韓国における地殻水平歪みと地震活動 Horizontal Crustal Strain and Seismic Activity inKorea
Changes in the crustal strain of Korea have been deduced from trilaration data with EDM performed in 1974-1994 and discussed in terms of regional seismic activity and tectonics. In the survey, 15 % of old triangulation points established in 1910s were reoccupied. Using data on the reoccupied points, strain changes in the last 80 years have been calculated. The average rate of the maximum shear strain (γ<SUB>max</SUB> ) is (0.12 ± 0.06) μ /yr, or about one third of the mean rate in Japan. The mean azimuth of the maximum compression axes (A<SUB>Z2</SUB>) is estimated to be (80.4 ± 13.0) °. This accords well with the P-axis distribution of the focal mechanisms of earthquakes in and around the Korean peninsula. The strain rate in the east Korea is significantly larger than that of the western area. Such a pattern of strain distribution is harmonious with that of seismic activity in Korea both historically as wall as today. Through Japan-Korea GPS joint observations made in 1995-1996, the strain rate in the area from the Yellow Sea coast of Korea to the Japan sea coast of western Japan is deduced as 0.026 μ /yr, or about one fifth of the strain rate in Korea by the ground survey. The strain analysis and the focal mechanism solution also revealed a clear discrepancy in the strain/stress axis between Korean peninsula and western Japan. The difference in the rate ofγ max by the ground survey in Korea and the Japan-Korea GPS can be explained by the largestrain rate in eastern Korea, an area of high seismic activity. The moment along the Tsushima fault is a possible candidate for producing the discontinuity in the strain/ stress axis.