Estimation of Near-Surface Density using Vertical Gravity Gradients in Central and Western Japan

The Vertical Gravity Gradient (VGG), derived from the difference between terrestrial and airborne gravity data, highlights shallow density contrasts. We estimated the near-surface density structure of Central and Western Japan using VGGs derived from terrestrial data within 3 km of airborne flight lines. We constructed an inversion model on a 1/7-degree grid to estimate surface density and a regional VGG trend, imposing spatial continuity con-straints on the trend. Hyperparameters were determined by minimizing the Akaike Bayesian Information Criterion (ABIC). The resulting model clearly identifies low-density zones in major plains, such as Kanto and Echigo, and distinct low-VGG anomalies along the Itoigawa-Shizuoka Tectonic Line (western margin of the Fossa Magna) and the Osaka-Lake Biwa region. The estimated surface density showed a weak correlation with seismic S-wave velocity models.