Chihshang fault is one of the sections of the longitudinal valley fault. The hane wall of the fault is the Lichi melange, and the foot wall is the Holocene alluvial. The fault begins from Chunjih village, Juisuei township in Hualien county, and extension to south, Rueisuei township in Taitung county. Except the outcrop of the south bank of Fuchih bridge, which shows that Lichi melange thrusts upon the late Pleistocene - Holocene river gravel bed, there is no obvious outcrop in the other areas. In accordance with the paleoseismologic study of Chihshang fault by Chen, W.S. et al. (2008), there are seven paleo-earthquakes occurred in thousand years.

　

Chihshang vault forms an obvious leaner scarp from Chingyuan village to Wanan village. This study chose the cliff at Paoan temple in Wanan village to drill three wells. Wanan core no. 1 reaches the fault plain at the depth of 29.2 m; Wanan core no. 2 reaches the fault plain at 46.4 m. The dip angle of the fault plain is 67º. After linking the location about the fault plain of the two boring log, it speculates that the dip angle of the fault is 65º (Figure 1), which is the same as the angle of the fault plain in two cores. It thus may speculate that the dip angle of Chihshang fault at Wanan area is about 65º.

　

According to the exposed location of the Lichi melanges under the wells and the surface, Chihshang fault is located on the upper part of the scarp. The trench of Wanan no. 1 and no. 2 are located on the foot of the scarp. The beds of the trench profile show two kinds of structures: Wanan no. 1 has seven shear structures, and the Wanan no. 2 has the fold structure. Based on the geography, the recent deformation (fault or fold structure) may be located on the foot of the scarp, which causes such an obvious leaner scarp.

 

Figur Figure 1. Profile of the core-boring of Wanan village, Chihshange township.

　

This study dig out the geographical profile on the south bank of Fuchih bridge at the Pien river, which is located to the south of Fuli. The outcrop shows that the Lichi mélanges, which is the hanging wall (the east side) of Chihshang fault, thrusts upon the river conglomerate. The thin terrace conglomerate of the Pien river unconformable deposited on the Lichi mélanges and the river conglomerate. The height ratio between the terrace and the riverbed of Pien river is 27 m (Fugure 2). The terrace conglomerate of Pien river is also offset by Chihshang fault, which causes a height difference between the two edges of the fault for 6.5 m. The ¹⁴C dating in the terrace conglomerate is 1180-940 yr BP.

　

According to the calculation of the height ratio of the terrace, the incised rate of the river is 23.5-27.00 mm/yr. By assuming the vertical slip rate of the fault is 5.5-6.9 mm/yr, this rate is obviously lower than the current vertical slip rate of Chihshang fault, which is about 20-22 mm/yr. This study determined that the fault located on the south bank of Pien river, which should belong to a branch fault of Chihshang fault and located on the hanging wall. Chihshang fault should be on the west side of the terrace. Based on the height ratio between terrace and riverbed, it speculates that the long-term uplift rate is about 23.5-27.0 mm/yr. Based on the near-surface fault dip angle of Chihshang fault is about 65º. The net slip rate of the fault plain is about 26-30 mm/yr. 

 

Figure 2. Geographical profile of Chihshang fault at the south bank of Fuchih bridge.