Seismic anisotropy reveals a large magmatic sill complex below the Toba Caldera

An understanding of the formation of large magmatic reservoirs is a key issue for the evaluation of possible strong volcanic eruptions in the future. Many geological observations of exposed past volcanic systems and geodynamic models have indicated that large magmatic reservoirs can build up over long periods of time, with small increments that promote vertical dykes and horizontally oriented sill intrusions. The typical size of such intrusions beneath presently active volcanoes is too small to be imaged with most geophysical methods. Here we show that large layered intrusion complexes at depth can be detected by measurements of the seismic anisotropy caused by the fine-scale layering. We have used data obtained from 42 stations between May and October 2008 on the Toba caldera complex (north Sumatra, Indonesia) to construct a 3D seismic model of the crust with using ambient noise tomography method. Approximately ~500 Rayleigh and Love waves were extracted from cross correlations of continuous records. Their group velocities were measured at periods between 5 and 19 s and, after a 2D regionalization, inverted into local 1D shear velocity profiles (V_SH from Love and V_SV from Rayleigh waves) using a Monte-Carlo method based on the Neighborhood Algorithm. All 1D profiles were combined into a final 3D model that shows a strong radial anisotropy below the Toba caldera at depths greater than 7 km. A plausible explanation of these observations is the presence of a large magmatic sill complex in the crust below 7 km in depth. Our data support the concept of the long-term incremental building up of magma bodies that leads to the largest volcanic eruptions.