ALBERT

Description: Mode conversions and reflections at upper-mantle seismic discontinuities may be contained in earthquake seismograms as weak secondary phases that often become visible only after special signal processing techniques are applied to the data. To extract fully the information these secondary phases carry about the three-dimensional structure of the Earth, new observational and interpretational methods have to be developed. However, new sources of possible systematic errors may lead to conflicting results. Studies carried out by various research groups on the thickness of the upper-mantle transition zone, the sharpness of upper-mantle discontinuities and the global existence of a 520 km discontinuity are examples where such discrepancies did arise. Although there is a general consensus that the depths to the 410 km and 660 km discontinuities vary by a few tens of kilometres at most, the question of wither the depth variations of the 410 km and 660 km discontinuities are correlated or anticorrelated is still unresolved. Similarly, different data sets and methods yielded different answers on the sharpness of the upper-mantle discontinuities at 410 km and 660 km depth. Finally, data apparently supporting the global existence of a seismic discontinuity at 520 km depth can be equally well explained by models that do not contain this discontinuity.

Description: P receiver functions from 23 stations of the SASE experiment in southern Africa are inverted simultaneously with SKS waveforms for azimuthal anisotropy in the upper mantle. Our analysis resolves the long-standing issue of depth dependence and origins of anisotropy beneath southern Africa. In the uppermost mantle we observe anisotropy with a nearly E-W fast direction, parallel to the trend of the Limpopo belt. This anisotropy may be frozen since the Archean. At a depth of 160 km the fast direction of anisotropy changes to 40° and becomes close to the recent plate motion direction. This transition is nearly coincident in depth with activation of dominant glide systems in olivine and with a pronounced change in other properties of the upper mantle. Another large change in the fast direction of anisotropy corresponds to the previously found low-S-velocity layer atop the 410-km discontinuity.

Description: Abstract We investigate the upper mantle discontinuities in the Central Mediterranean region by applying the P and S receiver function techniques on waveforms recorded at broadband stations located around the Tyrrhenian basin. P and S‐wave velocity profiles (down to 300km depth) are calculated with joint inversion of P and S receiver functions. We could identify the Moho, Lithosphere‐Asthenosphere Boundary and an underlying low‐velocity layer between ~60 and ~200km depth. The low‐velocity layer is interpreted as asthenospheric material, and its lower boundary is identified below the western Ionian and Tyrrhenian basins as a sharp Lehmann discontinuity. Although the stations are located on different lithospheric domains we find a strong correlation between Moho and the Lithosphere‐Asthenosphere Boundary depths, which suggests ubiquitous coupling of the crust and lithospheric mantle, consistently with the southward opening of the Tyrrhenian basin. The Tyrrhenian and western Ionian basins present thinning of the transition zone of ~14km, as inferred from a reduced P660s‐P410s differential time. Below the southern Apennines we observe a standard differential time that implies an average mantle transition zone thickness. We explain these mantle transition zone thickness variations as due to temperature heterogeneity linked to the area's subduction history. Finally, under central Europe (the location of the deep S‐to‐p conversion points) two strong signals from non‐standard discontinuities within the mantle transition zone are observed. These signals can be explained as being generated at the boundaries of high seismic velocity layers that are spatially correlated with stagnant slabs in the transition zone detected by seismic tomography.

Description: Observations of shear wave splitting in SKS seismic phase play a key role in the current efforts to understand kinematics and dynamics of mantle flow, but azimuthal anisotropy as a depth-localized phenomenon still is poorly known. Here we analyse stratification of seismic azimuthal anisotropy beneath central and northern Anatolia (a microplate within the Alpine belt) by inverting P -wave receiver functions jointly with shear wave splitting in SKS seismic phase. The analysis is based on recordings of stations of the North Anatolian Fault (NAF) passive seismic experiment. In the resulting model in a depth interval from 120 to 200 km fast direction of anisotropy is nearly parallel to the plate motion direction (~E–W), whilst a normal direction (close to S–N) is found in the low velocity zone (LVZ) between 60 and 90 km. Our preferred interpretation of these data suggests that the flow in upper mantle is nearly parallel to the Anatolian plate motion direction in the depth range from the LAB to 200 km, but in part of the LVZ fast direction of anisotropy is normal to the direction of shear in the mantle. This relation between anisotropy and shear is known from laboratory experiments with peridotite-type rock containing melt. A similar relation between anisotropy and flow in the LVZ is found in Fennoscandia. These findings may have far-reaching implications for interpreting mantle anisotropy elsewhere.

