SEISMIC COUPLING AND OUTER RISE EARTHQUAKES

Abstract

Variation of interplate seismic coupling at subduction zones is a major factor controlling the size of the largest underthrusting events. This variation also has a profound effect on the regional intraplate stresses in the vicinity of the subduction zone. Outer rise seismicity is strongly correlated with variations in interplate coupling, reflecting the stress state of the interplate coupled zone. Over 200 outer rise earthquakes with known focal mechanisms are used to investigate the relationship between stresses in the outer rise and interplate seismic coupling. These events occur within the downgoing (i.e., oceanic) plate near the bathymetric trench axes and generally fall into the categories of tensional (normal) or compressional (thrust) with their tensional or compressional stress axes oriented approximately horizontal and perpendicular to the trench. In uncoupled subduction zones, only tensional 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 outer rise events are found. These events are related both spatially and temporally to the distribution of large underthrusting earthquakes and are thus an integral part of the earthquake cycle. In the strongly coupled regions, tensional outer rise events follow large underthrusting events as the outer rise is temporarily in tension due to the underthrusting motion. Compressional outer rise events take place as compressional stress slowly accumulates oceanward of locked sections of the interplate zone. In four instances, compressional outer rise earthquakes have been followed by large underthrusting events which have occurred 2, 4, 7, and 19 years after the associated outer rise event. The remaining compressional outer rise events are located in regions that are either known seismic gaps or in regions where the seismic potential is unknown. The occurrence of compressional outer rise earthquakes suggests that compressional stress is accumulating in the adjacent interplate region and that there is the potential for a future large underthrusting event in the region. Thirty compressional outer rise events have been located in trench segments of Middle and South America, the Kurile Islands, the Tonga and Kermadec islands, the New Hebrides Arc, and the Solomon Islands regions. In both the southern Kamchatka and northern New Hebrides regions the outer rise seismicity indicates that the stress regimes in the outer rise have changed with time from tensional, following a previous large underthrusting event, to compressional at present. Thus three stages of the cycle from underthrusting to tensional outer rise regime to compressional outer rise regime are present, requiring only the occurrence of the next underthrusting event to complete the cycle. The occurrence of compressional outer rise events is useful for assessing the seismic potential of a region on an intermediate time scale.