Constraining the 410-km Discontinuity with Triplication Waveform

The detailed structures near the 410-km discontinuity provide key constraints of the dynamic interactions between the upper mantle and the lower mantle through the mantle transition zone via mass and heat exchange. The 410-km discontinuity topography inside the slab could be used to infer the existence of the metastable olivine wedge, further investigate the possible mechanism for deepfocus earthquakes. Multipathing, i.e., triplicated, body waves that bottom near the 410-km discontinuity carry rich information of this discontinuity, such as interface depth and wave speed jump across it. In this study, we investigated the frequency dependent resolution of triplicated waveforms sampling the 410-km discontinuity and explore the tradeoffs between wave speed and discontinuity depth. Additionally, we proposed the array-normalization technique. Finally, with the non-gradient-based inversion package we have developed, we derived a 1-D depth profile of the wave speed below the Tatar Strait of Russia. The inverted model shows an uplift interface at 400±5 km, with a significant wave speed jump of ∼ 7%-8%, which is 2%-3% larger than that of the IASP91 model. We proposed this interface to be an overlapping of the uplifted 410-km discontinuity and the slab upper interface. Our preferred slab upper interface, from the simultaneous inversion of the interface depth and the wave speed using high frequency waveforms (∼ 0.5 Hz), is ∼ 50-70 km shallower than the Slab2.0 model and the +2-3% wave speed contours of the regional tomography model.