Variations in Tectonic Activities of the Central and Southwestern Foothills, Taiwan, Inferred from River Hack Profiles

  • Author(s): Yen-Chieh Chen, Quocheng Sung, Chao-Nan Chen, and Jiin-Shuh Jean
  • DOI: 10.3319/TAO.2006.17.3.563(T)
  • Keywords: Strain accumulation, Hack profile, Taiwan Bautu, Chi-Chi earthquake

A longitudinal profile of a river under static equilibrium shows no degradation or aggradation and can be ideally described as a straight line on a semi-logarithmic graph. This type of profile is called a “Hack profile”. If a river runs across uprising active structure systems, its Hack profile becomes convex. Accumulated tectonic strain varies positively with the intensity of the upwarping in Hack-profile convexity. In this paper, we compare curvature changes in Hack profiles of a series of rivers running through faults in the central and southwestern Foothills of Taiwan. Longitudinal profiles of these rivers were derived from two versions of topographic maps (1904 and 1985) and recent DTM data (2000). Prior to comparisons, we calibrated the 1904 topographic map, named “Taiwan Bautu”, by “offsetting” horizontal coordinates north and westward approximately 440 m and then “linear transforming” the elevation values. The Tungtzchiau fault of the central Foothills has remained inactive since 1935. Here relatively high uplift activity near the Wu River is indicated by significantly convex Hack profiles. This strain accumulation can be attributed to a lack of small magnitude earthquakes along the fault over the past 70 years. In the southwestern Foothills, relatively high uplift activity of similar intensity to the central Foothills is indicted near the Neocho River. Significant profiles with concave segments below the ideal graded profiles, at the lower reaches of rivers where continuous small magnitude strain release events have occurred, can only be found along the Sandieh, Neocho and Bazhang rivers in the southwestern Foothills. All these findings indicate that fault systems in the central Foothills tend to be locked and these faults could yield large earthquakes similar to the Chi-Chi event.

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