Low-Angle Reverse Faulting During Two Earthquakes on the Northern Part of the Chelungpu Fault, Deduced From the Fengyuan Trench, Central Taiwan

Abstract

A trench and two boreholes located at the western margin of a young terrace, east of Fengyuan city, show the style and frequency of surface faulting on the northern part of the Chelungpu fault, central Taiwan. Both the 1999 earthquake fault and penultimate earthquake event are characterized by low-angle reverse faulting with overturning of younger strata. The penultimate event occurred between ca. 1400 yr BP (1340 - 1170 cal yr BP) and 2000 yr BP (2120 - 1800 cal yr BP). The age of the lower part of the terrace gravel is ca. 4000 yr BP; thus the two faulting events occurred during the past ca. 4000 years. The amount of horizontal shortening and vertical displacement since the penultimate event are significantly larger (3.7 - 3.9 m for horizontal and ca. 5 m for vertical) than those (2.7 m for horizontal and ca. 2 m for vertical) of the 1999 earthquake. The age of the penultimate earthquake at the Fengyuan site is the oldest earthquake recorded among the known penultimate penultimate events on the Chelungpu fault so far and suggests that this part of the Chelungpu fault may be a fault segment independent of the central and southern parts of the fault.

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