Using ground-failure features for paleoseismic analysis
Stephen F. Obermeier, Randall W. Jibson · Antarctica A Keystone in a Changing World · 1994
Liquefaction features can be used in many field settings to estimate the recurrence interval and magnitude of strong earthquakes through much of the Holocene.The relatively high shaking level required for their formation makes them particularly valuable as records of strong paleo-earthquakes.This state-of-the-art summary for interpretation and analysis of liquefaction effects takes into account input from both geologic and geotechnical engineering perspectives.Discussed are an overview of the processes involved in formation of the features, criteria for determining whether sediments have been deformed by seismically induced liquefaction, case studies in various geologic settings, and description of the methods for estimating magnitude of prehistoric earthquakes.Also discussed are some types of sediment deformations that can be misinterpreted as having a seismic origin.1966).Where the cap is thin (< 1-2 m) is the locale where the increased pore-water pressure in the underlying sand-water mixture can most easily break through to the surface.Alternatively, the same pore-water pressure increase may not be large enough to breach entirely through a thicker cap.Characteristics of the lower layer that enhance the liquefaction-fluidization process are (1) a thick, loose sand that, once liquefied, provides a large volume of water available for upward flow, and (2) a permeability that is high enough to allow water to flow quickly to the base of the cap, but that is not so high as to dissipate excess pore pressures between seismic cycles of shearing (Castro, 1987, p. 177-179;Dobry, 1989).This simple model fully explains most field observations.However, seemingly contradictory manifestations of the liquefaction and fluidization process also are encountered in the field.For example, sand dikes cutting gravel layers that are much more permeable than the sand dikes have been reported by Tuttle et al. (1992).Numerous field examples of the influence of the physical setting on liquefaction-induced structures are discussed below. Factors affecting liquefaction susceptibility and effects of fluidizationThe most important factors controlling development of liquefaction-induced dikes and sills are considered in this section. grain sizeBoth field and laboratory data show that loosely packed, cohesionless sands that are clean (i.e., with no clay or bonding of grain-to-grain contacts) can readily liquefy and form great numbers of clastic dikes, sills, and sand blows.Figure 8 shows often-cited curves with size boundaries for fine-and medium-grained sands that liquefy most easily and form large fluidization features.On the basis of my field observations of effects of the 1811-12 New Madrid earthquakes and the 1886 Charleston earthquake, for these sand sizes, a nonliquefiable cap of only slightly lower permeability can still provide adequate confinement to form large sand blows.Deposits of sand and gravel containing more than 30 to 50 percent gravel can liquefy, but liquefaction-induced features in such coarse deposits are sparse in comparison to sands.Such diminishment results from a number of reasons.The high gravel content increases the internal friction resistance, making initial liquefaction more difficult.In addition, gravel-rich deposits are generally more densely packed than sands in field situations.This increased resistance to shearing combined with the high permeability found in many gravel-rich deposits makes liquefaction much more difficult than for sands (Wong et al., 1975, p. 582).In deposits having only a small percentage of fines (amount smaller than about 0.06 mm), the gravels are in point-to-point contact rather than being encased in a matrix of sand and silt.If more than 80 percent of the sediment is coarser than 0.7 mm in diameter, the permeability may be too high to develop a condition of initial liquefaction if there is no cap to inhibit drainage (National Research Council, 1985, p. 94).However, very gravelly sediments can form large liquefaction-induced features if the source deposits are loose, thick, and capped by a stratum of low permeability.For example, extensive liquefaction occured during the Borah Peak earthquake (M 7.3) where very coarse sediment (Fig. 8) was confined by a lowpermeability cap (Andrus et al., 1991).Large amounts of gravel-bearing sand were vented onto the ground surface.Significant lateral spreading also occurred.The source strata contained at least 70 percent gravel.Peak earthquake accelerations probably were high at this 6/9