Depth of the 660-km discontinuity near the Mariana slab from an array of ocean bottom seismographs,

[1] High frequency records of deep Mariana earthquakes from a dense array of ocean bottom seismographs deployed in the Mariana arc and back-arc regions are stacked and searched for the phases P660p and S660p to constrain the depth of the 660-km discontinuity near the Mariana slab. Results of the high-resolution study suggest that around 18°N the 660-km discontinuity lies at about 710-730 km (±14 km) depth within or in the vicinity of the slab core. In the region seismicity ceases at $620 km depth. This implies that, although tomographic images show the Mariana slab penetrating into the lower mantle, deep seismicity in the region terminates$100 km above the base of the transition zone. These findings and similar observations in Tonga argue that factors other than the phase transition at the base of the upper mantle may control the maximum down-dip extent of the deep seismogenic region in the slab

Double seismic discontinuities at the base of the mantle transition zone near the Mariana slab

[1] We use P-to-S converted phases from teleseimic data recorded at island and ocean bottom stations in Mariana to investigate the upper mantle structure in the region. We find evidence for double seismic discontinuities at the base of the transition zone near the Mariana slab. A shallower discontinuity is imaged at depths of $650 -715 km and a deeper interface lies at$740 -770 km depth. The large lateral extent at near constant depths for both features is consistent with horizontal interfaces rather than small-scale scatterers. The amplitude ratios of the seismic signals suggest that the shear velocity contrast across the two interfaces is comparable. These characteristics support the notion that the discontinuities are the results of phase transformations in olivine (ringwoodite to post-spinel) and non-olivine component (ilminite to perovskite), respectively, for the pyrolite model of mantle composition