Geological and geophysical approaches for the definition of the areas prone to liquefaction and for the identification and characterization of paloeliquefaction phenomena, the case of the 2012 Emilia epicentral area, Italy

In May-June 2012, a seismic sequence struck a broad area of the Emilia-Romagna region in Northern Italy. The sequence included two mainshocks, the first one occurred on 20 May at 02:03 UTC with a ML 5.9 hitting an area between Finale Emilia and San Felice sul Panaro; the second occurred on 29 May at 07:00 UTC with a ML 5.8, about 12 km southwest of the previous mainshock. The focal mechanisms of these events consistently show a compressional kinematics with E-W oriented nodal planes. The whole aftershocks area extended in an E-W direction for more than 50 km, and included five ML >= 5.0 events and more than 1800 ML >1.5 events. The epicentral area of the 2012 Emilia sequence falls in the southern part of the Po Plain, about 40 km north of the foothills of the northern Apennines thrust and fold mountain belt. The instrumental seismic history shows low seismicity in coincidence of the area hit by the 2012 sequence and the same appears true for the location of the main historical events. The closest and more relevant historical earthquake is certainly the I=VIII MCS (Mw 5.5) 1570 earthquake that hit the area of Ferrara, about 30 km east of the 2012 sequence, with the occurrence of liquefaction phenomena in the area of Ferrara and surroundings as well as of open fractures and changes of the water flows in channels. Widespread secondary geological effects were produced by the 2012 Emilia sequence and are mainly related to liquefaction phenomena. Different teams from several institutions (e.g. INGV, ISPRA, University of Insubria, Emilia Romagna Geological Survey, etc.), promptly surveyed the area to collect coseismic geological evidence and all the survey teams verified that no primary surface rupture occurred. Furtehrmore, the most common geological features observed during the post-earthquake survey were related to liquefaction process induced by the 20 and 29 May mainshocks (based on several local eyewitnesses and on the magnitude and timing of the main events). As a whole, a total of more than 1300 sites with geologic coseismic effects were identified and surveyed over more than 1200 km2, but these effects are not evenly distributed over the area but mostly appear concentrated and aligned. The 2012 coseismic liquefactions clearly highlighted the relevant impact of liquefaction on the natural and human environment in alluvial plains. Thus, understanding the parameters that control liquefaction and defining the characteristics of the areas more favorable to liquefaction is of critical importance for a correct land use planning in alluvial plains. In this contribution we present some preliminary results concerning the study of the liquefactions occurred during the 2012 Emilia seismic sequence with particular emphasis on: a) the definition of the areas most prone to liquefaction hazard, b) the identification and characterization of potential paloliquefaction events. As for the first objective, a qualitative comparison between the geomorphic characteristics of the area and the location of liquefaction sites shows a good correlation of the latter with the location of the paleo-river channels, outflow channels, and crevasse splays of the Secchia, Panaro and Reno rivers as well as with the levees of the Cavo Napoleonico channel (artificial channel, connecting the Reno and Po rivers, built at the beginning of the XIX century). Moreover, in this contribution we will present our quantitative approach defined to obtain a detailed analysis (GIS based) of the geometric relationships between the observed liquefactions and peculiar geomorphic features of the 2012 epicentral area, used to define the zones with high, medium or low liquefaction hazard. Differently, for the recognition and characterization of paleoliquefaction events we adopted a multidisciplinary approach involving sedimentology, mineralogy and magnetic properties of the 2012 liquefied sand blows together with geophysical profiling (electric tomography), coring and dating (both absolute and archeologic) at specific sites. The basic idea is that if a site was prone to liquefaction in 2012, it should have experienced it also during past events (prehistorical or lost in the historical record). The objective is to find, the source layer that liquefies as well as traces of sand blows of the past.