In November 2017, an $Mw$ 5.4 earthquake with a shallow (⁠$∼4km$⁠) hypocenter occurred in the Pohang area, one of the most developing industrial cities in South Korea. This shock resulted in the costliest earthquake damages in modern Korean history. Immediately after the event, it was not easy to recognize the source fault due to the lack of clear coseismic surface ruptures but also the absence of previously mapped active faults around the epicenter. Based on satellite‐ and unmanned aerial vehicle (UAV)‐derived images, with ground resolution higher than 0.5 m, and field observations, we investigate surface deformation such as sand blows and ground failures associated with the Pohang earthquake as well as bedrock fault exposures in the epicentral area. To better constrain the source fault and deformation mechanism, we combine both moment tensors with aftershock relocations and surface displacements estimated by Interferometric Synthetic Aperture Radar (InSAR). Our results indicate that (1) various types and distributions of secondary ground deformation were controlled by blind oblique‐slip rupture; characterized by right‐lateral and reverse‐slip components, on a reactivated pre‐existing segmented normal‐fault system, and (2) local ground conditions could trigger or amplify ground deformations and their related earthquake damage. Our study highlights the importance of mapping the extent of coseismic deformation as well as the trace of active faults on improving seismic hazard assessments in the Korean Peninsula.

Sedimentological characteristics and zircon provenance dating of the Babulu Formation in the Fohorem area, Timor-Leste, provide new insights into depositional process, detailed sedimentary environment and the distribution of source rocks in the provenance. Detrital zircon sensitive high-resolution ion microprobe (SHRIMP) U–Pb ages range from Neoarchean to Triassic, with the main age pulses being Paleozoic to Triassic. In addition, the maximum deposition ages based on the youngest major age peak (ca 256–238 Ma) of zircon grains indicate that the basal sedimentation of the Babulu Formation occurred after the early Upper Triassic. The formation consists predominantly of mudstone with minor sandstone, limestone and conglomerate that were deposited in a deep marine environment. These deposits are composed of six lithofacies that can be grouped into three facies associations (FAs) based on the constituent lithofacies and bedding features: basin plain deposits (FA I), distal fringe lobe deposits (FA II) and medial to distal lobe deposits (FA III). The predominance of mudstone (FA I) together with intervening thin-bedded sandstones (FA II) suggest that the paleodepositional environment was a low energy setting with slightly basin-ward input of the distal part of the depositional lobes. Discrete and abrupt occurrences of thick-bedded sandstone (FA III) within the FA I mudstone suggests that sandstone originated from a collapse of upslope sediments rather than a progressive progradation of deltaic turbidites. This combined petrological and geochronological study demonstrates that the Babulu Formation in the Fohorem area of the Timor-Leste was initiated as a submarine lobe system in a relatively deep marine environment during the Upper Triassic and represents the extension of the Gondwana Sequence at the Australian margin.