THE EFFECT OF SEAMOUNT SUBDUCTION ON SEISMIC COUPLING

It has long been speculated that the subduction of seamounts may have profound mechanical consequences, particularly on the generation of la rge subduction zone earthquakes. We consider this problem as a special case of the seismic coupling model of Scholz and Campos. The model pr edicts that the subduction of a large seamount will increase the norma l stress across the. subduction interface and hence will enhance seism ic coupling. In the case of coupled arcs, this increased coupling wilt result in a large increase in the recurrence intervals of earthquakes . Such events will therefore appear rare in the instrumental record, a s is observed. For decoupled arcs, the subduction of a large seamount should cause them to become locally coupled. Three of the latter local ities have been found with satellite altimetry data, two on the Tonga- Kermadec Trench and one on the Izu-Bonin Trench. The diagnostic featur e is a shallowing or obstruction of the trench axis adjacent to a chai n or group of large seamounts on the subducting plate. In each case, t he small to moderate plate-interface seismicity typical of decoupled s eismic zones is almost completely absent in a 100-200 km are segment, indicating that the subduction zone is locally locked, as predicted by the model. The occurrence of large interplate earthquakes at both Ton ga sites confirms that they are seismically coupled.