GEOLOGICAL DATA FOR SEISMIC HAZARD ASSESSMENT (PALEOSEISMOLOGY)

Paleoseismology is used for: 1) the identification of seismogenic structures by the recognition of ground effects during past earthquakes; 2) precise dating of fossil earthquakes by trenching. For instance, trenching across the identified capable faults (seismogenic structures) allows to recognize displacement(s)/magnitude (e.g. thickness of colluvial wedge/s) and age (using age dating of the encountered sediments) of past event(s); 3) the estimation of the maximum potential earthquake for a given seismogenic structure through its parameters such as displacement per event and/or rupture length. 4) the calibration of probabilistic hazards analyses. Regarding the features per event, the “classic” correlations between surface faulting characteristics and magnitude derived by Wells and Coppersmith, recently improved by Mohammadioun and Serva. Regarding the idea that is behind the concept of “seismic landscape”, each relevant earthquake source (capable fault) creates a signature on the geology and especially on the geomorphology of an area, unequivocally related with the magnitude of its earthquake potential (earthquake size and frequency). It is a reasonable working hypothesis that earthquake should influence the evolution of an area in such a way as to enable an observer to recognize features in the local geologic landscape that are diagnostic of a point, or an interval, on the magnitude scales. The main controlling factors for this “seismic landscape” are a) the geodynamic setting in which the seismic source is located, including style of faulting, rate of tectonic activity, and the thickness and rheology of the seismogenic layer, b) the local climatic, geomorphic, sedimentary and anthropic environment and quaternary history. Within a region, a seismogenic structure capable to produce M 7 earthquakes appear to display, in a geological time interval, a characteristic landscape and one that is different from the landscape of a structure capable of only M 6 earthquakes. Then it should be possible to define a scale of seismic landscapes, conceptually equivalent to a magnitude scale, that provides input values for the seismic hazard assessment in terms of location, geometry, seismic potential and rates of activity of the relevant earthquake sources.