日本列島の重力異常陰影図

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Undulations of gravity anomalies are caused by horizontal and vertical variation of subsurface density. Distributions of gravity anomalies have been generally presented by contouring. Contouring method, however, is not suitable for representing fine undulations in such case that their amplitudes are less than a contour interval. For the purpose to display detailed gravity anomaly undulations, we employed a shaded relief map method and applied to gravity datasets of the Japanese Islands.In order to extract gravity anomalies due to crustal structures, contribution due to subducting plates was subtracted from observed ones. The contribution was calculated by using a 3D subducting plate model. We call the resultant gravity anomalies as Slab Residual Bouguer Anomalies (SRBA) here. By using SRBA datasets, shaded relief maps were generated.These shaded relief gravity anomaly maps indicate detailed variations in gravity anomalies like satellite images. In the maps we can recognize many lineament-like patterns. Although some of them correspond to known geologic faults, many of them do not. These patterns presumably indicate the existence of “hidden” faults which are covered by Quaternary sediments or volcanic products.Descriptions of characteristic patterns which appeared on the shading relief maps and their geological and geophysical implications will be discussed in the next paper.