Upper-mantle Velocity Research Articles

Seismic imaging of the eastern Algerian margin off Jijel: integrating wide-angle seismic modelling and multichannel seismic pre-stack depth migration

This study presents the results of a deep seismic survey across the north Algerian margin, based on the combination of 2-D multichannel and wide-angle seismic data simultaneously recorded by 41 ocean bottom seismometers deployed along a north-south line extending 180 km off Jijel into the Algerian offshore basin, and 25 land stations deployed along a 100-km-long line, cutting through the Lesser Kabylia and the Tellian thrust-belt. The final model obtained using forward modelling of the wide-angle data and pre-stack depth migration of the seismic reflection data provides an unprecedented view of the sedimentary and crustal structure of the margin. The sedimentary layers in the Algerian basin are 3.75 km thick to the north and up to 4.5-5 km thick at the foot of the margin. They are characterized by seismic velocities from 1.9 to 3.8 km s(-1). Messinian salt formations are about 1 km thick in the study area, and are modelled and imaged using a velocity between 3.7 and 3.8 km s(-1). The crust in the deep sea basin is about 4.5 km thick and of oceanic origin, presenting two distinct layers with a high gradient upper crust (4.7-6.1 km s(-1)) and a low gradient lower crust (6.2-7.1 km s(-1)). The upper-mantle velocity is constrained to 7.9 km s(-1). The ocean-continent transition zone is very narrow between 15 and 20 km wide. The continental crust reaches 25 km thickness as imaged from the most landward station and thins to 5 km over a less than 70 km distance. The continental crust presents steep and asymmetric upper- and lower-crustal geometry, possibly due to either asymmetric rifting of the margin, an underplated body, or flow of lower crustal material towards the ocean basin. Present-time deformation, as imaged from three additional seismic profiles, is characterized by an interplay of gravity-driven mobile-salt creep and active thrusting at the foot of the tectonically inverted Algerian margin.

3-D structure of the Rio Grande Rift from 1-D constrained joint inversion of receiver functions and surface wave dispersion

The Southern terminus of the Rio Grande Rift region has been poorly defined in the geologic record, with few seismic studies that provide information on the deeper Rift structure. In consequence, important questions related to tectonic and lithospheric activity of the Rio Grande Rift remain unresolved. To address some of these geological questions, we collect and analyze seismic data from 147 EarthScope Transportable Array (USArray) and other seismic stations in the region, to develop a 3-D crust and upper mantle velocity model. We apply a constrained optimization approach for joint inversion of surface wave and receiver functions using seismic S wave velocities as a model parameter. In particular, we compute receiver functions stacks based on ray parameter, and invert them jointly with collected surface wave group velocity dispersion observations. The inversions estimate 1-D seismic S-wave velocity profiles to 300 km depth, which are then interpolated to a 3-D velocity model using a Bayesian kriging scheme. Our 3-D models show a thin lower velocity crust anomaly along the southeastern Rio Grande Rift, a persistent low velocity anomaly underneath the Colorado Plateau and Basin and Range province, and another one at depth beneath the Jemez lineament, and the southern RGR.

Upper mantle seismic anisotropy beneath the West Antarctic Rift System and surrounding region from shear wave splitting analysis

SUMMARY We constrain azimuthal anisotropy in the West Antarctic upper mantle using shear wave splitting parameters obtained from teleseismic SKS, SKKS and PKS phases recorded at 37 broad-band seismometres deployed by the POLENET/ANET project. We use an eigenvalue technique to linearize the rotated and shifted shear wave horizontal particle motions and determine the fast direction and delay time for each arrival. High-quality measurements are stacked to determine the best fitting splitting parameters for each station. Overall, fast anisotropic directions are oriented at large angles to the direction of Antarctic absolute plate motion in both hotspot and no-net-rotation frameworks, showing that the anisotropy does not result from shear due to plate motion over the mantle. Further, the West Antarctic directions are substantially different from those of East Antarctica, indicating that anisotropy across the continent reflects multiple mantle regimes. We suggest that the observed anisotropy along the central Transantarctic Mountains (TAM) and adjacent West Antarctic Rift System (WARS), one of the largest zones of extended continental crust on Earth, results from asthenospheric mantle strain associated with the final pulse of western WARS extension in the late Miocene. Strong and consistent anisotropy throughout the WARS indicate fast axes subparallel to the inferred extension direction, a result unlike reports from the East African rift system and rifts within the Basin and Range, which show much greater variation. We contend that ductile shearing rather than magmatic intrusion may have been the controlling mechanism for accumulation and retention of such coherent, widespread anisotropic fabric. Splitting beneath the Marie Byrd Land Dome (MBL) is weaker than that observed elsewhere within the WARS, but shows a consistent fast direction, possibly representative of anisotropy that has been ‘frozenin’ to remnant thicker lithosphere. Fast directions observed inland from the Amundsen Sea appear to be radial to the dome and may indicate radial horizontal mantle flow associated with an MBL plume head and low upper mantle velocities in this region, or alternatively to lithospheric features associated with the complex Cenozoic tectonics at the far-eastern end of the WARS.

