A METHODOLOGY FOR MEDIUM-SCALE SEISMIC SUSCEPTIBILITY MAPS: AN EXAMPLE FROM THE MODENA APENNINES (NORTHERN ITALY)

Abstract

This paper describes a methodology for the implementation of medium-scale (1:50,000 scale) seismic susceptibility maps for assessing
seismic hazard in territorial planning. It illustrates the research carried out in the Modena and Reggio Emilia Provinces with an example in the Modena mid-Apennines (area between Zocca and Rocca Malatina).
The research, supported by bibliographical investigations on Quaternary active faults and on earthquake-induced surface effects, was
organised according to the following studies: 1) Study of the geological characteristics. Elaboration of “Geological maps”. 2) Study of
the litho-technical characteristics for the identification of the areas which may show homogeneous litho-technical behaviour in the
occurrence of earthquakes. Elaboration of “Litho-technical maps” and of “Simplified litho-technical maps” 3) Study of geomorphological characteristics for the determination of geomorphological situations which may give local responses by causing relative amplification of seismic waves and/or earthquake-induced instability. Elaboration of “Geomorphological maps” and of “Simplified geomorphological maps”; 4) Study of seismic susceptibility characteristics for the identification of the areas prone to seismic amplification and/or earthquake-induced instability on the basis of previous data. Elaboration of “maps of seismic susceptibility”. In these maps, the litho-technical features potentially causing amplification are shown with zones classified from 1 to 5 according to the increase of amplification (amplification from Low to High); the morphological features causing amplifications (scarps higher than 20 m, narrow and long ridges) are shown with linear symbols. The features causing earthquake-induced instability are shown with zones indicated with letters from A to D according to the following classes of instability: A) stable areas and intermediate stability areas; B) potentially unstable areas with possible problems regarding the bearing capacity of soils; C) potentially unstable areas prone to mass movements; D) unstable areas prone to mass movements. The Quaternary faults are also shown in the map. Therefore, in the maps of seismic susceptibility the classes of susceptibility can be shown as zones with an alphanumeric code defined through the combination of numbers, from 1 to 5, and letters, from A to D (or with different areal symbols or different corresponding colours).
The collected information was stored by means of Geographic Information Systems (GIS). The procedures for the implementation of
the “Maps of seismic susceptibility” were obtained by means of GIS operations starting from the geological map, geomorphological map and the data base of the inventories of Quaternary active faults and earthquake-induced surface effects. These maps give a sufficiently detailed picture of seismic hazard and susceptibility in the study area and can be easily consulted and understood by technicians from administration boards. The research herein described is qualitative and not quantitative and therefore, considering also the scale adopted, it can be considered as a grade 1 zonation (cf. TC4, 1999).