Probing the frontal deformation zone of the Chihshang Fault with boreholes and high-resolution electrical resistivity imaging methods: A case study at the Dapo site in eastern Taiwan

• Published in: Journal of applied geophysics 2018-06, Vol.153, p.127-135
• Main Authors: Chang, Ping-Yu, Huang, Wen-Jeng, Chen, Chien-Chih, Hsu, Han-lun, Yen, I-Chin, Ho, Gong-Ruei, Lee, Jian-Cheng, Lu, Shih-Ting, Chen, Po-Tsun
• Format: Article
• Language: English
• ISSN: 0926-9851; 1879-1859
• DOI: 10.1016/j.jappgeo.2018.04.006

Not only direct fault ruptures but also later mass movement may result in complicated frontal deformation of the faults. Consequently, the deformation front or the contacts between the unconsolidated materials from the hanging wall and footwall of the thrust fault may indicate the toe of the mass movement instead of the actual fault zone. In this study, we used a combination of surface electrical resistivity imaging methods and borehole records in order to investigate the geometries of the structures in the frontal deformation zone of the Chihshang Fault at the Dapo elementary school. From the cores, we observed three different geological components at the Dapo site: the conductive Lichi mélange of the hanging wall, the colluvial gravels and the underlying fluvial-gravel layer at the footwall. The resistivity images from two parallel survey lines reveal that the position where the fault trace was thought to be is actually the toe of the slumping body's surface ruptures consisting of materials from the Lichi mélange. On the basis of the resistivity images, we also found that the actual fault plane is located on the southeastern side of the resistivity survey line near the hilltop. As a result, we conclude that mass movement induced by the inter-seismic creeping, not direct faulting, is the main factor affecting the frontal deformation zone of the Chihshang fault at the Dapo site. •The ERI surveys and boreholes were used to study the Chihshang fault at Dapo.•We found that the tongue-like body of Lichi mélange is a slumping mass, not a part of the hanging wall.•The resistivity results suggest that the actual fault plane is in the uphill portion of the survey lines.•The initiation of current slumping of the tongue-like body took place after about 2000 yrBP.•The relationships of the slumping body and fault trace show the recurrence rate of creeping faults.