The Longitudinal Valley Fault runs about 150 kilometres along the eastern side of Taiwan, tracing the narrow valley that separates the island's Central Range from the Coastal Range. It is essentially the on-land trace of the boundary where the Philippine Sea Plate collides with and rides over the edge of the Eurasian Plate.
The fault is an oblique reverse structure combining thrust and left-lateral slip, and it is one of the fastest-moving faults on Earth, shortening at rates that can exceed 20 to 30 millimetres per year. Part of that motion is released by steady aseismic creep, visible in bent walls and buckled roads, while the rest is stored for earthquakes.
Large ruptures have struck the valley in modern times, notably the 1951 Hualien-Taitung earthquake sequence and the 2003 Chengkung earthquake of magnitude 6.8. Because the towns of Hualien and Taitung sit directly on the fault, its rapid slip makes it one of Taiwan's most closely monitored seismic sources.