Assessment of Spatial Coherency Using Tri-directional Ground Motions

Structures and infrastructure are subject to spatial varying seismic ground motions. The commonly available empirical lagged coherency models are for the record components along the same orientation at spatially distributed recording stations; the lagged coherency for record components in two orthogonal horizontal directions was reported recently. It appears that the lagged coherency between a vertical and a horizontal record component has not been systematically investigated. The assessment of the lagged coherency among tri-directional ground motion components at a single or multiple recording stations is presented in this study using ground motion records from dense arrays (i.e., LSST and SMART-1 arrays) in Taiwan. The results indicate that the lagged coherency between vertical record components at different recording stations is systematically lower than that between horizontal record components in the same direction; and as a first-order approximation the lagged coherency between any two orthogonal record component components can be modeled as a linear function of frequency and is insensitive to separation between the recording stations. A simple-to-use lagged coherency model is suggested and illustrated for simulating array of tri-directional ground motions.