Low-velocity Layer Research Articles

Crustal structure in the Weiyuan shale gas field, China, and its tectonic implications

We applied joint inversion of multifrequency receiver functions and surface wave dispersion to image crustal S-wave velocities and discontinuities beneath 29 temporary seismic stations with ~5 km station spacing in the Weiyuan shale gas field. The results show similarities and differences in the crust between the anticline and syncline areas and a widespread low-velocity layer in the synclinal sedimentary rocks. Similar crustal S-wave velocities and high velocities in the middle-lower crust and a thickness of ~12 km in the upper crust are observed in both the anticline and syncline areas. The sedimentary thickness changes mainly from 3 km to 4 km, and the average crustal thickness is ~42.4 km in the anticline area, while sedimentary thickness varies mainly from 4 km to 5 km and the average crustal thickness is ~41.0 km in the syncline area. These results suggest that the Weiyuan anticline should be a thin-skinned structure and that the study region maintains a typical cratonic structure in the crust. The observed 1–2 km-thick low-velocity layer at depths of 3–4 km in the syncline area may be related to Ordovician and Silurian shale gas strata with high porosity and low density. The observed characteristics of alternating high−/low-velocity layers and long-term hydraulic fracturing and water injection may have induced the occurrence of ML > 4.0 earthquakes in the Weiyuan shale gas field since 2018. Our study suggests that dense temporary seismic data can not only delineate small-scale structural characteristics but also provide valuable references for oil-gas exploration and earthquake mitigation.

РАСЧЕТ АПРИОРНЫХ СТАТИЧЕСКИХ ПОПРАВОК В МОВ С УЧЕТОМ ПОЛОЖЕНИЯ ЗАРЯДА ОТНОСИТЕЛЬНО ПОДОШВЫ ЗОНЫ МАЛЫХ СКОРОСТЕЙ

While processing ground-based seismic operations by the reflection method using an explosive excitation source, the standard method of compensation technique for the inhomogeneity of the low velocity layer (LVL) is based on the use of blastholes parameters: shot depth, TSP and velocity of underlying stratum, determined by the apparent velocity of refracted wave. The necessary condition for the applicability of this method is the arrangement of the charge below the base of the LVL. Therefore, to substatiate this method, it is necessary to evaluate the position of the charge relative to the roof of the LWL for each source. If the necessary condition for calculating statics computations is not met, it is necessary to solve the problem of estimating the LVL’s thickness and velocity. The presented work contains the problem of analyzing the position of the charge relative to the LVL’s position and estimating the layer parameters which is solving on the basis of joint analysis of the parameters of wells and wave refracted on the LVL base. The solution of the problem is illustrated by the example of real data processing.

Kinetic effects on the 660-km-phase transition in mantle upstreams and seismological implications

Summary The effects of reaction kinetics of bridgmanite and ferropericlase transforming to ringwoodite on elastic properties in upwelling mantle are investigated using data of kinetic experiments and internally self-consistent thermodynamic modelling of density and seismic velocities. The kinetic experiments show inhibited grain growth of ringwoodite. At the initiation of ringwoodite growth, bridgmanite completely transforms to a metastable pyrope-bearing garnet. Ringwoodite then gradually grows from the metastable assemblage of ferropericlase and garnet. The changes in mineralogy result in a low-velocity zone directly above the 660 km seismic discontinuity due to the lower seismic velocities and densities of ferropericlase and garnet compared to ringwoodite and bridgmanite. The modelling of the effects of reaction kinetics and its effect on seismic structure at ∼660 km depth shows more sensitivity to grain size than to temperature and upwelling rate. Modelling 1-D synthetic seismograms of PP (SS) underside reflections off the kinetically inhibited backward reaction to ringwoodite shows advanced traveltimes of underside reflections off ∼660 km depth of 0.2–0.8 s (1.2–1.6 s) for upwelling rates of 50 cm yr–1 and initial grain sizes between 5 and 20 cm due to the low-velocity zone above the 660 km discontinuity. The finite width of the low-velocity layer results in frequency-dependent behaviour of PP and SS underside reflection amplitudes, with higher amplitudes towards shorter periods. The effect on the traveltimes of P-to-s conversions used for receiver function is small (<0.7 s) but the inhibited backward reaction leads to strong amplitude changes and significant waveform variations. The effects of reaction kinetics in mantle upwellings might serve as an additional means to map regions of large-scale upwellings and to constrain grain size in the lower mantle.

