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controlling recurrence relationship for the site; and finally, selecting the design level of earthquake based upon an acceptable probabi li ty of exceedence and the projec t's exposure period se lee ted for th e de sign. F. Characterization of Ground Motion Expected at the Site The characteristics of the ground motion expected at the site are func tions of the earthquake source mechanism, the epicentral distance, and the geometry and physical properties of the geologic structures traversed by the body and surface waves as they propagate from the source to the site. Based on this myriad of geologic and tee tonic influences, seismograms recorded at all distances from the epicenter are very variable and difficult to interpret. They are especially complex in the near field where the ground motions are strongly influenced by the dynamics of the fault rupture, and where the source properties become more important in defining the ground motion than the site properties. At distances of greater than a few fault rupture widths from the source, site conditions begin to have a greater influence on the ground motion. Seismic waves traveling from rock to the ground surface through surficial deposits will be modified by a factor which is a func tion of the strength of the seismic wave motion and the depth, geologic structure, and dynamic properties of the soils. The inclination of the soil layers and bedrock can influence reflection and refraction processes, consequently, the complexity of the waves transmitted to the ground surface. Topography also affects the ground motion in that peaks or jutting outcrops may significantly amplify the seismic parameters. The many parameters affecting ground motion at a site are not fully understood. These parameters deal with the physics of the earthquake source and the propagation of the released energy to the site. While current procedures for characterizing ground motion are primarily empirical, an awarness of the state of the art IDJst be maintained and incorpora ted in the charac terizations. Ground motion can be described in a nunber of ways for use in engineering calculations for evaluating the effects of earthquake ground motion on structures. Data used to develop site ground motion are site dependent or site independent. Site-dependent data are obtained by using site-matched ground motion records or analytically determined ground response of the site. Site- independent data are standardized charac terization parameters obtained from statistical analyses of existing earthquake records chosen without the site-matched requirement. Although site- dependent data are preferable, sufficient site data may not be available. Therefore, it may be necessary to combine site- dependent data with site-independent data. Formats used to charac terize the ground motion are described below. 1. Peak Ground Motion Parameters Ground motion can be charac terized by peak acceleration, velocity, and displacement. values of ground The values are used -13- .