The observed time history and spatial dependence of the surface ground motions of the September 21, 1999, Chi-Chi, Taiwan earthquake over the entire island are presented. For the first time, this report presents an analysis of time-dependent spatial ground motions at both near-source and local distances using strong motion records. The two-dimensional ground motions of the Chi-Chi earthquake were reconstructed from more than 400 free-field strong motion records after being corrected for absolute times. The reconstructed seismic wavefields can be considered as a new type of observed data, named 3-D seismograms here. Results of this study offer some insight into the study of earthquake source and wave propagation based on 3-D numerical modelling and greatly compliment our ability to distinguish wave generations due to sources and/or structures. ¿ 2000 American Geophysical Union