Accretionary Wedge, Arc Magmatism and Fluid Migration in Northern Sumatra: Insight From Seismic Attenuation Tomography

The first three-dimensional (3-D) P and S wave attenuation (Qp and Qs) tomography of the crust and upper mantle of the northern Sumatra subduction zone is determined. We adopt an improved calculation scheme to precisely measure attenuation factor t* values from velocity amplitude spectral ratios among different stations that recorded the same earthquake. Our tomographic results show that the forearc accretionary wedge in Sumatra exhibits a significant high-attenuation (low-Q) zone along the trench. The seismic attenuation characteristics of the forearc accretionary wedge are probably influenced by variations in temperature and water content. The middle and upper crust beneath active arc volcanoes shows low-Q and high-Qp/Qs, while the lower crust exhibits less low-Q and low-Qp/Qs, probably reflecting hot volcanic roots with different water saturations from the upper to lower crust. Beneath the Toba volcano that had a super-eruption ∼74,000 years ago, a distinct low-Qp/Qs zone is revealed, which may reflect a transport pathway of fluids and/or local melts ascending from a slab window. The subducting Indo-Australian slab beneath the forearc island chain exhibits a low-Q and low-Qp/Qs belt, reflecting a moderate water saturation probably associated with backthrust faulting. High-Q and high-Qp/Qs zones appear along the slab surface, reflecting large amounts of fluids releasing from the slab dehydration, which may increase pore pressure and cause intense intraslab seismicity.