Geologic records of moderate-to-large pre-historical earthquakes in South Korea

South Korea, one of the most densely populated areas, has been considered a tectonically safe region as there were no destructive earthquakes in modern society. Indeed, there is no report of any earthquake event with surface ruptures in a historical period. After the 2016 Mw 5.5 Gyeongju earthquake, the largest instrumentally recorded event in South Korea, the need for research on large-earthquakes has highlighted and multidisciplinary research projects were conducted for paleoseismological investigations. Here we introduce the newly discovered geologic records of pre-historical large-earthquakes. Firstly, paleoearthquake surface ruptures were newly identified along the entire section of the Yangsan Fault, one of the most major strike-slip structures in the Korean Peninsula. For a 50-km-long fault section, a fault theme map with a scale of 1:25,000 was produced and paleo-earthquake records were found at multiple sites mainly based on excavation surveys. The results provide an opportunity to interpret the temporal and spatial scenarios of the paleo-earthquakes along the fault. Secondly, stratigraphic records of paleo-earthquake surface rupture were found at a few localities near another major strike-slip fault system; the Gongju Fault System, and we obtained paleoseismic data. These records imply that moderate-sized earthquakes often occurred on minor faults, not on the main trace of the major faults. Our results indicate that the crustal deformation of the Korean Peninsula, which belongs to the very slow-deforming regions, is accommodated by moderate-to-large earthquakes with a recurrence time ranging from thousands to tens of thousands of years. Considering that it is not easy to detect any crustal deformations using micro seismicity and/or geodetic data due to too slow deformation, we note that it is necessary to obtain more paleoseismological data to well understand tectonic deformations as well as to assess hazards associated with future earthquakes.