Spreading is a type of ground failure occurring on gently inclined slopes and characterised by extensional displacement along a gliding plane (Figure 1). It is possible to distinguish two main types of spreading: rock spreading and soil spreading. Rock spreading is defined as deep-seated gravitational slope deformation, and examples of this are observed in different geological and geographic settings. Soil spreading is associated with the liquefaction of a specific layer representing the gliding plane.
Liquefaction of soil involves the sudden loss of shear strength triggered by a dramatic and temporary increase of pore-water pressure. Seismic liquefaction is caused by earthquake induced-cyclic loading and is the most studied type of liquefaction affecting undrained and unconsolidated silty-sands and sandy deposits. Soil spreading, and related liquefaction, has been observed at terrestrial sites and inferred also in subaqueous settings.
Figure 1 Example of spreading morphology
Awareness on liquefaction of soils and related ground failure has increased among the geological, geotechnical and engineering geology communities since the 1964 disastrous Alaska and Niigata earthquakes. Since then, liquefaction and spreading were also documented in other settings worldwide. A large portion of the literature investigating spreading and liquefaction processes comes from the geotechnical and engineering communities.
Research on liquefaction related ground failure has notably improved in the aftermath of recent liquefaction events such as the 2012 Emilia Earthquake (Italy), the 2011 Great Japanese Earthquake, and the 2010-2011 Canterbury Earthquake Sequence (New Zealand). In particular, for the latter, extensive liquefaction and spreading have affected the urban area of Christchurch and its surroundings, causing a damage for 20 billion NZD.
The geological community, on the other hand, has focused on identifying geomorphic and sedimentological controls of surface manifestation of liquefaction, while only few studies have addressed the geological control of spreading. Some authors, have investigated spreading by using micro-topographic parameters as proxies for specific landforms susceptible to it. For example, Caputo assumed that the geometry of genetic landforms determines surface gravitational stresses, which may predispose certain parts of the land surface to tensional strain (via brittle failure and cracking), once a subsurface layer liquefies and loses shear strength. The latter is most strongly expressed in spreading where free faces already exist, but is also hypothesised to occur where more gentle topographic variation prevails and not necessarily in proximity to free faces.
In the particular case of spreading, even a small gravitational component affects the loss in shear stress of the sediment prone to liquefy, facilitating the liquefaction of the material, and promoting the spreading. Liquefaction features can be considered off fault markers of medium to high earthquake intensity (M> 5.5). This type of inference is valuable in active tectonic regions, as well as in intraplate settings characterized by moderate seismic hazard and no evidence of fault rapture.
Therefore, over the years more studies on seismic liquefaction were used for palaeoseismic purposes, in order to improve the seismic hazard of specific regions. However physical controls on the mechanical and rheological aspects of spreading related liquefaction are poorly known. Liquefaction-induced spreading has been poorly investigated in the offshore environment. Here, spreading can represent an hazard to marine infrastructures and it played a role in well known disastrous events such as the Grand Banks slope failure.
Spreading morphology has been documented and analysed in several other papers in the subaqueous setting, especially in glaciated margins. Micallef et al characterised the morphometric signature of spreading in the Storegga Slide (Norway), and attempted to identify the triggering factors, and understand the spreading process using a limit equilibrium model.
Nonetheless, some questions remain unsolved: what are the associations between spreading morphology, sedimentary architecture of the slide and liquefaction susceptibility? What are the mechanics of evolution of spreading? Is it possible to investigate and predict how spreading evolves? These questions have not clearly been explored due to the lack of sub-seafloor characterisation of spreads and a suitable understanding of the geotechnical characteristics of the failed sediments.
This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 101003388