Topographic Response to Simulated Mw 6.5–7.0 Earthquakes on the Seattle Fault

ABSTRACTWe explore the response of ground motions to topography during large crustal fault earthquakes by simulating several magnitude 6.5–7.0 rupture scenarios on the Seattle fault, Washington State. Kinematic simulations are run using a 3D spectral element code and a detailed seismic velocity model for the Puget Sound region. This model includes realistic surface topography and a near-surface low-velocity layer; a mesh spacing of ∼30 m at the surface allows modeling of ground motions up to 3 Hz. We simulate 20 earthquake scenarios using different slip distributions and hypocenter locations on a planar fault surface. Results indicate that average ground motions in simulations with and without topography are similar. However, shaking amplification is common at topographic highs, and more than a quarter of all sites experience short-period (≤2 s) ground-motion amplification greater than 25%–35%, compared with models without topography. Comparisons of peak ground velocity at the top and bottom of topographic features demonstrate that amplification is sensitive to period, with the greatest amplifications typically manifesting near a topographic feature’s estimated resonance frequency and along azimuths perpendicular to its primary axis of elongation. However, interevent variability in topographic response can be significant, particularly at shorter periods (<1 s). We do not observe a clear relationship between source centroid-to-site azimuths and the strength of topographic amplification. Overall, our results suggest that although topographic resonance does influence the average ground motions, other processes (e.g., localized focusing and scattering) also play a significant role in determining topographic response. However, the amount of consistent, significant amplification due to topography suggests that topographic effects should likely be considered in some capacity during seismic hazard studies.

Geodynamic Characteristics in the Southwest Margin of South China Sea

The strike-slip fault system in the southwestern margin of South China Sea (SCS) lies on the transition zone between the continental shelf and slope of SCS, which is an important ocean–continent boundary. By using submarine heat flow data, a seismic shear wave tomography model, and gravity potential field data, this paper investigates the distribution of submarine heat flow in the southwestern margin of SCS, the thermal–rheological structure of the crust and mantle, the temperature–viscosity characteristics of the upper mantle Vs low-velocity layer, the tangential stress field of the rheological boundary layer at the lithosphere base, and the convective velocity structure of the mantle asthenosphere. Our new results show that the deep geothermal activity in the southwestern margin of SCS is intense, and the high heat flow area of the mantle with Qm/Qs >70% is distributed along an NNE-trending strip. Moreover, both the east and west sides of the strike–slip fault zone correspond to two low-value areas with a viscosity coefficient of 1021–1022 Pa⋅s at Moho depth, and beneath the Nansha Block are strong and cold blocks with a viscosity coefficient of 1024–1025 Pa⋅s. The northward and eastward shear stress components τN and τE of the rheological boundary layer at the base of the lithosphere mantle decrease with depth. At 65-km depth, both τN and τE are greater than 5.5 × 108 N/m2. At 100-km depth, both τN and τE are less than 1 × 108 N/m2. The calculation results based on the seismic shear wave model of the upper mantle and the gravimetric geoid model indicate that the depth of 120–250 km is the low-velocity layer, and the average temperature of the mantle at 180-km depth can be up to 1,300°C. Moreover, the average effective viscosity coefficient is close to 1018 Pa⋅s, which satisfies the temperature and viscosity conditions for partial melting or convective migration of mantle material. The mantle convection calculation results show that the average flow rate is 8.5 cm/a at 200-km depth and 2.2 cm/a at 400-km depth.

A Versatile Solver of the Normal Modes for Horizontal Stratified Complicated Models

Abstract Because of the current widespread attention and interest in surface waves, whether in theoretical study or in applications, the significantly different application scales in diverse fields and the various complex media types with special properties necessitate an effective analysis tool for computing normal modes of surface waves for different models to meet all kinds of application demands. By further improving and generalizing the algorithm of generalized reflection and transmission coefficients, we propose a versatile solver of normal modes for horizontal stratified complicated models. Under a series of tests with commonly encountered models of various types and of abnormal medium properties (e.g., low-velocity layers included, fluid sandwiched), whether on the model adaptability, on the capability of the complete computation of the normal modes, or on the stability or computing efficiency, this solver fully demonstrates that it is a powerful tool for the theoretical calculation and inversion of surface waves.

