USE OF LIQUEFACTION-INDUCED FEATURES FOR PALEOSEISMIC ANALYSIS - AN OVERVIEW OF HOW SEISMIC LIQUEFACTION FEATURES CAN BE DISTINGUISHED FROMOTHER FEATURES AND HOW THEIR REGIONAL DISTRIBUTION AND PROPERTIES OF SOURCE SEDIMENT CAN BE USED TO INFER THE LOCATION AND STRENGTH OF HOLOCENE PALEO-EARTHQUAKES

Liquefaction features can be used in many held settings to estimate th e recurrence interval and magnitude of strong earthquakes through much of the Holocene. These features include dikes, craters, vented sand, sills, and laterally spreading landslides. The relatively high seismic shaking level required for their formation makes them particularly va luable as records of strong paleo-earthquakes. This state-of-the-art s ummary for using liquefaction-induced features for paleoseismic interp retation and analysis takes into account both geological and geotechni cal engineering perspectives. The driving mechanism for formation of t he features is primarily the increased pore-water pressure associated with liquefaction of sand-rich sediment. The role of this mechanism is often supplemented greatly by the direct action of seismic shaking at the ground surface, which strains and breaks the clay-rich cap that l ies immediately above the sediment that liquefied. Discussed in the te xt are the processes involved in formation of the features, as well as their morphology and characteristics in field settings. Whether lique faction occurs is controlled mainly by sediment grain size, sediment p acking, depth to the water table, and strength and duration of seismic shaking. Formation of recognizable features in the held generally req uires a low-permeability cap above the sediment that liquefied. Field manifestations are controlled largely by the severity of liquefaction and the thickness and properties of the low-permeability cap. Criteria are presented for determining whether observed sediment deformation i n the field originated by seismically induced liquefaction. These crit eria have been developed mainly by observing historic effects of lique faction in varied field settings. The most important criterion is that a seismic liquefaction origin requires widespread, regional developme nt of features around a core area where the effects are most severe. I n addition, the features must have a morphology that is consistent wit h a very sudden application of a large hydraulic force. This article d iscusses case studies in widely separated and different geological set tings: coastal South Carolina, the New Madrid seismic zone, the Wabash Valley seismic zone, and coastal Washington State. These studies enco mpass most of the range of settings and the types of liquefaction-indu ced features likely to be encountered anywhere. The case studies descr ibe the observed features and the logic for assigning a seismic liquef action origin to them. Also discussed are some types of sediment defor mations that can be misinterpreted as having a seismic origin. Two ind ependent methods for estimating prehistoric magnitude are discussed br iefly. One method is based on determination of the maximum distance fr om the epicenter over which liquefaction-induced effects have formed. The other method is based on use of geotechnical engineering technique s at sites of marginal liquefaction, in order to bracket the peak acce lerations as a function of epicentral distance; these accelerations ca n then be compared with predictions from seismological models.