Relation between tectonic stress, great earthquakes and earthquake swarms

Abstract

Along the northern boundary of the Philippine Sea plate are a remarkable earthquake swarm area and several epicenters of great earthquakes which occurred in the past. The temporal variation of the swarm activity shows a remarkable relation to the occurrence of great earthquakes: the swarm activity falls off when a great earthquake occurs at the boundary between the Philippine Sea plate and the continental block that includes the swarm area, but it rises when a great earthquake occurs at a boundary between the Philippine Sea plate and one of the other continental blocks. On the basis of the detailed mechanism studies on these great earthquakes, a simple model is constructed to explain this relation between the swarm earthquake and the great earthquakes. In this model, several independent crustal blocks are opposing the motion of the Philippine Sea plate; each block is stressed by the motion of the Philippine Sea plate through a friction-coupled interface where great earthquakes are supposed to occur. When a great earthquake occurs at the boundary between the plate and the crustal block that includes the swarm area, the stress in this block is released and the swarm activity falls off. On the contrary, the stress in this block increases when a great earthquake occurs, as a rebound, at the boundary between the Philippine Sea plate and one of the other crustal blocks, because such rebound removes the force opposing the motion of the Philippine Sea plate, propels its motion, and therefore increases the stress in the block that includes the swarm area. Since the rise of the activity is found to precede the earthquake, probably because of some premonitory creep-like deformation at depths, a practical means for predicting an impending great earthquake in this region may be provided by monitoring the swarm activity.

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