Abstract:

Seismites are tectonites or fault rocks that were deformed in a brittle manner by seismic faulting in the Earth’s lithosphere. They broadly include seismic liquefactites (soft sediments such as water-saturated sandy soils) and brittle fault rocks such as incohesive breccia, cataclasite, ultracataclasite, pseudotachyllyte and fault gouge (solid state sedimentary, igneous and metamorphic rocks). The sedimentary rocks that developed liquefaction-induced soft-sediment deformation structures, which have long been erroneously called ‘seismites’ in the literature, should be named as liquefactites. A large number of studies have shown that soil liquefaction and soft-sediment deformation structures can be caused not only by seismic shaking but also by any other instability events in various geological settings (e.g., water waves, tides, floods, ice floes, upward escape of water, differential stress-induced lateral displacement, landslides, and mudflow). Furthermore, not all earthquakes can cause liquefaction, depending on their moment magnitudes, the distances from the seismic fault, the thickness, water content and properties of the sediments (e.g., grain size distribution, porosity, and grain surface roughness) and the vertical overburden stress. Only the earthquake-induced liquefaction deposits can be called seismic liquefactites, which should be mainly distributed about a regionally important seismic fault associated with brittle fault rocks such as gouge, breccia, cataclasite and pseudotachylyte. A sporadic occurrence of small scaled soft-sediment deformation structures and sandy liquefaction structures (e.g., sand blows) are not sufficient evidence to support the presence of paleo-earthquakes.

Key words: seismites, liquefaction, seismic liquefactite, soft sediment deformation structure, fault rocks