Intraplate deformation in the Samoa-Gilbert-Ralik area: A prelude to a change of plate boundaries in the Southwest Pacific?

Abstract

Intense intraplate seismic activity has recently been identified in the west-central Pacific, along a line extending from Samoa through the Gilbert Islands and the Ralik Fracture Zone to the Caroline Islands. Over the past 50 years, this seismic line has contributed an estimated 15-30% of the entire intraplate seismicity for the Pacific plate (Hawaii excepted). Focal mechanisms available for ten of the largest events during the 1981-1983 Gilbert Islands swarm, with horizontal compressional stress oriented at right angles to the direction of motion of the plate, cannot be reconciled with simple models of intraplate stresses. We speculate that they express large scale deformation of that part of the Pacific plate, related to incipient detachment of a microplate and relocation of the Pacific-Australian subduction zone some 1000 km to the north of its present position.