On 7 March 2024, shortly before noon local time (10:41:41 GMT), an earthquake of magnitude ML 3.5 occurred in southern Bohemia (Czechia). The epicentre (49.42° N, 14.05° E) was located southeast of Mirotice town with the hypocentral depth of 24.5±1.5 km. Subsequent analysis identified four minor aftershocks with ML 0.5 occurring within one hour of the main shock, followed by a very weak event (ML -0.1) about 12 h later. The latest aftershock (ML 0.4) was recorded more than three weeks later, on 29 March 2024 at 17:51 GMT.

The second earthquake (magnitude ML 3.1) occurred on 24 April 2025 at 17:32:47.3 GMT. This event originated essentially at the same location (49.42° N, 14.04° E) and at a comparable hypocentral depth of 24.5±1.5 km. The main shock was followed by four weak aftershocks (maximum ML 0.3), two occurring immediately after the main shock and two recorded one day later.

Due to sparse seismicity and limited seismic stations, the earthquake locations and focal mechanisms (Fig. 2) were calculated from stations belonging to various networks: the Czech Regional Seismic Network (CZ, 1973), Saxon Seismic Network (SX, 2001), German Regional Network (GR, 1976), and two Czech local seismic networks: WEBNET (WB, 1991) and MONET (M1, 2017). Additionally, we incorporated data from temporary station network (Schlömer et al., 2022) from the ongoing AdriaArray experiment (Kolínský et al., 2025; Vecsey et al., 2025), as well as stations from local industrial networks (T1, 1991; D1, 2014 and H1, 2017).

The epicentral coordinates of both events were obtained from the bulletin of the Institute of Physics of the Earth. Hypocentre locations were determined using the Hypo3d program (Firbas and Werl, 1988), based on the arrival times recorded at 23 stations. The calculations used a simple 1D gradient velocity model, regionally calibrated using observed travel times from deep seismic refraction experiments (Hrubcová et al., 2005) and quarry blasts. The reliability of the depth estimates was further assessed by comparing the observed travel times of the depth phase (sP) at epicentral distances of 140–210 km with theoretical travel times derived from several alternative velocity models. Additional support for the accuracy of the depth determination was provided by the consistency of the optimal focal mechanism solutions derived from both full-waveform and first-arrival inversions.

The 2024 and 2025 Mirotice earthquakes were preceded by an earlier event in 2012. This earthquake occurred on 17 March 2012 at 00:56:31.8 GMT and was located at nearly the same epicentral position, at 49.401° N, 14.026° E. According to catalog of Institute of Physics of the Earth (CZEQ, 2024), the event originated at a depth of 19 km and had a small magnitude of ML 1.7.

Focal mechanisms were determined through full waveform moment tensor inversion using the Bayesian ISOLA method (Vackář et al., 2017), based on waveform data from 12 broadband seismic stations at epicentral distances of 11–70 km. The inversion employed velocity model derived from nearby CEL09 refraction and wide-angle reflection profile (Fig. 1) of the CELEBRATION experiment (Hrubcová et al., 2005), which improved the waveform fit. The results indicate that both events were characterized by normal faulting on a NW–SE striking fault plane with strike 169° and dip 50° and 67° for respective earthquake (Fig. 2c; Table 1). Similar focal mechanism was obtained from P-wave first-motion polarities recorded at 47 stations with slightly different velocity model (strike 164°, dip 51°).

The earthquakes occurred in the southern Bohemian Massif, which formed between 500–250 Ma during the convergence of Gondwana and Baltica. Its development was shaped by Neoproterozoic active margin of northern Gondwana, Cambro–Ordovician extension, Devonian subduction, and the Carboniferous underthrusting of the Saxothuringian microcontinent (Edel et al., 2018; Schulmann et al., 2014; Soejono et al., 2020). The Bohemian Massif is divided into four west-to-east lithotectonic units: the Neoproterozoic to Palaeozoic Saxothuringian Domain, the Ediacaran to Lower Palaeozoic Teplá–Barrandian Unit (TBU), the Variscan crustal Moldanubian Domain, and the Neoproterozoic to Palaeozoic Brunovistulicum (Hajná et al., 2017; Hanžl et al., 2019; Schulmann et al., 2014). The closure of the Saxothuringian ocean and subsequent continental underthrusting created the Central Bohemian Plutonic Complex (CBPC) magmatic arc at the Teplá–Barrandian–Moldanubian boundary (Schulmann et al., 2014; Žák et al., 2014).

The earthquake epicentral area is located within the Central Bohemian Plutonic Complex (CBPC), specifically in the Mirotice complex, near the NE–SW trending boundary (∼25 km east) between the Teplá–Barrandian Unit and the Moldanubian Domain (Fig. 1). The evolution of CBPC is characterized by the intrusion of Late Devonian plutons of magmatic arc affinity and voluminous syntectonic early Carboniferous arc plutons during regional vertical transpression into Neoproterozoic and Early Palaeozoic metasedimentary successions of subhorizontal structural record (Žák et al., 2014; Tomek et al., 2015). This resulted in deep-seated steeply dipping NE–SW oriented boundary separating CBPC in the east from contrasting lithologies of the Teplá–Barrandian Unit in the west.

The transpression regime was later replaced by ductile normal shearing along the Červená Shear Zone, which facilitated exhumation of high grade Moldanubian Domain and formed eastern boundary of CBPC (Žák et al., 2014; Fig. 1). In the study area, the Moldanubian is characterized by mostly shallow to subhorizontal structures and lithological contacts (Schulmann et al., 2014). The eastern margin of the region is further disrupted by the large scale NNE–SSW trending Rödl–Blanice Fault System, which developed during late Variscan tectonic evolution and was subsequently reactivated during the Cretaceous and Miocene (Prachař, 2022).

