Seismic Shear Wave Velocity Research Articles

Does VS30 reflect seismic amplification? Observations from the West Bohemia Seismic Network

The present study addresses verification of average seismic shear-wave velocity from the surface to a depth of 30 m (VS30) as a suitable proxy for a seismic amplification. For this purpose, we used instrumentally homogeneous and spatially dense seismic network WEBNET (West Bohemia Seismic Network), designed to monitor an intraplate earthquake swarm activity in the West Bohemia/Vogtland region (Czech Republic/Germany). Using a Multichannel analysis of surface waves (MASW) shear-wave velocity models and parameters VS30 and H800 (depth of Vs > 800 m/s) were obtained at 17 WEBNET sites. VS30 were compared with (i) H800 and (ii) determined bedrock densities. To understand the relation between VS30 and site amplification, VS30 values were correlated with amplitudes of two earthquakes: (1) Mw 6.4 Petrinja, Croatia (12/2020) and (2) Mw 4.2 SE of Vienna, Austria (03/2021) both recorded by the WEBNET. The correlation analysis examined four categories of seismic waves in nine frequency windows and described the relation between amplification and VS30 using newly defined regression model. The results show that for the regression model, the frequency window with the highest correlation is in the 1–3 Hz range, and this dependence is statistically best observed in the full wave record. The amplification generally decreases with increasing VS30. However, a large scatter in amplification within Eurocode 8 category B is observed. Based on the observations a new general approach is put forward to finely indicate the relation between amplification and VS30 and the use of other site proxies is discussed.

A lower crust shear zone facilitates delamination and continental subduction under the Apennines

Physical properties and structure of the lithosphere are the first step to constrain the evolution of mountain belts. Here we show detailed shear wave velocity profiles of the lithosphere in the Apennines that clarify a controversial aspect of continental subduction: the intricate mechanism of crust delamination from the downgoing plate. From the analysis of complete and dense teleseismic Receiver Function data set, we find that the delamination of the continental lithosphere is favored by the development of a low seismic shear wave velocity zone in the middle-lower crust. We observe a double Moho below the external portions of the present mountain range, suggesting the progressive formation of the shallow interface. The delamination edge is located in the forearc, far eastward than expected, implying that the re-equilibration of the thermal unbalance, generated by the mantle substitution, may last 10-7 Myr.

Geological Mapping Optimisation Using Satellite Gravity, Satellite Imagery, and Seismic Shear Velocity (Vs30) to Monitor Lithological Condition in Geothermal Area of Mount Salak, West Java

Conventional geological mapping has several challenges such as limitation of area coverage, tough terrain, unstable weather condition, high-cost survey, also limitation of expert geologist that capable to do geological mapping. Remote sensing is an alternative solution to geological mapping through the combination of satellite gravity, satellite imagery, and seismic shear wave velocity. This research aims to optimise geological mapping activity in the Mount Salak geothermal area from ineffective area coverage mapping and inefficient budgeting allocation. Integration of rock density from satellite gravity, lithological distribution from satellite imagery, and seismic shear wave velocity from Vs30 are giving more detailed lithological units interpretation with specific physical conditions. There are two main area of Mount Salak that should be preserved geothermal resource sustainable. Northeast area needs to preserve heat source and groundwater recharge area, that consist of dense vegetation as landcover, relatively high rock density value (2.30 - 2.50 gr/cm3), high seismic shear wave velocity (600 – 900 m/s), also dominantly covers by Andesitic-Basaltic Lava. Southwest area preserve reservoir and caprock of geothermal conceptual model, where covers by dense vegetation with several bare land as geothermal wells and powerplants, relatively low rock density value (2.00 – 2.20 gr/cm3), moderate seismic shear wave velocity value (450-750 m/s), also dominated by several pyroclastic product such as Lapilli and Tuff. This research shows good indication in geothermal resource preservation in the Mount Salak, that keep natural aspect of geothermal conceptual model still preserved well.

