Field-based, basin-wide relations are discovered among coseismic ground motion, water-level change and liquefaction induced by the 1999 (Mw = 7.5) Chi-Chi earthquake, Taiwan. The relations imply thresholds of 4.4 m/s2 and 0.6 m/s in the spectral acceleration and velocity at 1 Hz, respectively, below which pore pressure does not change appreciably and above which pore pressure increases exponentially with increasing acceleration and velocity. Liquefaction occurs when the 1-Hz spectral acceleration and velocity and the peak ground acceleration exceed ~16 m/s2, ~2.4 m/s, and ~2.5 m/s2 (~0.26 g), respectively. Broad consistency between the present and previous results suggests that the relations may be transferable to other areas and may provide fundamental constraints for modeling coseismic pore-pressure increase and liquefaction.