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

Abstract

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 macroscale of the Riedel shear model.

Surface deformation of 0206 Hualien earthquake revealed by the integrated network of RTK GPS

“…The measured data were generally self-consistent with nearby measurements. Field measurements (Hsu et al , 2019Huang et al 2019;Lin et al 2019) showed that the fault has left lateral motion of around 70 cm near the Chihsingtan village, which was comparable to our result in the same area. The campaign and continuous GPS data were also comparable to our current results as seen in both Figs.…”

“…As inferred from the distribution of the aftershock, which ranged from the offshore Milun Fault to the Great Hualien area on land, the seismogenic fault should be a strike-slip fault with a westward dip in a northeast-southwest direction. The shock induced the activities on the Milun Fault and the Lingding Fault and caused a large-scale of surface ruptures along the fault line (Hsu et al , 2019Huang et al 2019;Lin et al 2019) -mainly concentrated at the north of Hualien County (Fig. 2 The Milun Fault is a left-lateral fault with a reverse component striking in a direction of north-south and extending from the southwest of the Chihsingtan beach to Hualien City along the west edge of the Milun Terrace.…”

“…During the 0206 earthquake, surface ruptures with significant left-lateral and reverse slip were observed along the Milun Fault. Hsu et al (2018Hsu et al ( , 2019, Huang et al (2019), andLin et al (2019) all observed that the amount of the surface offset decreased from north to south: the max. horizontal offset is 77 cm and the max.…”

Introduction to the special issue on the 2018 Hualien, Taiwan, earthquake

“…They showed that surface ruptures follow the traces of the Milun fault and northern Linding fault. The mapped surface ruptures are typically appeared in en échelon arrays or distributed fractures rather than a through-going fault, which are comparable to the results of Hsu et al (2019) and Huang et al (2019). They considered that the appearances of the along-strike variations of the surface rupture for the 2018 Hualien earthquake are different from the surface rupture patterns documented for the 1951 Hualien-Taitung event.…”

“…Independently Hsu et al (2019) conducted field surveys along the Milun fault and examined the coseismic ruptures of the 2018 Hualien event. In addition to the rupture geometry, they determined the principal displacement zone and Terr.…”