Investigating the structure of the Milun Fault from surface ruptures of the 2018 Hualien Earthquake

A deadly M_w 6.4 earthquake occurred in the Hualien area of eastern Taiwan on 6 February 2018. It caused severe damage to infrastructure and creating surface ruptures in several areas mostly near the Milun Fault in Hualien City. In this study, we investigated the distribution of co-seismic surface ruptures by measuring the orientations of the ruptures, classifying the fracture patterns, and measuring the fracture geometries to calculate the principal displacement zone (PDZ) and the regional stress directions. As a result, local PDZ is observed to rotate anti-clockwise along the Milun Fault from north to south. Considering the deformation behaviors of the fractures and their relative positions along the Milun Fault, the shear zone in Qixingtan area is a horsetail structure derived from the right side of the end of the left-lateral strike-slip fault. In addition, the 170° trending fault splay in the middle segment and the fault branch corresponding to the Beipu Fault are elements of the Riedel shear structure associated with the left-lateral moving Milun Fault. Our results show that Riedel shear structures are common within co-seismic surface rupture zones in this area, and the variations in the orientations of Riedel structures reflect the influence of the pre-existing Milun Fault. We can also determine the most recent geometry, kinematics, and displacement characteristics of the Milun Fault through co-seismic surface ruptures and macro-scale Riedel shear structure analysis. This study provides a good example of understanding the relationship between the outcrop scale and the macro-scale of the Riedel shear model.