Using Array Analysis To Study Small-Scale Heterogeneities In The Mid-lower Mantle

A large number of small-scale heterogeneities are distributed in mid-lower mantle of the Earth.They have complex shapse and wide geographical distribution.They are generally considered to be related to large-scale tectonic activities such as plate subduction or upwelling.However,some studies have suggested that the mid-lower mantle heterogeneities may represent some global features,such as the change in mantle composition or viscosity discontinuities.Although the origin of these smallscale heterogeneities are controversial,it is foreseeable that studying their physical properties and distribution will provide important clues to our understanding of the mineral composition and dynamic evolution history of the Earth.First,this study focuses on the deep earthquakes beneath the Sea of Okhotsk in northwestern Pacific subduction zone,collecting broadband seismic records from dense seismic networks in Europe,China,and USArray.All stations are divided into subarrays with apertures of 5°?20° according to their geographic locations.The information of the S-to-P converted phases are extracted by using array analysis method.Then,based on the migration methods,a series of scatterers distributed in the depth range of 750?1650 km beneath the central and northern Kuril subduciton zone are determined.Waveform modelling results suggest the existence of a subhorizontal heterogeneity at a depth of 880 km with a thickness of 9-16 km,a length of 300 km and an S-wave velocity decrease of 2.3-3.7%to peridotitic mantle,agreeing with oceanic crust undergoing post-stishovite phase transition,which may represent the remnants of the subducted oceanic crust.Two explanations are proposed for the origin of these scatterers:(1)oceanic crust arising from southward dipping intra-oceanic Izanagi subduction during the Mesozoic,(2)anomalies entrained by the Pacific slab.Both explanations imply that the mid-lower mantle scatterers in this region are closely related to subduction.Second,this study present observations of a new PP precursor with the epicenter distance larger than 100°,called PdpP phase("d" indicate the depth of heterogeneities),The PdpP arrives 20-50s before PP but with a slowness between PP and Pdiff,which is not present for reference Earth models.We collect the vertical-component records of China National Seismic Network of large earthquakes(Mw>5.8),that occurred between 2010 and 2020 within the epicentral distance range from 80° to 130°,and identify PdpP from earthquakes that occurred in the Tonga-Fiji region.We find that the PdpP phases are often observed as a wave train with gradually increasing slowness,which reflect a series of discontinuities(410 km and 660 km)or scatterers from the mantle transition zone to mid-lower mantle.We locate the scatterers by calculating the semblance coefficient.To further determine physical properties and distribution of the heterogeneities generating the PdpP,we perform 2D waveform simulations.Our results suggest that the PdpP phases may originate from small-scale scatterers spread over a depth range of 700?1500 km,and generating the PdpP phase require the scale of the scatterers must be at least 10-20 kilometers with the horizontal distribution range greater than 4000 km,and the depth distribution range greater than 700?1000 km,which may represent a chemically distinct region related to the accumulation of subducted lithosphere.In addition,we have also collected the records from the earthquakes occurred at other region,and the PdpP signals are observed from different directions.Since the PdpP has a strong amplitude and high sensitivity to the distribution of scatterers,the observations from different region may provide some clues for our understanding of the global distribution of mid-lower mantle scatterers.