SURFACE RUPTURES DURING MODERATE EARTHQUAKES: IS THAT SO RARE?

The PDF file includes the slides (slightly modified after the conference) presented at the GSA  Connects Online conference (26-30 october 2020).

The reference of the abstract is : 

Stephane Baize, Oona Scotti, Jean-François Ritz, Matthieu Ferry, Christophe Larroque, Emmanuel Mathot, Laurence Audin, Fiia Nurminen, Paolo Boncio, Bertrand Delouis, SURFACE RUPTURES DURING MODERATE EARTHQUAKES: IS THAT SO RARE?Geological Society of America Abstracts with Programs. Vol 52, No. 6,, 2020, doi: 10.1130/abs/2020AM-361143:

Moderate magnitude earthquakes (M=<6) are dangerous, in terms of seismic motion and associated damage, as shown by moderate Le Teil (Mw=4.9; France) and Sparta (Mw=5.1; Eastern USA) earthquakes.

Analysis of historical databases gives low probabilities that they break the ground surface, for instance 10% for Mw=5 earthquakes (Moss and Ross, 2011). These statistics question the completeness of information from on-fault palaeoseismology, which is a problem where seismic hazard is dominated by moderate earthquakes. However, it is noted that these statistics include relatively old cases, often before 2000, a time when modern technologies (InSAR, LiDAR) were not or little available. As illustrated during the Le Teil earthquake in France or elsewhere in the world (ex. Ecuador, Australia, USA), remote sensing and geological field studies are crucial for the characterization of surface ruptures whose maximum amplitude is a few decimeters: using extensively those tools in the future will surely populate the catalogues with more and more surface rupturing events.

It will be important, in the light of future geological and geodetic studies, to update the databases, to refine the statistics and thus improve the robustness of the hazard analyses, in particular those concerning the risk of «surface rupture», one of two aspects of the growing seismic hazard in the world (lifelines, critical facilities).

In France like in other Stable Continental Regions, deformation rates are very low, hence a very rare occurrence of earthquakes rupturing the ground surface. These traces in the landscape, subject to erosion and anthropogenic activity, are then erased and, to solve this difficulty, we chose to implement the techniques of prospecting the subsoil (geophysics, trenches, drilling) to reconstruct the calendar of past earthquakes in recent sediments potentially lying against the Le Teil ruptured fault, which we believe remains the most promising approach.