Paleoseismicity in France: Fault trench studies in a region of moderate seismicity

Abstract

France is a country of moderate to low seismicity with a ten-century record of historical seismicity. Paleoseismicity appears to be the only available tool which is able to extend the record of seismic activity beyond this short time-window. However, moderate to low seismicity should be associated with low slip rates and, thus, geomorphic evidence for detecting active fault traces should be weak, impeding the development of paleoseismic fault studies. To test the paleoseismic method in France, we have developed a programme on paleoseismology during the past few years. First, we performed a critical reappraisal of existing data on French active tectonics, then we selected several suitable sites, and finally we trenched on some sites. Much of the revised evidence from recent and active tectonics was not selected because their origins were either dubious or non-tectonic. In particular, many reported sites from the Alps, seen in moraines, actually correspond to glacial processes (‘glacitectonics’). In addition, some clearly observed scarps could result from gravitational processes such as post-glacial rebound. Among the selected sites, three were studied in detail; they correspond to the Nimes, Moyenne Durance, and Argentera fault zones. The most reliable information was provided by trenching observations on the Moyenne Durance Fault. The trench site exhibited a N35 °E-striking knee fold, parallel to the Moyenne Durance Fault, covered unconformably by colluvial deposits. Radiocarbon dates indicate that folding occurred during the latest Pleistocene or earliest Holocene. The observed stratigraphical relations strongly suggest that folding resulted from a single coseismic slip event associated with a strong earthquake of magnitude Mw = 6.4–6.9. Our data also indicate long recurrence intervals for such earthquakes, at least of the order of 25 ka. Paleoseismic data collected on the other studied fault zones confirm that nearly Mw = 7 events may be generated by French active faults and that the recurrence intervals should be over 10 ka. Thus, the paleoseismic study of active faults is a robust tool to reconstruct seismic history where slip rates are low to moderate; however, specific exploration methods should be developed to localize fault traces more accurately and study hidden faults.