The largest compositional discontinuity within the Earth is the core-mantle boundary (CMB), at a depth of 2889 km. This boundary and the adjacent transition zones in the lowermost mantle and outermost core play a critical role in the Earth's thermal and chemical evolution. Estimates of the heat flux out of the core indicate that the 200-km-thick D" region at the base of the mantle is a major thermal boundary layer. The D" region, like its counterpart boundary layer in the lithosphere, is laterally heterogeneous and possibly chemically stratified. This complexity obscures the exact dynamic behavior of the D" region. However, it is clear that the large density contrast (4.3 g em 3) across the CMB provides favorable con­ ditions for density stratification on both sides of the boundary. The D" region may be the refuse pile for "heavy" mantle heterogeneities, while the outermost core may have a concentration of "light" components. Boundary-layer instabilities in D" have been invoked as sources of both localized mantle plumes and major upwelling currents in whole-mantle convection systems. Dynamically supported topography on the CMB and lateral temperature gradients in D" may produce coupling between the core and mantle, influencing the excitation of the geodynamo. The outer­ most core is generally believed to be radially and laterally homogeneous, although chemical stratification has not been ruled out. Seismological, geodynamical, and geomagnetic investigations are unveiling the three­ dimensional complexity of the deep Earth in the vicinity of the CMB.

https://es.ucsc.edu/~thorne/TL.pdfs/YL_CMB_AnnuRev1987.pdf

The Core-Mantle Boundary

Both spectral and time domain studies indicate that the frequency dependence and regional variation of the attenuation of P waves parallels that of S waves with t * β = 4 t * α . Scattering Q cannot be generally separated from intrinsic Q in the mantle. Forward scattering can generate time-dependent variations in the frequency content and complexity of body waves that affect the measurement of t * . Assuming that lateral heterogeneity biases the apparent Q of surface waves, the frequency dependence of Q can be explained by a relaxation model of intrinsic Q . In this model, Q is constant with frequency up to a cutoff frequency 1/(2πτ m ) Hz, where τ m = 0.1 to 0.2 sec. Regional variations in mantle attenuation are consistent with radiometric or magnetic age and tectonic activity, regions of higher relative attenuation coincident with younger, tectonically active crust. Continental cratons are underlain by mantle having small attenuation at all depths. High attenuation usually correlates with slow travel times, lower Pn velocity, and inefficient Pn and Sn propagation. Measures of differential frequency content (δt * and δ Q ) generally correlate better with differential travel time than measures of differential amplitude and m b . The regional pattern and intensity of both travel-time anomalies and t * measurements suggest that both share a common origin due to the regional variation of the thermal structure of the upper 200 to 400 km of the mantle.

/pdf/the-effect-of-attenuation-on-seismic-body-waves-172lvi42r8.pdf

The effect of attenuation on seismic body waves

Present Earth core models derived from the retrieval of global Earth structure are based on absolute travel times, mostly from the International Seismological Centre (ISC), and/or free-oscillation eigenfrequencies. Many core phase data are left out of these constructions, e.g., PKP differential travel times, amplitude ratios, and waveforms. This study is an attempt to utilize this additional information to construct a model of core P wave velocity which is consistent with the different types of core phase data available. In conjunction with our waveform modeling we used 150 differential time measurements and 87 amplitude ratio measurements, which were the highest-quality observations chosen from a large population of Global Digital Seismograph Network (GDSN) records. As a result of fitting these various data sets, a one-dimensional P wave velocity model of the core, PREM2, is proposed. This model, modified from the Preliminary Reference Earth Model (PREM) (Dziewonski and Anderson, 1981), shows a better fit to the combined data set than any of the existing core models. Major features of the model include a sharp velocity discontinuity at the inner core boundary (ICB), with a large jump (0.78 km/s), and a low velocity gradient at the base of the fluid core. The velocity is nearly constant over the lower 100 km of the outer core. The model features a depth-dependent Qα structure in the inner core such that a constant t* for the inner core fits the amplitude ratios and waveforms of short-period waves moderately well. This means the top of the inner core is more attenuating than the deeper part of the inner core. In addition, the P velocity in the lowermost mantle is reduced from that of PREM as a baseline adjustment for the observed separations of the DF and AB branches of PKP at large distances.

A P wave velocity model of Earth's core

Summary. Broadband displacement and velocity records of PKP phases are formed by the simultaneous deconvolution of the instrument responses of waveforms recorded by the short- and long-period channels of the Global Digital Seismograph Network. Revisions of standard earth models are inferred from the comparison of synthetic and observed waveforms. The synthetics incorporate a Q-model designed for the bandwidth of the data and a source model that accounts for the variations of waveform about the focal sphere due to the effects of finiteness and complexity of the rupture process. The interpretation of the data waveforms allows the position of the D-cusp to be fixed at 121 f 1" and the C-cusp at 154 f 2" for a surface source. The cusp positions are consistent with a P-velocity of 10.82 f 0.02 km s-' on the underside of an inner core boundary at 1216 km and a P-velocity of 10.30 f 0.05 kms-' on its upper side, resulting in a P-velocity jump of Aa! = 0.52 k 0.07 km s-'. Based on the agreement of synthetic and observed waveforms that have been diffracted beyond the Ccusp there is no zone of small or negative P-velocity gradient or of strong anelasticity at the bottom of the outer core. Second-order features in the observed waveforms near the D-cusp suggest that the S-velocity may be nearly zero at the top of the inner core. Q, must increase with depth in the inner core in order to satisfy the pulse width of the PKP-DF phase at distances greater than 150". In order to satisfy average properties of the inner core inferred from travel times and normal modes, these combined results require strong gradients in P-velocity, S-velocity and anelasticity in the upper 200-300 km of the inner core.

