Rheological modelling for constrain seismotectonic parameters

The comparison between seismicity cut-off depth and the corresponding BDT (Brittle-Ductile Transition) depth, obtained from an accurate rheological modelling, has been performed for numerous test sites all over the Aegean Region including the south-west Balkans. The investigated area spans the broader deformational region associated to the Nubia-Adria and Eurasia plate boundary interactions. This complex geodynamic framework is characterised by different tectonic regimes and mechanical behaviours of rock volumes, varying from the continental collision, affecting the western Balkans from Montenegro to north-western Greece, to the oceanic subduction occurring offshore the western Peloponnesus (and south of Crete Island) and the lithospheric extensional realm characterising the internal Aegean region. The Kefallinia Transfer Fault (transpressional), the North Aegean Trough (transtensional) and the North Anatolian Fault (purely strike-slip) represent major wrench zones affecting the large sectors of the region. Our research focuses on the correlation between the seismological and the rheological transitions, so that the latter can actually provide an effective independent constraint on the seismogenic layer thickness and hence for defining some crucial seismotectonic parameter (e.g. fault width). In order to have more accurate results only relocated seismic sequences and background seismicity, from catalogues and local datasets, have been considered. For the purpose of this work, indeed, the importance of a limited hypocentral depth error is crucial. In the same areas, based on rheological modelling, we reconstruct the local strength envelopes focusing our attention on the BDT depth occurrence. Comparing the obtained rheological information with the considered seismicity, in all the investigated sites representative different tectonic regimes (extensional, transcurrent and compressional), we obtained that 90% of the relocated events occurs within the corresponding rheological transition and, when magnitudes data are available, that almost the total amount of the seismic scalar moment is always released above the BDT depth. Accordingly, this results strongly validate the use of the rheological transition to define the seismogenic volumes and the maximum width for the active faults affecting these regions. Finally, in order to possibly improve the seismic hazard assessment analyses for these areas, relying on the defined constraints (the source width and indirectly the surface rupture length), we applied well known empirical relationships to estimate the possible maximum magnitude for the recognized seismogenic sources affecting the investigated test sites.