Sebastian Rost

Seismological constraints on a possible plume root at the core–mantle boundary

Recent seismological discoveries have indicated that the Earths core–mantle boundary is far more complex than a simple boundary between the molten outer core and the silicate mantle. Instead, its structural complexities probably rival those of the Earths crust. Some regions of the lowermost mantle have been observed to have seismic wave speed reductions of at least 10 per cent, which appear not to be global in extent. Here we present robust evidence for an 8.5-km-thick and ∼50-km-wide pocket of dense, partially molten material at the core–mantle boundary east of Australia. Array analyses of an anomalous precursor to the reflected seismic wave ScP reveal compressional and shear-wave velocity reductions of 8 and 25 per cent, respectively, and a 10 per cent increase in density of the partially molten aggregate. Seismological data are incompatible with a basal layer composed of pure melt, and thus require a mechanism to prevent downward percolation of dense melt within the layer. This may be possible by trapping of melt by cumulus crystal growth following melt drainage from an anomalously hot overlying region of the lowermost mantle. This magmatic evolution and the resulting cumulate structure seem to be associated with overlying thermal instabilities, and thus may mark a root zone of an upwelling plume.

Melting of the Earth's inner core.

The Earth’s magnetic field is generated by a dynamo in the liquid iron core, which convects in response to cooling of the overlying rocky mantle. The core freezes from the innermost surface outward, growing the solid inner core and releasing light elements that drive compositional convection. Mantle convection extracts heat from the core at a rate that has enormous lateral variations. Here we use geodynamo simulations to show that these variations are transferred to the inner-core boundary and can be large enough to cause heat to flow into the inner core. If this were to occur in the Earth, it would cause localized melting. Melting releases heavy liquid that could form the variable-composition layer suggested by an anomaly in seismic velocity in the 150 kilometres immediately above the inner-core boundary. This provides a very simple explanation of the existence of this layer, which otherwise requires additional assumptions such as locking of the inner core to the mantle, translation from its geopotential centre or convection with temperature equal to the solidus but with composition varying from the outer to the inner core. The predominantly narrow downwellings associated with freezing and broad upwellings associated with melting mean that the area of melting could be quite large despite the average dominance of freezing necessary to keep the dynamo going. Localized melting and freezing also provides a strong mechanism for creating seismic anomalies in the inner core itself, much stronger than the effects of variations in heat flow so far considered.

Fine-Scale Ultra-Low Velocity Zone Layering at the Core-Mantle Boundary and Superplumes

Ultra-low velocity layering at the Earth’s core-mantle boundary (CMB) has now been detected using a variety of seismic probes. P- and S-wave velocity reductions of up to 10’s of percent have been mapped in a thin (5–50 km) layer, which commonly underlies reduced seismic shear wave speeds in the overlying few 100 km of the mantle. Ultra-low velocity zones (ULVZ) contain properties consistent with partial melt of rock at the very base of the mantle. Strong evidence now exists for a significant density increase in the layer (∼5–10% greater than reference models), which must be included in dynamical scenarios relating ULVZ partial melt to deep mantle plume genesis. 3-D geodynamical calculations involving an initially uniform dense layer in the lowermost few 100 km of the mantle result in thermo-chemical piles that are geographically well-correlated with seismic tomography low velocities, when past plate motions are imposed as a surface boundary condition. The hottest lower mantle regions underlay edges of the dense thermo-chemical piles. A scenario is put forth where these piles geographically correlate with ultra-low velocity zones, and subsequent mantle plume genesis.

