As part of the Epos-France research infrastructure and its Transverse Thematic Action on Seismicity, Axis 3 “Macroseismics, historical and contemporary seismicity” covers all activities relating to the acquisition and management of macroseismic data and the evaluation of intensities. Among other things, it focuses on ancient earthquakes for which instrumental data are lacking. Here is an example of the work carried out.

The Ubaye earthquake of 1959

On May 16, 2023, an earthquake of magnitude 3.6 occurred in the Alps, near the town of Saint-Paul-en-Ubaye. Localized, according to sources, at a depth of between 2 and 3 km, it was felt as far away as Barcelonnette, Embrun and the Queyras, with a maximum macroseismic intensity of IV (on a scale of XII). On April 5, 1959, a strong earthquake had already occurred in the Alps near Saint-Paul-en-Ubaye. The quake was felt from Toulon to Chambéry and as far as Digne-les-Bains and Italy (testimonies from the border province of Cueno). The highest intensities (VII-VIII) were concentrated within a 2 km radius of the epicenter given by SisFrance. Intensities VII were observed some ten kilometers from the epicenter.

Two children were injured by falling tiles or partitions, and the damage was estimated at 200 million francs, or around 368 million euros today. This was one of the largest earthquakes in the area, along with the Embrun earthquake (1936), which occurred some ten kilometers to the northwest. It was also the biggest in mainland France after the Lambesc earthquake in 1909.

The magnitude M was estimated at between 5.25 and 5.5. There is no reliable instrumental depth for this earthquake. Estimating depth from the macroseismic intensity field requires a good knowledge of intensity decay in the epicentral zone (within a radius of 20 km). However, due to the presence of uninhabited mountain ranges within this perimeter, there is a lack of evidence. The macroseismic depth, estimated at 7 km, is probably poorly constrained for this 1959 earthquake, which was probably more superficial.

Three aftershocks were felt in the region in April 1959, then three in July, including one near Saint-Paul-en-Ubaye. The other two were felt only in Embrun (July 19, 1959) and Barcelonnette (July 17, 1959), making their location highly uncertain.

Within a 20 km radius of the epicenters of the April 5, 1959 and May 16, 2023 earthquakes, the oldest known earthquake dates back to 1844 (felt only in Barcelonnette). However, it is highly probable that strong earthquakes have occurred in the past in this region, which was attached to the Duchy of Savoy between 1416 and 1713, and whose archives are kept in Turin. An exploration of these archives could add to our historical knowledge of the region.

To know more

Champ macrosismique du séisme du 5 avril 1959 de Saint-Paul-en-Ubaye © Epos-France

Macroseismic field of the Saint-Paul-en-Ubaye earthquake of April 5, 1959 © Epos-France

Zoom sur la zone épicentrale du champ macrosismique du séisme du 5 avril 1959,<br />
de Saint-Paul-en-Ubaye © Epos-France

Zoom in on the epicentral zone of the macroseismic field of the Saint-Paul-en-Ubaye earthquake of April 5, 1959 © Epos-France