lerma Valley itself.

The hazard implications are immediate. The region has a documented history of destructive earthquakes, including the magnitude 6.1 Salta earthquake of 2010, the 2015 El Galpón event of magnitude 5.8, and historic shocks such as the 1692 Esteco and 1825 Anta earthquakes. The newly identified low-velocity layer in the south, attributed to the fine-grained Tajamar Formation, is particularly concerning: these unconsolidated silts are known to saturate with water during the austral monsoon and become susceptible to liquefaction during strong shaking. Detailed knowledge of sediment thickness and velocity is exactly what engineers need to model site amplification, the phenomenon by which soft basins magnify seismic waves and concentrate damage, as the 2010 and 2015 earthquakes demonstrated.

Beyond hazard, the study offers a template for understanding passive orogens and intraplate deformation worldwide. The southward-deepening Moho may reflect crustal underplating or lithospheric flexure driven by ongoing convergence, processes also documented across the central Andes and in collision zones such as the Tibetan Plateau. By integrating receiver functions, ambient noise tomography, H-k stacking, and two independent inversion schemes, the team showed how multiple imperfect constraints can converge on a robust structural model, even where each method alone is ambiguous. For a valley that had been seismically invisible for decades, the Lerma Valley now stands as one of the better-imaged corners of the Andean foreland, and as a natural laboratory for the inherited structures that quietly shape how continents deform.

Subject of Research: Crustal structure and seismic imaging of the Lerma Valley basin in northwestern Argentina

Article Title: Deciphering the crustal structure of the Lerma Valley (NW Argentina): a multi-method seismic investigation

Article References: Criado-Sutti, E. J. M., Olivar-Castaño, A., Krüger, F., Montero-López, C., Aranda-Viana, G., Zeckra, M., & Heimann, S. (2026). Deciphering the crustal structure of the Lerma Valley (NW Argentina): a multi-method seismic investigation. Solid Earth, 17(5), 711-733. https://doi.org/10.5194/se-17-711-2026

Image Credits: AI Generated

DOI: 10.5194/se-17-711-2026

Keywords: Lerma Valley, crustal structure, receiver functions, ambient noise tomography, Moho, Andes, Argentina, seismic hazard, detachment zone, vp/vs ratio, Hamiltonian Monte Carlo, foreland basin

The core of the work is a reliability analysis method for the permanent displacement of soil slopes subjected to sequential ground motions. Permanent displacement, the irreversible sliding deformation that accumulates when seismic forces momentarily exceed the resisting strength of the slope material, is a standard yardstick for judging whether a slope remains serviceable after an earthquake. The authors argue that judging this quantity from mainshock shaking alone systematically misrepresents the hazard, because aftershocks add cumulative damage to soil that has already been degraded by the first event. Their solution couples a carefully constructed stochastic description of sequence-type ground motions with the probability density evolution method, a computational framework for nonlinear stochastic dynamics originally developed by Li and co-workers and previously extended to complex slope reliability problems.

Building the seismic input was the first technical hurdle. The team assembled 96 recorded mainshock–aftershock pairs drawn from 15 seismic events in the NGA-West2 strong-motion database maintained by the Pacific Earthquake Engineering Research Center. Selection criteria were strict: the aftershock had to be the largest-magnitude event within twelve months of the mainshock, both events needed magnitudes of at least 5.0 and peak ground accelerations of at least 0.05 g, both records had to come from the same component of the same station, the site shear-wave velocity in the upper 30 meters had to fall between 100 and 700 meters per second, and rupture distances could not exceed 80 kilometers. Each sequence was configured as a mainshock followed by a twenty-second quiet interval and then the aftershock, preserving the fully non-stationary character of real sequences.

Rather than treating these recordings as isolated deterministic inputs, the researchers modeled them as zero-mean, fully non-stationary stochastic processes using an evolutionary power spectral density framework. Frequency-domain energy distribution curves were fitted for each ground motion using an optimal square approximation criterion and least-squares regression, yielding a parameter vector that defines each evolving spectrum. This random-function dimension-reduction approach generates representative ground-motion time histories together with their associated probabilities, forming a complete probabilistic set that can feed directly into the probability density evolution method. That is a crucial advantage over Monte Carlo simulation, which demands enormous sample counts and often leaves the probability structure of the sample set incomplete.

