Analysis of continuous GPS data [2000][2001][2002][2003] at 50 stations of the Korean GPS Network (KGN) suggests that the southern part of the Korean peninsula is tectonically more stable than other regions in the Eastern Asia. The average velocity was ∼1.5 mm/yr and the average overall strain rate was around −0.3 × 10 −9 str/yr. The obtained velocity field indicates the presence of anticlockwise rotation of the whole region with respect to Daejeon station (DAEJ), in the central part of South Korea. It also showed that KM-OB (Kyonggi Massif and the Okchon Basin) and YM-TB (Yeongnam Massif and Taebaeksan Basin) have left-lateral shearing movements. Both movements were confirmed by the deformation analysis of the KGN horizontal velocity field using the infinitesimal plate theory. The results show that South Korea moves toward the WNW direction with a velocity of 0.9 mm/yr with a slow anticlockwise rotation. The strain field in South Korea indicates the existence of both compression and tension. The compression and extension axes have WSW-EWE and NNW-SSE directions, respectively.

The new techniques of space geodesy, especially the Global Positioning System (GPS), allow the monitoring of ongoing crustal defonnation and provide a powerful tool for the analysis of the regional seismotectonic regime and characterization of cUITent strain accumulation and release.

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We use three years data from six permanent GPS stations, in the Korean peninsula and Jeju Island, to examine the relative displacement between Jeju Island and the Korean peninsula and whether the Jeju Island lies on the Amurian Plate or not. Out of these stations, three sites, considered to be on the stable interior of the Amurian Plate (the Korean peninsula), showed relative velocities less than 1.3 mm/yr. On the other hand, the maximum velocity of the Jeju Island stations was 2.1 mm/yr with respect to DAEJ station, and the average relative velocity between the island stations and those in the main land are less than 1.7 mm/yr. By comparing these results with other velocities within the Amurian Plate, we found that the observed velocities are intra-plate. Our results are supported by geological evidences and seismological records, which claimed that the Korean peninsula and the Jeju Island are on the same plate. Hence, the Amurian Plate boundary line is considered to lie somewhere to the south of Jeju Island.