Abstract
Maps of free air and Bouguer gravity anomalies in the western Pacific are compiled on the basis of the sea gravity data obtained during a period 1963-1980 by use of the Tokyo Surface Ship Gravity Meter.
Outer gravity high seaward of trench and negative gravity moat around seamounts can be interpreted as caused by anomalous thickness of the lithosphere.
Mutual interference between trench and seamount or rise is discussed from the view point of thickness of the lithosphere. Seamounts or rises can be classified into three types;
1) seamount which can easily subduct as represented by Kashima No. 1 Seamount,
2) sizable seamounts which take a long time to subduct,
3) rises which will never subduct.
It is suggested that a new subduction zone seaward of the old trench is formed if the rises of the type 3 encounter a trench. In such a case a relic of trench and a fore-arc ridge may be left in topography. After seamount of type 2 eventually subduct, they give rise to an acute bending of trench accompanied by a zone of negative gravity anomaly in the landward side of the trench.
