A decade of seismicity in metropolitan France (2010–2019): the CEA/LDG methodologies and observations

Abstract

We summarize ten years of the French seismicity recorded by the Geophysical and Detection Laboratory (LDG) of the French Alternative Energies and Atomic Energy Commission (CEA) network from 2010 to 2019. During this period, 25,279 natural earthquakes were detected by the LDG and located within metropolitan France and its immediate vicinity. This seismicity contributes to more than 47% of the natural earthquakes instrumentally recorded since 1962 (mainly due to the improvement of network capacity), and includes about 28% of the most significant earthquakes with a magnitude ML ≥ 4.0. Recent seismic events therefore significantly expand the available national catalogues. The spatial distribution of 2010-2019 earthquakes is broadly similar to the previous instrumental pattern of the seismicity, with most of the seismic activity concentrated in the French Alps, the Pyrenees, the Brittany, the upper Rhine Graben and the Central Massif. A large part of the seismic activity is related to the occurrence of individual events. The largest earthquakes of the last ten years include the November 11, 2019 Le Teil earthquake with ML 5.4 and maximal epicentral intensities VII to VIII, which occurred in the Rhone valley; the April 28, 2016 La Rochelle earthquake with ML 5.2 and epicentral intensity V, which occurred at the southernmost extremity of the Armorican Massif in the vicinity of the Oléron island; and the April 7, 2014 Barcelonnette earthquake with ML 5.1 and epicentral intensity VII, which occurred in the Ubaye valley in the Alps. In 2019, two other moderate earthquakes of ML 5.1 and ML 4.9 stroke the western part of France, in Charente-Maritime and Maine-et-Loire department, respectively. The recent moderate earthquake occurrences and the large number of small earthquakes recorded give both the potential to revise some regional historical events and to determine more robust frequency-magnitude distributions, which are critical for seismic hazard assessment but complex due to low seismicity rates in France. The LDG seismic network installed since the early 1960s also allows a better characterization of the temporal structure of seismicity, partly diffused and in the form of mainshock-aftershocks sequences or transient swarms. These aspects are important in order to lower the uncertainties associated to seismogenic sources and improve the models in seismic hazard assessment for metropolitan France.

Swarms and mainshock–aftershocks sequences are both triggered by fluids in the Ubaye Region (Western Alps)

“…This difference in behaviour between Serenne and Ubaye Valley might suggest a difference either in the geological context or in the driving processes. computed by the LDG (Duverger et al 2021) for all events in our focal mechanism catalogues. In both the Ubaye Valley and the Serenne areas, the median of the reconstructed fluid-pressure values does not depend on the event magnitude.…”

A strategy to build a unified data set of moment magnitude estimates for low-to-moderate seismicity regions based on European–Mediterranean data: application to metropolitan France

“…1), corresponding to the SI-Hex extended zone used in Cara et al (2017). We collect instrumental events available since 1962, corresponding to the beginning of the instrumental period for metropolitan France (Duverger et al 2021). In addition, we collect Mw estimates for events that occurred in a larger area required for the unification of the Mw estimates discussed later in Section 3.2.…”