High resolution seismic reflection PP and PS imaging of the bedrock surface below glacial deposits in Marsta, Sweden

Multi-component high resolution seismic reflection data were acquired at the Uppsala University field test site in Marsta, Sweden, in March 2019 with the aim of obtaining an improved understanding of the subsurface structure in the area. An advantage of the site is that a number of boreholes have been drilled there for both hydrogeological and geophysical purposes, allowing surface geophysics to be compared with downhole information. The presence of a low velocity layer above the water table generates significant processing challenges due to trapped waves. A further complication with the data is that the upper part of this layer was frozen at the time of the survey, resulting in the sediments just below the surface having a significantly higher velocity than those below. By analyzing and processing both the vertical and radial component data it was possible to build a velocity model that is consistent with the observed data. A strong PS converted reflection allows the bedrock to be imaged on the radial component stacks to much higher resolution than on the vertical component stacks. Both common conversion point binning and pre-stack depth migration were used to process the radial component data. We confirmed that our processing strategy was effective by generating synthetic data that were processed in a similar manner as the real data. The PS images indicate a step in the bedrock of about 2 m, with depth to it increasing from about 15 m to 17 m close to one of the boreholes. This step is not observed on the PP stacks, due to their lower resolution.

Stratification in planetary cores by liquid immiscibility in Fe-S-H

Liquid-liquid immiscibility has been widely observed in iron alloy systems at ambient pressure and is important for the structure and dynamics in iron cores of rocky planets. While such previously known liquid immiscibility has been demonstrated to disappear at relatively low pressures, here we report immiscible S(±Si,O)-rich liquid and H(±C)-rich liquid above ~20 GPa, corresponding to conditions of the Martian core. Mars’ cosmochemically estimated core composition is likely in the miscibility gap, and the separation of two immiscible liquids could have driven core convection and stable stratification, which explains the formation and termination of the Martian planetary magnetic field. In addition, we observed liquid immiscibility in Fe-S-H(±Si,O,C) at least to 118 GPa, suggesting that it can occur in the Earth’s topmost outer core and form a low-velocity layer below the core-mantle boundary.

A Partial Molten Low‐Velocity Layer Atop the Mantle Transition Zone Beneath the Western Junggar: Implication for the Formation of Subduction‐Induced Sub‐Slab Mantle Plume

AbstractDeep subduction of plate generally could induce a partial molten low‐velocity layer (LVL) atop the mantle transition zone (MTZ) around the subduction zone. But the effects of the LVL to its upper lithosphere are less known, especially at the oceanward direction of subducted slab. Here, we image the seismic velocity structure atop the MTZ by matching the synthetics and the observed triplicated P‐ and SH‐wave waveforms recorded by Xinjiang seismic network from four earthquakes in the southwestern Siberia. Our observations reveal a partial molten LVL atop the MTZ beneath the western Junggar (WJ) in the southwestern Central Asian Orogenic Belt. The SH‐wave velocity drop for the LVL is −5.6% which is larger than −4.4% of the P‐wave when its thickness is 29 km. It is inferred that the subducted Paleo‐Asian oceanic plate induces hydrous MTZ materials underneath the subducted slab to upwell to the upper mantle, and form the partial molten LVL. Combined with previous geophysical and geological results, we propose a new geodynamical process involving the lithosphere and MTZ underneath the subducted slab: the melt in the LVL is likely to upwell through slab window, which could be triggered by mid‐oceanic ridge subduction or self‐sustained buoyancy. This mode could account for the contemporaneous break‐off fossil oceanic plate, ocean island basalts‐type rocks, extensive A‐type granitoids, adakite, and porphyry Cu‐Au in the WJ during the late Carboniferous to early Permian.

Velocity Anomalies Around the Mantle Transition Zone Beneath the Qiangtang Terrane, Central Tibetan Plateau From Triplicated P Waveforms

AbstractThe Qiangtang terrane exhibits many unique features compared with surrounding areas in the Tibetan plateau. It has been largely remodeled by the northward downwelling of the Indian lithosphere and the vertical upwelling of the Eurasian asthenosphere. Here, we investigate the P‐wave velocity around the mantle transition zone (MTZ) beneath the Qiangtang terrane using high‐frequency triplicated waveforms recorded by the China Digital Seismic Network. We found a low‐velocity layer (LVL) atop the MTZ and a high‐velocity anomaly (HVA) in the lower part of the MTZ. They are characterized by thickness of 36 km with velocity drop of −2.0% and 155 km with velocity increment of 1.2%, respectively. The 410 and 660 km discontinuities are depressed by different depths of 12 and 30 km, suggesting a thickened MTZ. In combination with previous multi‐disciplinary observations, we infer that the LVL is a hydrous partial melt affected by dehydration and high temperature, and the HVA is due to water and low temperature conditions. The downwelling cold continental lithosphere may have accumulated in the lower MTZ and induced the hydrous hot MTZ materials to upwelling until the shallow depths of the upper mantle beneath the Qiangtang terrane.

