A Lower Mantle Slab Below the East Asia Margin Constrained by Seismic Waveform Complexity

The precise location, morphology, and physical properties of the slab in the lower mantle provide critical information for deciphering the subduction history. Here, we study the waveform complexity of the data recorded by the dense China National Seismograph Network and delineate a lower mantle slab structure below the East Asia margin. For the events that occurred in the Solomon Island area, we observe fast and strongly multipathed S waveforms at the stations near the Sichuan Basin. Furthermore, the observed multipathing patterns vary among different events, which provide robust constraints for the depth of this lower mantle slab. Through detailed waveform modeling, we confirm that a southeastern‐dipping low‐mantle slab exists at a depth of 1,000 km below the East Asia margin with a shear wave velocity perturbation of 2.5%, a length of 1,000 km, and a thickness of 200 km. The spatial correlation between the imaged lower mantle slab and the plate reconstruction model suggests that this slab may be a fragment of the Izanagi Plate from the late Cretaceous subducted beneath the proto South China Sea. However, this southeastern‐dipping structure may invoke the hypothesis of intraoceanic subduction within the Izanagi Plate since the late Cretaceous, which might exist further south of the western Pacific. Plain Language Summary The precise location and morphology of a slab in the lower mantle play important roles in plate reconstruction. However, the slab imaged in most tomographic models has a blurry feature, which may not reflect the real structure. The waveform information can be used to refine the tomographic images. In particular, while seismic waves sample a sharp edge of an anomaly, such as a slab, they generate multipathing with distorted waveforms. By using the dense China National Seismograph Network, we examine the multipathing effects and locate a lower mantle slab below East Asia. By modeling the waveforms, we show that a lower mantle slab has a depth of 1,000 km with a thickness of 200 km, a length of 1,000 km, and a shear wave velocity perturbation of 2.5%. One intriguing feature of this slab is its southeast dip, which is opposite to the typical ancient subduction direction along the western Pacific. To reconcile such contrary observations, we suggest two possible origins of the observed lower mantle slab: the Izanagi plate from the late Cretaceous subducted beneath the Eurasian Plate with a period of stagnation at the mantle transition zone or t