Relocation of the 20 January 2007 ML 4.8 Odaesan Earthquake Sequence

Abstract

A moderate-sized earthquake (ML 4.8) occurred in the mid-east Korea Peninsula on 20 January 2007. It was the largest inland earthquake to occur in South Korea since the installation of the Korea National Seismic Network in 1998. The earthquake caused minor damage to adjacent structures and temporaryfailure of the electrical power supply. Although only four aftershocks were noticed in previous studies, a careful review of continuous data revealed that the main event was accompanied by at least 74 micro foreshocks and aftershocks. A subset of 25 events was selected for further analysis to determine precise earthquake locations, focal mechanism solutions, and the current status of regional tectonic stress, as well as to answer questions raised about the sequence. The results of the precise earthquake relocationreveal that the Odaesan earthquake occurred on a vertical and circular fault plane. Although the Woljeongsa Fault is noticed from the local geological map, the relocated hypocenters indicate that the Odaesan earthquake and its aftershocks did not relate to the Woljeongsa Fault. We also observed anunusual lack of large-magnitude aftershocks, a relatively large stress drop during the main event, and no previous earthquake record in the region. Observations made in the study consistently indicate the sequence is resulted from the activation of a new small-scale fault or the reactivation of a previously unmapped fault as a consequence of long-term regional tectonic loading. Focal mechanism solutions of this and other events in Korea indicate that the Odaesan earthquake was a typical consequence of crustal deformation due to current tectonics reflecting ENE？SW compression and NNW？SE extensionnear South Korea in the stable Eurasian continent.