Earthquake Source Spectra and Site Attenuation in Northwestern France

ABSTRACT 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.