The upper mantle and the transition zone of the Baikal rift zone (BRZ) are studied. The observations are analyzed using P-wave receiver functions. It is found that in the BRZ central and northeastern part, the P410s converted seismic phase is preceded by a precursory wave with negative polarity which is formed in the low S-wave velocity layer at a depth of 350–410 km. A similar precursory wave with low S-wave velocity and negative polarity is formed at a depth of 600–660 km. The low-velocity layers are interpreted as resulting from the hydration of wadsleyite and ringwoodite during the subduction of the Pacific lithosphere. A similar study of the mantle in Central Mongolia found no expected signs of hydration. Modeling of the lithosphere–asthenosphere system in Central Mongolia by joint inversion of the body wave receiver functions and surface wave dispersion curves reveals a very thin lithospheric lid beneath Khangai and a thick layered asthenosphere to a depth of 200 km with a lithospheric inclusion between low-velocity layers.

Deep structure beneath the central part of the Balkan Peninsula was studied using P and S receiver function technique. Data from seismic stations from the Bulgarian National Seismological Network and several stations from neighbouring countries were used. Depth of Mohorovicic discontinuity has been estimated between 28–30 km in northern and central Bulgaria to 50 km in southwestern of Bulgaria. The 410 km mantle boundary is uplifted by 10 km relative to nominal depth in the area of Rhodopean Massif. In northern Bulgaria, the boundary is lowered by 10 km. Indications of a low-velocity layer are present at a depth exceeding 410 km. The thickness of the asthenosphere is estimated as 50 km and the depth of lithosphere-asthenosphere (LAB) boundary varies between 40 and 60 km. The results of this study have been published in Vinnik et. al., Izvestiya, Physics of the Solid Earth, 2021, Vol. 57, No. 6, pp. 849–863. This research has been carried out as part of a joint project supported by the National Science Foundation of Bulgaria (grant no. KP-06-RUSIA/27.09.2019) and the Russian Foundation for Basic Research (RFBR, grant no. 19-55-18008 Bolg_a).

By using P and S wave receiver functions and P and S wave travel time residuals, we have found velocity models for 16 seismograph stations in Eastern Anatolia. Our study is focused mainly on the mantle lithosphere, asthenosphere and transition zone. The volcanism and uplift of the Eastern Anatolia Plateau are thought to be related to the Bitlis slab break off and delamination of the continental lithosphere. Sinking cold slab and lithospheric drips can reduce temperature in the mantle transition zone (MTZ) by up to a few hundred degrees C. However, our analysis of seismic data provides no robust evidence of significant cooling of the transition zone. In the mantle immediately above the 410-km discontinuity there is a pronounced low S wave velocity layer that may be a source of the volcanism in the study region. Another low velocity layer is present at the base of the MTZ. The obtained S wave velocity models of the upper mantle can be divided into three groups. In the first group, the lithosphere—asthenosphere boundary (LAB) is at a depth of ~ 60 km. In the second group, the LAB is at a depth from 90 to 100 km. In the third group, the mantle lithosphere is practically absent. On a scale of our analysis there is no clear correspondence between the obtained mantle velocity models and the volcanism (< 23 Ma) exposed at the surface. Only the models of the first group are well represented in the neighboring Central Anatolian Plateau.

The velocity structure of the mantle under the Baikal Rift Zone (BRZ) is investigated with the P‑wave receiver functions (PRFs) for a group of 10 seismograph stations. The BRZ presents one of the world’s most active continental rift zones. The peculiarities of the BRZ include Cenozoic magmatism in the upper mantle of the southwestern part of the BRZ, which disappears in the central and northeastern parts. The analysis of seismic data reveals other indications of the significant lateral heterogeneity of the mantle beneath the BRZ. At half of the stations there is evidence of a sharp rise of the S velocity with depth at a depth of around 330 km, similar to the descriptions for the “X” or the “300-km” discontinuity. In the central and northeastern regions at depths from about 350 to 410 km there is a well pronounced low S velocity layer, which is practically missing in the southwestern part. The origin of this layer is apparently related to the upwelling and dehydration of wadsleyite in the transition zone. At depths from 500–600 to 660 km in the central and northeastern regions there is another low velocity layer that may be explained by the accumulation of garnetite in the process of subduction of the lithosphere of the Pacific. This layer is poorly pronounced in the southwestern region. The difference between the travel times of the P410s and P660s seismic phases (differential time) in the southwestern region (23.5 s) is close to the data for the standard model (Kennett, Engdahl, 1991). In the central and northeastern regions, the observed differential time is larger than the nominal time by 1.0 s. The rise of the differential time may be related to the cooling and/or hydration of the transition zone by the slabs of the subducted oceanic lithosphere. The obtained seismic data suggest a large role of processes of hydration and dehydration in the central and northeastern regions, however, this role is comparatively small in the southwestern region.

