Lianxing Wen

Hemispherical variations in seismic velocity at the top of the Earth's inner core.

Knowledge of the seismic velocity structure at the top of the Earths inner core is important for deciphering the physical processes responsible for inner-core growth. Previous global seismic studies have focused on structures found 100 km or deeper within the inner core, with results for the uppermost 100 km available for only isolated regions. Here we present constraints on seismic velocity variations just beneath the inner-core boundary, determined from the difference in travel time between waves reflected at the inner-core boundary and those transmitted through the inner core. We found that these travel-time residuals—observed on both global seismograph stations and several regional seismic networks—are systematically larger, by about 0.8 s, for waves that sample the ‘eastern hemisphere’ of the inner core (40° E to 180° E) compared to those that sample the ‘western hemisphere’ (180° W to 40° E). These residuals show no correlation with the angle at which the waves traverse the inner core; this indicates that seismic anisotropy is not strong in this region and that the isotropic seismic velocity of the eastern hemisphere is about 0.8% higher than that of the western hemisphere.

Seismic evidence that the source of the Iceland hotspot lies at the core–mantle boundary

Although Morgan proposed in 1971 that hotspots such as Iceland were the result of hot, rising mantle plumes, it is still debated whether plumes originate from a thermal boundary just above the core–mantle boundary or at the base of the upper mantle. Although seismic evidence of plumes in the upper mantle is accumulating, narrow plume conduits in the deep mantle have yet to be detected. Details of plume formation in the lower mantle have therefore remained largely unconstrained. Here, however, we present seismic evidence for the presence of a localized patch of material with ultra-low seismic wave speed, located at the core–mantle boundary beneath the Iceland hotspot, and propose that this zone represents the hot, partially molten source region of the Iceland mantle plume. Through the modelling of seismic waveforms, we constrain the seismic velocity structure at this patch of the core–mantle boundary using a numerical–analytical interfacing code designed to reproduce the complex interference of shear-wave phases transmitted through, and refracted at, the boundary. Although this structure is difficult to constrain precisely, our preferred model consists ofadome which is 250 km wide, 40 km high and contains P- and S-wave velocity (wave-speed) reductions of 10% and 30%, respectively.

Seismic evidence for a thermo-chemical boundary at the base of the Earth's mantle

Abstract We report seismic evidence for a unique 300 km thick layer at the base of the mantle beneath the south Atlantic ocean, with a steeply dipping edge, anomalously low shear wave velocities linearly decreasing from 2% (top) to 10–12% (bottom), and a maximum P velocity decrease of 3% relative to the preliminary reference Earth model (PREM). These characteristics can be best explained by partial melt driven by a compositional change produced early in the Earth’s history and a vertical thermal gradient within the layer. This boundary layer may provide an explanation for the distinctive isotope geochemical DUPAL anomaly observed at some surface ocean islands.

The global seismographic network surpasses its design goal

This year, the Global Seismographic Network (GSN) surpassed its 128-station design goal for uniform worldwide coverage of the Earth. A total of 136 GSN stations are now sited from the South Pole to Siberia, and from the Amazon Basin to the sea floor of the northeast Pacific Ocean—in cooperation with over 100 host organizations and seismic networks in 59 countries worldwide (Figure 1). Established in 1986 by the Incorporated Research Institutions for Seismology (IRIS) to replace the obsolete, analog Worldwide Standardized Seismograph Network (WWSSN),the GSN continues a tradition in global seismology that dates back more than a century to the network of Milne seismographs that initially spanned the globe. The GSN is a permanent network of state-of-the-art seismological and geophysical sensors connected by available telecommunications to serve as a multi-use scientific facility and societal resource for scientific research, environmental monitoring, and education for our national and international community.

Seismic evidence for a rapidly varying compositional anomaly at the base of the Earth's mantle beneath the Indian Ocean

Abstract Seismic observations recorded by an African seismic array reveal a low velocity anomaly at the base of the mantle beneath the Indian Ocean, with steeply dipping edges, rapidly varying thicknesses and geometries, and anomalously low shear wave velocities decreasing from −2% at 200 km above the core–mantle boundary to −9% to −12% at the core–mantle boundary (relative to the preliminary reference Earth model). These characteristics unambiguously suggest that it is a compositional anomaly and its velocity structures can be well explained by partial melt driven by a compositional change produced early in the Earth’s history. This chemical anomaly geographically coincides with the DUPAL geochemical anomaly observed in island volcanoes around the Indian Ocean and may provide an explanation for its distinctive isotope characteristics observed at the Earth’s surface.

