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BRAY, RATHJE, AUGELLO AND MERRY D Seismic Design for Lined Solid-Waste Landfills 207GEOSYNTHETICS INTERNATIONAL S 1998, VOL. 5, NOS. 1-2 in 50 years) cited in Subtitle D, as well as other levels such as the 10% exceedance in 50years. TheUSGSmapsprovideausefulcheck tosite-specific seismicity evaluations as well as reasonable values for some designs. Although these map values may be appropriate forsome designs, a site-specific seis- micity study is often warranted. For example, the use of one ground motion parameter (e.g.MHA) as a design basis is overly simplistic. Subtitle D is potentially misleading, with its focuson MHA asdescribing the seismic hazard at a site, because the frequency content and duration of a ground motion are equally important. Thepotential seismic hazardatasite typically resultsfromseveral earthquake events, each with their particular characteristics and potential impact on the landfill. The haz- ard often consists of a near-field (< 10 to 20 km), high intensity ground motion with significant shortperiod energyand a shortduration ofstrong shakingand anintermedi- ate-field (20 to 100 km), lower intensity ground motion (i.e. lower MHA value) with significant long period energy and long duration. The design of MSWLFs, which are typically longer period systems, will often be governed by a less intense motion with significant long period energy and a long duration of strong shaking. In developing a suite of design ground motions that capture reasonable scenario earthquake events, empirical attenuation relations are helpful for characterizing ground motions with respect to magnitude, distance, site conditions, and fault type, as well as other factors. Many updated relations are available in the Seismological Re- search Letters (1997). Although the most complete characterization of a ground motion is through its time history, it isoften useful to characterize key aspects ofa groundmotion with simplified parameters. Probabilistic groundmotion mapsandattention relationships are usefulfor estimating MHA as a ground motion intensity parameter (Figure 2a). The frequency content may be characterized by using the mean period,Tm , which is a more stable pa- rameter than the predominant period,Tp , with the relationship shown in Figure 2b (Rathje et al. 1998), and the significant duration,D5-95 (i.e. the time between 5and95% ofthe Arias Intensity ofthe acceleration-time history), may be estimated with the rela- tionship shown in Figure 2c (Abrahamson and Silva 1996). A number of other ground motion issues may be important for a particular project, for example, near-field direc- tivity effects at sites close to a major fault (e.g. Somerville etal. 1997).The reader may refer to the seismological literature for a discussion of these issues. 2.4 Dynamic Waste Fill Properties Recent studies have increased the state of knowledge regarding shear wave veloci- ties,Vs , of solid-waste fill. A large number of spectral analysis of surface waves (SASW) tests and one suspension logging test were performed at the Operating Indus- tries, Inc. (OII) landfill as a part of its closure study (Idriss et al. 1995). Additionally, SASWtests were performed at five other landfills inSouthern California (Kavazanjian et al. 1996).SASWmeasurements were taken at 43profiles atthese sixsites. Aweight- ingscheme wasemployed suchthatequal weightingwasgiventoeachlandfill site.Fig- ure 3 presents a median curve along with a recommended range for the shear wave ve- locity of solid-waste versus depth. This range of values is somewhat lower than the Vs profile recommended by Kavazanjian et al. (1995), but it is still significantly higher than the range of Vs values initially proposed (Singh and Murphy 1990).