High resolution shallow geophysical methods for the investigation of the liquefaction phenomena: case study of the ML 5.9 May 20th, 2012 Emilia earthquake (Italy)

Abstract

A massive surface fracturing and sand ejection took place following the main shock of May 20th, 2012 earthquake (ML=5.9 and hypocenter depth of 6.3 km), in the Emilia-Romagna Region, Northern Italy. These phenomena were induced by the liquefaction of a water-saturated, medium to coarse grained sand layer, laying at depths between 8 and 12 m below ground level. The occurrence of these co-seismic effects caused the damage of several buildings, many roads and sidewalks. In this work we describe the use of selected non-invasive high resolution geophysical methods to test their capability to characterize the subsurface of two sites were liquefaction events took place and possibly to map their extension at depth, especially those that reached the ground surface. The study is based on methods mapping the electric and electromagnetic
properties of the subsurface materials, like Electrical Resistivity and Induced Polarization Tomography (ERT-IPT) and Ground Penetrating Radar (GPR). We present two case studies located at Mirabello village (ca 15 km to the SW of Ferrara), where both methods were applied. In test site No. 1, a two-dimensional (2D) ERT-IPT and one GPR profile were carried out, while in test site No. 2 a 3D ERT-IPT survey together with two GPR profiles were carried out. The obtained 2D/3D inversion models provided suggestive details about the surface ruptures extension at depth and in some sections succeeded in detecting the liquefied sand layer. The details are more evident on the IPT images rather than on the ERT ones, although clues about lateral spreading and densification of superficial sediments could be also deduced from the 2D ERT models.
Similar results were also observed in the low frequency (70-100 MHz) GPR sections; however, the general applicability of this method is questionable because of the limited penetration depth due to the presence of fine grained soils and of a shallow water table depth. Based on the obtained results, which we consider as preliminary, the combined use of both ERT and IPT techniques can help achieving better lithostratigraphic models that can guide technicians in the proper selection of sites to be directly investigated by boreholes.

DOI:

Trovalo in Annali Online Dell'Università di Ferrara, Vol. 2 (1)