Rayleigh-Wave Dispersion Analysis and Inversion Based on the Rotation.

Rotational observation is essential for a comprehensive description of the ground motion, and can provide additional wave-field information. With respect to the three typical layered models in shallow engineering geology, under the assumption of linear small deformation, we simulate the 2-dimensional radial, vertical, and rotational components of the wave fields and analyze the different characteristics of Rayleigh wave dispersion recorded for the rotational and translational components. Then, we compare the results of single-component inversion with the results of multi-component joint inversion. It is found that the rotational component has wider spectral bands and more higher modes than the translational components, especially at high frequencies; the rotational component has better anti-interference performance in the noisy data test, and it can improve the inversion accuracy of the shallow shear-wave velocity. The field examples also show the significant advantages of the joint utility of the translational and rotational components, especially when a low-velocity layer exists. Rotational observation shall be beneficial for shallow surface-wave exploration.

Azimuthal multiple signal classification of dispersive and aliased surface waves recorded in 3D seismic acquisition

Irregular acquisition geometry causes discontinuities in the appearance of surface wave events, and a large offset causes seismic records to appear as aliased surface waves. The conventional method of sampling data affects the accuracy of the dispersion spectrum and reduces the resolution of surface waves. At the same time, “mode kissing” of the low-velocity layer and inhomogeneous scatterers requires a high-resolution method for calculating surface wave dispersion. This study tests the use of the multiple signal classification (MUSIC) algorithm in 3D multichannel and aliased wavefield separation. Azimuthal MUSIC is a useful method to estimate the phase velocity spectrum of aliased surface wave data, and it represents the dispersion spectra of low-velocity and inhomogeneous models. The results of this study demonstrate that the mode kissing affects dispersion imaging, and inhomogeneous scatterers change the direction of surface-wave propagation. Surface waves generated from the new propagation directions also are dispersive. The scattered surface wave has a new dispersion pattern different from that of the entire record. Diagonal loading is introduced to improve the robustness of azimuthal MUSIC, and numerical experiments demonstrate the resultant effectiveness of imaging aliasing surface waves. A phase-matched filter is applied to the results of azimuthal MUSIC, and phase iterations are unwrapped in a fast and stable manner. Aliased surface waves and body waves are separated during this process. Overall, field data demonstrate that the azimuthal MUSIC and the phase-matched filters can successfully separate aliased surface waves.

From Subduction to LLSVP: The Core‐Mantle Boundary Heterogeneities Across North Atlantic

AbstractThe distribution of Ultra‐low Velocity Zones (ULVZs) is generally associated with the Large Low Shear Velocity Zones (LLSVPs). However, subducted slabs may be important for the characteristics of the ULVZs as well. Here we use ScS precursor (SdS) and postcursor (ScscS) phases, as well as Scd phases recorded by USArray to investigate the lowermost mantle structure across the north Atlantic, which spans the African LLSVP and the subducted Farallon slab. Our results show a ULVZ located near the edge of the African LLSVP, with thicknesses ranging from 11–16 km and S wave velocity reductions of 12%–24%. At the same time, a thin high‐velocity layer (HVL) with thicknesses of 20–30 km, Vs increases of 3%–4%, is also detected outside the LLSVP in the northwest region. Further west there is a clear D" layer at 250 km above the core mantle boundary (CMB), overlaid by a low velocity layer with Vs reduction of 3%. This transition profile from D" to HVL to ULVZ provides an excellent opportunity for us to understand the heterogeneities near the CMB region. We propose that interactions between the subducted Farallon slab with ambient mantle and LLSVP material, together with thermal effects, may result in this transition between different seismic structures along the CMB.