Simultaneous inversion of P and S receiver functions and of dispersion curves of Rayleigh waves for 16 seismograph stations provides insight into structure beneath the Caucasus and the Caspian Basin up to a depth of 700 km. Crustal thickness of the Caucasus ranges from 30 to 50 km. An anomalously high velocity ratio of the P and S waves (2.0 and more) is observed systematically in the upper crust. The upper mantle at most locations can be split into the upper high S wave velocity layer (4.5–4.8 km/s, litospheric mantle) and the underlying low S velocity (4.0–4.2 km/s) asthenosphere. The depth to the lithosphere—asthenosphere boundary (LAB) ranges from 90 to 145 km. Under the East Caucasus the depth to the 410 km boundary is close to the standard (IASP91) value, whereas the 660 km boundary is lowered on the average by 10 km. The sinking of the 660 km boundary may be caused by cooling and/or hydration of the lower transition zone by the subducted Neo-Tethys plate. Under the western margin of the Caspian Basin structure of the upper mantle resembles a subduction zone: the low-velocity ( Vs < 4.2 km/s) asthenosphere which lies immediately beneath the Moho boundary is underlain at a depth of 140 km by a layer of (subducted) high-velocity lithosphere. The S wave receiver functions indicate that the 410 km boundary beneath the Caspian Basin is lowered by about 10 km. This may be an effect of elevated by 100°C temperature. An uplift of the 410 km boundary is found beneath the Scythian platform.

Abstract—Analysis of P- and S-receiver functions for 19 seismic stations on the Balkan Peninsula has been performed. Half of the stations are in Bulgaria. The crustal thickness varies from 28–30 to 50 km. The ratio of longitudinal and shear wave velocities in the upper crust reaches 2.0 in some places. In the southwest of the study area, the 410-km seismic boundary is uplifted by 10 km relative to nominal depth. The elevation may be caused by hydration and/or cooling of the mantle transition zone under the influence of the Hellenic subduction zone. A low S-wave velocity layer related to the 410-km boundary may be located atop this boundary. In the northwestern part of the study area this layer is present in spite of the absence of the 410-km boundary. A similar paradox has been previously noted in central Anatolia. Indications of a low-velocity layer are present at a depth exceeding 410 km. The simultaneous inversion of the receiver functions of the two types (P and S) and the Rayleigh wave phase velocities reveals a large (7–9%) decrease in the S-wave velocity in the upper mantle of southern Bulgaria and northern Greece. The thickness of the low-velocity layer (asthenosphere) is about 50 km. The lithosphere-asthenosphere boundary (LAB) is at depths of 40 to 60 km. In terms of tectonics, this zone is characterized as the South Balkan extension system. To the north of 43° N, the S-wave velocity in the upper mantle is usually at least 4.4 km/s and the LAB is not detected or is detected at a depth of over 80 km. The SKS analysis of azimuthal anisotropy reveals lateral zoning in the upper mantle that is correlated to velocity zoning. Probably, the mechanically weak low-velocity mantle of the South Balkan system is easily deformed, and the azimuth of the fast direction of anisotropy (20°) indicates the direction of extension. At the northern stations, the fast direction (about –30°) may be a reflection of an older process.