LITHO1.0: An updated crust and lithospheric model of the Earth

AbstractWe present the LITHO1.0 model, which is a 1° tessellated model of the crust and uppermost mantle of the Earth, extending into the upper mantle to include the lithospheric lid and underlying asthenosphere. The model is parameterized laterally by tessellated nodes and vertically as a series of geophysically identified layers, such as water, ice, sediments, crystalline crust, lithospheric lid, and asthenosphere. LITHO1.0 is created by constructing an appropriate starting model and perturbing it to fit high‐resolution surface wave dispersion maps (Love and Rayleigh, group and phase) over a wide frequency band (5–40 mHz). We examine and discuss the model with respect to key lithospheric parameters, such as average crustal velocity, crustal thickness, upper mantle velocity, and lithospheric thickness. We then compare the constructed model to those from a number of select studies at regional and global scales and find general consistency. It appears that LITHO1.0 represents a reasonable starting model of the Earth's shallow structure (crust and uppermost mantle) for the purposes in which these models are used, such as traveltime tomography or in efforts to create a 3‐D reference Earth model. The model matches surface wave dispersion over a frequency band wider than the band used in the inversion. There are several avenues for improving the model in the future by including attenuation and anisotropy, as well as making use of surface waves at higher frequency.

Constraints on the geodynamic evolution of the Africa–Iberia plate margin across the Gibraltar Strait from seismic tomography

Geophysical studies point to a complex tectonic and geodynamic evolution of the Alboran Basin and Gulf of Cadiz. Tomographic images show strong seismic waves velocity contrasts in the upper mantle. The high velocity anomaly beneath the Alboran Sea recovered by a number of studies is now a well established feature. Several geodynamic reconstructions have been proposed also on the base of these images. We present and elaborate on results coming from a recent tomography study which concentrates on both the Alboran and the adjacent Atlantic region. These new results, while they confirm the existence of the fast anomaly below the Alboran region, also show interesting features of the lithosphere-asthenosphere system below the Atlantic. A high velocity body is imaged roughly below the Horseshoe Abyssal plain down to sub-lithospheric depths. This feature suggests either a possible initiation or relic subduction. Pronounced low velocity anomalies pervade the upper mantle below the Atlantic region and separate the lithospheres of the two regions. We also notice a strong change of the upper mantle velocity structure going from south to north across the Gorringe Bank. This variation in structure could be related to the different evolution in the opening of the central and northern Atlantic oceans.

Finite‐frequency Rayleigh wave tomography of the western Mediterranean: Mapping its lithospheric structure

We present a 3‐D shear wave velocity model for the crust and upper mantle of the western Mediterranean from Rayleigh wave tomography. We analyzed the fundamental mode in the 20–167 s period band (6.0–50.0 mHz) from earthquakes recorded by a number of temporary and permanent seismograph arrays. Using the two‐plane wave method, we obtained phase velocity dispersion curves that were inverted for an isotropic Vs model that extends from the southern Iberian Massif, across the Gibraltar Arc and the Atlas mountains to the Saharan Craton. The area of the western Mediterranean that we have studied has been the site of complex subduction, slab rollback, and simultaneous compression and extension during African‐European convergence since the Oligocene. The shear velocity model shows high velocities beneath the Rif from 65 km depth and beneath the Granada Basin from ∼70 km depth that extend beneath the Alboran Domain to more than 250 km depth, which we interpret as a near‐vertical slab dangling from beneath the western Alboran Sea. The slab appears to be attached to the crust beneath the Rif and possibly beneath the Granada Basin and Sierra Nevada where low shear velocities (3.8 km/s) are mapped to >55 km depth. The attached slab is pulling down the Gibraltar Arc crust, thickening it, and removing the continental margin lithospheric mantle beneath both Iberia and Morocco as it descends into the deeper mantle. Thin lithosphere is indicated by very low upper mantle velocities beneath the Alboran Sea, above and east of the dangling slab and beneath the Cenozoic volcanics.

