GNGTS 2013 - Atti del 32° Convegno Nazionale

sediments. Consequently, an intensive microzoning activity has been developed in the last four years in those territories, such as L’Aquila crater and Ferrara province, placed in the epicentral areas of the “unexpected earthquakes” of April 6 th , 2009 and May 20 th and 29 th , 2012. Plural scientific approaches have been testing on Italian territory to address microzoning studies for urban planning. These activities must be focused on 1) near field conditions and 2) on several seismic records from the last strong motion events. Moreover, they shall be compliant with the maps of maximum expected seismic intensities based on the Italian historical earthquake catalogues and databases that are sound and complete enough to support the prediction of earthquakes with a return period higher than 475 years. In this paper, some critical aspects of microzoning strategies commonly used in Italy have been pointed out and new proposals are provided to be discussed. Recent evolutions of Italian seismic microzonation rules. The need to identify seismic homogeneous zones in urban areas was born after the 1997 Umbria-Marche and 2002 San Giuliano di Puglia earthquakes. These events caused differentiated damages, casualties and disruptions, that enforced the evidence that local seismic effects play a relevant role also in near field conditions. The fruitful debate developed within the Italian scientific community on the best practices and most meaningful parameters for the seismic Italian territory classification and zonation, gave birth in 2006 (OPCM N. 3519) to the seismic Italian hazard map for different return periods. Moreover, the National Office for Civil protection was commissioned by the government to develop and issue novel guidelines and best practices for microzoning studies. The international guidelines from Technical Committee for earthquake geotechnical Engineering, TC4, ISSMGE (1999) were taken as a reference. Then, the Italian microzoning guidelines and criteria were published in 2008 (DPC, 2008) based on three subsequent levels of detail in microzoning studies: level 1, mapping homogeneous geological units with respect to seismic behavior by means of surface geological relieves; level 2, mapping numerical indexes for homogeneous susceptible areas by means of simplified approaches according to standard procedure prescribed by DPC; level 3, mapping homogeneous seismic responses drawn from site specific experimental surveys and one, two- and even three-dimensional numerical analyses when needed. Meanwhile the scientific community was discussing how these guidelines and criteria can be applied over the whole national territory for decreasing the seismic risk, the first “unexpected” main shock of L’Aquila earthquake occurred on April 6 th , 2009 at 3:32 local time, causing 306 fatalities, more than 60,000 people displaced and heavy damages to civil structures and buildings: the old town of L’Aquila was strongly damaged and Onna city was completely destroyed under the near field 5.8 M l (local magnitude) earthquake with 9 km hypocenter depth. After that, the Italian microzoning guidelines and criteria were applied by the Working group MS–AQ (2010) to those portions of L’Aquila crater where the macroseismic intensity map (Fig. 1a) reported values higher than VMCS and within the epicenter area where it pointed out relevant differentiated amplification effects (Galli and Camassi, 2009). These differentiated responses were also recorded by accelerometer stations (Fig. 2c). Meanwhile discussion on these topics were on, other three “unexpected” main shocks of Emilia Romagna earthquake hit the provinces of Ferrara and at a lesser extent of Modena: May 20 th at 4:03 local time with M l INGV 5.9 and 6.3 km depth and May 29 th at 9:00 with M l INGV 5.8 and 10.2 km depth, and M l INGV 5.3 and 6.8 km depth at 12:55 local time events (QUEST Working Group 2012). These seismic events occurred in an area where the hazard map (OPCM N. 3519) (Fig. 1b) predicted low hazard level although the intensity maps did not (Fig. 1a). Furthermore, a differentiated damaged level was plotted by Quest Working Group (2012) (Fig. 1c) within 20 km from the epicenter. Finally, 21/06/2013 at 10:33:57 UTC a seismic event of M l INGV 5.2 was recorded in Lunigiana at a 5.1 km depth. In field surveys performed by Quest Working Group (2013) by the end of June 2013 showed maximum intensity degree of 5-6 and 6 MCS within epicentral area. Although this last earthquake was not as severe as the others two, uncommonly, this earthquake 326 GNGTS 2013 S essione 2.2