A second, equally consequential decision concerned how to measure the intensity of a ground-motion sequence. Conventional practice leans on peak ground acceleration, but a single peak value says little about the sustained, damage-accumulating character of repeated shaking. The team therefore screened 21 candidate intensity measures spanning three families: peak-based parameters such as peak ground velocity and displacement, spectral measures such as response spectral acceleration and acceleration spectral intensity, and cumulative measures including Arias intensity, significant duration, and several root-mean-square and cumulative-absolute quantities. Correlation analysis against the simulated slope responses produced a clear winner: cumulative absolute velocity, or CAV, showed the strongest correlation with permanent displacement, with a correlation coefficient of 0.872, and captured the cumulative aftershock contribution better than any peak-based alternative.

With the seismic input and intensity measure defined, the mechanical side of the analysis proceeded on a two-layer soil slope model 150 meters long and 30 meters high, built in the finite-difference platform FLAC3D with a mesh refined to roughly half a meter in the potential sliding zone. The soil, a clay from southwestern China, was represented by a Mohr–Coulomb constitutive model with a tensile cutoff, and local damping with a 5 percent critical damping fraction, corresponding to a damping coefficient of 0.157, reproduced energy dissipation during wave transmission. Because FLAC3D avoids assembling a global stiffness matrix, it is computationally efficient for the many nonlinear dynamic simulations that a probabilistic treatment requires. Ninety-six scaled sequences were run at three CAV levels of 12, 28, and 40 meters per second, generating the response ensemble needed for density evolution.

The results reveal how strongly and how variably sequences load a slope. Average permanent displacements rose from 0.067 meters at a CAV of 12 meters per second to 0.328 meters at 28 and 0.633 meters at 40 meters per second, values comparable to those produced by mainshock-only shaking at peak ground accelerations of 0.4, 0.5, and 0.6 g in earlier work. Even with CAV as the intensity measure, the displacement results remained scattered, a reminder that frequency content and duration also matter and that no single parameter fully predicts slope response. That residual variability is precisely why the authors insist on a probabilistic treatment: the probability density evolution method propagates the uncertainty through the dynamics rather than averaging it away.

The probability distributions that emerged are strikingly non-standard. The probability density curves of permanent displacement showed bimodal or multimodal shapes that cannot be described by normal or lognormal distributions, underscoring how sequence effects reshape the statistics of slope response. Reliability estimates depended heavily on the chosen failure threshold. At a CAV of 12 meters per second, displacements ranged from 0 to 0.25 meters and reliability was 48.5 percent against a low-level threshold of 0.05 meters. At 28 meters per second the range widened to 0 to 1.00 meters and reliability fell to 45.5 percent at the medium threshold of 0.25 meters, while at 40 meters per second the range stretched to 0 to 2.00 meters and reliability dropped to 31.7 percent at the high threshold of 0.50 meters.

Comparisons against a conventional PGA-based analysis highlight the practical payoff of the new approach. At the 50 percent cumulative probability level, CAV-based displacements were approximately 0.07, 0.32, and 0.55 meters for the three intensity levels, against roughly 0.20, 0.50, and 0.70 meters for the corresponding PGA cases, and the PGA-based curves showed heavier large-displacement tails. Overall, switching from PGA to CAV reduced the dispersion of predicted displacements by about 40 percent on average, producing tighter, more informative reliability estimates. The authors caution that their results apply to clayey soil slopes of the kind found in southwestern China and to shallow crustal earthquake sequences, and that soil-parameter and geometric variability were not yet included, an extension they expect will further improve engineering applicability.

For disaster prevention, the message is clear: the aftershock is not a footnote but a first-order player in landslide hazard. By embedding sequence effects into a rigorous probabilistic framework and identifying cumulative absolute velocity as the intensity measure that best tracks cumulative damage, the study gives engineers a defensible way to quantify how likely a slope is to exceed critical displacement limits when the ground refuses to stay still. As seismic safety requirements tighten worldwide, methods that treat mainshock–aftershock sequences as the coupled, cumulative phenomena they truly are may well become the standard against which slope stability is judged.

