Seismic response to slab rupture and variation in lithospheric structure beneath the Savu Sea, Indonesia

Variations in seismic moment release and stress state across the transition from subduction of oceanic crust to arc–continent collision in the Banda Arc are constrained by focal mechanism solutions from the CMT earthquake catalogue. In particular the slab under the western Savu Sea is unusual in that intermediate depth (70–300 km) events indicate that at this depth range the slab is largely in down-dip compression. This contrasts with the intermediate depth, down-dip tension that typifies the Sunda slab to the west and the far eastern Banda slab to the east. Down-dip compression beneath the Savu Sea reflects subduction of transitional crust of the Scott Plateau, more buoyant than the Indian Ocean crust subducting further west. In this region, enhanced magma flux is indicated by unusually narrow volcano spacing in the overlying arc, and suggests that down-dip compression reflects not only more buoyant transitional crust but also a reduction in slab–wedge coupling induced by enhanced magma flux. East of the Savu Sea, the near complete absence of intermediate depth seismicity is attributed to a slab window that has opened where Australian continental crust has collided with the arc. Differences in seismic moment release around this slab window indicate asymmetric rupture, propagating to the east at a much faster rate than to the west.