Ultra-low-velocity anomaly inside the Pacific Slab near the 410-km discontinuity

Abstract Under equilibrium conditions, the mineral olivine (α) transforms to wadsleyite (β) near 410 km depth1,2. The phase transition is a global boundary known as the “410-km discontinuity”, where seismic wave speed and density increase by 3–10 percent3-6. We present new observations of triplicated P-waves from dense seismic arrays in northeastern China that constrain the structure of the Pacific slab near the 410-km discontinuity beneath the northern Sea of Japan. Our analysis of P-wave travel times and waveforms at periods as short as T = 2 s indicates the presence of an ultra-low-velocity layer within the cold slab, with a P-wave velocity that is at least 20% lower than in the ambient mantle and an apparent thickness of ~ 16 km along the wave path. This ultra-low-velocity layer could contain unstable material (e.g., poirierite) with reduced grain size where diffusionless transformations are favored7-12