Detection Of Small-scale Seismic Heterogeneities Of The Lowermost Mantle

The core-mantle boundary(CMB)and the lowermost mantle,as the most complex and geodynamically active region in the Earth,have always been the research focus in the Earth science.Specifically,the core-mantle boundary is the most profound internal layer inside the Earth,separating the solid silicate mantle from the liquid iron outer core.Meanwhile,the lowermost mantle on top of this boundary plays an important role in dynamic processes,such as the fate of subducted slabs,the source of mantle plumes as well as material and heat exchange between mantle and core.Although global tomographic images have revealed large-scale seismic heteogeneities in this region,however,due to the low tomographic resolution,detailed small-scale structures are poorly known.Nevertheless,characterising these small-scale structures is critical to a better understanding of the dynamic evolutions and compositions of the lowermost mantle.In this study,we utilized PKP precursors and PKKP diffractions for resolving the anomalous structures in the lowermost mantle and discussed the geodynamic significance within the Earth.Firstly,we analyzed PKP precursors from earthquakes in the Aleutian Islands and Kamchatka Peninsula recorded by seismic arrays in Antarctica.With the migration method and modeling the precursor envelope,we find that strong scatterers are distributed between 30° and 40°N in latitude and extend ~400 km above the CMB.The scatterers exhibit larger P-wave velocity perturbations of 1.0~1.2% in the center(160-180°E)and ~0.5% to the west and east(140-160°E,180-200°E).ScS-S differential travel-time residuals reveal similar features: the middle and the two sides show S-wave velocity variations of about 2.0~3.0% and 1.0~1.5%,respectively.These seismic scatterers are likely the remnants of ancient subducted slab material.The lateral variations may be caused either by(1)different slabs or(2)variations in slab composition resulting from segregation process.Secondly,we selected seismograms from earthquakes that occurred in South America recorded by two array stations(WRA and ASAR)in Australia.We determined the slowness and back-azimuth parameters of coherent precursor phases using array processing methods.Our results show that these precursors are caused by seismic scatterers beneath both the southeast of Australia and eastern Pacific Ocean in the lower mantle.Furthermore,results from waveform modeling using AxiSEM show that there exist small-scale,patchy pockets of ultra-low velocity zone(ULVZ)near the eastern boundary of Pacific LLSVP.These ULVZs exhibit P-wave velocity reductions of 6~10% and thickness of 20~40 km,with ~30 km in width.Besides,we developed the migration method to obtain the spatial distribtion of scatterers in the lowermost mantle beneath eastern Pacific Ocean.The strongest scatterers are approximately distributed in(-89°E,-10°N),which is consistent with results from waveform modeling.We suggest that these strong scatterers in the lowermost mantle are probably ULVZs which may be related with partial melting or chemically compositional heterogeneity.Lastly,we examined PKKPab diffraction waves from earthquakes in the western Pacific region that are recorded by USArray to probe the velocity structure along the core-mantle boundary.We measured the ray parameters of PKKPab diffraction waves by Radon transform analysis that is suitable for large-aperture arrays and further estimated the P-wave velocity variations by converting these ray parameters.We find that the CMB regions beneath the northwestern edge of Africa LLSVP and southern Sumatra Islands exhibit velocity reductions of approximately 4%~8.5% relative to PREM,suggesting the presence of low/ultra-low velocity zones.In addition,the velocity changes of diffraction waves beneath northern Atlantic Ocean may indicate different effects that LLSVP plays on ULVZs according to the distance away from it.In total,we propose a possible link between the small-scale seismic scatterers and ULVZs detected in this study.During the slab penetration onto the CMB,the segregated oceanic crusts of subducted slabs could form the small-scale scatterers distributed throughout the lower mantle,which could generate the observed PKP precursors.Furthermore,the introduction of the distinct chemical compositions in the oceanic crusts could possibly result in ULVZs on the CMB as well.