—The velocity structure of the lithosphere is studied beneath the Khibiny and Lovozero massifs in the northeastern part of the Baltic shield. The massifs belong to the world’s eight largest alkaline intrusions and are widely known for their unique mineralogy. The crustal and upper-mantle velocity structure beneath these two rock blocks is reconstructed down to a depth of ~300 km from P - and S -receiver functions ( PRF and SRF , respectively) based on the data on the remote earthquakes recorded by broadband seismic sensors of the Apatity (APA) and Lovozero stations (LVZ). For identifying the probable differences in the velocity structure of the Khibiny and Lovozero massifs and the distinction of this structure of Baltic Shield, the data for each station were divided into two groups containing records of the converted waves formed (a) within a closest intrusive body to a station and (b) outside the intrusive body, i.e. within the shield. The four resulting data sets were processed separately. The modeling results do not show significant differences between the analyzed structures. A sharp crust–mantle boundary is established at a depth of 40 km for the models corresponding to the shield’s structure; a similar structure of the Moho zone is established for the region of the Khibiny massif. A gradient crust-mantle transition zone with Moho at a depth of ~48 km is established in the region of ​​the Lovozero massif. In all the studied structures, the S -wave velocities in the upper mantle are somewhat higher than standard IASP91 model values; a low-velocity zone in the upper mantle is observed at the depths from 90 to 140 km. For identifying the probable anomalies at the 410- and 660-km phase transition boundaries, the calculated PRF s were stacked. The estimated delays of the converted waves from these boundaries are somewhat shorter than standard delays ​​relative to the IASP91 model and lie within the expected limits ​for paleotectonic structures.

The article Lake Van (Southeastern Turkey) Experiment.

The crustal structure in the interior of Greenland is largely unknown because of its remote location below the up to 3.4‐km‐thick ice sheet. We present a model of the crustal velocity structure in central‐eastern Greenland based on simultaneous inversion of P and S receiver functions for data acquired at 23 broadband stations between the coast and the center of the ice sheet. The area is believed to mainly include Precambrian basement and includes a part covered by Tertiary volcanic rocks and some sedimentary basins. Our results show a westward deepening Moho from less than 20 km at the coast to 50 km below central Greenland. Crustal S wave velocities are generally 3.75 km/s through the whole crust which may be relatively small for Precambrian areas, and Vp/Vs is generally around 1.73, although slightly higher in central Greenland. In the coastal area we observe anomalously low velocities at the top of the crust. In the volcanic area south of Scoresbysund Fjord this layer has very high Vp/Vs (>2), which indicates a high mafic content and the presence of water‐filled cracks in the basaltic material. In the north, outside the volcanic area, Vp/Vs is normal and the low‐velocity layer probably is instead related to the presence of sedimentary basins. At stations in the center of our study area we find low Vs and high Vp/Vs in the lower crust. Based on the Moho topography, our results do not support Airy type isostasy as explanation of the high topography in eastern Greenland.

Summary 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.

In this study, we use teleseismic P and S receiver functions (i.e., S-to-P and P-to-S converted signals) and their joint inversions to determine seismic discontinuities in the crust and upper mantle. Eight permanent broadband stations from the KOERI (Kandilli Observatory and Earthquake Research Institute) that are distributed along the eastern Pontides orogenic belt (EPOB), NE, Turkey, comprise our database. Inversion is performed by using a simulated annealing technique with and without travel time residuals. Our inversion results reveal the Moho depth, a high S velocity lid, a low-velocity zone, and the underlying upper mantle layer. The studied area is divided into two regions based on the station locations: (a) a northern region and (b) a southern region. The inversion results from the northern area produce crustal models that indicate that the uppermost crust is represented by a low P and S wave velocity (Vp = ~ 5.0 km/s and Vs = ~ 2.8 km/s). These velocities are clear evidence of (1) the sedimentary and volcanic rocks that widely crop out in the region and (2) a thinner uppermost crust, whereas the velocities of the southern region (Vp = ~ 6.0 km/s and Vs = ~ 3.1 km/s) indicate a thicker uppermost crust (~ 7 km). Our calculated Vp/Vs velocity ratio in the lower crust is approximately 1.90 and 1.80 in the northern and southern regions, respectively. These ratios are generally attributed to mafic rocks. Beneath the northern stations, the crustal thicknesses are 30, 33, 37, and 40 km from east to west, while the depths of the Moho are 46, 42, 39, and 44 km beneath the southern stations. Some velocity histograms show a transition from the high S velocity mantle lid to the low-velocity zone, which is known as the lithosphere–asthenosphere boundary. A representative value of the boundary’s depth is around 83 km in the north and ~ 88 km in the south. The hypothesis of some researchers regarding the existence of southward subduction beneath the eastern Pontides orogenic belt during the Late Mesozoic–Cenozoic is supported by our 2-D and 3-D velocity-depth models.

