Shallow fault rupture of the Milun Fault in the 2018 M _w 6.4 Hualien earthquake: A high-resolution approach from optical correlation of Pléiades satellite imagery

We use high-resolution Pléiades optical satellite imagery to study the distribution and magnitude of fault slip along the Milun fault surface rupture, which broke during the 2018 Hualien earthquake (M _w 6.4) in eastern Taiwan. Correlation of pre- and postearthquake stereo Pléiades images reveals detailed 3D surface displacements along the 8-km-long Milun fault, with maximum ∼1 m left-lateral offsets across the fault. Along the northern section of the Milun fault, our correlation results indicate a localized deformation zone, with offset values slightly larger than the maximum offsets reported in the field (∼77 cm). To the south, the left-lateral offsets become increasingly distributed, producing arctangent shapes in displacement profiles crossing the fault. In places, the deformation zone reaches widths of 200 m and can be explained by a shallow east-dipping fault rupture extending from 2 to 3 km depth to 70–120 m below the surface. A very shallow coseismic rupture on the Milun fault is consistent with a shallow locking depth interpreted from previous geodetic analyses from the interseismic period. Despite a few highly discontinuous and irregular surface ruptures reported along the southern section of the fault, our results suggest the main fault slip (up to 1 m) stopped at very shallow depths below the surface, in which ∼60% of the deformation may be accommodated as off-fault deformation (OFD). In this upper ∼100 m of the crust, OFD may be promoted by a significant change in material strength, as the fault crosses from bedrock and/or consolidated sediments into weaker, water-rich, poorly consolidated alluvial sediments.