Rapid peak seismic response prediction of two-story and three-span subway stations using deep learning method

A deep learning-based rapid peak seismic response prediction model for the most common two-story and three-span subway stations is proposed in this study. The established model predicts the peak seismic responses of subway stations with a data-driven fashion and using limited information. The prediction model extracts the features of ground motions using one-dimensional convolutional neural network (1D-CNN) and then integrates the information of subway stations (i.e., the seismic fortification intensity, buried depth, and shear wave velocity) through a fully connected neural network for regression, resulting in peak seismic responses, namely the peak floor acceleration (PFA) and maximum inter-story drift ratio (MIDR). The model is trained using 19,200 samples obtained from the nonlinear time-history analyses (NLTHAs) of the designed 48 typical subway station structures. Furthermore, the external model verification was performed on 960 additional samples. For the predictions of PFA and MIDR, the coefficient of determination (R2) values are 0.967 and 0.986, respectively, and the damage states of subway stations are further evaluated, achieving an accuracy of 95.0%. These indicates that the model has good predictive performance and generalization ability. Moreover, the prediction model demonstrates a significantly higher computational efficiency compared to numerical simulation methods.

Magma and hydrothermal sources below the northern part of Paramushir Island (Kuril Arc) inferred from ambient noise tomography

Paramushir is the northernmost island of the Kuril Arc comprising several Holocene volcanoes, of which two are presently active: Ebeko and Chikurachki. We present the first crustal-scale three-dimensional seismic shear-wave velocity model of the northern part of Paramushir derived with the use of data of 20 temporary stations operated in 2021–2022 and one permanent station. The continuous seismic data were used to obtain the Rayleigh wave dispersion curves for periods ranging from 0.5 s to 12 s through computing ambient seismic noise cross-correlation functions. Then the 3D shear wave velocity distribution was constructed by seismic tomography. The synthetic tests demonstrate that the model has sufficient resolution down to 7–10 km depth. We observe a series of low-velocity anomalies at the depths of 4–6 km below the volcano-origin Vernadsky Ridge. They may likely represent magma reservoirs that are responsible for Holocene eruptions along the ridge. These anomalies are overlain by the high-velocity anomalies associated with the rigid cover consisting of an upper-crustal granite-granodiorite body and consolidated magma intrusions. Along the eastern flank of Ebeko we reveal a thin low-velocity anomaly at a depth of ∼1 km that can be interpreted as a water-saturated layer. The contact of this layer with hot magma intrusions leads to the formation of a large amount of steam that is ejected during the frequent phreatic eruptions of Ebeko and from numerous fumaroles in the summit area. On the other hand, this layer, which is following down to the Pacific Coast, might be promising for the geothermal energy exploitation.

A comparative study of seismic tomography models of Southwest China

The margin of the Tibetan Plateau of Southwest China is one of the most seismically active regions of China and is the location of the China Seismic Experimental Site (CSES). Many studies have developed seismic velocity models of Southwest China, but few have compared and evaluated these models which is important for further model improvement. Thus, we compared six published seismic shear-wave velocity models of Southwest China on absolute velocity and velocity perturbation patterns. The models are derived from different types of data (e.g., surface waves from ambient noise and earthquakes, body-wave travel times, receiver functions) and inversion methods. We interpolated the models into a uniform horizontal grid (0.5° × 0.5°) and vertically sampled them at 5, 10, 20, 30, 40, and 60 km depths. We found significant differences between the six models. Then, we selected three of them that showed greater consistency for further comparison. Our further comparisons revealed systematic biases between models in absolute velocity that may be related to different data types. The perturbation pattern of the model is especially divergent in the shallow part, but more consistent in the deep part. We conducted synthetic and inversion tests to explore possible causes and our results imply that systematic differences between the data, differences in methods, and other factors may directly affect the model. Therefore, the Southwest China velocity model still has considerable room for improvement, and the impact of inconsistency between different data types on the model needs further research. Finally, we proposed a new reference shear-wave velocity model of Southwest China (SwCM-S1.0) based on the three selected models with high consistency. We believe that this model is a better representation of more robust features of the models that are based on different data sets.

