Constraining the point source parameters of the 11 November 2019 Mw 4.9 Le Teil earthquake using multiple relocation approaches, first motion and full waveform inversions

Abstract

The point source parameters of the 11 November 2019 Le Teil earthquake (Mw 4.9) in France are revisited. Previous hypocenter determinations either located it on the wrong side of the causative fault, or had to restrain the depth ranges to obtain epicenters compatible with the fault geometry. Here, relocation is carried out with three different approaches: (1) using as a master event an aftershock well-recorded by the post-earthquake temporary seismic network; (2) optimizing the velocity model in two subareas; and (3) using a quarry blast. All three approaches concur in a common epicentral area located in the middle of the rupture, confirming its bilateral character. Hypocentral depth is best constrained at $1.5 \pm 0.5~\mathrm{km}$, a range confirmed by waveform inversion. The dip of the SE-dipping nodal plane corresponding to the fault is not well constrained by waveform inversion, but the focal mechanism at rupture initiation determined with the first motions suggests that fault dip may be constant (50°–60°) from the surface down to 1–1.5 km depth. Optimized velocity models point to a strong velocity contrast, mainly in the $V_p/V_s$ ratio, between the NW and the SE sides of the Cevennes fault system, which clearly correlates with the geology.

Keywords

• Relative location
• Le Teil earthquake
• Master event
• Focal mechanism
• Earthquake location
• Waveform inversion
• Relative location