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.