Three-Dimensional Surface Layer Model for Strong Motion and Liquefaction Prediction

Abstract eng:
For predicting strong motion and liquefaction evaluation over a wide area, it is important to modeling 3D surface layer considered the spatial distribution of parameters (S-wave velocity, N-value, lithofacies) and a reproducibility of the dynamic characteristics. In this paper, we show the 3D surface layer model and strong ground motion and liquefaction prediction, using Nagoya City data as reference. Modeling the 3D surface layer, we used the 34,600 points of borehole logs, the 499 points of PS logging and the 25,287 grain size analysis database. We classified lithofacies of borehole logs into seven strata. And we interpolated altitude values of stratum boundary. Using Kriging method as the interpolation algorithm, we modeled the stratum boundary at intervals of 50m grids according to latitude and longitude. And, we modeled cut/fill boundary data by calculating a difference among the DEM based on the latest topographic map and that of based on the 1940's aerial photograph and that of based on the old edition of topographic map at 1950’s. In this way, we got the boundary grids of 8 strata. The lithofacies and N-values of borehole logs were interpolated for each stratum. The modeling procedure is as follows : 1) Depth of the descriptions of lithofacies and N-values in the borehole logs were standardized to obtain regularly aligned datasets for vertical intervals of 1m in the altitudes, 2) The values of lithofacies indexes were converted to grain size distribution (the composition ratio of gravel fraction, sand fraction and fine fraction) based on the grain size analysis database. 3) The irregularly scattered values of grain size distribution and N-values were horizontally interpolated to obtain regularly gridded datasets in each altitude, 4) The datasets were stacked vertically. 5) The values of grain size distribution at every grid were converted to the values of lithofacies indexes. The 3D lithofacies and N-value model shows good agreement with the geological profile from geotechnical engineering maps. The 3D lithofacies and N-value model was converted to 3D S-wave velocity structure model, using empirical conversion formula of S-wave velocity. Using the 3D surface layer model, strong ground motions and liquefaction were simulated for the hypothetical Tonankai earthquake. The distribution of the simulated seismic intensity in Nagoya City area shows good agreement with that of observed building damage at Tonankai earthquake in 1944. Similarly, simulated distribution at "high" liquefaction potential shows good agreement with the liquefaction confirmed area at Tonankai earthquake in 1944.

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