Slab residual gravity anomaly: gravity reduction due to subducting plates beneath the Japanese Islands

Abstract

Gravity contributions due to subducting plates (slabs) beneath the Japanese Islands were calculated using a three-dimensional slab model. The slab model is constructed from the depth of deep seismic planes. A density contrast between the slabs and the surrounding asthenosphere is assumed to be 0.065 g/cm 3 and the thicknesses of the Pacific and the Philippine Sea plates are postulated to be 90 km and 40 km, respectively. Some other cases, such as the different plate thicknesses, the different density contrasts, layering structure within the slab, etc., are also examined in the course of the present study. The belts of the highest gravity contributions appear along the eastern and southern coasts of Japan. The contributions gradually decrease toward the Japan Sea, with the deepening of the subducting plates. The gravity highs reach over 220 mGal in the northeastern and over 80 mGal in the southwestern Honshu (main island of Japan), respectively. The Slab Residual Gravity Anomaly ( SRGA) is obtained by subtracting the slab contribution from the observed gravity anomaly. The SRGA represents the gravity contributions mainly due to the crustal structures. The SRGA over the Japanese Islands reveals a stronger negative correlation between topography and the gravity anomaly.