Antarcticaʼs hypsometry and crustal thickness: Implications for the origin of anomalous topography in East Antarctica

The hypsometry of Antarctica revealed by BEDMAP2 data is characterised by deglaciated modal elevations of ∼−450 m and ∼650 m for West and East Antarctica, respectively, and an East Antarctic plateau that is topographically anomalous by ∼400–600 m with respect to global continental modal elevation estimates. Superimposed on the East Antarctic plateau are the Gamburtsev Subglacial Mountains, the Dronning Maud Land Mountains and the Vostok Highlands with modal elevations ∼400 m in excess of the East Antarctic mode. To ascertain whether East Antarcticaʼs anomalous topography can be attributed to Airy-type crustal compensation, a continental-scale crustal thickness model was derived from the inversion of GOCO03S satellite gravity data constrained by seismic crustal thickness estimates. The average crustal thickness of East Antarctica is ∼40 km (for West Antarctica ∼24 km), a value typical of continental shields, and while crustal thicknesses of >50 km locally beneath the Gamburtsev Subglacial Mountains and Dronning Maud Land can account for their differential modal elevation above the plateau, crustal thicknesses elsewhere across East Antarctica offer no suggestion of crustal-level continental-scale support for the broader plateau. Enderby Land, for example, resides on the plateau and is characterised by a modal elevation of ∼750 m and crust ∼40 km thick, whereas off the plateau in East Antarctica, the Aurora and Wilkes Subglacial Basins have modal elevations of ∼−50 m and ∼50 m, respectively, yet similarly thick crust. The lack of crustal support for the elevated broader East Antarctic plateau, coupled with seismic images showing fast upper mantle velocities beneath the plateau, suggest a mid-to-lower mantle source for East Antarcticaʼs anomalous topography.

Shear velocity structure and mineralogy of the transition zone beneath the East Pacific Rise

Models of seismic velocity as a function of depth through the upper mantle provide some of the strongest constraints on the mineralogy and composition of the mantle. Although receiver function studies have provided new information on the depths of upper mantle discontinuities they do not provide as much information on seismic gradients between discontinuities and absolute velocities within the upper mantle. The waveforms and travel times of upper mantle turning waves provide the strongest constraints on vertical variations in upper mantle velocity although in the past they suffered from the lack of dense profiles of data sampling a single part of the upper mantle that would minimize effects of 3D variations in velocity. Here we model three dense profiles of triplicated upper mantle broadband S and SS waves recorded by USArray, Canadian and NARS-Baja stations located in western North America. Earthquakes along the East Pacific Rise were recorded along profiles within 5° back azimuth windows and with stations at a maximum of 0.5° separation. The distance range covered is from 35° to 60° and thus the waves sample the mantle from the shallow upper mantle to depths near 1000 km. The data were inverted using a conjugate gradient algorithm that utilizes the reflectivity synthetic technique. The results show a much smaller gradient within the transition zone than the PREM model with larger jumps in velocity across the 410 km and 660 km depth discontinuities. These results are consistent with velocities predicted for a pyrolite composition mantle transition zone. Compositional models with lower olivine content, such as piclogite, are not consistent with our seismic model.

Seismic inhomogeneities in the upper mantle beneath the Siberian craton (Meteorite profile)

The upper-mantle structure was studied from first-arrival data along the Meteorite profile, run using underground nuclear explosions. Unlike the layered, slightly inhomogeneous models in the previous works, emphasis was laid on lateral inhomogeneity at the minimum possible number of abrupt seismic boundaries. We used forward ray tracing of the traveltimes of refracted and overcritical reflected waves. The model obtained is characterized by considerable velocity variations, from 7.7 km/s in the Baikal Rift Zone to 8.0–8.45 km/s beneath the Tunguska syneclise. A layer of increased velocity (up to 8.5–8.6 km/s), 30–80 km thick, is distinguished at the base of seismic lithosphere. The depth of the layer top varies from 120 km in the northern Siberian craton to 210 km in its southeastern framing. It has been shown that, with crustal density anomalies excluded, the reduced gravity field is consistent with the upper-mantle velocity model.

