Earthquake source characterization in stable continental regions: Application to the Armorican Massif, France

The seismic moment M₀, the associated moment magnitude M_w, and the corner frequency f_c are essential parameters for earthquake studies and seismic risk management. In the context of stable continental regions (SCRs), remote from active plate boundaries, the assessment of these parameters is made difficult by the low energy release associated with each earthquake and the low density of seismological networks.

In the north west of France, the Armorican Massif and its surroundings are part of a SCR, where the densification of the seismological network, completed in 2019, now allows for a reassessment of the regional seismicity. Though characterized by very small strain rates, the region currently displays a high rate of low-to-moderate earthquakes (up to a few M_w lower or equal to 4.0 – 5.0). For such small earthquakes, these assessments are particularly sensitive to the signal-to-noise ratio, to the seismic structure of the region, to its attenuation properties, and to the azimuthal distribution of the regional network with respect to the focal mechanisms.

We attempt to determine the M₀, and the f_c, of 106 earthquakes, detected in northwestern France between 2015 and 2023, with local magnitudes M_L ranging from 2.0 to 5.3, using spectral methods. We obtained a linear relationship between M_L and M_w for M_w ranging from 1.5 to 5.0. Our analysis also highlighted the importance of the frequency dependence of attenuation on the assessment of the f_c. This study will show the relation between M₀ and f_c in the region.