Subject of Research: Probabilistic reliability analysis of soil slope permanent displacement under mainshock–aftershock earthquake sequences

Article Title: Reliability analysis method for soil slopes permanent displacement under mainshock–aftershock sequences

Article References: Wang, T., Zhang, C., Zhang, J., Chen, S., & Dai, Z. (2026). Reliability analysis method for soil slopes permanent displacement under mainshock–aftershock sequences. Natural Hazards and Earth System Sciences, 26(9), 4611-4619. https://doi.org/10.5194/nhess-26-4611-2026

Image Credits: AI Generated

DOI: 10.5194/nhess-26-4611-2026

Keywords: aftershocks, mainshock–aftershock sequences, slope stability, permanent displacement, cumulative absolute velocity, probability density evolution method, reliability analysis, landslides, ground motion intensity measures, FLAC3D, seismic hazard, soil slopes

The tectonic backdrop is dramatic. Off the Pacific coast, the Nazca Plate slides obliquely beneath the South American Plate, and the collision of the buoyant Carnegie Ridge with the margin pushes a sliver of crust, the Northern Andean Sliver, northeastward at 5.8 to 9.5 millimeters per year relative to stable South America. For decades, geoscientists assumed that most of this sliver motion was absorbed along its great eastern boundary faults, such as the Chingual-Cosanga-Pallatanga-Puná system in Ecuador and the Cayambe-Afiladores-Sibundoy system in Colombia. But more recent GPS-based block models have revealed a more fragmented picture: the sliver itself is broken into several independently moving microblocks, each bounded by faults that must absorb their share of the deformation. One of these, the Quito-Latacunga microblock, sits in the Inter-Andean valley, home to Quito and hundreds of thousands of people living near active faults and volcanoes.

Geodetic models predicted about 3 millimeters per year of right-lateral strain along the microblock’s northern edge, near the border between Ecuador and Colombia. The problem was that no one knew which faults actually carried that motion. GPS stations in the region are spaced tens of kilometers apart, far too sparse to resolve whether the strain is concentrated on one through-going structure or smeared across many. The distinction is critical for hazard assessment. A single, long, mature fault with a high slip rate could produce very large earthquakes at relatively short intervals, whereas a family of smaller, segmented faults would limit rupture size but spread the hazard across a wider area, complicating everything from building codes to emergency planning.

The catalyst for the new work came on 25 July 2022, when a magnitude 5.6 earthquake struck about 10 kilometers northeast of El Angel and roughly 15 kilometers southeast of the Chiles-Cerro Negro volcanic complex. The earthquake was remarkably shallow, with a hypocentral depth of less than 5 kilometers, and its focal mechanism indicated almost purely strike-slip motion. Using interferometric synthetic aperture radar, or InSAR, from both the European Sentinel-1 C-band satellites and the Japanese ALOS-2 L-band sensor, the research team mapped the coseismic deformation in unprecedented detail. The interferograms revealed a surface rupture oriented roughly east-northeast, with line-of-sight displacements of 5 to 13 centimeters. In the eastern section, a discrete rupture trace extended over 5 kilometers, while to the west the deformation splintered into en-echelon strands and diffuse off-fault deformation, a signature of a fault zone that is young, immature, and geometrically complex.

Field reconnaissance three months after the earthquake confirmed the InSAR mapping in a limited way: subvertical cracks a few centimeters wide were found on the walls of an irrigation channel precisely where the radar data placed the rupture. Dense vegetation in the El Angel ecological reserve and landslide-blocked roads made the terrain difficult to survey, and the thick, organic-rich Andisol soils, which can deform ductilely, likely absorbed much of the shallow slip. But the earthquake had done its job as a scientific beacon, pointing the team toward a previously underappreciated fault, now named the 25 July fault, that cuts across glacial moraines near the microblock’s northern edge.

To place this rupture in context, the researchers turned to remote sensing at multiple scales. They used 4- and 5-meter digital terrain models from lidar and GeoSAR surveys, satellite imagery, and, crucially, new high-resolution terrain models built from Pleiades tri-stereo satellite images processed with the NASA Ames Stereo Pipeline. These data revealed a swarm of southwest-to-northeast-trending lineaments within a zone roughly 70 kilometers wide, oblique to the better-known north-trending thrust faults of the region. Two structures stood out: the Reservoir fault and the Polylepis fault, both north of and subparallel to the 2022 rupture. Along the Reservoir fault, the Pleiades models showed fault scarps up to 3 meters high, sag ponds where streams had been dammed, landslides, and, most tellingly, glacial lateral moraines and stream channels offset right-laterally by measured amounts ranging from about 5 to 37 meters.

