Earthquake Source Spectra and Site Attenuation in Northwestern France

Northwestern France is a stable continental region submitted to a high rate of low‐to‐moderate‐magnitude earthquakes for which source characteristics are not well known. The deployment of permanent seismological networks during the last decade allows, for the first time in this region, to study their source spectra to determine their seismic moment $M0$⁠, associated magnitude $Mw$⁠, corner frequency $fc$⁠, and stress drop $Δσ$⁠. In France, two national agencies, RéNaSS and CEA‐LDG, provide local magnitudes. The spectra of 108 events of $2.0≤MLRéNaSS≤5.3$ that occurred between 2015 and 2024 are analyzed in the frequency range [0.01–50] Hz under the assumption of an omega‐square source model. The effect of attenuation on $fc$ is tested by correcting the observations with two different absorption‐band models, and an empirical parameter $κ‾0$ is assessed at each station to account for the near‐site loss of energy above ∼5 Hz. Linear relationships $MLRéNaSS∝1.13Mw−0.21$ and $MLLDG∝1.20Mw−0.19$ are obtained for $1.7≤Mw≤5.0$⁠. A Brune $Δσ$ in the range [5.4–9.4] MPa is derived for the largest magnitude (⁠$Mw$ 5) of the data set, which is similar to results obtained in other intraplate regions. When combined with the $M0−fc$ relations, the regressions between local magnitudes and $Mw$ indicate that the slope of the $MLLDG−Mw$ relation is governed by attenuation effects, whereas the $MLRéNaSS−Mw$ relation may be explained either by attenuation effects or by an increase of $Δσ$ with $M0$⁠. However, a more precise description of the attenuation is required before concluding about a possible deviation from self‐similarity between small‐ and large‐magnitude earthquakes.