Towards a bayesian seismotectonic zoning for use in Probabilistic Seismic Hazard Assessment (PSHA)

The mathematical representation of seismic sources is an important part of probabilistic seismic hazard assessment. It reflects the association of the seismicity with the tectonically-active geological structures evidenced by seismotectonic studies. Given that most active faults are not characterized well enough, seismic sources are generally defined as areal zones, delimited with finite boundary polygons, within which the geological features of active tectonics and the seismicity are deemed homogeneous (e.g., focal depth, seismicity rate, and maximum magnitude). Besides the lack of data (e.g., narrow range of recorded magnitudes), the application of this representation generates different problems: 1) a large sensitivity of resulting hazard maps on the location of zone boundaries, while these boundaries are set by expert decision; 2) the zoning can not represent any variation in faulting mechanism; 3) the seismicity rates are distributed throughout the zones and we lose the location of the determinant information used for their calculation. We propose an exploratory study for an alternative procedure in area source modeling. First, different data (e.g., geomorphology, geology, fault orientations) will be combined by using automated spatial partitioning (investigation of both supervised and unsupervised methods) in order to obtain several information classes, which may be defined as areal source zones. Then, a given hypocenter belonging to a given ''zone'', from now on called seismicity model, will be expressed by a probability computed from the 2D (spatial) probability density function (pdf) for the active tectonic model used as an a priori and updated with specific data from seismicity catalogs (e.g., focal mechanism) or other new data sources (e.g., geomorphology, subsurface exploration). This hypocenter will thus be allowed to contribute to several models, with weights given by the value of the pdf for each model. The annual rate of occurrence, for a given model, will be calculated by the weighted average of the different hypocenter contributions contained in this model. Future applications will couple the seismicity models to Ground Motion Prediction Equations. In consequence, the results will provide the full spectrum of variability in the hazard and will highlight zones badly constrained and deserving to be more studied.