Dating those offset landforms required two independent geochronological tools. The team sampled large, fresh andesitic and basaltic boulders embedded in the moraine crests and measured cosmogenic helium-3 in pyroxene grains, which records how long the boulders have been exposed to cosmic rays since glaciers deposited them. The exposure ages clustered between roughly 11.9 and 20.4 thousand years, consistent with deglaciation at the end of the last glacial maximum in the tropical Andes. Dividing the measured offsets by these ages yields slip rates on the Reservoir fault between about 0.8 and 6.1 millimeters per year, depending on which offset and age combinations are used. Notably, the higher end of this geologic range exceeds the 3 millimeters per year predicted by the geodetic block model, an intriguing discrepancy that the authors attribute, at least in part, to the influence of the nearby volcano.

Roadcut exposures along the Reservoir fault provided a paleoseismic record. A vertical fault zone cuts through post-glacial sediments and volcanic soils, with three stacked colluvial wedges marking three separate surface-rupturing earthquakes. Radiocarbon dating of organic-rich Páramo soils, analyzed with Bayesian modeling in OxCal, brackets the events at about 7,030, 4,890, and 4,320 years before present, implying an average recurrence interval of roughly 2,300 years for large ruptures. Scaling relations linking rupture length and displacement to magnitude suggest these prehistoric earthquakes were between magnitude 6.3 and 7.0. Significantly, the Reservoir, Polylepis, and 25 July faults all fall within the proposed epicentral area of the destructive 15 August 1868 earthquake, estimated at magnitude 6.4 to 6.8, which damaged the town of El Angel. The fresh-looking surface expression and recent landslides along the Polylepis fault lead the team to speculate that it may have hosted that event, though dedicated paleoseismic trenching will be needed to confirm it.

Perhaps the most provocative element of the study is the proposed link between faulting and volcanism. The Chiles-Cerro Negro volcanic complex has been inflating since 2014, with InSAR documenting uplift rates of up to 2.9 centimeters per year and ground spreading away from a north-south axis through the volcano. The researchers argue that this inflation could mechanically load the adjacent faults, inducing right-lateral shear on the 25 July fault and potentially triggering earthquakes through elevated pore fluid pressures, a mechanism previously invoked for a 2014 magnitude 5.6 event south of the volcano. If volcanic unrest periodically accelerates slip, the geologic slip rates over the Holocene could legitimately exceed the decadal geodetic averages, resolving the apparent mismatch between the two datasets. The same logic may explain why ancient terrane-bounding suture zones near Chiles-Cerro Negro are not being reactivated, while in southern Colombia, near the Galeras volcano, the Romeral shear zone is being reactivated as active right-lateral reverse faults such as the Buesaco and Aranda faults, which slip at 0.7 to 2.6 millimeters per year.

Taken together, the results portray the northern boundary of the Quito-Latacunga microblock as a crustal-scale distributed shear zone rather than a discrete plate-boundary-style fault. No sharp velocity gradient appears in the InSAR data, seismicity is diffuse rather than aligned, and the mapped faults are discontinuous, segmented, and spread over tens of kilometers. For the communities of El Angel, Tulcán, Ibarra, Pasto, and Túquerres, the message is sobering but actionable: the hazard is real, distributed, and comes from multiple sources, each capable of magnitude 7 events, and possibly modulated by the breathing of the volcanoes above them. The study’s open dataset, including the Pleiades terrain models, offset measurements, and dating results, now provides a foundation for the denser GPS networks, systematic paleoseismic trenching, and refined hazard models that this tectonically restless corner of the northern Andes urgently needs.

Subject of Research: Distributed right-lateral faulting and earthquake hazard at the northern boundary of the Quito-Latacunga microblock in Ecuador and Colombia

Article Title: Distributed right-lateral strain at the northern boundary of the Quito-Latacunga microblock

Article References: Harrichhausen, N., Marconato, L., Audin, L., Lacan, P., Baize, S., Jomard, H., Alvarado, A., Hollingsworth, J., Blard, P.-H., Mothes, P. A., Rolandone, F., & Ortiz Martin, I. D. (2026). Distributed right-lateral strain at the northern boundary of the Quito-Latacunga microblock. Solid Earth, 17(5), 763-787. https://doi.org/10.5194/se-17-763-2026

Image Credits: AI Generated

DOI: 10.5194/se-17-763-2026

Keywords: Ecuador, Colombia, Quito-Latacunga microblock, strike-slip faulting, InSAR, paleoseismology, cosmogenic dating, Chiles-Cerro Negro, Northern Andean Sliver, seismic hazard, tectonic blocks, volcano-tectonic interaction