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The Longitudinal Valley fault is a key element in the active tectonics of Taiwan. It is the principal structure accommodating convergence across one of the two active sutures of the Taiwan orogeny. To understand more precisely its role in the suturing process, we analyzed fluvial terraces along the Hsiukuluan River, which cuts across the Coastal Range in eastern Taiwan in the fault's hanging wall block. This allowed us to determine both its subsurface geometry and its long‐term slip rate. The uplift pattern of the terraces is consistent with a fault‐bend fold model. Our analysis yields a listric geometry, with dips decreasing downdip from about 50° to about 30° in the shallowest 2.5 km. The Holocene rate of dip slip of the fault is about 22.7 mm/yr. This rate is less than the 40 mm/yr rate of shortening across the Longitudinal Valley derived from GPS measurements. The discrepancy may reflect an actual difference in millennial and decadal rates of convergence. An alternative explanation is that the discrepancy is accommodated by a combination of slip on the Central Range fault and subsidence of the Longitudinal Valley floor. The shallow, listric geometry of the Longitudinal Valley fault at the Hsiukuluan River valley differs markedly from the deep listric geometry illuminated by earthquake hypocenters near Chihshang, 45 km to the south. We hypothesize that this fundamental along‐strike difference in geometry of the fault is a manifestation of the northward maturation of the suturing of the Luzon volcanic arc to the Central Range continental sliver.

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The 1999 Chi-Chi earthquake was caused by rupture of the Chelungpu fault, one of the most prominent active thrust faults of Taiwan. This largest of Taiwan's historical fault ruptures broke the surface for over 90 km at the western base of the rugged mountain range. A short right-lateral tear extended southwestward from the southern end of the Chelungpu fault, and a complex assemblage of shallow folds and faults ran northeastward from the northern end. Vertical offsets averaged about 2 m along the southern half of the Chelungpu fault and about 4 m along the northern half, and offsets of 5 to 7 m were typical along the northern part of the major thrust. The sinuous nature of the surface trace is consistent with seismographic data that indicate a dip of about 30Њ. The 1999 rupture draws attention to the fact that this active fault system is highly segmented and that this segmentation influences the characteristics of seismic ruptures. Active faults to the south, north, and west of the Chelungpu fault have distinctly different characteristics. Faults to the south and north broke the surface during earthquakes in 1906 and 1935. The active Changhua fault to the west, a blind thrust similar in length to the Chelungpu, has not ruptured in the historical period and should be considered a prime candidate for generating a future earthquake.