Correlations between seismic b-value and heat flow density in Vlora-Lushnja-Elbasani-Dibra Fault Zone in Elbasani area, central Albania

In this study, the correlations between the heat flow density and seismotectonic b-value in the Elbasani area of Albania were investigated to understand the how low-velocity layers underneath the Elbasani area in central Albania struggling the large-scale Vlora-Lushnja-Elbasani-Dibra Fault Zone affect the heat flow data. For this purpose, the heat flow and regional distribution of b-value were imaged for different locations and depths. To achieve the analysis, the Albanian Seismological Catalogue for the period between 1 July 1968 and 26 December 2022, including 1830 earthquakes with a local magnitude of 0.5 ≤ Ml ≤ 5.2 that occurred at the depths below 70 kilometres, was considered. The b-value was calculated as 1.03 ± 0.06 by considering the magnitude of the completeness value as Mc-value = 2.6. This result shows that the b-value of earthquake distribution in the Vlora-Lushnja-Elbasani-Dibra (VLED) fault zone is well represented by the Gutenberg and Richter (G-R) scaling law with the b-value close to 1.0. The regional variations of the b-value show that b-values smaller than 0.9 were observed in the western and south-western parts of the Llinxha-Kozan thermal water belt. The depth distribution of the b-value indicates that there exists a sharp decrease in the b-values from 1.15 to 0.7 in the depths varying from 5 to 20 km. The highest heat flow values were observed on the Dumrea diapiric dome and in the central part of the Elbasani area. Thus, our analysis indicates significant and robust correlations between the geothermal and earthquake distribution. The discontinuity of Moho interface is deep in the regions where high b-values were observed. Low b-values are found at the depths ranging from 20–25 km and 35–40 km in the Dumrea evaporite massif area. We have evidence that high b-values and large heat flow values are related to the low seismic velocity layers underneath the Elbasani area. The low-velocity zone (LVZ) in Albania occurs in the Earth’s crust and in the upper mantle. It is characterized by an unusually low seismic shear wave velocity compared to the surrounding depth intervals. It is well known that the low-velocity layers in the Elbasani area are determined in the upper crust, at shallow depths of 2–4 km, and in the middle crust at a 10–14 km depth. Hence, it is suggested that the Moho interface in the eastern part of the Elbasani area is relatively deep (45 km) compared to the western part of Albania (35 km), and the magnitude of earthquakes is smaller where the high heat flow values were observed.

Estimating core-mantle boundary temperature from seismic shear velocity and attenuation

The temperature at Earth’s core-mantle boundary (CMB) is a key parameter to understand the dynamics of our planet’s interior. However, it remains poorly known, with current estimate ranging from about 3000 K to 4500 K and more. Here, we introduce a new approach based on joint measurements of seismic shear-wave velocity,VS, and quality factor,QS, in the lowermost mantle. Lateral changes in bothVSandQSabove the CMB provide constraints on lateral temperature anomalies with respect to a reference temperature,Tref, defined as the average temperature in the layer immediately above the CMB. The request that, at a given location, temperature anomalies inferred independently fromVSandQSshould be equal gives a constraint onTref. CorrectingTreffor radial adiabatic and super-adiabatic increases in temperature gives an estimate of the CMB temperature,TCMB. This approach further relies on the fact thatVS-anomalies are affected by the distribution of post-perovskite (pPv) phase. As a result, the inferredTrefis linked to the temperatureTpPvat which the transition from bridgmanite to pPv occurs close to the CMB. A preliminary application toVSandQSmeasured beneath Central America and the Northern Pacific suggest that forTpPv= 3500 K,TCMBlies in the range 3,470–3880 K with a 95% likelihood. Additional measurements in various regions, together with a better knowledge ofTpPv, are however needed to determine a precise value ofTCMBwith our method.

