Could We Have Pre-located the Northeastern Portion of the 1999 Chi-Chi Earthquake Rupture Using Geological and Geomorphic Data?

Abstract

The Chi-Chi earthquake rupture that mostly follows the mountain front in central Taiwan bends east/northeast into hills in the Tachia River valley. We found no major pre-existing fault systems in this hill area. However, based on river terrace morphology and sequences, the lower Tachia River valley appears to have been uplifted more rapidly than the lower Taan River valley in the past ten thousands of years, which is consistent with the bend of the earthquake rupture. The earthquake rupture (and associated uplift) also coincides with: (1) straight mountain-front escarpment south of the Tachia River valley; (2) two fault/fold-scarps on the Holocene these terraces in the Tachia River valley; (3) local development of non-laterized terraces in the Taan River valley; (4) abnormal rise and narrowing of the Taan/Tachia drainage divide. These geomorphic anomalies, however, are either local or diffusive; features (1) to (3) might have been interpreted as of fluvial origin. Also, the link between these features and the movement of the Chelungpu-Sanyi fault, the causative fault of the earthquake, might have been uncertain. We thus argue that the pre-location of this portion of the Chi-Chi earthquake rupture would have been difficult, even though we could have roughly delineated a potential coseismic deformational zone in the region.

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