Application of active-source surface waves in urban underground space detection: a case study of Rongcheng County, Hebei, China

Active-source surface wave exploration is advantageous because it has high imaging accuracy, is not affected by high-speed layers and has a low cost. It has unique advantages in the investigation of shallow surface structures. For the development and utilization of urban underground space, there are two important parameters in the shallow surface, namely, shear wave velocity (vs.) and site predominant period, which determine the elevation and aseismic grade of the building design. The traditional method mainly obtains the above two parameters through the testing and measurement of drilling samples. However, this method is extremely expensive and time consuming. Therefore, this paper uses the multichannel surface wave acquisition method to extract the fundamental dispersion curve of the single shot data using the phase shift method and obtains the vs. characteristics in the uppermost 40 m by inversion. According to the vs. profile, the following two conclusions were obtained. First, the study area can be roughly divided into 5 layers, among which the layers 0-8 m, 14-20 m, and 20-30 m are low-velocity layers, corresponding to miscellaneous fill, a water-bearing sand layer and a sand layer. Therefore, the vs. is relatively low, and the layers at 8-14 m and 30-40 m are high-velocity layers that are mainly composed of clay, with a relatively better compactness and relatively high vs. values. In addition, the low-speed anomaly suddenly appears in the high-speed area at 20-40 m. Combining this with geological data suggests that it is an ancient river channel. Second, according to the vs. value, the V_se(equivalent shear wave velocity) was calculated. The construction site soil is classified as Category III, with good engineering geological conditions. In addition, according to vs., we calculate the site predominant period to be 0.56-0.77 s. Therefore, in the overall structural design of foundation engineering, the natural vibration period of the structure should be strictly controlled to avoid the predominant period of the site.

The 2017 Ischia Earthquake (Southern Italy): Source Mechanism and Rupture Model From the Inversion of a Near-Source Strong Motion Record

With the aim to investigate the rupture complexity and the radiated wave field of the 2017, Mw 3.9, Ischia earthquake, south-west of Naples (Italy), we used finite-fault modeling to invert the near-source (< 1-km epicentral distance) horizontal-component velocity records of the accelerometric station (IOCA) and searched for the best-fit kinematic rupture parameters. This analysis showed that the rupture nucleated at about 600 m west of IOCA and 1.1-km depth, along a 1 km, NW-SE striking fault (i.e., thrust with right-lateral component), with a rupture velocity of about 0.7 km/s. The retrieved rupture model coupled with multipath reverberations effects related to a thin, low-velocity near-surface volcanic sedimentary layer, well explains the observed long ground motion duration and the large amplitudes recorded all over the island. Finally, the apparent source time function (STF), obtained from inverse modeling using a theoretical Green’ function (GF), is validated by implementing an empirical GF (EGF) analysis.

Body wave dispersion characteristics of regional deep earthquake at Southern Ryukyus subduction zone

The convergent plate boundaries known as the subduction zone is the location where the cold, hydrate, and old oceanic plate subduct under the hot, dry, and young continental plate. The top of the oceanic plate, i.e. the oceanic crust usually is less than 6 km thick carries the hydrate material to the hot continental mantle, which is commonly an indication of the occurrence of the deep earthquake. From the recent studies, the deep regional earthquake body wave signal that occurred at the subduction zone was found dispersed and or shown as a waveguide. Thus, by examining these signals, we could further estimate the body wave characteristics at the subduction zone. A narrow bandpass signal and the maximum of Hilbert transform envelope are being used to estimate the delay time of the dominant frequency energy at the Southern Ryukyus subduction zone. It was found that the Southern Ryukyus has a larger delay with the dominant frequency at 2 Hz, thus, the signal travel within the oceanic crust where it acts as a low-velocity layer compared to the surrounding continental and oceanic mantle, inversely it is known as a high-quality material (preserve the high-frequency energy).

Dispersive seismic waves in a coal seam around the roadway in the presence of excavation damaged zone

