11:30 AM - 11:45 AM
[SIT15-09] Subslab hot upwelling, slab window and Toba volcano super-eruption
Keywords:Sumatra, slab, volcano, mantle upwelling
Seismic and volcanic activities are very intense in Sumatra, where a huge earthquake (Mw 9.2) took place in 2004, and the Toba volcano had a super-eruption about 74,000 years ago. Previous seismic studies have suggested that slab tearing occurs beneath North Sumatra, but the slab morphology and structure were poorly constrained. Due to the lack of resolution on the slab, the presence of a slab tear has not been validated, and the main factors responsible for slab tearing are hard to discern. In this study, we use a large number of P-wave travel-time data of local and teleseismic events recorded at 19 permanent and 147 portable seismic stations to determine a high-resolution 3-D tomographic model of the crust and upper mantle beneath North Sumatra. Our results reveal a well-constrained low-velocity (low-V) anomaly in the subducting slab, which reflects a slab window. Several subslab low-V zones are imaged, one of which is connected with the slab window. Our relocated earthquakes show that no intra-slab event occurs in the slab window. In contrast, a protruding cluster of seismicity appears along the subducted Investigator Fracture Zone, indicating that high reliefs of the Investigator Fracture Zone possibly enhance the coupling between the subducted slab and the overriding plate. Shear stress resulting from the differential coupling in the slab may have created an area of mechanical weakness in correspondence of the fossil-spreading ridges, which eventually led to the formation of the slab window. This window allows the hot upwelling material below the slab to enter the mantle wedge through the slab, which merges with the corner flow in the mantle wedge and so led to the super-eruption of the Toba volcano. The previously damaged parts (e.g., fracture zones and spreading centers) of the oceanic plate could help promote the growth of the slab window.