The 75 year history of the American Geophysical Union has accompanied great advances in our understanding of the physics and chemistry of the transition zone between the Earth's core and mantle. The core-mantle boundary (CMB) is the most significant internal boundary within our planet, buried at remote depths and probably forever hidden from direct observation; yet this region is very important to our understanding of the dynamic Earth system. The thermal and chemical processes operating near the CMB have intimate relationships to fundamental events in Earth history, such as core formation, and continue to play a major role in the planet's evolution, influencing the magnetic field behavior, chemical cycling in the mantle, irregularities in the rotation and gravitation of the planet, and the mode of thermal convection of the Earth. The D″ region, comprising the lowermost 300 km of the mantle, is known to be highly heterogeneous in material properties on large and small scales, presumably due to thermal and chemical variations, while the outermost core is much more uniform. The substantial knowledge that we now have about this remote region is testimony to the remarkable progress made in geophysical remote sensing, prompted by prodigious increases in data, computational power, and experimental methodologies used to investigate deep earth structure and processes. In the past decade in particular there have been unprecedented multidisciplinary advances in understanding the CMB region, and there are excellent prospects for developing a comprehensive understanding of this region in the next few decades. Fundamental issues yet to be resolved include the causes of layering extensively observed in D″, whether or not downwelling slabs accumulate at the base of the mantle, whether plumes arise from the CMB region to feed hotspots at Earth's surface, the extent of core-mantle chemical reactions, the relative importance of topographic versus electromagnetic coupling across the CMB, and the degree to which mantle structure influences the geomagnetic field and its reversals. This overview highlights the progress and future directions in geophysical investigations of the CMB region.

The core-mantle boundary region

Body wave, surface wave, and normal mode data are used to place constraints on the frequency dependence of Q in the mantle. With a simple absorption band model it is possible to satisfy the shear sensitive data over a broad frequency range. The quality factor Qs (ω) is proportional to ωα in the band and to ω and ω−1 at higher and lower frequencies, respectively, as appropriate for a relaxation mechanism with a spectrum of relaxation times. The parameters of the band are Q(min) = 80, α = 0.15, and width, 5 decades. The center of the band varies from 101 seconds in the upper mantle, to 1.6×103 seconds in the lower mantle. The shift of the band with depth is consistent with the expected effects of temperature, pressure and stress. High Qs regions of the mantle are attributed to a shift of the absorption band to longer periods. To satisfy the gravest fundamental spheroidal modes and the ScS data, the absorption band must shift back into the short-period seismic band at the base of the mantle. This may be due to a high temperature gradient or high shear stresses. A preliminary attempt is also made to specify bulk dissipation in the mantle and core. Specific features of the absorption band model are low Q in the body wave band at both the top and the base of the mantle, low Q for long-period body waves in the outer core, an inner core Qs that increases with period, and low QP/QS at short periods in the middle mantle. The short-period Qs increases rapidly at 400 km and is relatively constant from this depth to 2400 km. The deformational Q of the earth at a period of 14 months is predicted to be 463.

/pdf/absorption-band-q-model-for-the-earth-4soka4pdia.pdf

Absorption band Q model for the Earth

Regional body wave and surface wave studies indicate that there is a low-Q upper mantle layer underlying a high-Q lithosphere. Great circle surface wave attenuation is used to refine the Q structure of the upper mantle and to demonstrate that these features are consistent with the global data. Body wave results are used to constrain the average Q of various regions of the mantle and core and the Q gradient in the lower mantle. Normal mode data are used to test the hypotheses that bulk dissipation is not required in the mantle and that the inner core has low Q. Both hypotheses are consistent with the data. The data are also consistent with a smooth increase of Q with depth over most of the lower mantle and a low-Q zone at the base of the mantle. The radial modes require bulk dissipation somewhere in the earth, probably in the inner core. A series of parametric models is presented which illustrate the sensitivity of the attenuation data to major features of the Q distribution.

Q of the Earth

Some glaciers and ice streams periodically lurch forward with sufficient force to generate emissions of elastic waves that are recorded on seismometers worldwide. Such glacial earthquakes on Greenland show a strong seasonality as well as a doubling of their rate of occurrence over the past 5 years. These temporal patterns suggest a link to the hydrological cycle and are indicative of a dynamic glacial response to changing climate conditions.

https://science.sciencemag.org/content/sci/311/5768/1756.full.pdf

Seasonality and Increasing Frequency of Greenland Glacial Earthquakes

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

Low-frequency icequakes produced by Columbia Glacier, Alaska, are caused by large icebergs which break off the glacier terminus into Prince William Sound. Occasionally, many ice seracs fall in rapid succession, producing complex, low-frequency icequake signals with durations sometimes exceeding 10 min. An empirical relationship V = 4000 T between the volume V of an iceberg in cubic meters and the duration T of the seismic signal in seconds, can be used to estimate the calving rate of Columbia Glacier from seismological data alone. During 1984 and 1985, a period of rapid retreat of the glacier, the calculated number of icequakes per month showed seasonal fluctuations that corresponded to variations in calving rate determined by photogrammetry. It is likely that most previously reported low-frequency icequakes in southern Alaska are produced by calving, tidewater glaciers.

Calving icebergs: A source of low-frequency seismic signals from Columbia Glacier, Alaska

Attenuation in the core is examined by using spectral ratios of seismic core waves. In the outer core a Qα of 5000–10,000 is obtained from the amplitude radio P7KP/P4KP. From the PKIKP/PKP amplitude ratio, Qα in the inner core has a value between 120 and 400. An unusual result is that P7KP is consistently higher in frequency than P4KP, which may indicate frequency-dependent reflection and transmission coefficients at the mantle-core boundary for rays that nearly graze the core.

Anthony Qamar

Papers

Calving icebergs: A source of low-frequency seismic signals from Columbia Glacier, Alaska

The damping of core waves