Excavation damaged zone (EDZ) is a fractured rock mass in the vicinity of the roadway. Knowing its elastic properties, such as seismic velocity, is crucial for the safety of underground mining. However, the seismic wavefield's complexity due to the low-velocity layer (e.g., a coal seam) makes it difficult to obtain reliable velocity estimates. Here, we study the influence of a roadway and EDZ on the seismic wavefield propagation inside the coal seam along the roadway close to the sidewall based on numerical simulation and in-seam seismic measurements. We focus on dispersive waves due to their dominant energetic contribution and, hence, application potential. We use the finite-difference method and viscoelastic model. First, we analyze seismic wave propagation within a simple rock-coal-rock model. Then, we add a cylindrically shaped roadway with 3-m thick EDZ to the model. We observe a strong so-called roadway mode. Polarization analysis shows that such a mode in a horizontal plane in the center of the coal seam is the mix of Love-type channel and Rayleigh surface tunnel waves. Moreover, the EDZ causes a significant decrease of the group velocity, which turns out to be even lower than the lowest group velocity of the surface tunnel waves which travels solely in a coal seam. We also observe that the roadway mode with the presence of the EDZ is less dispersive, which can be useful with imaging the EDZ by those modes. Those roadway mode features were confirmed by the real seismic data from an underground experiment in the Rydułtowy coal mine in Poland. The rock mass deformation in the vicinity of the roadway strongly correlates with the roadway mode dispersion characteristic. Our study shows the great potential of using roadway mode in imaging and monitoring of underground excavations in the presence of a low-velocity layer.

A low-velocity layer atop the mantle transition zone beneath the western Central Asian Orogenic Belt: Upper mantle melting induced by ancient slab subduction

Increasing evidence shows that the low-velocity layer (LVL) atop the mantle transition zone (MTZ) may play a critical role in Earth's evolution. The existence and formation of the LVL in continental interiors remain ambiguous. This study investigates the P-wave velocity structure atop the MTZ by matching P- and sP-wave triplicated waveforms simultaneously in the western Central Asian Orogenic Belt far away from continental margins. Two groups of nearly orthogonal triplications are allocated using records of four earthquakes at stations from the China National Seismic Network and the Incorporated Research Institutions for Seismology. Our observations resolve an LVL with 21–23 km thickness and a P-wave velocity drop of approximately −5.7% to −5.8% atop the MTZ. The two nearly orthogonal directions show little variation in the velocity structure of the LVL. It is inferred that the LVL results from partial melting induced by water and/or other volatiles released from the subducted Paleo-Asian oceanic slab and, hence, has existed since the Paleozoic, with possible variations in melt fraction. The long-lived partially molten LVL may play an important role in the intraplate magmatism and mantle dynamics in the Central Asian Orogenic Belt. Our results provide strong seismic evidence for the LVL existing in the continental interior, indicating that the LVL may be pervasive in Eurasia because of vast subductions of ancient plates; this provides new insights into the material circulation in the ancient oceanic subduction zones.

Mantle rain toward the Earth's surface: A model for the internal cycle of water

The internal or deep water cycle controls the volume of the oceans at the surface of the Earth. The advent of subduction 2–3 billion years ago initiated the transport of water back to the Earth's interior. With one ocean mass injected into the deep mantle over the last 2–3 billion years, some mantle regions must have become saturated and thus turned into a deep source of water. The mantle transition zone (MTZ) between 410 and 660 km depths is unlikely to be a source of hydrous melt, because its minerals can integrate several thousand ppm of water. On the contrary, the low-velocity layer (LVL) lying above the 410 km-discontinuity is one such source. As proposed by the “Transition-Zone-Water-Filter Model”, the LVL is ubiquitously formed by the global uplift of the hydrous MTZ as a counter flow of subduction of slabs into deeper regions. The seismic signature of the LVL is compatible with the presence of 0.5–1% melt. This melt is produced by dehydration melting during upwelling of the mantle transition zone (MTZ) containing 2200(300) ppm wt H2O, which corresponds to 0.6 ocean mass stored today in the MTZ. Hydrous silicate melt can be gravitationally stable just above the 410 km discontinuity. We propose, here, that at the upper limit of the LVL it becomes buoyant, especially where the mantle is particularly hot and/or hydrous. Once it becomes buoyant, the melt can percolate rapidly upwards through the mantle. As a consequence, the olivine-bearing mantle (OBM) could be almost saturated in water, due to the presence of upwelling hydrous melt. On its path, the melt may be responsible for the seismic low-velocity zones at mantle depths of between 80 and 150 km. It could also be a source for refertilisation of the lithospheric mantle. Based on this model, there should be ~1.0 oceanic mass (OM) stored in the upper mantle today. Secular cooling of the mantle implies an increased capacity of the OBM minerals to store water. The related decrease of oceans' mass at the Earth's surface is estimated to ~20% per one billion years.

Deep Structure and Dynamics of the Central Balkan Peninsula from Seismic Data

—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.