Geological Features That Exacerbated the 921 Earthquake's Damage

Extremely Shallow Hypocenter: The earthquake's hypocenter was only 8 kilometers deep, classifying it as a very shallow earthquake. The energy propagated to the surface over a very short distance, causing extremely violent shaking acceleration (maximum intensity of 7) on the ground in central Taiwan. Rare, Massive Slip Amount: Geologically, the northern section of the Chelungpu Fault (around areas like Fengyuan and Pifeng Bridge in Taichung) experienced concentrated stress. This resulted in a vertical uplift of the ground surface by 9 to 12 meters, and the displacement along the underground rupture plane may have reached 14 meters, setting a rare global record for a massive slip at the time. Asymmetrical Damage Range: Due to it being an east-dipping reverse fault, the earthquake's energy was primarily borne by the eastern hanging wall (areas like Nantou, Dongshi in Taichung, and the mountainous regions of Puli). Consequently, surface destruction, building collapses, and landslides were significantly more severe in the hanging wall areas compared to the footwall.