ALBERT

Description: An accurate magnitude estimation is necessary to properly evaluate seismic hazard. Unfortunately, magnitudes of small earthquakes are subject to large uncertainties due to high-frequency propagation effects which are generally not properly considered. To address this issue, we developed a method to separate source, attenuation and site parameters from the elastic radiative transfer modeling of the full energy envelopes of seismograms. The key feature of our approach is the treatment of attenuation -both scattering and absorption- in a simple but realistic velocity model of the Earth's lithosphere, including a velocity discontinuity at the Moho. Our separation method is based on a 2-steps inversion procedure. First, for each source-station pair, we retrieve optimal frequency-dependent attenuation parameters from the fitting of observed energy envelopes in the 0.375-24Hz band. In a second step, we correct for regional propagation effects to determine site amplification and source displacement spectra. From the latter, we estimate the moment magnitude Mw. The inversion procedure is applied to the 2019 ML 5.2 Le Teil and 2014 ML 4.5 Lourdes earthquakes, which both occurred in Southern France. The inversion results confirm a significant variability in the attenuation parameters (scattering and intrinsic absorption) at regional scale and a strong frequency dependence. We determine moment magnitudes Mw 5.07±0.17 for the Le Teil earthquake and 4.13±0.13 for the Lourdes earthquake, in good agreement with previous estimates. In the future, we intend to automate our method and apply it routinely to smaller earthquakes for which traditional methods are not readily applicable due to the complexity of waveforms.