Geophysical Monograph Series American Geophysical Union

Surface-ship and pendulum gravity measurements have been combined in new free-air gravity anomaly maps of parts of the northwest Pacific Ocean basin and its margin. The most promi­nent features of these maps are the broad belts of positive anomalies which occur seaward of deep-sea trenches and landward of island arcs at the margins of the northwest Pacific basin and the broad belts of positive and negative anomalies which flank the eastern end of the Hawaiian ridge in the interior of the basin. The positive anomalies seaward of trenches can be easily isolated from the longer wavelength regional gravity field of the northwest Pacific upon which they are superimposed. These studies show that (1) positive free-air gravity anomalies of about +40 to +50 mgal occur seaward of trenches and (2) negative residual gravity anomalies of about -10 mgal occur over the Pacific basin, except over Hawaii and the Gulf of Alaska where they are positive.