Gravity field around Japan — sea gravimetry by the Geological Survey of Japan

Abstract

Abstract Using a LaCoste & Romberg sea gravimeter, more than 60,000 gravity measurements were obtained from 11 research cruises around Japan, as conducted by the Geological Survey of Japan from 1974 to 1979. From the misties∗ of free‐air anomalies the r.m.s. accuracy of the measurements was estimated to be 2.8 mgal (with the satellite navigation system and an EM‐log) and 0.7 mgal (with a Doppler sonar system). From this result it was suggested that the observed gravity error was mainly due to the ship's velocity measurements. Mean free‐air anomalies of 15’ × 15’, 30’ × 30’, and 1° × 1° were calculated using these measurements and compared with the previous data A free‐air anomaly map in the area of 22°N to 46°N and 123°E to 148°E was compiled based mainly on these data. The map fully covers four island arc and trench systems—the Northeast Japan, the Ogasawara (Bonin), the Southwest Japan, and the Ryukyu Arc and Trench—and they are associated with paired belts of large‐amplitude positive and negative anomalies. There are broad positive anomaly belts landward of the trenches, which are due to the high‐density subducting Iithospheric slabs. Seaward of the active volcanic belts, belts of large‐amplitude positive anomalies are observed. The basement in these areas seems to be uplifted, and the crust has a higher mean density. Negative anomalies are observed at the margins of the Japan Basin and the Yamato Basin. These anomalies are due to the edge effect of the roots of the neighboring continental or subcontinental crust. Isostatic anomalies in these basins are probably almost constant.