Long-Term Gravity Variation Observed by an Absolute Gravimeter with a Rotating Vacuum Pipe

Abstract

Since July 1994, we have made long-term observations to detect secular gravity changes by an absolute gravimeter with a rotating vacuum pipe (AGRVP) at the Esashi Gravity Station (EGS), Iwate Prefecture, Japan. The AGRVP can measure absolute gravity values every 15 minutes with a standard deviation of about 2 × 10⁻⁷ms⁻² (20μGals). We compared mean values of more than 400 data obtained by AGRVP at EGS for different periods from July 1994 and found that gravity values increased gradually except for the period from January to February 1995. The Esashi Gravity Station is located within a zone where the Pacific plate is subducting under the Eurasian or the North American plate. Precise leveling data, sea level data and finite element models suggest that there is a subsidence at a rate of several millimeters per year in the area of Pacific Ocean side of the northern part of Honshu. The observed mean gravity values, on the other hand, increase at a rate of about 1.0 × 10⁻¹⁰ms⁻²/day (3.6 × 10⁻⁸ms⁻²/year). The rate of 3.6 × 10⁻⁸ms⁻²/year (3.6μGals/year) is equivalent to crustal subsidence at the rate of about 11mm/year if we assume the height variation only takes part in the gravity change with the free air gradient.