Prediction Equations for Estimating Shear‐Wave Velocity from Combined Geotechnical and Geomorphic Indexes Based on Japanese Data Set

Abstract

© 2015, Seismological Society of America. All rights reserved. We develop empirical predictive equations for shear-wave velocity (V S ) conditional on standard penetration test resistance (N-value) and other explanatory variables using data sets from Japan. The V S and N data sets are from the Kyoshin network (K-NET), which include 16,845 collocated measurements of V S and N at 1102 sites. We begin with baseline equations considering soil type and overburden pressure in addition to N. For coarse-grained soils, V S is more sensitive to overburden pressure than to N, whereas for fine-grained soils, V S is more sensitive to N. We find that residuals (difference between data and model) for a given borehole tend to be correlated (e.g., samples within a boring are consistently high or low); hence, we use mixed-effects regression to compute a quantity akin to an average borehole residual referred to as the between-boring residual. Sites associated with older (pre-Holocene, >10,000 yrs) geology tend to have null to slightly positive between-boring residuals, whereas these residuals are negative for more recent materials. Accordingly, we provide adjustments to the baseline equations conditional on geologic condition. Between-boring residuals exhibit spatial correlations; however, due to lack of knowledge as to the cause of these correlations, we do not recommend such effects for inclusion in the model. When applied to predict the time-averaged V S in the upper 30 m (V S30 ) in the absence of direct measurement, the proposed equations provide significantly better accuracy than widely used local geomorphology-based proxies (0.26 versus 0.42 of standard deviation of the natural log of residuals). This suggests that penetration resistance data can improve predictions of V S30 compared with geomorphology- based proxies alone, when site-specific V S measurements are unavailable.