Global seismicity characteristics of subduction-to-strike-slip transitions

There are at least 30 major plate boundary segments worldwide where the pla te boundary changes from subduction to strike-slip; these include six tripl e junctions and 24 two-plate boundaries. This study investigates earthquake seismicity in the 24 two-plate subduction-to-strike-slip transition (SSST) regions by utilizing recently published earthquake relocations, ternary di agrams of focal mechanisms, and moment rate calculations. To facilitate cro ss-regional comparisons, we categorize the geometry of SSST plate boundarie s in terms of (1) their radius of curvature, (2) their sense of curvature, that is, whether they are convex or concave as viewed from the downgoing pl ate, and (3) their tectonic complexity, that is, the variability of crustal thickness and the segmentation of the plate boundary trace. We observe thr ee main trends in SSST regions: (1) there is a conspicuous scarcity of stri ke-slip earthquakes along plate boundary segments that plate motion models indicate are strike-slip boundaries; (2) in these apparent strike-slip segm ents, both the rate of occurrence of earthquakes of any kind and the moment release rate are low compared to adjacent subduction segments; and (3) the re were few observable differences in seismicity between convex and concave boundaries. The observation that transform zones exhibit moment rate defic iencies, that is, have few large-magnitude earthquakes in the historical re cord, may have important implications for seismic hazard assessment in SSST regions. In particular, is motion along these boundaries aseismic with lit tle seismic hazard, or is motion expressed in very large magnitude, infrequ ent, but potentially devastating earthquakes? In at least three such region s, New Zealand, the Philippines, and the Dominican Republic, paleoseismic e vidence and the historical record of seismicity suggest that very large, in frequent earthquakes do occur.