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: Earthquakes in Switzerland and surrounding regions during 2019 and 2020. (2025). Original publication

Image Credits: AI Generated

DOI: Not provided

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 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

The research is rooted in performance-based earthquake engineering, a framework that treats structures not as objects that simply stand or fall, but as systems whose performance can be predicted probabilistically. At the heart of this framework sits the intensity measure, a scalar descriptor of ground shaking, such as peak ground acceleration or spectral acceleration at a given period, that serves as the bridge between hazard analysis and structural response prediction. The quality of an intensity measure is judged by its efficiency, meaning how tightly it correlates with the engineering demand parameters that describe structural and operational damage, and by its sufficiency, meaning how well the predicted response remains independent of other ground-motion characteristics. An intensity measure that is both efficient and sufficient allows engineers to build accurate probabilistic seismic demand models with fewer costly simulations, which is precisely where the new study makes its contribution.

The particular system the researchers examined, known as the simply-supported-bridge-vehicle coupled system, is among the most common bridge forms on high-speed railway networks, especially on challenging routes such as the Sichuan-Tibet Railway, where lines must cross regions laced with active strike-slip faults. Simply supported spans rest on bearings that allow rotation and, to a degree, translation, which makes them economical and constructible but also vulnerable when the ground beneath them lurches in two directions at once. When a train is present, the problem becomes even more intricate, because the vehicle, the track, and the bridge form a dynamically coupled system in which the running safety of the train depends on the deformation of the deck, and the vibration of the deck is in turn influenced by the moving masses of the vehicles above it. Past investigations by these and other research groups have shown that near-fault pulse-type ground motions can compromise derailment resistance and that fault rupture itself imposes permanent, quasi-static displacements that no amount of dynamic damping can absorb.

Strike-slip faulting introduces a uniquely punishing combination of effects. As the fault ruptures, the ground on either side shears horizontally past the other, and a structure crossing the fault trace is forced to accommodate the offset. In the near-fault zone, two signature phenomena dominate: the fling step, a permanent, often unidirectional displacement pulse produced by tectonic deformation, and forward directivity, a strong long-period velocity pulse that arrives when the rupture front propagates toward the site at nearly the speed of the shaking itself. These effects are inherently directional, aligned with the fault-parallel and fault-normal orientations, so the structural response depends critically on the angle at which the bridge crosses the fault. Compounding the challenge, recorded ground motions close to strike-slip surface ruptures are scarce, forcing analysts to work with limited datasets in which the choice of intensity measure carries outsized consequences for the reliability of the resulting risk estimates.

To tackle this problem, the team developed a modified intensity measure selection framework for cloud analysis, a widely used statistical technique in which a family of ground motion records, each scaled or unscaled, is run through the structural model and the resulting demands are regressed against candidate intensity measures in logarithmic space. The innovation lies in the normalization of the intensity measures, which sharpens the comparison of efficiency across candidates whose raw numerical ranges differ by orders of magnitude. By normalizing before evaluating statistical performance, the framework reduces distortions that can arise in regression diagnostics and produces a fairer ranking of alternatives. The researchers then applied the framework across a battery of candidate measures drawn from the standard toolbox of earthquake engineering, including peak ground velocity, peak spectral displacement, peak spectral velocity, and measures defined from individual ground motion components as well as geometric-mean combinations, testing each against six representative engineering demand parameters spanning the bridge and the running vehicles.

The verdict from thousands of coupled dynamic analyses is strikingly clear: under the coupled fling-step and forward-directivity demands of crossing strike-slip faulting, velocity- and displacement-based spectral measures outperform the acceleration-based measures that have traditionally dominated fragility studies. Specifically, the peak spectral displacement, SDmax, and peak spectral velocity, SVmax, emerged as the top performers for constructing probabilistic seismic demand models of the coupled system. This makes physical sense. Long-period velocity pulses and permanent displacement offsets, the hallmarks of near-fault strike-slip motion, resonate most directly with displacement-type demands such as bearing displacement, pier drift, and the deck deformations that govern train running safety. Peak ground acceleration, by contrast, emphasizes high-frequency content that is relatively less consequential for these long-period, quasi-static-dominated failure modes, and its correlation with demand weakens accordingly.

Equally important is the finding about directionality. The study shows that intensity measures computed from the fault-parallel component of ground motion perform consistently well in integrated assessments across all six engineering demand parameters, reflecting the dominant role of the shearing displacement imposed along the fault trace. When the researchers turned to specific engineering scenarios defined by the fault-bridge crossing angle, they identified scenario-specific optima: for a 90-degree crossing, the geometric mean of peak spectral displacement across the two horizontal components, SDmax,GM, proved best, while for a shallower 45-degree crossing, the fault-parallel peak spectral displacement, SDmax,FP, took the top spot. In both cases the chosen measures delivered a balanced combination of efficiency and sufficiency, giving engineers a defensible, defensible-to-auditor basis for record selection and fragility construction tailored to the actual geometry of a proposed crossing.

The practical implications reach well beyond academic statistics. High-speed rail corridors in tectonically active regions, from southwest China to Turkey, California, and Taiwan, increasingly must traverse fault zones because alternative routings are economically or geographically impossible. The 1999 Kocaeli and Duzce earthquakes in Turkey and the Chi-Chi earthquake in Taiwan famously collapsed or displaced simply supported spans whose unseated girders traced the fault rupture across their alignments. By identifying which ground-motion descriptors most faithfully capture the demand imposed on a coupled bridge-train system, the new framework enables more economical and more trustworthy fragility assessment, supporting decisions about bearing seat widths, restrainers, isolation systems, and operational speed limits during seismic events. Because cloud analysis with unscaled records is computationally expensive, the improved efficiency of the recommended measures also translates directly into fewer simulations required for a given confidence level, a meaningful saving when each coupled vehicle-track-bridge analysis involves extensive nonlinear computation.

Methodologically, the study also contributes a reusable template. The normalization-based cloud analysis framework is not tied to any particular bridge form, and the authors’ evaluation metrics, which weigh efficiency, sufficiency, and practicality across multiple demand parameters simultaneously, can be redeployed for continuous girders, cable-stayed spans, and suspension bridges crossing faults, where prior work by the same community has documented severe track-bridge interaction and long-span dynamic amplification. The research was supported by the National Natural Science Foundation of China, the Department of Education of Guangdong Province, the Foshan Science and Technology Bureau, and Hunan University of Science and Technology, and drew on the strong-motion database of the Pacific Earthquake Engineering Research Center’s Next Generation Attenuation-West2 project. As high-speed rail networks push deeper into seismically hostile mountains and basins, the humble task of choosing the right number to describe a ground motion, once treated as a technical footnote, now stands revealed as one of the decisive levers for keeping trains, bridges, and passengers safe when the ground itself refuses to hold still.

Subject of Research: Selection of optimal seismic intensity measures for railway simply-supported-bridge-vehicle coupled systems subjected to crossing strike-slip faulting.

Article Title: Analysis and selection of seismic intensity measures for railway simply-supported-bridge–vehicle coupled systems subjected to crossing-strike-slip faulting

Article References: Zhou, T., Li, Z., Jiang, L., & Wen, T. (2026). Analysis and selection of seismic intensity measures for railway simply-supported-bridge–vehicle coupled systems subjected to crossing-strike-slip faulting. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02676-6

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02676-6

Keywords: seismic intensity measures, railway bridge, strike-slip fault, vehicle-bridge interaction, probabilistic seismic demand, fling step, forward directivity, cloud analysis, running safety, high-speed railway, fault crossing, earthquake engineering