The Uzège (Southeastern France) 22 March 1186 Earthquake Reappraised

Abstract

Uzège, or Pays d’Uzès, is the area surrounding Uzès, a town in the administrative department of Gard (southeastern France). The Languedoc-Roussillon region of France, which Gard belongs to, does not have a high level of local seismicity, though it can affected by large earthquakes originating in the Pyrenees and in Provence. The new seismic zoning of France, issued on 1 May 2011, grades the seismicity of the Gard department as moderate to weak (Plan Séisme 2011). The extant SisFrance (2010) Gard earthquake catalog starts with a shock felt in Uzès in 1186 (intensity and epicentral location unknown), includes only one slightly damaging local earthquake (1946, epicentral intensity 6–7 on the MSK- 1964 scale), and associates the maximum macroseismic intensity on record in the area with an extra-regional event, the 1909 Lambesc (Provence) earthquake, the strongest event located in metropolitan (i.e., mainland) France during the last century (epicentral intensity 8–9 on the MSK-1964 scale). But this picture could be incomplete. A recent study of the Uzès-Nîmes Roman aqueduct, Volant et al. (2009), has hypothesized a connection between some archaeological evidence of damage undergone by a section of this artifact and the possible occurrence of one or maybe even two major earthquakes (M ≥ 6, type of magnitude undefined) in the 3rd or 4th century AD. We are now able to show that Uzège was the setting of the most significant local earthquake of the last millennium, though the actual import of this event has remained ignored until now, owing to the lack of historical evidence in situ. Thanks to the accidental discovery of a previously unknown contemporary source, preserved very far from the place where it was originally written, it is now possible to significantly revise the 1186 earthquake dataset by providing reliable evidence of relevant damage incurred by settlements less than 10 km northeast of Uzès.