Constraints On Small-scale Heterogeneity In The Lowermost Mantle From Observations Of High Frequency Scattered Seismic Waves

The interior structure of the solid Earth is layered in spherical shells,the lower mantle is adjacent to the most active liquid outer core.The liquid outer core releases colossal(8-16 TW)energy and causes rapid variation of Earth's angular momentum,which can be coupled to the mantle through the lowermost mantle and affect many physical processes of the Earth,such as the change of length of day(LOD)and evolution of geomagnetic field.Therefore,the properties of lowermost mantle and topography of core-mantle boundary(CMB)provide critical constraints on the core-mantle coupling(Roberts and Aurnou,2012),and those parameters could be inferred from studying the various scales of heterogeneities of the lowermost mantle(Garnero,2000).These large lateral variations are interpreted to be of thermal origin related to mantle convection,or of thermochemical origin which may have been a result of early differentiation.Compositional or chemical heterogeneities at small-scales is mostly due to the high thermal conductivity of the mantle material that would dissipate purely thermal anomaly on the kilometer scale within a few million years.Volumetric heterogeneities and complex dynamic processes on large(->1000 km)and intermediate scales(?>100 km)in the lowermost mantle have been established with seismological,geochemical,mineral physical approaches.However,there are controversies regarding the level of heterogeneity in lowermost mantle at small scales(a few kilometers to tens of kilometers),with lower bound estimates ranging from 0.1%to a few percent.We take advantage of the small amplitude PcP waves at near podal distances(0-12°)to constrain the level of small-scale heterogeneity within 250 km above the CMB.First,we compute short period synthetic seismograms with a finite difference code for a series of volumetric heterogeneity models in the lowermost mantle,and find that PcP is not identifiable if the small-scale heterogeneity in the lowermost mantle is above 2.0%.We then use a functional form appropriate for coda decay to suppress P coda contamination.By comparing the corrected envelope of PcP and its precursors with synthetic seismograms,we find that perturbations of small-scale(?8 km)heterogeneity in the lowermost mantle is?0.2%-0.5%beneath regions to the China-Myanmar border area,Okhotsk Sea and South America.Whereas strong perturbations(?1.0%)are found beneath Central America.In the regions studied,we find that this particular type of small-scale heterogeneity in lowermost mantle is weak,yet there are some regions requiring heterogeneity up to 1.0%.Where scattering is stronger,such as under Central America,more chemically complex mineral assemblages may be present at the core-mantle boundary.Most global tomography models show high velocity anomalies near the core-mantle boundary beneath EastAsian,which are called 'China High'(Dziewonski et al.,1993).From their cross sections through those global tomography models showing 'China High' layer about 300-400 km thick,and have strong topography top interfaces.The global tomography usually have limited resolution due to the sparse raypath coverage,in order to detected the fine structure of 'China High' should be used the short period body wave signal.We constrained the Vp characteristics of the high velocity anomalies beneath EastAsian from the observations of PcP precursors and PdP phase.Through our and previous observed results shown that the reflective signal PdP are weak,likely due to the strong topography of D" discontinuity.The topography of D" discontinuity may be attributed to the buckling of ancient subducted slabs boundary or subducted oceanic slab material transition to the post-perovskite,resulting in a tomography reflecting surface at the phase change within slab material(Lay,2015).In order to distinguish the D" discontinuity is caused by ancient subducted slabs or a post-perovskite phase change surface,we constrained the small-scale heterogeneity with D" layer from the observation of PcP precursors.By comparing the corrected envelope of PcP and its precursors with synthetic seismograms,we find that perturbation of small-scale heterogeneity in D" layer is?0.1?0.2%beneath the EastAsian,and the scale length of the heterogeneity is 4 to 10 km.If the original ancient subducted slabs still accumulate in the D" layer,the small-scale heterogeneity should be much stronger.Thus the phase transition boundary may be the origin of D" discontinuity,and the high velocity anomalies in D" may be related to extensive distribution of post-perovskite beneath EastAsian.We analyzed the travel time and amplitude characteristics of Pdiff coda by the numerical and ray theoretical waveform modeling approach.We found that the Pdiff coda should generates from the scattering in the whole mantle at distance?<115°,and it could be explained by scattering in the lowermost mantle at distance>115°.In the future,we will collect Pdiff coda observations and analyze their characteristics.And by fitting to the synthetic waveform to constrain the small-scale inhomogeneity in the lower mantle.