Localization and characterization of the southern Ulsan fault (UF) using geo-electrical imaging: Implication for seismic hazard assessment in an urbanized area

In seismic hazard assessment, subsurface geophysical surveying has gained popularity in recent years towards fault mapping and determining seismic deformation parameters such as fault offset, recurrence, and depth of fault, locating proper trench sites based on the subsurface information. In the present study, electrical resistivity tomography (ERT) was used to (1) locate the trace of the southern segment of the Ulsan fault, (2) to test the applicability of ERT techniques for active fault mapping in a close to the highly urbanized and complex geological environment with a slow tectonic activity. We have applied the ERT technique at five sites. At one place, we used the Wenner array, Schlumberger array, Dipole-Dipole array, and Seismic-profiling techniques to know which method provides a better result in complex geological conditions like Korea. Out of these methods, the Dipole-Dipole array provided high-resolution results and was used for the other two sites. The ERT result shows high and low resistivity zones interpreted as bedrock (mainly Tertiary and Cretaceous formations) and coarse fluvial sediment layer, respectively. The maximum vertical displacement recorded along the fault varies from 6 m to 12 m. Based on the ERT results, two trenches were excavated to directly investigate the deformation pattern associated with the southern segment of the Ulsan fault. The ERT and trench survey results support that the southern Ulsan fault has slipped multiple times since Quaternary. Using this multi-approach, ~5 km long active fault map was prepared for the southern Ulsan Fault. It is found from this study that the integrated approach is highly beneficial where contrasting sub-lithological units exist in terms of their physical properties, even though human activity or the ongoing urbanization process has modified the surface morphology. This study argues for judicious use of ERT techniques to delineate the shallow subsurface geology across various active faults in the Korean peninsula and similar tectonic settings.