abstract:This paper tracks the time history and spatial variation of the long-period (1 - 10 sec) strong ground motions recorded in the Taipei basin during the magnitude 7.1 earthquake of 31 March 2002, offshore of eastern Taiwan. The two-dimensional ground motions of this event were reconstructed from 89 free-field strong motion records over an area of 40 square km in northern Taiwan. The observed basin ground motions show complex waveforms, extended durations and multiple propagation directions in later phases. The dominant basin ground motions are identified to shake in its radial directions after the S-wave arrivals. Within the analyzed period band, results show that seismic waves amplifications were observed inside the sedimentary basin and its major amplifications were located on the eastern edge, the thick sediment portion of the basin. Across the Taipei basin, large ground motions were still maintained at the Linkou Tableland. Employing moving-window and waveform stacking techniques to analyze array seismograms revealed that high amplitude later phases have lower apparent velocities than the incident S-waves and cross the basin in multiple directions. We interpret these long period later arrivals to be surface waves, which are generated by a body wave interacting with the thick soft sediment of basin.