Longitudinal Valley of Eastern Taiwan
The Central Range is separated from the Coastal Range by a long and narrow valley between the cities of Hu
alien and Taitung in eastern Taiwan, the Longitudinal Valley. It is approximately 150 kilometers long and 3
to 6 kilometers wide, averaging 4 kilometers m width. The valley is the suture between the Eurasian
continent on the west and the Luzon Island Arc on the Philippine Sea plate on the east, the two important
tectonic elements of Taiwan. This valley is a tectonic through valley drained by three streams with
topographically indistinct divides.
The Longitudinal Valley is linear and narrow, with negligible relief, odd drainage patterns, and high
seismicity. All these features indicate that the Longitudinal Valley is structurally controlled (T. L. Hsu,
1976a; York, 1976) and related to faulting (Alien, 1962; T. L. Hsu, 1962b). This valley contains an
exceptional thickness of alluvium and is fringed with an extensive series of alluvial fans, mostly from
streams draining the Central Range. These spectacular morphological features are clearly shown on the
geologic map of Taiwan and on aerial photography or satellite imagery.
The fault origin of the Longitudinal Valley has been recognized for a long time. Active faulting is indicated
by high seismicity, many historical great earthquakes, and fault scarps that cut alluvial deposits of the
valley floor (Alien, 1962). Biq (1965) postulates that the Longitudinal Valley is a ramp valley in
accordance with the hypothesis that each side of the valley is bounded by a high-angle upthrust. These
thrusts generally had sinistral strike-slip components as well. The fault on the eastern side is called the
Coastal Range fault, and is east-dipping; whereas the western side is bounded by the Central Range fault,
which is a west-dipping thrust uplifting the Central Range to its present height. These two major tectonic
lines have been very active in recent times and are very significant in the Quaternary tectonics of Taiwan.
Nevertheless, due to rapid erosion rate and thick alluvial cover, exposures of the actual fault surface can
rarely be observed (T.L. Hsu, 1976; York, 1976). The few fault scarps or scarplets and other evidence of
faulting are so scattered and dissected that little information about the overall sense of fault
displacement can be gained, accounting for the present debate about the exact nature of the fault tectonics
in the Longitudinal Valley.
The most powerful proof concerning the nature of fault movement of the Coastal Range fault is that of the
major earthquakes of October 22 and November 25, 1951, each associated with well-documented faulting on the
surface (T. L. Hsu, 1962b). Both fault breaks are east-dipping upthrusts with a sinistral strike-slip
component. A number of other fault scarplets have also been found discontinuously along the trace of the
Coastal Range fault between Hualien and Taitung (T. L. Hsu, 1976a). Arguments still exist, however, between
geologists and geophysicists on the existence of the Central Range fault along the western margin of the
Longitudinal Valley. More investigations are needed to study this intricate tectonic problem.
Judging from the extreme linearity of the Longitudinal Valley, Alien (1962) visualized that the main
boundary faults could be largely vertical transcurrent fractures. Recent studies of fracture patterns by
Barrier and Angelier (1986) support the idea that the faults along the Longitudinal Valley are mainly thrust
faults, under a compressional stress in the direction between 090 and 140, with only twenty percent of the
movement representing the component of sinistral displacement. The sinistral movement of the faults is also
shown by the offset of streams in the Longitudinal Valley (Biq, 1971; York, 1976). Re-triangulation of the
northern part of the Longitudinal Valley (C. Y. Chen, 1974) also shows an average left-lateral movement of
3.65 meters between surveys made in 1971 and 1909 to 1942, a sinistral displacement rate of more than 6
cm/year. The significance of the Longitudinal Valley is further discussed in the last chapter of this text.