Geophysical Journal International

SummaryWe present a new, regionally adjusted local magnitude (ML) model for Switzerland and surrounding regions. The model is derived based on Wood-Anderson displacement amplitudes (AWA) calculated from 150,000 high-quality waveforms from 15,000 earthquakes between 2000 and 2025, recorded by more than 700 seismic instruments. This dataset is substantially richer than those used in previous ML studies in Switzerland, with a large number of near-source recordings and data from low-magnitude events, which were notably sparse in earlier works. AWA attenuation over hypocentral distance is parametrised through linear and logarithmic distance terms along with hinge distance points, which allow proper modelling of the attenuation characteristics at long distances and changes in attenuation associated with post-critical reflected phases. Regional differences in attenuation between the Alpine region in southern Switzerland and the northern Foreland are smoothly modelled through a ray-path-specific regional adjustment parameter, allowing the model coefficients and the hinge distances to vary spatially. The coefficients of the parametric attenuation curves are estimated using mixed-effects regressions, and the model is anchored to yield a magnitude 3 for an AWA of 10 mm measured at a hypocentral distance of 17 km. The station terms are calculated with respect to Swiss reference rock conditions. The new ML model reduces uncertainty by 33 per cent compared to the current ML scale used by the Swiss Seismological Service and does not exhibit any residual trends with respect to hypocentral distance, earthquake depth, local site conditions, or event magnitude. Empirical radiation pattern corrections are derived, further reducing the uncertainty by 8 per cent for strike-slip events. Alternative models, based on non-parametric and cell-based 2D approaches, are derived independently to validate the parametrisation of the parametric model. The new model – MLS26 – yields lower magnitudes for smaller events (with catalogue magnitudes lower than about 2.5) and for events located in the northern Foreland, whereas the magnitudes of the larger Alpine events remain similar. The reduced magnitudes of smaller events decrease the b-value of the input earthquake catalogue from 1.00 to 0.93, corresponding to a reduction of about 7 per cent. MLS26 scales one-to-one with moment magnitude (MW) for MLS26 > 4, while for smaller events, it scales with the logarithm of the seismic moment.

SummaryThis study aims to understand the recurrent seismicity that occurs in a limited volume along a major fault in the French western Alps, the Vuache Fault, which crosses the geological Jura in a flat-and-ramp zone. In 1996, an M5.3 earthquake occurred near Annecy (France), located at a depth of approximately 2 km. In this article, we analyze the seismicity that has occurred since then and calculate the seismic velocity variations at local permanent seismic stations. The magnitude and focal mechanism of the 1996 M5.3 earthquake indicate that the process was tectonic in origin. However, the duration of the Omori decay of its aftershocks, their migration, the variations in spring flow, the existence of repeated swarms, the seasonal variations in seismic velocity and their relationship to rainfall and seismicity show that the continuation of this seismicity is linked to the pressurization of fluids in a deep, fractured aquifer connected to the surface. This aquifer appears to be limited to the sedimentary formations. Earthquakes, especially during the M5.3 aftershock migration, reach the depth of the Triassic gypsum. The aftershock sequence occurred in two different phases: an initial phase lasting around ten days, during which the earthquakes did not show any migration but rather a random spatial distribution, and a second phase showing a clear migration, suggesting a fluid diffusion process. During this last phase, the hydraulic diffusivity of the aquifer was calculated and estimated at around 2 m²/s. The same order of magnitude was obtained by using the correlation between seismic velocity variations and rainfall. This is a high value, close to those found during man-made fluid injections. This aquifer forms a confined, pressurized reservoir between two thrusts in the regional flat-and-ramp structure. The scenario described in this article could be found, at various scales, in flat-and-ramp regions where tectonic stresses are sufficient to generate seismicity.

SummaryModeling seismic wave attenuation and dispersion in fluid-saturated porous media is essential for reservoir characterization; however, significant challenges remain in accurately capturing the effects of anisotropy. Unified theoretical frameworks that combine Biot and squirt flow mechanisms have often been limited by two key factors: they are typically based on the isotropic assumptions and rely on oversimplified physical models for squirt flow. Consequently, a time-domain numerical implementation for advanced, physically-based anisotropic squirt models has been lacking. This study presents a unified theoretical and numerical framework that, for the first time, integrates Biot’s theory of anisotropic poroelasticity with a state-of-the-art model for anisotropic squirt flow based on one-dimensional fluid pressure diffusion in cracks partially connected to spherical pores. The core innovation is a time-domain implementation achieved through a semi-analytical conversion of the complex, frequency-dependent frame moduli into a Generalized Zener Model representation. With parameters optimized via a genetic algorithm, the system is expressed as a set of differential equations with memory variables, enabling efficient finite-difference time-domain (FDTD) simulations in complex heterogeneous media. Our numerical results demonstrate that the model accurately captures frequency- and angle-dependent velocity dispersion and attenuation in VTI media due to squirt flow in the seismic-to-sonic frequency band. The FDTD algorithm is rigorously validated against analytical solutions, and simulations in heterogeneous media highlight its capability to capture spatially-varying anisotropic attenuation effects. This framework bridges a critical gap between advanced rock physics theory and practical wavefield simulation, providing an accurate forward modeling tool for interpreting seismic data to characterize complex reservoir rocks.

SummaryDuring the Early Medieval Ages, unusually strong and rapid geomagnetic field variations have been reported in several European regions; however, archeomagnetic data from Central Europe remain scarce. To help filling this gap, we present new archeointensity results from eight archeological sites in Germany, Austria, and Poland, dated between 500 and 1200 AD. The investigated materials mainly consist of potsherds, together with two in situ baked clay structures that also provided archeodirectional information. Archeointensities were determined using the MT4 protocol, a Thellier-type technique including cooling-rate and anisotropy corrections. For two sites, the multi-specimen domain-state-corrected paleointensity protocol was additionally applied. Rock magnetic experiments indicate that the main remanence carriers are low-coercivity magnetite and high-coercivity ε-maghemite and hematite. The presence of these phases suggests incomplete transformation to hematite during firing. To further assess the archeointensity determinations, the Bias Corrected Estimation of Paleointensity (BiCEP) method was applied, particularly for specimens showing curved Arai plots. This analysis confirmed the reliability of the Thellier results for most investigated structures, whereas no reliable BiCEP outcome could be obtained for one structure from Chobienia (Poland). For four structures, the classical Arai plot evaluation agrees with the BiCEP results and, for one site, also with the independently obtained multi-specimen results. In another case, the comparison between Thellier and BiCEP estimates allowed a more realistic assessment of intensity uncertainty. One site mean value (~ 50 µT) around 600 AD yields a lower geomagnetic field intensity than other contemporaneous European records. Overall, the data suggest an increase in field intensity between 600 and 800 AD, with values becoming more consistent with previously published regional results after this period. However, given the relatively large uncertainties and the still limited number of available studies, additional archeointensity data from 500-800 AD are needed to determine whether the observed regional differences reflect genuine geomagnetic field heterogeneity during this period. Furthermore, new chronological constraints were obtained through archeomagnetic dating approaches applied to the two available full-vector records.