Comprehensive Survey Delineating the active Lichi Fault in Southeastern Taiwan: Shallow Geometry and Holocene activity

The Lichi Fault forms the southernmost segment of the Longitudinal Valley Fault (LVF) in eastern Taiwan, yet its shallow geometry and Holocene activity remain poorly constrained, particularly south of the Pinan River. We integrate resistivity image profilings, borehole observations, radiocarbon dating, and existing geological and geophysical data to delineate the concealed fault geometry along seven profiles spanning the southern Longitudinal Valley. The results identify the Lichi Fault as an east-dipping thrust system that juxtaposes the Lichi Mélange against the Pinanshan Conglomerate or Quaternary alluvial deposits. Borehole and stratigraphic evidence, together with radiocarbon ages, demonstrate deformation of Holocene sediments and confirm continued fault activity during the Holocene. The mapped fault extends for approximately 25 km from the Pingting Terrace to the Taitung City area, where its trace is largely concealed beneath alluvial deposits and urban development. Segment-specific confidence assessments further demonstrate that fault-trace reliability varies spatially with the quality and consistency of available evidence. The results provide new constraints on the geometry, segmentation, and activity of the southern LVF and establish an integrated framework for evaluating concealed active faults in engineering geological investigations.