Rayleigh wave constraints on the structure and tectonic history of the Gamburtsev Subglacial Mountains, East Antarctica

The Gamburtsev Subglacial Mountains (GSM), located near the center of East Antarctica, remain one of the most enigmatic mountain ranges on Earth. A lack of direct geologic samples renders their tectonic history almost totally unconstrained. We utilize teleseismic Rayleigh wave data from a 2 year deployment of broadband seismic stations across the region to image shear velocity structure and analyze the lithospheric age of the GSM and surrounding regions. We solve for 2‐D phase velocities and invert these results for 3‐D shear velocity structure. We perform a Monte Carlo simulation to improve constraints of crustal thickness and shear velocity structure. Beneath the core of the GSM, we find crustal thickness in excess of 55 km. Mantle shear velocities remain faster than global average models to a depth of approximately 250 km, indicating a thick lithospheric root. Thinner crust and slower upper mantle velocities are observed beneath the Lambert Rift System and the Polar Subglacial Basin. When compared with phase velocity curves corresponding to specific tectonothermal ages elsewhere in the world, average phase velocity results for the GSM are consistent with regions of Archean‐Paleoproterozoic origin. Combined with radiometric ages of detrital zircons found offshore, these results indicate a region of old crust that has undergone repeated periods of uplift and erosion, most recently during the Mesozoic breakup of Gondwana. Lower crustal seismic velocities imply a moderately dense lower crust beneath the core of the GSM, but with lower density than suggested by recent gravity models.

The deep seismic structure of the Ethiopia/Afar hotspot and the African superplume

SUMMARY The Ethiopia/Afar hotspot has been frequently explained as an upper mantle continuation of the African superplume, with anomalous material in the lower mantle under southern Africa, rising through the transition zone beneath eastern Africa. However, the significantly larger amplitude low velocity anomaly in the upper mantle beneath Ethiopia/Afar, compared to the anomalies beneath neighboring regions, has led to questions about whether or not alongstrike differences in the seismic structure beneath eastern Africa and western Arabia are consistent with the superplume interpretation. Here we present a new P-wave model of the hotspot’s deep structure and use it to evaluate the superplume model. At shallow (< ∼400km) depths, the slowest velocities are centered beneath the Main Ethiopian Rift, and we attribute these low velocities to decompression melting beneath young, thin lithosphere. At deeper depths, the low velocity structure trends to the northeast, and the locus of the low velocity anomaly is found beneath Afar. The northeast-trending structure with depth is best modeled by northeastward flow of warm superplume material beneath eastern Africa. The combined effects of shallow decompression melting and northeastward flow of superplume material explain why upper mantle velocities beneath Ethiopia/Afar are significantly slower than those beneath neighboring East Africa and western Arabia. The superplume interpretation can thus explain the deep seismic structure of the hotspot if the effects of both decompression melting and mantle flow are considered.

Mantle constraints on the plate tectonic evolution of the Tonga–Kermadec–Hikurangi subduction zone and the South Fiji Basin region

The Tonga–Kermadec–Hikurangi subduction zone is a major plate boundary in the Southwest Pacific region, where the Pacific plate subducts westward underneath the Australian plate. Considerable controversy exists regarding the Cenozoic evolution of this subduction zone, its connection with the Vitiaz–Solomon trench, the opening of adjacent backarc basins (South Fiji Basin, Norfolk Basin), and the obduction of ophiolites in New Caledonia and New Zealand. We analyse three tectonic reconstructions from 45 Ma to the present that represent three end-member tectonic scenarios. The first model involves an approximately west-dipping Tonga–Kermadec–Hikurangi subduction zone that rolls back clockwise, and an approximately east-dipping New Caledonia–Northland subduction zone that rolls back anticlockwise until ca 21 Ma. The other models involve only the Tonga–Kermadec–Hikurangi subduction zone that rolls back clockwise in one, and anticlockwise in the other model. We tied the three reconstructions to an Indo-Atlantic moving hotspot reference frame, indicating how the subduction zones migrated with respect to the lower mantle at 45–0 Ma, thereby predicting the location of slab material in the mantle. The predicted slab locations have been compared with a global P-wave tomography model (UU-P07) that gives insight into mantle structure and fossil slab locations. Minor agreement is achieved for the anticlockwise model, which fails to predict most of the lower mantle Tonga–Kermadec–Hikurangi slab anomaly below the South Fiji Basin and North Fiji Basin. Moderate agreement is achieved for the clockwise model, which predicts most of the lower mantle Tonga–Kermadec–Hikurangi slab anomaly below the South Fiji Basin, but not its northernmost extent below the North Fiji Basin. The model involving two oppositely dipping subduction zones shows good agreement, predicting slab anomalies at the right depth and geographical location for the Tonga–Kermadec–Hikurangi and Solomon–Vitiaz subduction segments, as well as for the New Caledonia–Northland subduction zone. Connection between the Tonga and Vitiaz subduction segments until ca 20–15 Ma is predicted and agrees with tomography. Predicted average upper mantle and lower mantle sinking velocities are 3.3–7.0 cm/yr and 0.8–1.7 cm/yr, respectively, with variation occurring at different segments along the subduction zone owing to variation in subduction velocity and subduction partitioning. Our analysis couples plate tectonics to mantle evolution and demonstrates that reconstructions tied to an Indo-Atlantic moving hotspot reference frame better agree with mantle structure than those tied to a Pacific fixed hotspot reference frame.

