On Saturday, September 14 at 8:47pm, a new earthquake hit the Var, Bouches-du-Rhône and Alpes-Maritimes departments without causing any damage. The tremors were felt in Nice, Toulon and Marseille. The epicenter of the earthquake was located at sea, 90 km off Cannes and Fréjus, at a depth of 9 km (+/- 3-4 km), with a magnitude of 3.8 (Mw)(1), according to Laboratoire Géoazur.
This earthquake took place in a particular geological context. The epicenter is located in the Ligurian Sea basin, the most northerly in the western Mediterranean, at a depth of 2,000m. This basin, formed 30 million years ago as the Corso-Sardinian block rotated eastwards, is one of the most seismically active areas in France. In 2022, 855 low-to-moderate magnitude earthquakes (less than 3.7) were recorded here, according to the Sismoazur catalog (2). However, this basin has also experienced events of greater magnitude, such as earthquakes of magnitude 6.6-6.9 (Mw) in 1887 and 6.0 (Mw) in 1963, as well as an earthquake of magnitude 4.9 (Mw) off Ajaccio in 2011. The devastating 1887 earthquake, responsible for over 600 deaths between the Côte d’Azur and Genoa, even generated a 2-metre tsunami(2).
Focus mechanisms calculated since 1963 in this zone reveal deformation characterized mainly by compression oriented north-south to north-west/south-east. Most of the seismic activity is concentrated in the northeast of the basin, between Nice and Genoa, probably linked to activation of the Ligurian fault system. On the other hand, the rate of seismicity decreases markedly towards the west, off the Var, and towards the south, in the central part of the basin and towards Corsica. The earthquake of September 14, 2024 occurred in a historically low-seismic zone off Fréjus.
These observations suggest that, although the Ligurian fault plays an important role, other faults in the central, southern and western parts of the basin are also active, confirming the tectonic complexity of the region.
Notes
- (1) Moment magnitude (Mw). The local magnitude (Ml) has been estimated at 4.4 by BCSF-Rénass and 4.3 by CEA-LDG. This local magnitude (Ml) is used for warning purposes and is measured at high frequency. It is more sensitive to the nature of the rocks crossed by the waves. The moment magnitude (Mw) is measured at a lower frequency. It is more stable and is a reference value for estimating the size of the rupture on the fault.
- (2) Taramelli, T. &Mercalli, G., 1888. Il terremoto ligure del 23 febbraio 1887, Annali dell’Ufficio Centrale Meteorologico e Geodinamico Italiano, II, 8(4), 331-626.
More information
Map of outbreak mechanisms between 2001 and September 2024 revised by Geoazur/OCA (in blue), or automatic for adjacent areas (in red) published on the Sismoazur website, https://sismoazur.oca.eu/#/focal_mechanism/oca/.
Seismicity map and geological map in the Ligurian Sea (seismicity according to LDG catalog, 1996-2019 for magnitude ML ≥2), from Larroque et al., 2021). Red lines represent the Ligurian fault system. Mechanisms at focus: 1 (1995/04/21, Mw 4.5; Courboulex et al., 1998), 2 (1989/12/26, Mw 4.2; Béthoux et al., 1992), 3 (2001/02/25, Mw 4.6; Courboulex et al., 2007), 4 (1963/07/19, Mw 6.0; Bossolasco et al., 1972), 5 (2011/07/07, Mw 4.9; Larroque et al., 2016). Larroque et al., 2021. https://doi.org/10.5802/crgeos.69