地殻構造調査結果を反映させた重力モデルにょる西太平洋海域の地殻の密度構造

摘要

A three-dimensional crustal density structure model of the western Pacific area is constructed using detailed marine gravity data, as well as 19 lines of seismic crustal structure data collected by the Hydrographic and Oceanographic Department, Japan Coast Guard during Japanese continental shelf surveys. The model consists of five layers: seawater, sediments, upper crust, lower crust and mantle, with densities of 1.03,2.3, 2.4, 3.0 and 3.3 g/cm~3,respectively. The boundaries between the seawater and the sediments and between the sediments and the upper crust are determined using existing bathymetric and sediment thickness data. The differences between the observed free air anomalies and the gravity anomalies in the initial five-layer model are divided into three components by their wavelengths: the density inhomogene-ity in the mantle, variations in depths of the Moho and the top of the lower crust. The depths of the top of the lower crust and the Moho are obtained with an inversion calculation from their anomaly contributions. According to the results, most large seamounts are associated with Mo-hos of about 15 ㎞ or greater in depth. However, some have almost no Moho lows below them, and others are even located over Moho highs. Fujibakama and Kaede Escarpments and their southeastward extensions are clearly visible in the free air gravity anomaly map, and steps in the top surface of lower crust layer of a few hundred to several hundred meters in height are obtained as results of the model calculation.