Crustal and upper mantle velocity structure beneath central Tibet by P-wave teleseismic tomography

SUMMARY A detailed 3-D P-wave velocity model beneath central Tibet was obtained using 26 741 arrival times from 1025 teleseismic events recorded by the portable stations of the Hi-CLIMB project. In the crustal correction, we consider both vertical and lateral velocity variations. Our teleseismic P-wave tomography result reveals that the Indian lithospheric mantle underthrusts no further than 32.5°N. In addition, the presence of low velocity anomalies under the Indus-Tsangpo suture suggests that subduction is not a simple and continuous process. We suggest that the delamination of Indian mantle lithosphere induces mantle upwelling beneath the rifts, which in turn created cracks or a break in the subducted plate. Moreover, the formation of active rifts near the profile is related to the mantle upwelling.

Structure and anisotropy of the Mexico subduction zone based on Rayleigh‐wave analysis and implications for the geometry of the Trans‐Mexican Volcanic Belt

We develop a three‐dimensional model of shear wave velocity and anisotropy for the Mexico subduction zone using Rayleigh wave phase velocity dispersion measurements. This region is characterized by both steep and flat subduction and a volcanic arc that appears to be oblique to the trench. We give a new interpretation of the volcanic arc obliqueness and the location of the Tzitzio gap in volcanism based on the subduction morphology. We employ the two‐station method to measure Rayleigh phase velocity dispersion curves between periods of 16 s to 171 s. The results are then inverted to obtain azimuthally anisotropic phase velocity maps and to model 3‐D variations in upper mantle velocity and anisotropy. Our maps reveal lateral variations in phase velocity at all periods, consistent with the presence of flat and steep subduction. We also find that the data are consistent with two layers of anisotropy beneath Mexico: a crustal layer, with the fast directions parallel to the North American absolute plate motion, and a deeper layer that includes the mantle lithosphere and the asthenosphere, with the fast direction interpreted in terms of toroidal mantle flow around the slab edges. Our combined azimuthal anisotropy and velocity model enables us to analyze the transition from flat to steep subduction and to determine whether the transition involves a tear resulting in a gap between segments or is a continuous deformation caused by a lithospheric flexure. Our anisotropy results favor a tear, which is also consistent with the geometry of the volcanic belt.

Lithospheric and upper mantle structure of the northeastern Tibetan Plateau

We use receiver functions calculated for data collected by the INDEPTH‐IV seismic array to image the three‐dimensional geometry of the crustal and upper mantle velocity discontinuities beneath northeastern Tibet. Our results indicate an average crustal thickness of 65 to 70 km in northern Tibet. In addition, we observe a 20 km Moho offset beneath the northern margin of the Kunlun Mountains, a 10 km Moho offset across the Jinsha River Suture and gently northward dipping Moho beneath the Qaidam Basin. A region in the central Qiangtang Terrane with higher than normal crustal Vp/Vs ratio of ∼1.83 can be the result of the Eocene magmatic event. In the Qiangtang Terrane, we observe a significant lithospheric mantle discontinuity beneath the Bangong‐Nujiang Suture at 80 km depth which dips ∼10° to the north, reaching ∼120 km depth. We interpret this feature as either a piece of Lhasa Terrane or remnant oceanic slab underthrust below northern Tibet. We detect a ∼20 km depression of the 660‐km discontinuity in the mantle transition zone beneath the northern Lhasa Terrane in central Tibet, which suggests this phase transition has been influenced by a dense and/or cold oceanic slab. A modest ∼10 km depression of the 410‐km discontinuity located beneath the northern Qiangtang Terrane may be the result of localized warm upwelling associated with small‐scale convection induced by the penetration of the sinking Indian continental lithosphere into the transition zone beneath the central Tibetan Plateau.