Crustal imaging across the North Anatolian Fault Zone from the autocorrelation of ambient seismic noise

Seismic images of active fault zones can be used to examine the structure of faults throughout the crust and upper mantle, and give clues as to whether the associated deformation occurs within a narrow shear zone, or is broadly distributed through the lower crust. Limitations on seismic resolution within the crust, and difficulties imaging shallow structures such as the crust-mantle boundary (Moho), place constraints on the interpretation of seismic images. In this study we retrieve body wave reflections from autocorrelations of ambient seismic noise. The instantaneous phase coherence autocorrelations allow unprecedented ambient noise images of the North Anatolian Fault Zone (NAFZ). Our reflection profiles show a Moho reflected P-wave, and additional structure within the crust and upper mantle. We image a distinct vertical offset of the Moho associated with the northern branch of the NAFZ indicating that deformation related to the fault remains narrow in the upper mantle.

Array seismology advances research into Earth's interior

The densification of regional seismic networks, the proliferation of temporary portable seismometer deployments, and the increasing ease with which traditional seismic array data may be obtained all facilitate a new wave of imaging Earths interior at the shortest-ever possible scale lengths using array methods. In fact, seismic array techniques are often necessary for retrieval of subtle, yet important, deep Earth seismic structures, particularly those containing fine-scale features. While only a handful of investigations over the past few decades have used traditional seismic array processing for imaging Earths deep interior, the recent data renaissance in our community enables utilization of methods once predominantly devoted to the near surface for peering deep into the planet, from upper mantle discontinuities to the solid inner core, from the earthquake source to near-surface receiver structure. Thus, it is anticipated that this branch of seismology will become increasingly important in the pursuit of deciphering Earth structure and the earthquake source in unprecedented detail, especially with the emergence of the U.S. National Science Foundation (NSF)-funded USArray of the EarthScope initiative (see http://www.earthscope.org).

A compositional origin to ultralow-velocity zones

We analyzed vertical component short-period ScP waveforms for 26 earthquakes occurring in the Tonga-Fiji trench recorded at the Alice Springs Array in central Australia. These waveforms show strong precursory and postcursory seismic arrivals consistent with ultralow-velocity zone (ULVZ) layering beneath the Coral Sea. We used the Viterbi sparse spike detection method to measure differential travel times and amplitudes of the postcursor arrival ScSP and the precursor arrival SPcP relative to ScP. We compare our measurements to a database of 340,000 synthetic seismograms finding that these data are best fit by a ULVZ model with an S wave velocity reduction of 24%, a P wave velocity reduction of 23%, a thickness of 8.5 km, and a density increase of 6%. This 1:1 VS:VP velocity decrease is commensurate with a ULVZ compositional origin and is most consistent with highly iron enriched ferropericlase.

Crustal thickness variations and isostatic disequilibrium across the North Anatolian Fault, western Turkey

We use teleseismic recordings from a dense array of seismometers straddling both strands of the North Anatolian Fault Zone to determine crustal thickness, P/S velocity ratio and sedimentary layer thickness. To do this, we implement a new grid search inversion scheme based on the use of transfer functions, removing the need for deconvolution for source normalization and therefore eliminating common problems associated with crustal-scale receiver function analysis. We achieve a good fit to the data except at several stations located in Quaternary sedimentary basins, where our two-layer crustal model is likely to be inaccurate. We find two zones of thick sedimentary material: one north of the northern fault branch, and one straddling the southern branch. The crustal thickness increases sharply north of the northern strand of the North Anatolian Fault Zone (NAFZ), where the fault nearly coincides with the trace of the Intra-Pontide Suture; the velocity ratio changes across the southern fault strand, indicating a change in basement composition. We interpret these changes to indicate that both strands of the NAFZ follow preexisting geological boundaries rather than being ideally aligned with the stress field. The thick crust north of the northern NAFZ strand is associated with low topography and so is inconsistent with simple models of isostatic equilibrium, requiring a contribution from mantle density variations, such as possible loading from underthrust Black Sea oceanic lithosphere.