Using Seismic Cross-Hole Method to Evaluate Soil Improvement for Outside Wall in Al-Abbas Holy Shrine in Holy Karbala, Iraq

The aim of this study is to use the seismic cross-hole survey in evaluating the injection work to fill the cavities spread in the soil underneath the old foundation of the outer wall of the Al-Abbas holy shrine. A seismic cross-hole survey was conducted for eight profiles, four of these are applied before grouting and four profiles are done after grouting at Al-Imam Musa Al-Kadhum and Al-Imam Sahib Al-Zaman gates. Using the longitudinal and shear wave velocities for different depths ranging from 1 to 15 m. The results of the survey indicate that the soil injection process was successful and effective in filling the cavities. The results of cross- hole survey after the grouting show the percent of variation is 0.7-77 percent for seismic shear wave velocity more than before grouting to variant depths.

Peak ground acceleration prediction using artificial neural networks for Kachchh, Gujarat, India

The uncertainty in the empirical ground motion prediction models (GMMs) for any region depends on several parameters. In the present work, we apply an artificial neural network (ANN) to design a GMM of peak ground acceleration (PGA) for Kachchh, Gujarat, India, utilizing independent input parameters viz., moment magnitudes, hypocentral distances, focal depths and site proxy (in terms of average seismic shear-wave velocity from the surface to a depth of 30 m (Vs30)). The study has been performed using a PGA dataset consisting of eight engineering seismoscope records of the 2001 Mw7.7 Bhuj earthquake and 237 strong-motion records of 32 significant Bhuj aftershocks of Mw3.3–5.6 (during 2002–2008) with epicentral distances ranging from 1.0 to 288 km. We apply a feed-forward back propagation ANN method with 8 hidden nodes, which is found to be optimal for the selected PGA database and input–output mapping. The standard deviation of the error has been utilized to examine the performance of our model. We also test the ground motion predictability of our ANN model using real recordings of the 2001 Bhuj mainshock, two Mw5.6 Kachchh aftershocks and the 1999 Mw6.4 Chamoli mainshock. The standard deviation of PGA prediction error estimates in log10 units is found to be ± 0.2554. Also, the model predictability of our ANN model suggests a good prediction of the PGA for earthquakes of Mw5.6–7.7, which are occurring in Kachchh, Gujarat, India.

Elasticity of Hydrated Al‐Bearing Stishovite and Post‐Stishovite: Implications for Understanding Regional Seismic VS Anomalies Along Subducting Slabs in the Lower Mantle

AbstractSeismic studies have found seismic scatterers with −2 to −12% shear velocity anomalies along some subducting slabs at 700–1900 km depth. The ferroelastic post‐stishovite transition in subducted mid‐ocean ridge basalt (MORB) has been linked to these seismic features, but compressional and shear wave velocities (VP and VS) and full elastic moduli (Cij) of Al,H‐bearing stishovite and post‐stishovite at high pressure remain uncertain. Here we have determined Raman shifts of optic modes and equation of state parameters of two hydrated Al‐bearing stishovite crystals, Al1.3‐SiO2 (1.34 mol% Al and 0.55 mol% H) and Al2.1‐SiO2 (2.10 mol% Al and 0.59 mol% H), up to ∼70 GPa in diamond anvil cells coupled with Raman spectroscopy and X‐ray diffraction. The experimental data are modeled using a pseudoproper Landau theory to derive full Cij and sound velocities across the post‐stishovite transition at high pressure. The Al and H dissolution in stishovite significantly reduces the transition pressure to 21.1 GPa in Al1.3‐SiO2 and to 16.1 GPa in Al2.1‐SiO2, where the transition is manifested by approximately 29% VS reduction. Considering that stishovite with approximately 1.3 mol% Al and 0.6 mol% H could account for 20 vol% in subducted MORB at the top‐lower mantle, the Al,H‐bearing post‐stishovite transition with a Clapeyron slope of 65 K/GPa would occur at about 1060 km depth with −7(4)% VS anomaly. The VS anomalies across the Al,H‐bearing post‐stishovite transition can help explain the seismically‐observed depth‐dependent VS anomalies along some subducting slabs in the top‐ to mid‐lower‐mantle depths including the Tonga subducting slab.

