Converted wave imaging of the Toba Caldera, Indonesia

Receiver functions calculated from data recorded by the 1995 PASSCAL Toba Seismic Experiment in northern Sumatra, Indonesia and the permanent station PSI reveal the presence of a low seismic velocity zone at between 8 and 14 km depth, which is interpreted to be the top of a magma reservoir. The Moho under the Toba caldera lies near 30 km with a localized thickened region that reaches 39 km beneath the eastern edge of the caldera with an abrupt shallowing to the northeast. This thickened crust may be where hot gabbroic material underplates the base of the crust to serve as the heat source for shallower volcanism. Alternatively, the thickened crust and Moho offset may be due to localized crustal thickening when this region was situated in a sharp restraining bend of a now extinct segment of the Sumatran fault.