Citizen Reports of Two Rare Deep Earthquakes Map Hidden Crustal Boundaries in Czechia

When the ground began to shake beneath the rolling countryside of southern Bohemia in March 2024, few residents of the Czech Republic expected anything unusual. Earthquakes are rare in this part of Central Europe, and the region around the small town of Mirotice is not known as a seismic hotspot. Yet within hours, thousands of people had logged onto a website run by the Institute of Geophysics of the Czech Academy of Sciences to describe what they had felt: rattling windows, swaying lamps, clinking dishes, and a strange rumbling noise that many compared to a passing freight train or a distant explosion. Those voluntary reports, it turns out, have now delivered something far more valuable than a catalogue of shaken nerves. They have mapped, with surprising precision, the deep and largely invisible architecture of one of Europe’s oldest geological terrains.

A team of Czech seismologists led by Pavla Hrubcová has analysed two earthquakes that struck near Mirotice, one of magnitude 3.5 in March 2024 and a smaller magnitude 3.1 event in April 2025, combining conventional seismological data with more than 1,500 citizen-filled macroseismic questionnaires for each event. Their study, published in the journal Solid Earth, demonstrates that ordinary people describing their experiences can effectively constrain deep geological structures in a region where earthquakes, and therefore seismic instruments’ chances of illuminating the subsurface, are few and far between.

What made these earthquakes scientifically remarkable was not their size but their depth. Both events originated at approximately 24.5 kilometres beneath the surface, an unusually large focal depth for the Bohemian Massif, where present-day seismicity is low and typically confined to the upper crust at depths of less than 12 kilometres. The 2024 event was followed by four minor aftershocks within an hour and further weak events over the following weeks; the 2025 earthquake produced a similar modest aftershock sequence. A small magnitude 1.7 earthquake in 2012, located at nearly the same position and at 19 kilometres depth, had already hinted that something unusual was happening deep beneath this quiet corner of Bohemia.

Pinpointing the events required assembling waveforms from a patchwork of seismic networks spanning the Czech Regional Seismic Network, Saxon and German networks, local Czech networks, temporary stations from the ongoing AdriaArray experiment, and even industrial monitoring arrays. Hypocentres were computed using a three-dimensional location program with a one-dimensional gradient velocity model calibrated against travel times from deep seismic refraction experiments and quarry blasts. The depth estimates were cross-checked using the sP depth phase recorded at epicentral distances of 140 to 210 kilometres, and by verifying that independent focal mechanism inversions yielded consistent solutions. Both earthquakes showed normal faulting on a steeply dipping, northwest-southeast striking fault plane, derived from full waveform moment tensor inversion using the Bayesian ISOLA method and a velocity model drawn from the nearby CEL09 refraction profile of the CELEBRATION 2000 experiment.

But the real revelation came from the public. Despite the epicentral area being rural and sparsely populated, with roughly 20 to 50 people per square kilometre, the Institute of Geophysics collected 1,512 questionnaires for the 2024 earthquake and 1,679 for the 2025 event, submitted through online forms and a smartphone application. Media interviews following unusual events in Czechia regularly encourage citizens to report their experiences, and the response rate was so reliable that only about 2 percent of questionnaires had to be discarded for inconsistent timing, duplication, or effects not clearly attributable to the earthquake. Notably, the weaker 2025 earthquake actually generated more reports than the stronger 2024 event, a testament to both public engagement and unusually efficient transmission of seismic energy from depth.

Respondents described shaking felt across distances of 70 to 100 kilometres from the epicentre, reaching the towns of Písek, Strakonice, and Tábor and the Šumava Mountains. For the 2024 event, 66 percent of respondents reported light shaking and 17 percent strong shaking, while around 90 percent of reports in both years included acoustic phenomena such as rumbling or booming sounds. Individual responses were converted into intensities on the European Macroseismic Scale EMS-98, with qualitative terms like few, many, and most quantified into percentage ranges. Reports that the earthquake was heard but not felt were treated separately as heard-only observations, since acoustic effects alone are not diagnostic for intensity assignment. Maximum intensities reached IV to V for the 2024 event and IV for the 2025 event, with occasional hairline cracks in plaster reported near the epicentre.

