Distinct compositional thin layers at mid-mantle depths beneath northeast China revealed by the USArray

We observe a clear seismic arrival at ∼35–45 s after the direct P wave in USArray recordings of two deep earthquakes occurring beneath northeast China. Velocity-spectrum and beam-forming analyses reveal that this arrival has a lower slowness value than the direct P wave and a back azimuth slightly different from the great-circle direction. The measured slowness and arrival time indicate that it is a transmitted S to P conversion from structures below the sources. We employ the common-conversion-point (CCP) stacking and diffraction migration methods to determine the location and geometric features of the seismic structures. The CCP stacking image indicates that the structure is a localized discontinuity at ∼1000 km with a dimension at ∼200 km by ∼50 km along the E–W and N–S directions, respectively. It is located at ∼150 km northeast to the two events. The 2D migrated images, on the other hand, indicate that the sources structure are reflectors dipping northeastwards by ∼17° at a slightly shallower depths. The reflectors have a length scale of ∼100 km ant their centers are ∼50 km away from the epicenters of the two earthquakes. Forward waveform modeling suggests that the dipping reflectors may be thin layers with a thickness of few kilometers. The layers have a lower shear velocity and a higher density than that of the surrounding mantle, which matches well with those predicted for mid-ocean-ridge basalt (MORB) at mid-mantle depths, according to a recent ab initio study. Combined with the results from previous studies, our observations here suggest that the former oceanic crust may be ubiquitously present in the lower mantle beneath subduction zones.