Earth's Enigmatic Interface

Identifying dehydration-induced shear velocity anomaly in the Earth's core-mantle boundary

The steep temperature gradient near the bottom of the mantle is known to generate a negative correlation between the shear wave velocity (V S ) and the depth in most regions of the D″ layer, as detected by seismological observations. However, increasing V S with depth is observed at the D″ layer beneath Central America, where the Farallon slab sinks, and the origin of this anomaly has not been well constrained. Here, we calculate the thermoelastic constants and obtain the elastic wave velocities of hydrous phase H with various Al contents and cation configurations, which may act as a water carrier to the D″ layer. We find its V S to be substantially lower than the post-perovskite-type bridgmanite. The dehydration of Al-enriched phase H and the redistribution of Al from the hydrous component to dry silicates would gradually raise the V S below the top of the D″ layer. The presence of 3.5 wt % water is sufficient to compensate for the thermal effects to match the seismic anomaly at the bottom of the mantle beneath Central America. The positive slope of V S versus depth in the D″ layer may fingerprint deep water recycling.

Seismic detection of rigid zones at the top of the core

Data from earthquakes in the Tonga-Fiji region recorded at a seismic array in northern Australia show evidence for rigid zones at the top of the outer core. The ScP waveforms can be modeled by thin (0.12 to 0.18 kilometer) zones of molten iron mixed with solid material with a small, but positive, S-wave velocity (0.6 to 0.8 kilometer per second) that enables the propagation of S-waves in the outermost core. The zones may be topographic highs of the core-mantle boundary filled by light core sediments and might be important for variation of Earth's nutation and for convection of the outer core.