抄録
Decay of a magnetic anomaly pattern to the north of the Nankai Trough, off Shikoku, Japan is discussed based upon studies of the seismic structure of the oceanic crust of the region. A simple subduction model of the lithosphere with a temperature gradient in the crust to reduce the resultant amplitude of the total magnetic field anomaly on the sea surface is not sufficient to explain the large decay rate of the total force anomaly versus landward distance from the Trough axis. Assumptions are made that some kind of destruction of the topmost part of the magnetic basement is causing the rapid decay. Effects of the geometrical disordering of the magnetic layer to reduce the resultant field intensity is studied by model experiments. Several models were tested to conclude that the destruction of the topmost part (ca. 1km) of the oceanic basement, the magnetic source of the anomaly field, may affect the decay rate in two ways; i. e., randomization of natural remanent magnetization as well as the subduction of the rest of the nondisturbed magnetic basement. Subduction of the Philippine Sea plate along the Nankai Trough area and strong friction may enhance the destruction of the magnetism. Some of the present-day destruction and subsequent dismembering of the part of the oceanic basement seem to be occurring at intermediate depths between the crust and subducting lithosphere.
