| 05 Mar 2026
Sequential approach to seismotectonic zonation for South-East France
Abstract. The south-east of France, encompassing the western Alps, the Jura Mountain range, the Rhône valley and the Provence region, is the most seismically active region in metropolitan France and, consequently, one of the most extensively surveyed and studied. However, seismicity remains low to moderate (less than 10 Mw 5 and 1 Mw 6 events per century since 1300 CE) and geodetic deformation rates appear relatively low (< 20 nanostrain yr-1). The resulting low-signal-to-noise ratios, together with the complex, dense fault networks inherited from a polyphased tectonic history, make this region particularly challenging for seismic hazard assessment. The geophysical and geological data available are extensive, yet inhomogeneous and insufficient to confidently characterize active faults, quantify on-fault deformation and associate seismic rates. Therefore, seismic source characterization through seismogenic area models is commonly adopted. These models are however highly sensitive to the data used to describe seismotectonic behavior. Our objective is to consider newly available geophysical data as complementary constraints to geological observations to further refine seismotectonic zonation models.
We present an innovative sequenced zoning methodology that disaggregates seismotectonic behavior into three components (namely – crustal structure, observed seismicity and surface deformation) each analyzing several key features. We thus derive three novel, independent seismotectonic zonation models, each representing a different perspective on the seismogenic process. Additionally, we associate confidence levels with zone limits to each subsequent zonation model, by assessing feature homogeneity among neighboring zones. Afterwards, we propose a synthetic model which integrates all seismotectonic features by merging the most recurrent and highest confidence zone limits from the three independent zonation models. This approach intends to minimize zone mapping uncertainties by quantitatively assessing seismotectonic observations, and to yield reproducible and updatable models representative of the current state of seismotectonic knowledge. We subsequently compare the resulting zonation models and discuss their implications for seismic source characterization.
The paper presents a new seismotectonic methodology for South-East France, the most seismically active region in metropolitan France. The study is based on the disaggregation of seismotectonic behaviour into three main components: the crustal/geological component, the seismological component, and the geodetic component. These three individual models are then integrated to produce a unified model that incorporates all the different datasets.
The manuscript has excellent scientific significance and overall quality, as well as a clear presentation of data, results, and conclusions. The amount of data used is very large and reflects the considerable effort carried out by the authors. The work represents an improvement over current approaches for mitigating seismic hazard in a critical region of France and in neighbouring areas such as north-west Italy, thanks to the integration of more recent datasets and the compilation of information that had not previously been considered.
In its current form, the manuscript would benefit from some clarifications, mainly listed below. I recommend publication once these points have been addressed:
-Clarification of the criteria adopted to build the unified model should be provided, and included also in the Methods section. It is not fully clear to me if the zone limit with the highest confidence level was selected to define the geometry of the unified zone. In general, I found it difficult to understand the role of the confidence levels associated with the zone boundaries, and how these were used later on. Perhaps more clarification on this matter should be provided.
- Some information should be provided on how the qualitative analysis of features of the structural model were quantified to calculate the normalized sum of differences.
- On a similar note, fracture numbers are defined as low, mean, high. What are these adjectives referring to? Which sources were used?
- The crustal model was built adopting one velocity, yet it is mentioned that two velocities for the Adriatic and European Moho are needed. Some justification on why you use only one velocity should be provided.
- I did not find an explanation of how fine and coarse analysis are build. In some areas, multiple signals of one type appear to be present, with variable predominance, yet in the coarse analysis a different dominant color is shown.
- To build the seismogenic thickness, different sources with different uncertainties are used and kept distinct. How were the overlapping zones between the different models handled?
- The ranges of confidence levels for the three models differ considerably. How does this impact the building of the unified model? Two of them appear rather uneven; does this affect the unified model?
Other minor comments are in the attached file.