Seismic detection of sublithospheric plume head residue beneath the Pitcairn hot-spot chain

We study array seismograms of French nuclear explosions from two islands in French Polynesia and use these to constrain the structure in the upper mantle beneath the islands. Seismograms of nuclear explosions on the hot-spotrelated Fangataufa atoll show discrete, large-amplitude P-coda phases which are not observed in recordings of explosions on Mururoa, an atoll V40 km to the north. The source for these P-coda phases is located beneath the Fangataufa atoll, indicating that structural heterogeneities are present in the oceanic upper mantle in this region on very small scale lengths. Synthetic seismograms for models of the upper mantle beneath Fangataufa require a layer with P-wave velocity elevated by V10% between depths of 51 and 85 km with a sharp termination in the north, possibly at the Austral Fracture Zone, to match these P-coda phases. Few mineralogic scenarios exist that can explain this structure, and the properties of this layer imply that extensive enrichment in garnet occurs in this depth range. The garnet-enriched layer is likely of similar origin to the well-known xenolithic ‘garnet megacryst’ suite found in kimberlitic regions. We propose a model for the formation of the Fangataufa and Mururoa atolls involving garnetenriched zones being generated at depth through magmatic processes at the Pitcairn plume head. Thus, the initiation of hot-spots could produce complex geochemical and structural heterogeneities at depth in the suboceanic mantle. 4 2003 Elsevier Science B.V. All rights reserved.

Seismic Detections of Small-Scale Heterogeneities in the Deep Earth

We report the detection of coherent scattered energy related to the phase PKPPKP (P′P′) in the data of medium aperture arrays. The scattered energy (P′•P′) is weak and requires array processing techniques to extract the signal from the noise. The arrival time window of P′•P′ is mostly free from other interfering body wave energy and can be detected over a large distance range. P′•P′ has been detected in the data of large aperture arrays previously, but the detection in the data of smaller arrays shows its potential for the study of the small-scale structure of the Earth. Here, we show that P′•P′ can detect scattering off small-scale heterogeneities throughout the Earth’s mantle from crust to core making this one of the most versatile scattering probes available. We compare the results of P′•P′ to a related scattering probe (PK•KP). The detected energy is in agreement with stronger scattering, i.e., more heterogeneous structure, in the upper mantle and in an approximately 800-km-thick layer above the core–mantle boundary. Lateral variations in heterogeneity structure can also be detected through differences in scattered energy amplitude. We use an application of the F-statistic in the array processing allowing us a precise measurement of the incidence angles (slowness and backazimuth ) of the scattered energy. The directivity information of the array data allows an accurate location of the scattering origin. The combination of high-resolution array processing and the scattering of P′•P′ as probe for small-scale heterogeneities throughout the Earth’s mantle will provide constraints on mantle convection , mantle structure , and mixing related to the subduction process.

Near surface structure of the North Anatolian Fault Zone fromRayleigh and Love wave tomography using ambient seismic noise

We use observations of surface waves in the ambient noise field recorded at a dense seismic array to image the North Anatolian Fault zone (NAFZ) in the region of the 1999 magnitude 7.6 Izmit earthquake in western Turkey. The NAFZ is a major strike-slip fault system extending ∼ 1200 km across northern Turkey that poses a high level of seismic hazard, particularly to the city of Istanbul. We obtain maps of phase velocity variation using surface wave tomography applied to Rayleigh and Love waves and construct high-resolution images of S-wave velocity in the upper 10 km of a 70× 30 km region around Lake Sapanca. We observe low S-wave velocities (< 2.5 km s−1) associated with the Adapazari and Pamukova sedimentary basins, as well as the northern branch of the NAFZ. In the Armutlu Block, between the two major branches of the NAFZ, we image higher velocities (> 3.2 km s−1) associated with a shallow crystalline basement. We measure azimuthal anisotropy in our phase velocity observations, with the fast direction seeming to align with the strike of the fault at periods shorter than 4 s. At longer periods up to 10 s, the fast direction aligns with the direction of maximum extension for the region (∼ 45). The signatures of both the northern and southern branches of the NAFZ are clearly associated with strong gradients in seismic velocity that also denote the boundaries of major tectonic units. Our results support the conclusion that the development of the NAFZ has exploited this pre-existing contrast in physical properties.