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.