Distribution of magma beneath the Toba caldera complex, north Sumatra, Indonesia, constrained by three-dimensional P wave velocities, seismicity, and gravity data

We estimate the one‐ and three‐dimensional P wave velocity structure beneath the Toba caldera complex, a 30 × 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 low‐velocity 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.