Description: P and S receiver functions (PRF and SRF) from 19 seismograph stations in the Gibraltar Arc and the Iberian Massif reveal new details of the regional deep structure. Within the high-velocity mantle body below southern Spain the 660-km discontinuity is depressed by at least 20 km. The Ps phase from the 410-km discontinuity is missing at most stations in the Gibraltar Arc. A thin (~50 km) low- S -velocity layer atop the 410-km discontinuity is found under the Atlantic margin. At most stations the S410p phase in the SRFs arrives 1.0–2.5 s earlier than predicted by IASP91 model, but, for the propagation paths through the upper mantle below southern Spain, the arrivals of S410p are delayed by up to +1.5 s. The early arrivals can be explained by elevated Vp / Vs ratio in the upper mantle or by a depressed 410-km discontinuity. The positive residuals are indicative of a low (~1.7 versus ~ 1.8 in IASP91) Vp / Vs ratio. Previously, the low ratio was found in depleted lithosphere of Precambrian cratons. From simultaneous inversion of the PRFs and SRFs we recognize two types of the mantle: ‘continental’ and ‘oceanic’. In the ‘continental’ upper mantle the S -wave velocity in the high-velocity lid is 4.4–4.5 km s –1 , the S -velocity contrast between the lid and the underlying mantle is often near the limit of resolution (0.1 km s –1 ), and the bottom of the lid is at a depth reaching 90–100 km. In the ‘oceanic’ domain, the S -wave velocities in the lid and the underlying mantle are typically 4.2–4.3 and ~ 4.0 km s –1 , respectively. The bottom of the lid is at a shallow depth (around 50 km), and at some locations the lid is replaced by a low S -wave velocity layer. The narrow S–N-oriented band of earthquakes at depths from 70 to 120 km in the Alboran Sea is in the ‘continental’ domain, near the boundary between the ‘continental’ and ‘oceanic’ domains, and the intermediate seismicity may be an effect of ongoing destruction of the continental lithosphere.

Description: 〈span class="paragraphSection"〉〈div class="boxTitle"〉Abstract〈/div〉Vp/Vs ratio where Vp and Vs are 〈span style="font-style:italic;"〉P〈/span〉- and 〈span style="font-style:italic;"〉S〈/span〉-wave velocities is an indicator of rock composition, but estimates of Vp/Vs for the lower continental crust remain sparse. We present estimates of Vs, Vp and Vp/Vs in the crust of the Archean-Paleoproterozoic Siberian craton that are obtained by simultaneous inversion of 〈span style="font-style:italic;"〉P〈/span〉 and 〈span style="font-style:italic;"〉S〈/span〉 receiver functions from GSN seismograph stations NRIL, YAK and TIXI. These stations are located in the region of the Siberian traps (NRIL), close to the Laptev Sea Rift (TIXI) and the Viluy rift system (YAK). The most conspicuous result of our analysis is a high Vp/Vs ratio (≥2.0) at depths from 20–30 to 40 km. A very high Vp in this layer (from 7 to 8 km s〈sup〉−1〈/sup〉) is indicative of magmatic underplating. We find broadly similar data in the western Mediterranean and in India. In a dry lower crust the Vp/Vs ratio is ∼1.8, which is hard to reconcile with the estimates 〉2.0. A coincidence in depths of zones of high electric conductivity and of anomalously high Vp/Vs in Siberia suggests that both may have the same origin: fluid-filled porosity. The porosity which is required by our seismic observations is on the order of 1 per cent. Origins of the fluids may be linked with processes of magmatic underplating.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉Sharpness of the 410-km boundary is of interest because it is sensitive to water content in the transition zone. We evaluate the width of the 410-km discontinuity with a new seismic method. Our estimates are inferred from the amplitude ratio of the P2p410s and P410s seismic phases that are detected in P-wave receiver functions. We applied this method to seismic recordings from arrays of broad-band stations deployed in central Fennoscandia, southern Africa and southern China. The obtained estimates of width of the 410-km discontinuity range from 10 to 22 km and always exceed the width of 7 km which is expected for anhydrous conditions. The enlarged width may be interpreted in terms of hydrous conditions, but we have found only one region (the eastern Yangtze Craton in China) where the broad 410-km discontinuity, as expected, is accompanied by a broad transition zone. Water in the transition zone may be a kind of a global phenomenon, but evidence of the enlarged width of the transition zone may be missing in most of our data because the reference seismic model is affected by water, as well.〈/span〉