Layered mantle convection: A model for geoid and topography

The long-wavelength geoid and topography are dynamic effects of a convecting mantle. The long-wavelength geoid of the Earth is controlled by density variations in the mantle and has been explained by circulation models involving whole mantle flow. However, the relationship of long-wavelength topography to mantle circulation has been a puzzling problem in geodynamics. We show that the dynamic topography is mainly due to density variations in the upper mantle, even after the effects of lithospheric cooling and crustal thickness variation are taken into account. Layered mantle convection, with a shallow origin for surface dynamic topography, is consistent with the spectrum, small amplitude and pattern of the topography. Layered mantle convection, with a barrier about 250 km deeper than the 670 km phase boundary, provides a self-consistent geodynamic model for the amplitude and pattern of both the long-wavelength geoid and surface topography.

A two-dimensional P-SV hybrid method and its application to modeling localized structures near the core-mantle boundary

A P-SV hybrid method is developed for calculating synthetic seismograms involving two-dimensional localized heterogeneous structures. The finite difference technique is applied in the heterogeneous region and generalized ray theory solutions from a seismic source are used in the finite difference initiation process. The seismic motions, after interacting with the heterogeneous structures, are propagated back to the Earths surface analytically with the aid of the Kirchhoff method. Anomalous long-period SKS-SPdKS observations, sampling a region near the core-mantle boundary beneath the southwest Pacific, are modeled with the hybrid method. Localized structures just above the core-mantle boundary, with lateral dimensions of 250 to 400 km, can explain even the most anomalous data observed to date if S velocity drops up to 30% are allowed for a P velocity drop of 10%. Structural shapes and seismic properties of those anomalies are constrained from the data since synthetic waveforms are sensitive to the location and lateral dimension of seismic anomalies near the core-mantle boundary. Some important issues, such as the density change and roughness of the structures and the sharpness of the transition from the structures to the surrounding mantle, however, remain unresolved due to the nature of the data.

The fate of slabs inferred from seismic tomography and 130 million years of subduction

The volume and location of subducted plate, since 130 Ma, are reconstructed from magnetic anomalies and updated finite rotation parameters describing relative motions in the ocean basins and between plates and hotspots. The area of subducted plate is calculated from the relative motions between overriding and overridden plates along the length of convergence in the hotspot reference frame in 5 Ma intervals. Ridge locations are reconstructed by rotating magnetic anomalies from their present to their former positions. The distances between trenches and ridges, at a certain point of time, in the spreading direction are thus estimated and are used to determine the ages of trenches, according to the spreading history recorded in present-day oceans. The thickness of the oceanic plate is obtained from the age based on the half-space cooling model. About 3.45 × 10^(10) km^3 of oceanic plate has been subducted during the past 130 Ma, with maximum accumulation beneath Southeast Eurasia. At spherical harmonic degree l = 2, where subduction flux peaks, excellent correlation is found between four seismic tomographic models at the top of the lower mantle (about 800–1100 km deep) and predicted slab locations. For some seismic models excellent correlation is also found at other degrees in that depth range. Some models also have good correlations with predicted slab locations in the deep mantle at l = 2. Direct comparison between subduction and seismic tomographic patterns at 800–1100 km shows that subduction history correlates with tomography very well in terms of location and amplitude of anomalies. Cold downwellings, which may be related to slabs, appear to be trapped in the mesosphere, or middle mantle. Correlations between seismic tomography and subduction over different time periods support this conclusion. There are also some correlations in the upper mantle and in the deep lower mantle, although they are generally not as significant as those in the 800–1100 km depth range. There may be a significant boundary in the mantle near the 800–1100 km depth. From a geodynamic and chemical point of view, the lower mantle may start at a depth closer to 1000 km than to 670 km.

Seismic evidence for ultralow‐velocity zones beneath Africa and eastern Atlantic

KS waveforms recorded at distances of about 110 o are extremely useful to constrain seismic velocity structure at the base of the mantle.

Intense seismic scattering near the Earth's core-mantle boundary beneath the Comoros hotspot

KS waves near this distance develop a com- plicated interference pattern with the phases SPaKS and