Fig. 5: A schematic model of the magmatic system below Axial Seamount.

The geometry of asymmetric magma plumbing and melt fractions in the upper crust is based on full waveform inversion models in Fig. 2 and estimated melt fraction in Fig. 4. The distribution of stacked sills in the mid-to-lower crustal melt-mush feeder conduit is according to seismic reflection images in ref. ¹². The Moho depth, west-dipping mantle flow (dashed streamlines) and ~2% melt (dark red) in the upper mantle are based on refs. ^4,54,78. Green, blue, and cyan polygons on the seafloor indicate 1998, 2011, and 2015 lava flows, respectively. The red star indicates the ___location of a revised prolate-spheroid inflation source for the 2015 eruption⁵², and the magenta cycles denote the hydrothermal vents. Black lines denote eruptive fissures identified during 2011 and 2015 eruptions. The yellow shaded area illustrates the magma throat beneath the eastern caldera wall that connects the main magma reservoir (MMR) to layer 2A. The magenta shaded area indicates the diking path constrained by seismicity¹⁸. A thermally controlled permeability boundary layer separates the MMR from hydrothermal fluid circulation (cyan dashed lines with arrows) in the dike and lava section above¹³. White dashed lines denote the isotherms. WMR: western magma reservoir. SMR: secondary magma reservoir. SSMR: shallow secondary magma reservoir.