Osservatorio Sismologico, Universita` di Messina

The results of geological and geomorphologic surveys on the salt marsh of Ganzirri (Pantano Grande), combined with geophysical researches and historiographical data, are reported here to define the genesis of the marsh and to evaluate the physical factors that influenced its recent evolution. The genesis of the Pantano Grande may be due to a state of equilibrium reached between differential lowering of the coastal plain, confined by normal faults, and generalized chain uplift. In particular, two normal faults are considered: the first borders the northern shore of the Pantano Grande, and the second bounds the Ionian coastal plain towards the south. Concerning the recent evolution of the Pantano Grande the importance of the sterile conglomerate outcrop, which borders the Ganzirri coastal plain, is stressed. The conglomerate is interposed between sediments that define the Pantano Grande basin, and the Ionian Sea, and influenced the water exchange between the marsh and the sea. Before the excavation of two canals that link up the Pantano Grande with the sea, the conglomerate, limiting the spread of benthic species, has definitively affected the ecological structure of the original marsh.

A modelling of the observed macroseismic intensity of historical and instrumental earthquakes in southern Spain is proposed, with the aim of determining the macroseismic parameters for seismic hazard evaluation in a region in which the characterization of intensity distribution of seismic events shows different levels of difficulty referable to the complex faults system of the area in study. The adopted procedure allows an analytical determination of epicenters and principal attenuation directions of earthquakes with a double level of verification with reference to the maximum shaking area and structural lineaments of the region, respectively. The analyses, carried out on a suitable number of events, highlight, therefore, some elements for a preliminary characterization of a seismic zonation on the basis of the consistency between seismic intensity distribution of earthquakes and corresponding structural framework.

An anisotropic attenuation law, based on an anisotropic characterization of intensity distribution for seismogenic zones, is proposed. This approach, that distinguishes itself for its consistency to the observed data, initially reconfigured by filtering procedures, is particularly suitable for seismic hazard evaluation.

For the seismic hazard evaluation of the region including the southern Calabro- Peloritanian Arc and southeastern Sicily, the determination of the macroseismic virtual intensity distributions has been carried out, characteristic of the seismogenic zones that fall within the area in study, starting with the structural framework of the region and from the analysis of the observed intensity effected through suitable filters. The macroseismic parameters, derived from such virtual distributions and used for seismic hazard evaluation, are not only a reference for eventual subsequently deeper knowledge referable to the need for a better characterization of the reference modelling, but distinguish themselves as an essential instrument for the definition of seismic hazard scenarios correlated to seismic events that take place in single seismogenic zones.