Temporal aN and aT in earthquake ground motion at a single point

This paper describes a continued study on three-dimensional temporal characteristics of earthquake ground motions at a single point. Based on an instantaneous tangential and normal acceleration decomposition of ground acceleration trajectory, a ground motion can be partitioned into a finite sequence of staggered time intervals of acceleration and deceleration. A formulation is developed to estimate speed and angular changes over a partitioned interval in terms of rates of positive and negative tangential and normal acceleration. Based on these concepts, general ground motion properties, peak ground acceleration, peak ground velocity, and peak ground displacement are examined. Several Northridge earthquake records are studied in detail. It is found that the highest peak of ground acceleration in these records corresponds to a high peak of deceleration, and a velocity maximum often precedes the peak of acceleration.