Horizontal Crustal Movements in Hokkaido and its Tectonic Implications

Abstract

Displacements and strains during the recent 80 years in Hokkaido are investigated by the analysis of triangulation or trilateration data. According to the distribution of displacements and strains, Hokkaido can be separated into three blocks, eastern, middle and southwestern Hokkaido.
In eastern Hokkaido, northwestward displacements and NW-SE contractions are dominant before the 1973 Nemuro-Oki earthquake, while seaward displacements and EW extensions are detected after the event. Summing up them, westward displacements are left, which suggests westward sliding of the outer Kurile arc along the volcanic front and colliding at the Obihiro Tectonic Line [TADA and KIMURA (1987)].
The Ishikari Low Land, which is considered to be a relic trench, may be the boundary between middle and southwestern Hokkaido along which E-W contractions are prevailing. This observation suggests that northern Honshu would not belong to the North American or the Okhotsk plate.
Displacements associated with four major earthquakes which occurred during the period between the first and the last surveys are computed. Results show that the 1940 Shakotan-Oki earthquake may not give much influence on Hokkaido. Theoretical displacements computed by models for the 1952 Tokachi-Oki (M8.1) and 1968 Tokachi-Oki (M7.9) earthquakes are inconsistent with the observation, which may require re-analyses of data. The model for the 1973 Nemuro-Oki earthquake can predict the observed displacements qualitatively. The observed displacements are greater than the computed ones by a factor of two or three, which means that old triangulation surveys might suffer from scale errors.