After an earthquake, the observation of damages on constructions and infrastructures often highlights substantial differences in different built-up areas, even at short distance among them. In some cases, noteworthy collapses and damages occurred in places lying at great distance from the epicentre.
In occasion of L'Aquila earthquake of 6 April 2009, these type of situations were found both in the municipal territory of L'Aquila and in some distant municipalities, such as S.Pio delle Camere, the fraction of Castelnuovo (about 30 km SE of the epicentre). The quality of buildings definitely plays a major role in the damage entity, but often causes are linked to the local seismic dangerousness, determined also by the different earthquake propagation, or by the instability of the soil.
These considerations are part of the studies of Seismic Microzonation (SM), though which it is possibile to pick out and characterize stable areas, stable areas susceptible to local amplification and areas subject to instability, such as landslides, surface fractures and soil liquefaction.
Historical studies of Seismic Microzonation
Issues handled in the studies of Seismic Microzonation have had a strong development at scientific level over the last 40 years, although the importance of characteristics of resistance and soil seismic stability was highlighted also in the past. Starting from XVIII century, with the rise of the Enlightenment vision of natural phenomena, it was clear to experts that local conditions of foundation soils considerably affected the earthquake effects. Even one cenrury ago, informative criteria of the Technical Regulations approved with royal decree on 18 April 1909, no. 193, following the catastrophic earthquake of Messina and Reggio Calabria of 1908, reported the prohibition of new constructions and reconstructions “on soil above or close to ruptures, loose or o apt to fall, or to communicate to buildings vibrations and stresses depending on different geological constitution or different resistance of the single parts”.
Internationally, a 1969 study by American scholars of the 1957 S. Francisco earthquake pointed out that within the same city, a few hundred meters apart, the same earthquake had caused significantly different shaking depending on the thicknesses and geomechanical characteristics of the soils present in the more superficial layers. Since then, many studies have been carried out on severe earthquakes (e.g., Friuli, 1976; Irpinia, 1980; Mexico City, 1985; Kobe, Japan, 1992; Izmit, Turkey, 1999; San Giuliano di Puglia, 2002), and data have been collected showing how local land characteristics can dramatically alter seismic action.
The objectives of Seismic Microzonation
Seismic microzonation studies aim to streamline knowledge about the alterations that seismic shaking can experience on the surface, providing helpful information for land use governance, design, emergency planning, and post-earthquake reconstruction.
In territorial planning, depending on the several scales and levels of intervention, Seismic Microzonation studies are conducted on those areas for which the regulatory framework allows or foresees their use for building or infrastructure purposes, their potential transformation for these purposes, or anticipates their use for civil protection purposes.
MS studies are of paramount importance in the planning process to:
- direct the selection of areas for new settlements
- define allowable interventions in each area
- plan investigations and levels of depth
- determine guidelines and procedures for intervention in urbanized areas
- define priorities for intervention.
All these are the topics of Seismic Microzonation (MS) studies, through which it is possible to identify and qualify stable zones, stable zones susceptible to local amplification, and zones subject to instability, such as landslides, fault surface ruptures, and dynamic soil liquefaction.
In emergency planning, whether at the municipal or provincial level, MS studies allow for better and more informed identification of an emergency plan's strategic elements and civil protection resources in general.
Awareness of the possible local effects induced by a seismic event on an area contributes to:
- select emergency areas and facilities and strategic buildings in stable areas
- identify "critical" sections of road and service infrastructure and relevant works for which specific safety evaluations might be needed in the event of a collapse.
In the reconstruction phase, Seismic Microzonation:
- contributes to the selection of areas for temporary housing
- provides elements to engineers and administrators on the suitability of reconstructing unusable buildings
- helps to choose new building areas.
In planning new works or interventions on existing works, Seismic Microzonation studies emphasize the presence of phenomena of possible amplification of shaking linked to the lithostratigraphic and morphological characteristics of the area and phenomena of instability and permanent deformation activated by the earthquake. Seismic Microzonation studies, therefore, can offer relevant elements for the design of works, with different impacts depending on the level of depth and characteristics of the works, addressing the choice of detailed investigations.
The Seismic Microzonation study is a cognitive tool with different potentials, which has different costs depending on the desired level of in-depth study:
- level 1 is a preparatory level to the actual MS studies, as it involves a collection of pre-existing data, processed to divide the territory into qualitatively homogeneous micro areas
- level 2 introduces the quantitative element associated with the homogeneous zones, using additional and targeted investigations where necessary, and defines an accurate MS map
- level 3 provides an MS map with insights into particular issues or areas.
When deciding on the execution of the study, the utility that can be gained from it should be kept in mind when determining the level of in-depth study to compare it with the costs to be faced. The improved knowledge produced by MS studies can contribute concretely, along with vulnerability and exposure studies, to optimizing resources available for interventions focused on seismic risk mitigation. The technical procedures for the execution and application of MS on the Italian territory are defined by the Guidelines for Seismic Microzonation, "Indirizzi e Criteri per la Microzonazione Sismica," recently approved by the Civil Protection Department and the Conference of Regions and Autonomous Provinces (MS Working Group, 2008).