Relationship between shallow and deep seismicity in the western Pacific region

Abstract

The two types of relationship between the shallow and deep seismic activities in island-arc regions are described: (1) A great shallow earthquake in a region is preceded and sometimes followed by the marked increase of deep seismic activity in the same down-dip seismic zone perpendicular to the trend of an arc structure. The increase of the deep seismic activity before the great shallow earthquake cannot be understood as an accidental event nor as a mere trigger of the great shallow earthquake, but as an essential forerunning phenomenon of it. This regularity is found in the Kurile-Kamchatka and northern Japan island-arc regions, particularly near the ends of these two arcs and their junctions, where great shallow earthquakes frequently occur. (2) The seismic activity tends to progress in time from a shallow to a deep region within a descending lithosphere. This regularity is found in the Mariana and Tonga arcs, where the seismic activity is nearly continuous from the surface to the deep region and no very great shallow earthquake occurred in the considered period. The rate of migration in these earthquakes along the descending lithosphere is estimated at about 50 km per year, nearly similar to that in shallow earthquakes. These regularities in the space-time distribution of shallow and deep earthquakes in island-arc regions can be explained from the viewpoint that the oceanic lithosphere is pushed downward under the continental lithosphere by external forces, probably due to the mantle convection. The present discussion suggests that the descending lithosphere does not act as a rigid plate but as a visco-elastic body under gradually increasing loading.