The 2019 Le Teil earthquake (Mw 5.0) @ PATA days 2022

France is not a seismic Country, at least if compared to other european regions nearby. Nonetheless, some earthquakes shook this land during historic times and also recently.

In a previous post, I’ve already described our visit, during the PATA days 2022, in the epicentral area of the 1909 Lambesc earthquake.

This is the turn of the strongest earthquake that hit Provance in the instrumental era: the enigmatic 2019 Le Teil event. An earthquake that shook France and the scientific community as well.

Maybe not so unexpected.. but still intriguing due to its kinematics, the extremely shallow hypocentral depth and the debated interpretation of the first trenches opened across the causative fault.

But let’s take it easy… and let’s procede step-by-step.

The earthquake was caused by the inversion of the La Rouvière fault: an inherited Oligocene normal fault supposed as inactive. Nonetheless, the earthquake hypocenter is very shallow – only 1 km of depth (Shallow events seem to be the rule rather than the exception in Provance!).

As a matter of fact the earthquake resulted also in surface faulting posing some pressing questions to a Country where high risk plants are still operative!

This picture is getting even more complicated if we consider that on the fault hanging wall, extensive quarry activities have removed a considerable volume of rock. Calculations have been made and some causative relation between the induced unloading and the triggering of the earthquake has been claimed.

At this point, we have a positively inverted normal fault and an earthquake possibly triggered by human activities. Still, the calculated present-day stress field would comply with compressive tectonics and a natural loading of the fault is not diregarded at all…

What should be done is investigating the past events on the same fault: i.e., trenching, dear paleoseismologist! 😉

They have done, obviusly, and he INSU-CNRS, IRSN, CEA and others researchers opened 15 (fifteen!) trenches on this fault. I think it’s the highest trench density per linear unit in the world, up to my knowledge!

Here we got the problems, because trenching revealed a really ambiguous stratigraphy with a thick stack of colluvial deposits laying against a bedrock fault. An architecture resembling that one expected by an active normal fault, actually.

Evidence of the deformation induced by the 2019 reverse faulting event are subtle in the trenches and the possible interpretation of a previous event has been strongly questioned in the field. Don’t you trust me? here is a poll by @ch_gruetze on the topic!