Horizontal Crustal Movements Revealed by Geodetic Measurements

-Applications of a New Method for Estimating Displacement Vectors-

Abstract

A geodetic network adjustment method under the constraint condition using the maximum shear strains has been developed as a new method for estimating displacement vectors. This adjustment method enables fixation of a network automatically without prior physical models or distortion of the network. Moreover, this method can be applied to networks on the ellipsoid and allows for the free selection of points which are under constraint. This method was applied to a small GPS network around Unzen Volcano and a 12 cm westward movement of the point on Mt. Fugendake was detected between November 1990 and February 1991. The method was also applied to the nationwide first order triangulation network of Japan using the data from the old first order triangulation (1882-1909) as well as the Primary Precice Geodetic Survey (1973-1989). As a result, crustal movements which are considered to have been caused by the plate motions were detected and a difference in patterns of displacement vectors was found between the northeastern and southwestern parts of the Japanese Islands. Furthermore, by converting the displacement vectors to those relative to a selected fixed point, crustal movements were studied in regions where a plate boundary or a tectonic line is assumed to exist. The results show notable features in crustal movements in the vicinity of the Suruga Trough in the Tokai district, the Itoigawa-Shizuoka Tectonic Line, the Oga-Ojika Tectonic Line in the Tohoku district and the Beppu-Shimabara Graben in Kyushu.