Seismic radiation from wind turbines: observations and analytical modeling of frequency-dependent amplitude decays

Abstract. In this study, we determine spectral characteristics and amplitude decays of wind turbine induced seismic signals in the far field of a wind farm (WF) close to Uettingen, Germany. Average power spectral densities (PSDs) are calculated from 10 min time segments extracted from (up to) 6 months of continuous recordings at 19 seismic stations, positioned along an 8 km profile starting from the WF. We identify seven distinct PSD peaks in the frequency range between 1 and 8 Hz that can be observed to at least 4 km distance; lower-frequency peaks are detectable up to the end of the profile. At distances between 300 m and 4 km the PSD amplitude decay can be described by a power law with exponent b. The measured b values exhibit a linear frequency dependence and range from b=0.39 at 1.14 Hz to b=3.93 at 7.6 Hz. In a second step, the seismic radiation and amplitude decays are modeled using an analytical approach that approximates the surface wave field. Since we observe temporally varying phase differences between seismograms recorded directly at the base of the individual wind turbines (WTs), source signal phase information is included in the modeling approach. We show that phase differences between source signals have significant effects on the seismic radiation pattern and amplitude decays. Therefore, we develop a phase shift elimination method to handle the challenge of choosing representative source characteristics as an input for the modeling. To optimize the fitting of modeled and observed amplitude decay curves, we perform a grid search to constrain the two model parameters, i.e., the seismic shear wave velocity and quality factor. The comparison of modeled and observed amplitude decays for the seven prominent frequencies shows very good agreement and allows the constraint of shear velocities and quality factors for a two-layer model of the subsurface. The approach is generalized to predict amplitude decays and radiation patterns for WFs of arbitrary geometry.

Multichannel Analysis of Surface Waves Accelerated (MASWAccelerated): Software for efficient surface wave inversion using MPI and GPUs

Multichannel Analysis of Surface Waves (MASW) is a technique frequently used in geotechnical engineering and engineering geophysics to infer 1D layered models of seismic shear wave velocities in the top tens to hundreds of meters of the subsurface. We aim to accelerate MASW calculations by capitalizing on modern computer hardware available in the workstations of most engineers: multiple cores and graphics processing units (GPUs). We propose new parallel and GPU accelerated algorithms for computing 1D MASW inversion, and provide software implementations in C using Message Passing Interface (MPI) and CUDA. These algorithms take advantage of sparsity that arises in the problem, and the work balance between processes considers typical data trends. We compare our methods to an existing open source Matlab MASW tool. Our serial C implementation achieves a 2x speedup over the Matlab software, and we continue to see improvements by parallelizing the problem with MPI. We see nearly perfect strong and weak scaling for uniform data, and improve strong scaling for realistic data by repartitioning the problem to process mapping. By utilizing GPUs available on most modern workstations, we observe an additional 1.3x speedup over the serial C implementation on the first use of the method. We typically repeatedly evaluate theoretical dispersion curves as part of an optimization procedure, and on the GPU the kernel can be cached for faster reuse on later runs. We observe a 3.2x speedup on the cached GPU runs compared to the serial C runs. This work is the first open-source parallel or GPU-accelerated software tool for MASW imaging, and should enable geotechnical engineers to fully utilize all computer hardware at their disposal.

