季節変動を補正した水準測量データに基づく東海地域の最近の地殻変動

Abstract

Seasonal fluctuation superposed on elevataion change data was found for the precise levelling along the northwest-southeast trending route of about 25km length between Kakegawa and Hamaoka in the Tokai area, central Japan. The seasonal change amounting 17mm in height difference between the end points exhibits the peak and trough in late January and late July, that is apparently correlated with the change in atmospheric temperature. Correcting the seasonal effect, we obtained a reliable data set of elevation change for the levelling route, along which measurement was repeated thirteen times since 1962.It has been known that the area is subjected to general subsidence and eastward tilt due to westerly subduction of the Philippine Sea plate from the Suruga Trough. The corrected data, however, revealed more detailed features of the recent crustal movement: (1) A rapid upheaval amounting 2cm at Kakegawa against Hamaoka took place probably in June 1976; and (2) except for the special event, Hamaoka referred to Kakegawa has constantly subsided at a rate of about 4mm a year. The constant rate of subsidence strongly suggests that shallow portion of the continental plate is subject to steady dragging by the subducting Philippine Sea plate. The abrupt crustal deformation in 1976 is interpreted by occurrence of creep dislocation at a deeper portion of the plate interface. Anomalous phenomena similar to the 1976 event may indicate relatively high risk of the pending Tokai earthquake as the deep-seated creep is expected to produce additional shear stress in the hypothetical rupture zone.