Analogue experiments applied to active tectonics studies: the case of seismogenic normal faults

Lithosphere can be divided into three main zones as a function of increasing depth: an aseismic updip zone, the seismogenic zone and a deep aseismic zone. Identifying the location of these zones is a key goal to understand how a specific seismogenic fault works. The evaluation of the seismogenic structures potential in tectonically active regions needs an accurate knowledge of the geometries and kinematic of the faults. In many cases, large seismogenic faults are not clearly and unambiguously expressed at the surface, whereas in other regions with higher deformation rates a clear geological surface evidence is often associated with large earthquakes. Therefore, the characterization of the seismogenic faults and of their mutual interactions it is not always straightforward; in this case, analogue modeling can provide an independent and useful tool for the interpretation of the surface geological data. Analogue modeling applied to earthquake geology is a quite innovative technique: when combined with other datasets (e.g.: seismic tomography, seismic profiles, well-logging data, field geology, morphotectonic and palaeo-seismological data) it can provide significant insights on the long term (i.e. Quaternary) evolution of a seismogenic fault. We carried out a set of analogue models at 1 : 100,000 scale that reproduce in 2D a normal fault with a relatively low dip angle (45°-50°). In our experimental approach different materials have been used to simulate the three main zones in which the lithosphere is separated. Dry sand and wet clay simulate different mechanical behaviour of rocks during seismic cycle. The dry sand, with its negligible cohesion and ductility, represents brittle rocks that deformed by localized faulting during earthquakes. Wet clay, with its slightly greater cohesion and ductility, mimics aseismic updip zone. Glass microbeads simulate aseismic plastic zone. Preliminary results are highlighting a mutual control among the three analogue materials, both in the spatial distribution of secondary structure and on the development of the overlying associated basins. We believe that the modeling approach applied to the study of active faults can provide useful insights to investigate the seismic potential of a structure with important appliances also for the seismic hazard assessment.