Back to the Source: Connecting the Seismological Observations of Le Teil Earthquake (Mw 4.9, 2019/11/11, France) to the Local Geology

The spatial distribution of seismicity in active fault zones depends primarily on the geometry of the fault networks. However, recent advances suggest that seismic activity may also be controlled by the rheology of the geological units surrounding fault zones. In the present work, we use seismological and geological analyses to investigate the influence of the local geology on the rupture nucleation and propagation of the 2019 Le Teil earthquake (France; M_L = 5.4; M_w = 4.9) and on the distribution of its aftershocks. The kinematic rupture model of the mainshock, obtained by joint inversion of seismological and INSAR data, shows that the rupture nucleated at 1 km depth and propagated mainly up-dip and bilaterally. Template matching detection identifies 115 aftershocks (−1.6 ≤ M_L ≤ 2.5) in the two months following the mainshock. Double difference location shows that the aftershocks delineate a 4 km-long NE-SW plane with a 60° dip to the SE, consistent with the extent and the geometry of the La Rouvière fault activated during the mainshock. By placing the seismicity in the local geological context, we conclude that the seismic activity was largely controlled by the rheological properties of the contrasting lithology. The mainshock nucleated in marly-limestone, but the coseismic slip was maximum above 0.6 km depth in a massive limestone more prompt to store elastic strain. The aftershocks occurred mainly in marls and marly limestone. They are interpreted as ruptures in competent beds of limestones of varying thickness, interbedded with marls.