The striking feature, however, was not how strong the shaking was but where it was felt. Rather than spreading in a roughly circular pattern around the epicentre, as would be expected in a geologically homogeneous region, the observations formed a distinctly trapezoidal patch, sharply bounded on the west by the contact between the Teplá-Barrandian Unit and the Moldanubian Domain, and on the east by the Rödl-Blanice Fault System. Crucially, this asymmetry could not be explained by population distribution. The more densely populated region around Pilsen to the west produced almost no reports, while the sparsely settled epicentral area yielded dense coverage. Normalising the observations by population density would only sharpen the contrast.

The geological explanation lies in the contrasting rocks on either side of those boundaries. The trapezoidal zone of strong reports is dominated by the felsic granites and granodiorites of the Central Bohemian Plutonic Complex and the high-grade metamorphic rocks of the Moldanubian Domain, crystalline rocks that attenuate seismic energy weakly and allow waves to propagate efficiently, possibly with resonant effects. West of the pluton, the early Palaeozoic sedimentary sequences of the Teplá-Barrandian Unit, with their steeply oriented, highly anisotropic deformation structures near the contact, appear to absorb seismic energy far more effectively, and observations drop off sharply. To the east, the attenuation coincides with the Rödl-Blanice Fault System, a crustal-scale discontinuity reactivated during the Cretaceous and Miocene. These subvertical structures, the authors argue, are not clearly resolved by conventional seismic investigations, yet the citizen reports delineate them with remarkable clarity.

The tectonic story behind these boundaries stretches back hundreds of millions of years. The Bohemian Massif, the largest exposure of the European Variscan orogenic belt, formed between roughly 500 and 250 million years ago during the convergence of the ancient continents Gondwana and Baltica. The closure of the Saxothuringian ocean and subsequent continental underthrusting created the Central Bohemian Plutonic Complex as a magmatic arc at the boundary between the Teplá-Barrandian and Moldanubian units. Deep seismic profiling has revealed that the Moldanubian crust is cold and mechanically strong, with a sharp Moho discontinuity reaching depths of up to 39 kilometres and surface heat flow below 50 milliwatts per square metre. That low geothermal gradient, the study suggests, is what allows brittle failure, and therefore earthquakes, to occur in the lower crust at depths where rocks elsewhere would deform plastically. The hypocentres of the Mirotice events coincide with a reflective interface identified along the CEL09 profile, which may represent a structural or rheological discontinuity controlling earthquake nucleation.

The implications reach well beyond Czechia. In regions of sparse seismicity and limited instrumentation, crowdsourced macroseismic observations offer a cost-effective way to gather large volumes of data that reflect geological controls on seismic wave propagation, complementing instrumental networks that may record only a handful of events per decade. The Mirotice study shows that the pattern of who felt an earthquake, and who did not, can trace deep, steeply dipping tectonic boundaries rooted in the lower crust, structures that pre-Permian thrusts in the Variscan belt are known to follow. Two modest earthquakes, felt by thousands of ordinary people and dutifully reported through a web questionnaire, have drawn a portrait of the deep crust that no amount of waiting for the next big event could have produced. In seismology’s quiet corners, it seems, the public itself has become the most sensitive array of all.

Subject of Research: Crowdsourced macroseismic observations of two rare deep earthquakes constrain crustal structure in the south Bohemian Massif

Article Title: Public response to two rare earthquakes: crowdsourcing constraints on crustal structure in south Bohemian Massif

Article References: Hrubcová, P., Machek, M., Vackář, J., Pohořalá, A., Kampfová Exnerová, H., Špaček, P., & Zedník, J. (2026). Public response to two rare earthquakes: crowdsourcing constraints on crustal structure in south Bohemian Massif. Solid Earth, 17(8), 979-989. https://doi.org/10.5194/se-17-979-2026

Image Credits: AI Generated

DOI: 10.5194/se-17-979-2026

Keywords: earthquakes, Bohemian Massif, crowdsourcing, macroseismic intensity, crustal structure, seismology, Czechia, Mirotice, intraplate seismicity, seismic attenuation, Variscan orogeny, citizen science