Identifying triggers for liquefaction-induced soft-sediment deformation in sands

Triggers for liquefaction-induced soft-sediment deformation structures in sands include seismic shaking, effects of water waves, rapid sediment accumulation and groundwater movements. Many soft-sediment deformation structures are attributed to a seismic trigger, but the evidence is often variable and inconclusive. Liquefaction and its effects are reviewed in the context of earthquakes, other triggers and experiments. The interpretation of liquefaction-induced soft-sediment deformation structures comprises two key stages: recognising liquefaction as the deformation mechanism, and determining the trigger for liquefaction. The characteristics of sediment that has undergone liquefaction include the pervasive, ductile character of deformation, preservation of stratification, a gradual upward increase in the extent or complexity of deformation, possible water-escape structures in the upper parts of a liquefied horizon, a horizontal upper surface, and a distinctive grain fabric. Approaches to determining the trigger for liquefaction include those based on criteria and those based on the sedimentological and palaeoenvironmental context. Few of the criteria applied to seismic triggers are diagnostic and several are not applicable on the scale of single outcrops. Criteria are poorly developed for non-seismic triggers. A methodology is proposed for analysing soft-sediment deformation structures within their overall sedimentological and palaeoenvironmental context in order to refine and improve criteria for distinguishing the action of ‘external’ (allogenic) triggers, including earthquakes, from ‘internal’ (autogenic) triggers.