The survey and mapping of sand-boil landforms related to the Emilia 2012 earthquakes: preliminary results

Abstract

Sand boils, which are also known as sand blows or sand volcanoes, are among the most common superficial effects induced by high-magnitude earthquakes. These generally occur in or close to alluvial plains when a strong earthquake (M >5) strikes on a lens of saturated and unconsolidated sand deposits that are constrained between silt-clay layers [Ambraseys 1988, Carter and Seed 1988, Galli 2000, Tuttle 2001, Obermeier et al. 2005], where the sediments are converted into a fluid suspension. The liquefaction phenomena requires the presence of saturated and uncompacted sand, and a groundwater table near the ground surface. This geological– geomorphological setting is common and widespread for the Po Plain (Italy) [Castiglioni et al. 1997]. The Po Plain (ca. 46,000 km2) represents 15% of the Italian territory. It hosts a population of about 20 million people (mean density of 450 people/km2) and many infrastructures. Thus, the Po Plain is an area of high vulnerability when considering the liquefaction potential in the case of a strong earthquake. Despite the potential, such phenomena are rarely observed in northern Italy [Cavallin et al. 1977, Galli 2000], because strong earthquakes are not frequent in this region; e.g., historical data report soil liquefaction near Ferrara in 1570 (M 5.3) and in Argenta 1624 (M 5.5) [Prestininzi and Romeo 2000, Galli 2000]. In the Emilia quakes of May 20 and 29, 2012, the most widespread coseismic effects were soil liquefaction and ground cracks, which occurred over wide areas in the Provinces of Modena, Ferrara, Bologna, Reggio Emilia and Mantova (Figure 1). These were the causes of considerable damage to buildings and the infrastructure. The soil liquefaction and ground cracks were accompanied by sand boils, which are described in this report. The spatial distribution and geomorphological setting of sand boils and ground cracks are also described here. A detailed three-dimensional (3D) reconstruction of these features is also presented, which was carried out using terrestrial photogrammetry. Since archeological times, fluvial ridges, and in general sandy deposits on low plains have been the preferred sites for human infrastructure, colonial houses, roads, etc. Therefore, it is very important to understand how the local topography/ morphology interacts in the liquefaction processes. Numerous distinctive seismic landforms were generated by the May 2012 strong earthquakes (seven with M >5), and in particular, sand boils and ground fractures. The sand-boil landforms, also known as sand craters or sand volcanoes, are formed by low mounds of sand that have been extruded from fractures [Tuttle 2001]. The cone is a generally shortlived structure that naturally collapses, starting from the center holes that mark the water retreat back into the fracture. Sand boils also occurred along larger cracks (with decimetric lateral and vertical displacements). Here, the upper scarps block the formation of craters and allow the deposition of a sandy layer several centimeters thick (e.g. ca. 4 cm in the San Carlo crack), on the lower side of the steep slope. These landforms are highly vulnerable to erosion. After a few weeks, they are washed out by rain, destroyed by human activity, or masked by growing crops. Thus, ground surveys that investigate these events have to be carried out as soon as possible [Panizza et al. 1981]. In this report, we present preliminary results using methods to map the detailed micro-morphology of some representative liquefaction features (Figure 2) that normally disappear for the aforementioned reasons, or that are recorded only in qualitative terms.