Obliquity limit of megathrust earthquakes

Xiaochuan Tian , Magali Isabelle Billen

The largest earthquakes occur on subduction-zone megathrusts, where plates commonly converge obliquely to trenches and generate trench-parallel strain. How this strain is partitioned between megathrust slip and upper-plate deformation remains uncertain, hindering earthquake recurrence estimates. Here we combine global slab geometry, plate motions, and earthquake focal mechanisms to quantify convergence and megathrust slip obliquities. The results show that megathrust earthquakes slip less obliquely than plate convergence, but rarely normal to the trench, so common end-member assumptions misrepresent strain accumulation. To explain this pattern, we develop a work-minimization model of strain partitioning that predicts dip-dependent limits on megathrust slip obliquity and preferred slip directions. This framework provides a mechanical basis for estimating strain accumulation on megathrusts undergoing oblique convergence, with implications for seismic-cycle and terrane-translation models.