Receiver functions for the central Tien Shan and northern Tarim in central Asia reveal a pronounced depression on the 410 km discontinuity beneath the Permian basalts in Tarim. The depression may be caused by elevated temperature. The striking spatial correlation between the anomaly of the MTZ and the Permian basalts suggests that both may be effects of the same plume. This relation can be reconciled with the possible motion of Tarim on the order of 1000 km by assuming that the mantle layer, which has moved coherently with the plate since the Permian, extends to a depth of 410 km or more. Alternatively, the lithosphere and underlying mantle are decoupled at a depth of ∼ 200 km, but a cumulative effect of the Tarim plate motion since the Permian is less by an order of magnitude. A similar explanation is applicable to the Siberian traps.

We have obtained P-wave and S-wave receiver functions for 10 broadband seismograph stations in the Baikal rift zone (BRZ) and inverted them for seismic velocity models of the crust and upper mantle. The thinnest crust (30–35 km) is found in the Baikal basin, the thickest in the East Sayan uplift (45–50 km). Intermediate values (40 km) are found in the BRZ at distances around 100 km from Lake Baikal. A high (at least 1.8) Vp/Vs ratio is observed in the middle and lower crust. It exceeds 2.0 at some stations. In our opinion, the highest Vp/Vs ratios are due to fluid-filled porosity with a high pore pressure. The seismic lithosphere – asthenosphere boundary (LAB) is manifested by a shear velocity drop from 4.5 km/s to 4.0–4.2 km/s. Beneath the Baikal basin, the LAB is located at a depth not more than 50 km, and the S velocity drop is maximal (10 %). A similar structure is found outside the basin, underneath a segment of the East Sayan uplift. At other locations in the BRZ, a typical depth of the LAB varies from 80 to 90 km. Having considered changes in the depth of the 410 km seismic discontinuity, we cannot find any evidence of an elevated temperature of a hypothetical thermal plume beneath the BRZ.

Siberian traps are the result of huge basalt eruptions which took place about 250 Ma ago over a vast territory of Siberia. The genesis of Siberian traps is attributed to a mantle plume with a center in the region of Iceland or beneath the central Urals in terms of their present coordinates. The eruption mechanism is associated with delamination—replacement of the mantle lithosphere by the deep magma material. The receiver function analysis of the records from the Norilsk seismic station (NRIL) allows comparing these hypotheses with the factual data on the depth structure of the region of Siberian traps. The S-wave velocity section place the seismic lithosphere/asthenosphere boundary (LAB) at a depth of 155–190 km, commensurate with the data for the other cratons. The mantle lithosphere has a high S-wave velocity characteristic of cratons (4.6–4.8 km/s instead of the typical value 4.5 km/s). The seismic boundary, which is located at a depth around 410 km beneath the continents is depressed by ~10 km in the region of the NRIL station. The phase diagram of olivine/wadsleyite transformation accounts for this depression by a 50–100°С increase in temperature. At the depths of 350–400 km, the S-wave velocity drops due to partial melting. A new reduction in the S-wave velocities is observed at a depth of 460 km. The similar anomalies (deepening of the 410-km seismic boundary and low shear wave velocity at depths of 350–400 and 460–500 km, respectively) were previously revealed in the other regions of the Meso-Cenozoic volcanism. In the case of a differently directed drift of the Siberian lithosphere and underlying mantle at depths down to 500 km, these anomalies are barely accountable. In particular, if the mantle at a depth ranging from 200 to 500 km is fixed, the anomalies should be observed at the original locations where they emerged 250 Ma ago, i.e. thousands of km from the Siberian traps. Our seismic data suggest that despite the low viscosity of the asthenosphere, the mantle drift at depths ranging from 200 to 500 km is correlated with the drift of the Siberian lithospheric plate. Furthermore, the position of the mantle plume beneath the Urals is easier to reconcile with the seismic data than its position beneath Iceland because of the Siberian traps being less remote from the Urals.