How volcanically active is an abyssal plain? Evidence for recent volcanism on 20 Ma Nazca Plate seafloor

The abyssal plains are generally assumed to be geologically inactive parts of the ocean plate interiors where processes (such as pelagic sedimentation or manganese crust and nodule formation) occur at very slow rates. In terms of intraplate volcanic activity, almost all is assumed to occur at hotspots, leading to little exploration in other intraplate regions. The Peru Basin is an abyssal plain known to host Mn-nodule fields. We present remotely-operated underwater vehicle (ROV) investigations of a small seamount adjacent to such a Mn-nodule field on 20Ma Nazca Plate crust, showing that it appears to have been recently volcanically and hydrothermally active. The seamount lies 1600km east of the nearest spreading axis (East Pacific Rise) and 600km from both the Galapagos Plateau (to the north) and the subduction zone off Peru (to the east), making off-axis, hotspot or petit-spot processes unlikely as a cause of the volcanism. The shallow mantle below the Nazca (and conjugate Pacific) Plate shows globally anomalous low seismic shear-wave velocities, perhaps reflecting higher-than-normal amounts of melt in the mantle below this region which may provide a source for the magmas. Our own regional mapping work and literature sources highlight several similar sites of probable young volcanism elsewhere in the Peru Basin which may also be related to this anomaly. The Nazca abyssal plain may be much more geologically active than previously thought. If so, this could have wider implications for, among other things, chemosynthetic ecosystem connectivity.

Electrical conductivity of metasomatized lithology in subcontinental lithosphere

AbstractA plausible origin of the seismically observed mid-lithospheric discontinuity (MLD) in the subcontinental lithosphere is mantle metasomatism. The metasomatized mantle is likely to stabilize hydrous phases such as amphiboles. The existing electrical conductivity data on amphiboles vary significantly. The electrical conductivity of hornblendite is much higher than that of tremolite. Thus, if hornblendite truly represents the amphibole varieties in MLD regions, then it is likely that amphibole will cause high electrical conductivity anomalies at MLD depths. However, this is inconsistent with the magnetotelluric observations across MLD depths. Hence, to better understand this discrepancy in electrical conductivity data of amphiboles and to evaluate whether MLD could be caused by metasomatism, we determined the electrical conductivity of a natural metasomatized rock sample. The metasomatized rock sample consists of ~87% diopside pyroxene, ~9% sodium-bearing tremolite amphibole, and ~3% albite feldspar. We collected the electrical conductivity data at ~3.0 GPa, i.e., the depth relevant to MLD. We also spanned a temperature range between 400 to 1000 K. We found that the electrical conductivity of this metasomatized rock sample increases with temperature. The temperature dependence of the electrical conductivity exhibits two distinct regimes. At low temperatures <700 K, the electrical conductivity is dominated by the conduction in the solid state. At temperatures >775 K, the conductivity increases, and it is likely to be dominated by the conduction of aqueous fluids due to partial dehydration. The main distinction between the current study and the prior studies on the electrical conductivity of amphiboles or amphibole-bearing rocks is the sodium (Na) content in amphiboles of the assemblage. Moreover, it is likely that the higher Na content in amphiboles leads to higher electrical conductivity. Pargasite and edenite amphiboles are the most common amphibole varieties in the metasomatized mantle, and our study on Na-bearing tremolite is the closest analog of these amphiboles. Comparison of the electrical conductivity results with the magnetotelluric observations constrains the amphibole abundance at MLD depths to <1.5%. Such a low-modal proportion of amphiboles could only reduce the seismic shear wave velocity by 0.4–0.5%, which is significantly lower than the observed velocity reduction of 2–6%. Thus, it might be challenging to explain both seismic and magnetotelluric observations at MLD simultaneously.

