Subducted lithosphere beneath the Kuriles from migration of PP precursors

Abstract

We seismically image both thermal and chemical heterogeneity of the mantle beneath the Kurile subduction zone using P-wave energy reflected from the underside of discontinuities, arriving as precursors to the seismic phase PP. We take advantage of new broadband seismic data provided by the High Lava Plains Seismic Experiment and EarthScope's USArray, collecting a dataset of 31 high-quality Sumatran earthquakes sampling beneath the Kuriles. We employ high-resolution array analysis techniques, including migration and vespagrams, to identify precursory arrivals and study lateral variations in discontinuity depth, sharpness, and impedance of the mantle transition zone. We find the 410 km boundary is at 395 km near the subducting Kurile slab, though the boundary is 410–425 km deep elsewhere. In regions away from subduction, we do not detect a laterally continuous underside reflection of P-waves from the 660 km discontinuity. However, in the vicinity of the subducting Kurile slab, we detect robust P660P reflections from interfaces near 620–670 km depth, signifying an increase in the impedance contrast at 660 km depth. We also detect deeper reflectors, down to 720 km depth, beneath the Kurile slab in a localized area. Cold, aluminum-depleted harzburgitic lithosphere residing at the base of the transition zone best explains the local enhancement of the 660 km discontinuity P-wave impedance contrast. Our new discontinuity measurements support the hypothesis of cold, depleted lithosphere stagnating at the 660 km discontinuity beneath the Kuriles subduction zone, and imply the 660 km boundary can locally impede mantle flow and produce chemical heterogeneity within the transition zone.