Mapping the Crustal Magmatic System and Regional Deformation of the Toba Region by Multiparameter Ambient Noise Adjoint Tomography

Abstract

In this study, we develop a multiparameter ambient noise adjoint tomography method, and apply it for the first time to image the crustal magmatic system and regional deformation of the Toba region. Using Rayleigh and Love waves at periods of 5–20 s extracted from ambient noise, we construct a new multiparameter 3D crustal model that includes shear-wave velocity, radial and azimuthal anisotropy. The isotropic component of our model reveals (a) over 30% Vs reductions beneath the Toba caldera with a melt fraction ranging from $14.5\%$ to $18.5\%$, and (b) two low Vs bodies located in the middle crust (10–20 km) beneath the Helatoba volcano and the upper crust beneath the Lubukraya volcano, suggesting a large transcrustal magmatic mush model beneath this volcanic arc region. Our anisotropic model shows $>10\%$ positive radial anisotropy $\left(V_{sh}>V_{sv}\right)$ in the middle crust of the volcanic regions, indicating the presence of horizontally layered melt sills. In the upper crust, we find predominantly weak negative radial anisotropy and significant azimuthal anisotropy, suggesting subvertical rock fabrics dominate upper crustal anisotropy. The orientation of fast velocity directions (FVDs) mostly aligns with the Sumatran Fault due to fault fabrics resulting from shearing deformation along the plate boundary. In the Tarutung region with rich geothermal resources, FVDs shift to being fault-perpendicular probably due to the alignment of stress-induced, fluid-rich microcracks. Our study provides new insights into crustal magmatic architecture and deformation regimes of the Toba region shaped by regional tectonics and magmatic processes.