Active Tectonic Risk Assessment—Problems with Soil and Soft Sediment Deformation Structures

Abstract

Many sites feature soils and sediments that have undergone syn- or post-depositional deformation. Typical forms of deformation include load casts, wedges, involutions and diapirs. In studies on geologically-young (Quaternary) soils and sediments in much of northwest Europe and some other areas, such forms are frequently attributed to the former action of non-glacial freezing and thawing during past cold periods. Similar features are found in arctic and high altitude areas. They are used to help reconstruct past climate and associated ground thermal regimes, including the extent of permafrost. The identification of the features as being relict with a low likelihood of such intense freeze-thaw processes being replicated has implications for site risk assessment and, consequently, design. Features that reflect syn- or post-depositional deformation elsewhere in the world, or in pre-Quaternary rocks, are frequently attributed to strong ground motion, and are used to help reconstructed past earthquake histories, both for regions and for individual potentially-active tectonic structures. There are direct implications for risk assessment and design. In theory, since the processes related to ice growth/decay and cyclic ground motion might be expected to be different, it should be possible to differentiate between them using diagnostic criteria. Unfortunately, either because different researchers are unconsciously biased by their training (perhaps causing the difference in interpretation depending on geographical region and the age of the features), or because the criteria are inadequately defined and are equally applicable to different processes, problems remain for fully understanding site risk. This paper compares the current state of diagnostic features used for interpreting deformations and uses these to explore possible interpretations and implications of a range of features found in apparently ‘cryogenic’ and ‘palaeoseismic’ settings. It ends with a call for a renewed focus on developing a robust and testable diagnostic toolkit for site investigation of soil and soft sediment deformation structures so that the features can be understood in terms of either “fossil” ice processes or active tectonic risk.