Anomalous co-seismic surface effects produced by the 2014 Mw 6.2 Ludian earthquake, Yunnan, China: An example of complex faulting related to Riedel shear structures

Riedel shear structures are often developed within strike-slip fault zones. This study focuses on the Riedel shear structures within the co-seismic surface rupture zone and the co-seismic landslides produced by the 2014 Mw 6.2 Ludian earthquake. All the geologic, seismic, and geodesic data indicate a WNW-ESE principal compressive stress acting on the Baogunao-Xiaohe fault zone (BXFZ). Based on the idealized Riedel shear model, together with evidences form the particular distribution of aftershocks and co-seismic surface effects, the 2014 Ludian earthquake could be with a complex faulting process related to the Riedel shear structures under this present tectonic stress, i.e., NNW-striking sinistral strike-slip fault plane (Y shear) which trends approximately N40 degrees W, and ENE-striking dextral strike-slip fault plane (R' shear) which is nearly perpendicular to former. Field surveys and structural analysis of the co-seismic surface rupture zone are performed based on Unmanned Aerial Vehicle (UAV), borehole drilling, and adit excavation, to the north of Wangjiapo Village, southeast of the epicenter. The results indicate that these co-seismic surface ruptures were produced following a pre-existing sinistral transtensional duplex structure in plan view, which mainly comprises i) typical Riedel shear structures characterized by en echelon cracks running northwest and ii) tensile cracks subparallel to the slope contours running northeast. And the inward-dipping geometry of faults under the sinistral transtentional duplexes converges downwards into a single shear zone, which is defined as a negative (normal faulted) flower structure in vertical crosssection. In addition, on the basis of the analyses of three large-scale landslides, this study demonstrates that the slopes in a state of marginal static stability just situated inside the region of main energy release, which was predominantly controlled by the Y shear fault, are the reasons why this moderate earthquake induced anomalous co-seismic landslides.