The ground motion data of the 1999 Chi-Chi, Taiwan, earthquake exhibit a striking difference in frequency content between the north and south portions of the rupture zone. In the north, the ground motion is dominated by large low-frequency displacements with relatively small high-frequency accelerations. The pattern is opposite in the south, with smaller displacements and larger accelerations. We analyze the fault dynamics in light of a fault lubrication mechanism using near-field seismograms and a detailed rupture model. The fault zone contains viscous material (e.g., gouge), in which pressure increases following the Reynolds lubrication equation. When the displacement exceeds a threshold, lubrication pressure becomes high enough to widen the gap, thereby reducing the area of asperity contact. With less asperity contact, the fault slips more smoothly, suppressing high-frequency radiation.