Archeoseismological and paleoseismological approach to characterizing recent tectonic activity of the Montagne du Vuache fault, a slow fault in an intraplate domain (Jura Mountains, France).

In intraplate domain, active faults mostly have slow slip rates (<mm/year). The Vuache Mountain Fault (MVF) is one of the few active faults in mainland France with a recent event (Epagny, Mw 5.3, 1996). Its northwestern section crosses the archeological site of Villards d'Héria (Jura, France), which was occupied between 50 BCE and 400 CE. In the Jura massif, the cumulative displacements linked to the activity of the MVF since ~65 Ma are easily identifiable in the morphology. However, due to the scarcity of quaternary geomorphological structures, it is difficult to characterize the recent activity of the MVF. We use a multidisciplinary approach focused on archeoseismological analysis to overcome this. This is based on the quantification of deformations in archeological objects according to the EAE classification. It differentiates i) direct deformation markers, which are colocalized with surface fault rupture and synchronous with the earthquake, and ii) markers of induced deformation, linked to an earthquake causing transient ground motion beneath the object concerned, and various deformations within it.

Analysis of these EAE has identified two seismic events that took place on and in the immediate vicinity of the archeological site. To chronologically constrain these seismic events, a stratigraphic section was made near a wall impacted by two EAE. This indicates a first seismic event, EQ1, occurring between ~50 BCE and 100 CE. This earthquake caused a coseismic rupture that reached the surface and directly shifted the foundations. These were subsequently reconsolidated by Gallo-romans. A second earthquake, EQ2, occurred between ~70 CE and the abandonment of the site (400 CE). This earthquake impacted the wall indirectly, causing it to tilt and creating a cone-shaped collapse level, that we assimilate to a earthquake-related colluvial wedge. If we apply the Environmental Seismic Intensity (ESI) Scale, one could derive intensities between VII and IX, because of the amplitude of surface rupture.

Our analysis shows two earthquakes very close together, which we believe to be a period of seismic activity with probably a main earthquake and an aftershock. The next step in our study is to characterise a return period for the MVF. To do this, we carried out a paleoseismological trench 3 km south of the archeological site. This 3-meter-deep trench reveals several stratigraphic horizons dating from ~1220 BCE to the present day. These horizons show disturbances that result in an apparent vertical shift of ~20 cm between horizons. One horizon seals these events. Here, we characterise two seismic events of the MVF with a recurrence period of ~2000 years.

Magnitude of identified events can be inferred from various empirical relationships. If we rely on classical ones, established from worldwide database and containing a heterogenous set of earthquakes, the ~20 cm right-lateral offsets would mean events with magnitude ~6.3-6.4. However, we stress that the Jura seismotectonics are mainly confined within the Mesozoic cover above the Triassic decollement level, meaning that the above-mentioned relationship could be inapplicable. In such shallow tectonic regime, magnitude ~5 events could actually cause this range of surface rupture.