Glacial isostatic adjustment of the Pacific Coast of North America: the influence of lateral Earth structure

SUMMARYThe Pacific Coast of Central North America is a geodynamically complex region which has been subject to various geophysical processes operating on different timescales. Glacial isostatic adjustment (GIA), the ongoing deformational response of the solid Earth to past deglaciation, is an important geodynamic process in this region. In this study, we apply earth models with 3-D structure to determine if the inclusion of lateral structure can explain the poor performance of 1-D models in this region. Three different approaches are used to construct 3-D models of the Earth structure. For the first approach, we adopt an optimal 1-D viscosity structure from previous work and add lateral variations based on four global seismic shear wave velocity anomalies and two global lithosphere thickness models. The results based on these models indicate that the addition of lateral structure significantly impacts modelled RSL changes, but the data-model fits are not improved. The global seismic models are limited in spatial resolution and so two other approaches were considered to produce higher resolution models of 3-D structure: inserting a regional seismic model into two of the global seismic models and, explicitly incorporating regional structure of the Cascadia subduction zone and vicinity, that is the subducting slab, the overlying mantle wedge and the plate boundary interface. The results associated with these higher resolution models do not reveal any clear improvement in satisfying the RSL observations, suggesting that our estimates of lateral structure are inaccurate and/or the data-model misfits are primarily due to limitations in the adopted ice-loading history. The different realizations of 3-D Earth structure gives useful insight to uncertainty associated with this aspect of the GIA model. Our results indicate that improving constraints on the deglacial history of the southwest sector of the Cordilleran ice sheet is an important step towards developing more accurate of GIA models for this region.

High resolution tomography of the Tanlu fault zone near Hefei with passive seismic and magnetotelluric linear array data

As the largest fault trending NNE-SSW to NE-SW in the eastern Eurasia Continent, the Tanlu fault zone (TLFZ) extends over 2,400 km within China, roughly from Wuxue, Hubei Province, to Russia. Since the Quaternary period, the TLFZ has been an earthquake-prone area in eastern China where several major earthquakes resulted by tectonic compression occurred, causing tremendous casualties and significant economic losses. Many studies on different segments of the TLFZ have been carried out in the past few decades. However, numerous key questions regarding the fault zone remain unanswered due to a lack of clear subsurface characterization and fault delineation. In this study, we present high-resolution tomographic results across the TLFZ to the east of Hefei, where one 22-km-long passive seismic array with densely spaced short-period nodes, and a 24-km-long magnetotelluric array were deployed adjacent to each other. We find the velocity and resistivity variations are highly consistent with known surface geology. Sharp property contrasts in both the seismic shear wave velocity and electrical resistivity profiles clearly delineate the Tanlu F1 fault (TLF-1) near Hefei. More interestingly, an upwelling with distinct high velocity is imaged within the Hefei Basin to the west of the TLF-1, whereas a slanted block with low-velocity and low-resistivity seems to cut into or thrust upon the high-grade to low-grade middle-pressure rocks in the Zhangbaling uplift right below the Tanlu F2 ductile shear fault (TLF-2). The presented results show a new approach to characterize deep subsurface structure of the TLFZ beyond 2-km depths using passive data, which it is often difficult for active seismic surveys with refracted and reflected waves to image.

Assessing the presence of volatile-bearing mineral phases in the cratonic mantle as a possible cause of mid-lithospheric discontinuities

