• Constrained seismic velocity, density, and basalt content at and below the 660-km discontinuity (d660) by S660S waveform inversions
• Found that slabs exhibit a smaller impedance jump across the d660 but a steeper gradient below the d660 compared to other regions
• Suggested that basalt accumulates at the d660, especially in subduction zones, but decreases significantly below it, forming a harzburgite-enriched layer