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