A number of possible hypotheses have been proposed to explain the origin of mid-lithospheric discontinuities (MLDs), typically characterized by ∼2-6% reductions in seismic shear wave velocity (VS) at depths of 60 km to ∼150 km in the cratonic sub-continental lithospheric mantle (SCLM). One such hypothesis is the presence of low-shear wave velocity, hydrous and carbonate mineral phases. Although, the presence of hydrous silicates and carbonates can cause a reduction in the shear wave velocity of mantle domains, the contribution of volatile metasomatism to the origins of MLDs has remained incompletely evaluated. To assess the metasomatic origin of MLDs, we compiled experimental phase assemblages, phase proportions, and phase compositions from the literature in peridotite + H2O, peridotite + CO2, and peridotite + H2O + CO2 systems at P-T conditions where hydrous silicate and/or carbonate minerals are stable. By comparing the experimental assemblages with the compiled bulk peridotite compositions for cratons, we bracket plausible proportions and compositions of hydrous silicate and carbonate mineral phases that can be expected in cratonic SCLMs. Based on the CaO and K2O contents of cratonic peridotite xenoliths and the estimated upper limit of CO2 content in SCLM, ≤∼10 vol.% pargasitic amphibole, ≤∼2.1 vol.% phlogopite and ≤∼0.2 vol.% magnesite solid solution can be stable in the SCLM. We also present new elasticity data for the pargasite end member of amphibole based on first principles simulations for more accurate estimates of aggregate VS for metasomatized domains in cratonic mantle. Using the bracketed phase compositions, phase proportions, and updated values of elastic constants for relevant mineral end members, we further calculate aggregate VS at MLD depths for three seismic stations in the northern continental U.S. Depending on the choice of background wave speeds of unmetasomatized peridotite and the cratonic geotherm, the composition and abundance of volatile-bearing mineral phases bracketed here can explain as much as 2.01 to 3.01% reduction in VS. While various craton formation scenarios allow formation of the amphibole and phlogopite abundances bracketed here, presence of volatile-bearing phases in an average cratonic SCLM composition cannot explain the entire range of velocity reductions observed at MLDs. Other possible velocity reduction mechanisms thus must be considered to explain the full estimated range of shear wave speed reduction at MLD depths globally.

Response of Earth's magnetic field to large lower mantle heterogeneity

A simplified two-fold pattern of convection in the Earth's core is often used to explain the non-axisymmetric magnetic flux concentrations in the present day geomagnetic field. For large lateral variations in the lower mantle heat flux, however, a substantial east–west dichotomy in core convection may be expected. This study examines the effect of a large lateral variation in heat flux at the outer boundary in cylindrical annulus experiments that achieve approximate geostrophy of the convection as well as in rapidly rotating spherical shell simulations. In either geometry, the imposed boundary heat flux is derived from the seismic shear wave velocity in the lowermost mantle. The pattern of large-scale convection in the simulations closely follows that in the annulus experiments, which suggests that the lateral buoyancy at the equator essentially determines the structure of core convection. In particular, the location of a coherent downwelling that forms beneath Canada in mildly driven convection entirely switches over to the Siberian region in strongly driven states. Spherical dynamo models in turn show that this eastward migration of convection causes the relative instability or even the disappearance of the high-latitude magnetic flux in the Western hemisphere. Finally, large radial buoyancy causes homogenization of convection, which may place an upper bound for the Rayleigh number in the core.

Characterisation of subglacial water using a constrained transdimensional Bayesian transient electromagnetic inversion

Abstract. Subglacial water modulates glacier-bed friction and therefore is of fundamental importance when characterising the dynamics of ice masses. The state of subglacial pore water, whether liquid or frozen, is associated with differences in electrical resistivity that span several orders of magnitude; hence, liquid water can be inferred from electrical resistivity depth profiles. Such profiles can be obtained from inversions of transient (time-domain) electromagnetic (TEM) soundings, but these are often non-unique. Here, we adapt an existing Bayesian transdimensional algorithm (Multimodal Layered Transdimensional Inversion – MuLTI) to the inversion of TEM data using independent depth constraints to provide statistical properties and uncertainty analysis of the resistivity profile with depth. The method was applied to ground-based TEM data acquired on the terminus of the Norwegian glacier, Midtdalsbreen, with depth constraints provided by co-located ground-penetrating radar data. Our inversion shows that the glacier bed is directly underlain by material of resistivity 102 Ωm ± 1000 %, with thickness 5–40 m, in turn underlain by a highly conductive basement (100 Ωm ± 15 %). High-resistivity material, 5×104 Ωm ± 25 %, exists at the front of the glacier. All uncertainties are defined by the interquartile range of the posterior resistivity distribution. Combining these resistivity profiles with those from co-located seismic shear-wave velocity inversions to further reduce ambiguity in the hydrogeological interpretation of the subsurface, we propose a new 3-D interpretation in which the Midtdalsbreen subglacial material is partitioned into partially frozen sediment, frozen sediment/permafrost and weathered/fractured bedrock with saline water.