The deep crustal structure of the Bohemian Massif has been investigated by several active seismic profiling (e.g., Tomek et al., 1997; Růžek et al., 2003, 2007; Hrubcová et al., 2005; Vavryčuk et al., 2004). These studies revealed higher P-wave velocities with lower vertical gradient in the Moldanubian crust, reflecting the dominance of crystalline rocks exposed at the surface. The Moho beneath this unit forms a sharp first-order discontinuity reaching maximum depths to 39 km (Hrubcová et al., 2005). This coincides with minimum surface heat flow values of <50 mWm-2 (Čermák, 1977), suggesting cold and mechanically strong Moldanubian lithosphere (Babuška and Plomerová, 2000).

In contrast, the Teplá–Barrandian Unit is characterized by higher near-surface velocity gradients, consistent with its Palaeozoic volcano-sedimentary sequences. The Moho beneath this unit is shallower as documented along the refraction CEL09 profile (Hrubcová et al., 2005, 2017). In addition, two middle crustal reflectors, characterized by modest velocity contrasts (∼0.15–0.3 kms-1), are identified along CEL09 at depths of ∼7–13 and 17–20 km across much of the region.

The 2024 and 2025 macroseismic observations provide insight into deep crustal structure of the Bohemian Massif. They are unevenly distributed around the epicentre, forming a trapezoidal pattern (Figs. 3 and 4). Their non-circular distribution is bounded by two major structures: the Teplá–Barrandian and Moldanubian contact in the west, expressed as a deep-seated lithological boundary, and the Rödl–Blanice Fault System in the east, expressed as a crustal-scale fault zone (Fig. 5). The enclosed area is dominated by intrusive felsic rocks, mainly granites and granodiorites of the Central Bohemian Plutonic Complex (CBPC), and high-grade Moldanubian metamorphic rocks. Their low seismic attenuation may promote efficient wave propagation and possibly resonant effects.

West of CBPC, the density of macroseismic observations sharply decreases. This area corresponds to the Teplá–Barrandian Unit, dominated by early Palaeozoic sedimentary formations with steeply oriented, highly anisotropic deformation structures near the contact (Fig. 5). This contrast suggests increased seismic attenuation within the Teplá–Barrandian Unit. The contrasting lithologies along this NW–SE-trending contact are also expressed in gravity and, particularly, in geomagnetic data, pointing to the deep character of this zone (Bucha and Blížkovský, 1994; Švancara et al., 2021).

To the east, attenuation coincides with the N–S-trending Rödl–Blanice Fault System, indicating a crustal-scale discontinuity. The macroseismic pattern therefore delineates subvertical structures that are not clearly resolved by conventional seismic investigations.

The epicentral region is sparsely and relatively evenly populated. However, and quite importantly, the main asymmetry in the macroseismic observations cannot be explained by population density because the more densely populated area toward Pilsen region contains few or no macroseismic reports. Normalization by population density therefore enhances rather than removes the observed contrast.

The asymmetric distribution may partly reflect the focal-mechanism radiation pattern, particularly the S-wave radiation toward the southwest and northeast. However, the sharp termination of observations along two old tectonic structures, independently of population density, indicates a strong structural control on seismic-wave propagation.

Strong reflections from the Teplá–Barrandian–Moldanubian contact represent another possible explanation, but they are unlikely given the geometry. The earthquakes originated at depths of ∼24 km, and ∼25 km horizontally from the contact, whose subvertical geometry is unfavourable for reflecting substantial shear-wave energy back toward the surface from such nearby sources.

Although smaller in magnitude, the 2025 earthquake (ML 3.1) was reported more frequently than the 2024 event (ML 3.5). Together with acoustic phenomena reported in ∼ 90 % of observations, this indicates efficient transmission of seismic energy from depth. It also suggests a relatively homogeneous crustal medium within the Moldanubian Domain, particularly the CBPC, facilitating both seismic-wave propagation and acoustic signals.

The attenuation observed within the Teplá–Barrandian Unit resembles regional patterns identified in deep seismic studies of the Bohemian Massif and Pannonian Basin (Beránek and Zátopek, 1981; Hrubcová et al., 2005, 2010). These studies showed efficient propagation through crystalline rocks of the Bohemian Massif and strong attenuation within sedimentary sequences of the Pannonian Basin.

The Mirotice earthquakes are rare in the region and unusual within the Bohemian Massif because of their large focal depths of ∼24 km. Their occurrence at such depths may reflect the low regional geothermal gradient (Čermák, 1977), which allows brittle failure to occur within the lower crust. Their hypocentres coincide with a reflective interface identified along the CEL09 seismic profile of the CELEBRATION 2000 experiment (Hrubcová et al., 2005), which passes close to the epicentral area (Fig. 1). This interface may represent a structural or rheological discontinuity controlling earthquake nucleation. A persistent deep-seated source is further supported by the small 2012 earthquake, which originated in the same area.

The two earthquakes have nearly identical focal mechanisms, consistent with their similar waveforms recorded at the nearby ZVC station (Fig. 2b). Both indicate normal faulting on a NW–SE-striking plane (Fig. 2c; Table 1). The inferred stress regime is characterized by a vertical maximum principal stress, a NW–SE-oriented intermediate principal stress, and a minimum horizontal principal stress perpendicular to the fault strike. This geometry is consistent with normal faulting at lower-crustal depths, where lithostatic loading controls the vertical stress, while the horizontal stresses retain the regional orientation. The regional stress field is influenced by far-field compression associated with Africa–Eurasia convergence with SHmax predominantly oriented NW–SE across central Europe (Müller et al., 1992; Heidbach et al., 2016).