Intraplate earthquakes and seismic coupling.

Abstract

Over two hundred outer-rise earthquakes are used to investigate the relationship between stresses in the outer-rise and interplate seismic coupling on a global scale. It is shown that the temporal and spatial variations observed in outer-rise seismicity can be explained in terms of the cyclic influences of large underthrusting earthquakes on the regional stresses. In uncoupled subduction zones, only tensional (normal) outer-rise earthquakes occur, which indicates that the outer-rise is dominated by tensional stresses associated with plate bending and/or slab pull forces. In strongly coupled subduction zones, both tensional and compressional (thrust) outer-rise events occur. Tensional outer-rise events follow large underthrusting events as the subducting plate is temporarily in tension due to the underthrusting motions. Compressional outer-rise events occur as compressional stress slowly accumulates oceanward of a locked section of the interplate zone. Compressional stress appears to accumulate in these locked segments through movements in adjacent subduction zone segments. In four instances, compressional outer-rise earthquakes have been followed by large underthrusting events. The occurrence of compressional outer-rise earthquakes suggests that compressional stress is accumulating in the adjacent interplate region and may even be useful for assessing the seismic potential of a region on an intermediate time scale. A similar model can also be applied to events at intermediate depths (40-300 km), downdip of the interplate region. In this case it is suggested that tensional stress will accumulate at the downdip edge of a locked zone due to slab pull forces. Thus downdip tensional earthquakes may be expected to occur below the coupled zone prior to a large underthrusting event. After a large underthrusting event the intermediate depth region may undergo a transition to downdip compressional stress due to the loading which occurs from the incremental motions associated with the underthrusting event. This model is tested using several large events which occurred in the southern Tonga region. While the correlation of intraplate seismicity at intermediate depths with subduction zone processes appears to be more complicated than that found in the outer-rise, intraplate events in the southern Tonga region do suggest that in some instances temporal effects related to the subduction process can influence the stress field at intermediate depths. (Abstract shortened with permission of author.)