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.