Partitioning of Seismic Moment Release in Tightly-Curved Subduction Zones

We examined mutually-orthogonal components of seismic moment release in the tightly-curved Banda, Caribbean, Hellenic, Mariana, and Scotia subduction zones. The summed seismic moments of energy released within each zone are used to determine the proportion of seismic moment released by three primary modes of slab deformation: down-dip compression or extension, along-strike shortening due to lateral bending that increases the curvature of the subduction zone both at the surface and at depth, and motions orthogonal to the slab surface. We used the focal mechanisms of the Global Centroid-Moment Tensor catalog (CMT) for all slab earthquakes ≥ 4.7 Mw in the study regions. For each event in each slab, we rotated the seismic moments from the N-S, E-W, up-down coordinate system to a coordinate system defined by the local strike and dip of the slab. We then summed the seismic moments in the down-dip, along-strike, and orthogonal to slab surface directions, yielding models of the seismic deformation of each slab in three-dimensions. We find that, as expected, the greatest seismic moment release, 50% of the total, occurs as faulting promoting down-dip motions. Faulting that promotes along-strike bending at these tightly-curved subduction zones releases 15-20% of the total seismic moment budget of these slabs. Faulting that deforms these slabs through slip orthogonal to the slab accounts for 30-35% of the total seismic moment release. This is a somewhat surprising result, indicating that distributed shear on slab faults that strike at high angles to the local strike of tightly-curved subducted slabs is an important process, perhaps serving to thicken slabs as they subduct.