Abstract
Conventionally, in wide-area prediction of strong ground motion or long-period ground motion by the finite difference method (FDM), it is difficult to evaluate the ground motion considering a top surface low-velocity layer with S-wave velocity as low as 100m/s from the limitations of the computer memory for calculation. We attempted to devise a modeling method for minimizing the amount of calculation in the FDM, and investigated a simplified numerical analysis method that can be considered from the seismic bedrock to the ground surface. To be specific, a so-called "Equivalent shallow ground model" obtained by averaging the ground physical properties of several layers shallower than the engineering bedrock was provided in a virtual manner, and the ground motion was calculated by the FDM using an “Integrated ground model” from the seismic bedrock to the ground surface. Furthermore, the ground motion obtained from the integrated ground model was pulled back to the engineering bedrock by using the equivalent shallow ground model based on the multiple reflection theory. Finally, the ground motion at the ground surface was recalculated from the engineering bedrock using the original (true) shallow ground model. In addition to this modeling method, in order to reduce further the calculation amount in the FDM, a so-called "Uniform shallow ground model" that reflects directly the ground physical property of engineering bedrock to the surface layer was provided virtually, and a method for calculating the ground motion in a manner similar to that shown above was also examined. As a result, in both examined methods, as compared with the conventional so-called "Separation analysis" separating the analysis region at the engineering bedrock, it has become possible to further evaluate the precise ground motion. By performing an “Integrated analysis” from the seismic bedrock to the ground surface, it is possible to improve the accuracy of long-period ground motion in addition to the short-period ground motion.
