Spatial seismicity variations along convergent plate boundaries

Summary. Five major convergent plate boundaries (South America, Izu–Bonin–Marianas, New Hebrides, Tonga–Kermadec and Indonesia) show strong variations in levels of background seismicity on scales ranging from tens to thousands of kilometres. These variations were tested using two statistical approaches and we conclude that small earthquakes are not distributed randomly along these zones. Two types of large‐scale seismicity variations (termed first order) were recognized. First, regions with dimensions on the order of 100 km with extremely high seismicity levels (first‐order actives). One such region was recognized in each of the zones studied. Second, large‐scale (500 to several thousand kilometres) differences in the level of background seismicity along a given plate boundary. Regions with consistent levels of background seismicity are termed first‐order segments. We examined each first‐order segment for smaller scale variations. Ten regions ranging in length from 40 to 170 km with anomalously high seismicity levels were recognized. Fifty‐three regions ranging in length from 25 to 355 km were found to have anomalously low seismicity levels. Thus, areas with anomalously high levels of activity are rare in subduction zones. These observations indicate that background seismicity in subduction zones is not randomly distributed along the strike of the zones. It seems likely that the observed variations reflect tectonic differences. In fact, many of the seismicity variations which we observed appear to be spatially related to features on the seafloor or on the overriding plate. If this is so, then they may provide a powerful tool for characterizing subduction zones and understanding the mechanisms of earthquake generation.