Chapter 7 using liquefaction‐induced and other soft‐sediment features for paleoseismic analysis

This chapter focuses on the methodology for determining whether observed sediment deformation had a seismic shaking or a nonseismic origin. The chapter emphasizes features developed from the process of liquefaction, which is the transformation of a granular material from a solid state into a liquefied state as a consequence of increased pore-water pressure. Geophysical methods including electrical resistivity and electromagnetic induction and ground-penetrating radar are refined sufficiently to be used with some success to locate buried liquefaction features. Paleoliquefaction investigations are useful to engineers and planners because of the high shaking threshold required to develop liquefaction features. The threshold is a horizontal acceleration on the order of 0.1 g for strong earthquakes, even in highly susceptible sediment. Features having a liquefaction origin can be developed at earthquake magnitudes as low as about 5 but a magnitude of about 5.5–6 is the lower limit at which liquefaction effects become relatively common. Seismic liquefaction effects described in the chapter are caused mainly by cyclic shaking of level or nearly level ground. Primary seismological factors contributing to liquefaction are the amplitude of the cyclic shear stresses and the number of applications of the shear stresses.