Temporal distribution of earthquake slip is one of the most important information for accessing the earthquake hazard. In many cases, ages of past earthquakes can be ob- tained from historical records or excavations of datable materials near active faults. Not all faults have historical records or suitable materials for age-dating, therefore, alterna- tive way of determining timing of fault slip is critically needed. The rapid tectonic activ- ities in the north-south-trending Longitudinal Valley along the eastern Taiwan provides an important opportunity to study the relationship between the collisional mountain building and the spatio-temporal pattern of deformation. However, due to the equally rapid rates of erosion and dense vegetation, geomorphic features related to tectonic activ- ities such as surface fractures and fault scarps can disappear quickly. We exploit digital elevation model (DEM) to look for indications of active tectonics preserved. Specifically, spatial distribution of knickpoints in the eastern flank of the Central Range are used to inferred the tectonic activities in the Longitudinal Valley.
Transient knickpoints in the fluvial longitudinal profiles represents signals of base level fall in river networks, which are later transmitted upstream to their current lo- cations. In Longitudinal Valley of eastern Taiwan, many knickpoints across different tributaries of the same fluvial system can often be found in clusters of similar altitudes, and several clusters are found in each of three main fluvial networks in Longitudinal Valley (Hualien river system in the north, Hsiuguluan river system in the central, and Pinan river system in the south), which indicate different temporal fault slip pattern in each fluvial network system. Non-dimensionalized knickpoint propagation rate is cal- culated and an non-dimensional age is assigned to each knickpoint cluster, and finally the recurrence interval calculated. Our investigation indicate that for the past thousand years, southern section of the Longitudinal Valley is the most active with earthquake recurrence interval approximately at two-thirds of the north and central segments of Longitudinal Valley.