松本盆地南部のＧＰＳ地殻変動観測（1992－2012）

Since 1992, geodetic measurements with GPS have been taken in the southern area of the Matsumoto basin, Nagano Prefecture, central Japan. The Matsumoto basin is located on the Itoigawa-Shizuoka Tectonic Line, which has recently been regarded as a plate boundary between the Eurasian plate and the North American plate. The GPS network of Shinshu University, established along the Itoigawa-Shizuoka Tectonic Line, consists of four observation stations: Shinshu University, Muroyama, Nakayama, and Matsumoto City Hall. The observed data were analyzed using broadcast ephemeris and default meteorological values with the double-difference technique. From the present GPS measurements, it was found that, during the period from 1992 to 2010, the distance between Muroyama and Shinshu University̶about 12km in the east-west direction̶contracted 4 cm (4 ppm). Subsequently, during the three years from 2010 to 2012, the temporal change of the baseline length between Muroyama and Shinshu University showed an extension of 4 cm (4 ppm). In particular, the steep change of the baseline length between them showed an extension of 1.5 cm following the massive Tohoku Earthquake, with a magnitude of 9.0 on March 11, 2011. The distance between Nakayama and Shinshu University̶about 6 km in the north-south direction̶extended 1 cm (2 ppm) during the period from 1992 to 2010. Subsequently, in 2011, it contracted 1.5 cm following the Naganoken Chubu Earthquakes, with magnitudes of 5.4 and 5.1, on June 30, 2011, which struck Matsumoto city. Crustal deformation in the southern area of the Matsumoto basin, Nagano Prefecture during the last twenty years, 1992-2012, is reported and discussed in the present study