Bedrock Motions Computed from Surface Seismograms

The method of characteristics is used to compute seismic bedrock motions which caused recorded transient motions at the ground surface. The method is based on a feed-forward process and may be characterized as a method of synthesis; it is termed the base motion synthesis method. Shear waves traveling normally through soil deposits of constant thickness resting on horizontal or inclined rigid bedrock are considered. The soil is assumed to react either as a linear viscoelastic material (Voigt solid), or as a strain-softening material. Ramberg-Osgood type shearing stress-shearing strain curves are used to model the nonlinear behavior of the soil. In this case, a centered implicit scheme is used in conjunction with the method of characteristics. Two examples are presented to illustrate the use and the accuracy of the base motion synthesis method. Nonlinear soil behavior is realistically treated and computational time is significantly reduced by using the base motion synthesis method.