/pdf/the-structure-of-the-inner-core-inferred-from-short-period-153pwwfb10.pdf

The structure of the inner core inferred from short- period and broadband GDSN data

This paper reviews seismological constraints on the elastic structure of the inner core and on its rate of rotation. In addition, a new model for inner core structure is evaluated. Nearly 2000 high-quality differential travel times of compressional waves that pass through the core are collected from seven different studies. The ray sampling of the inner core is inadequate to uniquely constrain the detailed three-dimensional variation of elastic anisotropy. However, a model that is consistent with existing travel-time observations has strong (>3%) anisotropy in the western hemisphere from depths of 100 km to its center. The symmetry direction for hexagonal anisotropy is near the spin axis and represents high wave speeds. The deepest 600 km appears anisotropic at all longitudes, but the upper 400 km of the quasi-eastern hemisphere (40°E to 160°E) exhibits far weaker anisotropy (≤1%). The isotropic (Voigt) average appears to not vary with longitude, suggesting there is no need for lateral variations in chemistry or temperature which would produce density variations that drive flow. Though still controversial, there is growing evidence from extensive data sets of high quality differential travel times that the inner core is rotating 0.2 to 0.6 °/yr faster than the mantle. Recent normal mode constraints favor no rotation, but are consistent with the slower rotation rates. A new, speculative model is discussed. This model has an isotropic upper inner core separated from an anisotropic lower inner core by a discontinuity at depths near 100 km in the western hemisphere and reaching depths of 400 km in parts of the eastern hemisphere.

Inner Core Anisotropy and Rotation

Summary. Several studies have suggested that there are lateral heterogeneities in the velocity structure at or near the base of the mantle. Such heterogeneities can be studied through the analysis of amplitude ratios of core phases PKPAB and PKPDF for given earthquake-station pairs. In the epicentral distance range 155-175" these arrivals are well separated in time, and PKPAB has near-grazing incidence at the mantle-core boundary 80 that it is highly sensitive to lateral changes in velocity structure in that region, while PKPDF with its near-normal incidence is less sensitive. The observed amplitude ratios vary by well over an order of magnitude, but the size of the ratio is found to be correlated with the region of the core-mantle boundary sampled by the PKPAB rays: beneath the eastern Pacific and western Atlantic, for which abundant data are available, well-defined, contiguous regions (millions of square kilometres in extent) are found which correlated with predominantly larger or smaller amplitudes. We interpret this result as indicating differences in the degree to which the velocity structure of the lowermost mantle is heterogeneous.

/pdf/lateral-heterogeneity-at-the-base-of-the-mantle-revealed-by-1bcdoyqzxv.pdf

Lateral heterogeneity at the base of the mantle revealed by observations of amplitudes of PKP phases

We present a systematic study of f -mode oscillations in neutron stars containing hyperons, ex-tending recent results obtained within the Cowling approximation to linearized General Relativity. Employing a relativistic mean ﬁeld model, we ﬁnd that the Cowling approximation can overesti-mate the quadrupolar f -mode frequency of neutron stars by up to 30% compared to the frequency obtained in the linearized general relativistic formalism. Imposing current astrophysical constraints, we derive updated empirical relations for gravitational wave asteroseismology. The frequency and damping time of quadrupole f -mode oscillations of hyperonic stars are found to be in the range of 1.47 - 2.45kHz and 0.13 - 0.51 sec respectively. Our correlation studies demonstrate that among the various parameters of the nucleonic and hyperonic sectors of the model, the nucleon eﬀective mass shows the strongest correlation with mode characteristics and neutron star observables. Estimates for the detectability of f -modes in a transient burst of gravitational waves from isolated hyperonic stars is also provided.

General relativistic treatment of 
f
-mode oscillations of hyperonic stars

Confining materials to two-dimensional forms changes the behavior of electrons and enables new devices. However, most materials are challenging to produce as uniform thin crystals. We present a new synthesis approach where crystals are grown in a nanoscale mold defined by atomically-flat van der Waals (vdW) materials. By heating and compressing bismuth in a vdW mold made of hexagonal boron nitride, we produce ultrathin crystals less than 10 nanometers thick with flat surfaces. Cryogenic measurements of the ultrathin bismuth demonstrate high-quality electronic transport exhibiting quantum oscillations and a 10x larger residual resistance ratio compared to thin films grown by molecular beam epitaxy. Our vdW mold-growth technique

pdf/ultrathin-crystals-of-bismuth-grown-inside-atomically-smooth-15m85ozx.pdf

Ultrathin crystals of bismuth grown inside atomically-smooth van der Waals materials

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Lateral heterogeneity at the base of the mantle revealed by observations of amplitudes of PKP phases

General relativistic treatment of f -mode oscillations of hyperonic stars

Ultrathin crystals of bismuth grown inside atomically-smooth van der Waals materials