Characteristics of sand volcanoes caused by 2017 Pohang Earthquake-induced liquefaction and their paleoseismological approach

A large number of sand volcanoes were observed in the vicinity of the epicenter due to the M-L 5.4 earthquake occurred on November 15, 2017 in Heunghae area, Pohang City. In this study, trench investigations were carried out on unconsolidated sediments to trace the vertical migration of the sand volcanoes and the liquefaction zone under the surface. The examined sections are markedly dominated by muddy sediments in the upper part, sandy sediments in the lower part, which have been deposited in a floodplain. The liquefied sand was injected along the irregular cracks of the silty clay layer at the upper part of the trench. The sand volcanoes show a similar grain size distribution to the sediments at the altitudes of 2.4-3.0 m showing the mode of 200-300 mu m. Considering the development of the soft-sediment deformation structure in this section, the sand volcano is considered to have been originated at the altitude of 2.6-3.0 m. The lenticular coarse-grained sand layer showing a remarkable difference in particle size at the altitude of 2.5 m is likely to have been injected from lower liquefied sand by paleo-earthquake event in this area. Therefore, it is considered that M>5 earthquake occurred during AD 1360-1640 based on the radiocarbon dating from the upper and lower layers of the lenticular coarse-grained sand layer. This result could be basic data to trace paleo-earthquake in the southeastern part of Korea.