The Investigator Fracture Zone (IFZ) subducts beneath Toba Caldera, the Earth's largest Quaternary caldera, in northern Sumatra, suggesting a possible relationship between them. Locations of sub‐crustal earthquakes based on arrival times of P and S waves at a seismograph network surrounding Toba reveal the geometry of the subducted slab and the IFZ beneath Toba. A vertical tear of less than 20 km in the slab across the IFZ, as previously suggested, cannot be ruled out but the large‐scale geometry of the slab is dominated by a broad bend of slab contours parallel to the concaveseaward indentation of the trench. The slab shape is probably a response to the trench curvature, can explain the change in trend of the volcanic arc near Toba, and may cause shallowing of the forearc basin near Nias Island. The decrease in radius of curvature of the slab contours is not accompanied by an observable decrease in dip angle, possibly resulting in lateral compressive stress in the slab. The high rate of seismicity along the subducted Investigator Fracture Zone, that intersects the slab obliquely to its plunge direction, is uncommon at subducted fracture zones and is likely caused by such lateral stress in the slab.

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We estimate the one-and three-dimensional P wave velocity structure beneath the Toba caldera complex, a 30 Â 100 km topographic depression in North Sumatra, using arrival time data of local earthquakes recorded by a 40-station seismic network that operated for 4 months. Inversions reveal the presence of P velocities up to 37% below normal that likely map the distribution of magma within this subduction-related volcanic system, considered the world's largest. In the upper 10 km of crust the largest low-velocity region underlies the southern two thirds of the depression and coincides with a gravity low centered over the resurgent dome. A smaller volume of low velocities is observed in the upper crust under the north end of the depression. Separating the two regions is a zone of locally high velocities, indicating that the shallow, subcaldera magma system is composed of two separate reservoirs, not a single one that extends the entire length of the caldera complex. Above each lowvelocity region is a postcollapse volcano that erupted mostly mafic lavas after the last major caldera collapse $74 kyr ago. A low-velocity column below one of these volcanoes can be traced into the uppermost mantle and corresponds with a planar distribution of low-frequency earthquakes in the 20-to 40-km-depth range. The low-frequency earthquakes apparently record the migration of melt in the mafic roots of this large-volume, crustal magma system.

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[1] For the first time, ambient noise tomography is used to clearly image the magma chamber beneath Lake Toba caldera, one of the largest Quaternary calderas on Earth. Using data from 40 seismic stations deployed between May and October 2008 around Lake Toba, empirical Green's functions are extracted from long term cross-correlations of continuous records. These functions are dominated by Rayleigh waves, whose group velocities can be measured in the period range from 2.5 to 12 seconds. Arrival times of these waves are picked for a given period and inverted using 2-D tomography to calculate lateral variations in velocity for the given period. This was done for six different periods, which all correspond to different sampling depths. Thus the six 2-D models presented together provide information on velocity variations with depth. The results show a low-velocity body coincident with the Lake Toba caldera, representing the magma chamber under the volcano. The chamber is observed to have a complex 3-D geometry, with at least two separate sub-chambers underlying the caldera. Other results include a deep low velocity body, possibly another magma chamber, south west of the lake with an upper limit of ∼7 km depth. The maximum depth to which this body reaches could not be resolved. The Sumatra Fault marks a velocity contrast, but only down to depths not greater than 5 km. The reliability of the results was further confirmed by checkerboard recovery tests. Citation: Stankiewicz, J., T. Ryberg, C. Haberland, Fauzi, and D. Natawidjaja (2010), Lake Toba volcano magma chamber imaged by ambient seismic noise tomography, Geophys.