Abstract

A moderate-sized earthquake (ML 4.8) occurred in the mideast Korea Peninsula on 20 January 2007. It was the largest inland earthquake to occur there since the inception of a modern seismic observation system. 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. Earthquake hypocenters were seen to be more clustered after the HypoDD relocation. A source radius of 1 km for the main event was estimated based on the distribution of precisely determined aftershock locations. Focal mechanism solutions of larger events in the sequence suggest either a left-lateral strike-slip fault trending west-northwest–east-southeast or a right-lateral strike-slip fault trending north-northeast–south-southwest as the responsible structure. Although the Woljeongsa fault striking north-northeast–south-southwest in the local geological map matches one of the proposed trends, precise earthquake relocation results gave a contradictory result, showing that a previously unknown west-northwest–east-southeast striking fault was responsible for the earthquake sequence. We also observed an unusual 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 nucleated along a less-developed fault. Focal mechanism solutions suggest the current status of tectonic stress governing earthquake generation in Korea is east-northeast–west-southwest compression and north-northwest–south-southeast extension.

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