Estimating rotational components of ground motion using data recorded at a single station

Seismic analysis, design, and performance assessment of buildings, bridges, and safety-related nuclear structures is based on two or three translational components of earthquake ground motion. Although rotational components (rocking and torsional) may contributesignificantly to translational and torsional response and damage, they are not consideredin design and assessment because their intensity and frequency content are not measured by accelerograms deployed in the free field. A method for developing rotational time series by first deconstructing the three translational time series of a ground motion recordedat one station into body waves is presented in this paper. The body waves are then reassembled to generate rotational time series. Point and line source representations of the fault rupture are considered. Results of the single station procedure are presented using rotational acceleration spectra and compared with the multiple-station-based geodetic method. The spectra are similar at periods greater than 1 s. The spectral ordinates computed using the geodetic method are significantly smaller at shorter periods, which is attributed to the underlying assumption of a plane surface for the recorded data at any time instant in the method.