Velocity and Attenuation Structure of the Tibetan Lithosphere Under the Hi-CLIMB Array From the Modeling of Pn Attributes

Using seismic data from regional earthquakes in Tibet recorded by the Hi-CLIMB experiment, Pn attributes are used to constrain the velocity gradient and attenuation structure of the Tibetan lithosphere under the Hi-CLIMB array. Numerical modeling is performed using the spectral-element method (SEM) for laterally varying upper-mantle velocity and attenuation, and the seismic attributes considered include the Pn travel-time, envelope amplitude, and pulse frequency. The results from the SEM modeling provide two alternative models for the upper-mantle beneath the Hi-CLIMB array in Tibet. The first model is derived from the 3D velocity model of Griffinet al. (Bull Seism Soc Am 101:1938–1947, 2011) with a constant upper-mantle velocity gradient, and laterally varying upper mantle attenuation. The second model has a laterally varying upper-mantle velocity gradient, and constant upper-mantle attenuation. In both cases, the Qiangtang terrane is distinguished from the Lhasa terrane by a change in Moho depth and upper-mantle velocities. The lower upper-mantle velocities, as well as higher Pn attenuation, suggest hotter temperatures beneath the Qiangtang terrane as compared to the Lhasa terrane. Although the fits to the Pn amplitude and pulse frequency data are comparable between the two models, the first model with the constant upper-mantle velocity gradient fits the travel times somewhat better in relation to the data errors.

Upper-mantle structure beneath the Southern Scandes Mountains and the Northern Tornquist Zone revealed by P-wave traveltime tomography

SUMMARY This study images upper-mantle structure beneath different tectonic and geomorphological provinces in southern Scandinavia by P-wave traveltime tomography based on teleseismic events. We present results using integrated data from several individual projects (CALAS, MAGNUS, SCANLIPS, CENMOVE and Tor) with a total of 202 temporary seismological stations deployed in southern Norway, southern Sweden, Denmark and the northernmost part of Germany. These stations, together with 18 permanent stations, yield a high density data coverage and enable presentation of the first high resolution 3D seismic velocity model for the upper mantle for this region, which includes the entire northern part of the prominent Tornquist Zone and the Southern Scandes Mountains. P-wave arrival time residuals of up to ±1 s are observed indicating large seismic velocity contrasts at depths. Relative regional as well as absolute global tomographic inversion is carried out and consistently show upper-mantle velocity variations relative to the ak135 global reference model of up to ±2–3 per cent corresponding to P-wave velocity differences of 0.4–0.5 km s–1 from depths of about 100 km to more than 300 km. High upper-mantle velocities are observed to great depth to the east in Baltic Shield areas of southwestern Sweden suggesting the existence of a deep lithosphere keel. Lower velocities are found to the west and southwest beneath the Danish and North German sedimentary basins and in most of southern Norway. A well defined, generally narrow and deep boundary is observed between areas of contrasting upper-mantle seismic velocity. In the southern part of the study area, this boundary is localized along and east of the Sorgenfrei–Tornquist Zone. It seems to follow the eastern boundary of a zone of significant Late Carboniferous–Permian volcanic activity from southwestern Sweden to the Oslo Graben area. To the north, it crosses shield units, Caledonides as well as areas of high topography. Supported by independent results of surface wave studies, we interpret this velocity boundary as a first order lithosphere boundary representing the southwestern edge of thick shield lithosphere. In basin areas to the southwest, low upper-mantle velocities are associated with asthenosphere beneath thinned lithosphere and velocity contrasts are likely to arise mainly from temperature differences. To the north structural and geodynamic relations are more complex and both temperature and compositional differences may play a part. Reduced upper-mantle velocity beneath southern Norway also seems, despite relatively low heat flow, to be associated with areas of thinned lithosphere, pointing towards increased temperatures and reduced density in the upper mantle. This feature extends over large areas and seems not directly correlated to the shorter wavelength high topography of the Scandes Mountains, but may contribute with some isostatic buoyancy on a regional scale. For this northern area, there is no obvious geodynamic explanation to reduced upper-mantle velocity. A number of candidates are available including deep transient thermal influence from basin areas to the southwest.

Detailed one-dimensional seismic velocity profiles beneath the Himalayan collision zone: evidence for a double Moho?

Seismicity in the Himalaya indicates that relatively deep earthquakes (focal depth 40∼100 km) occur in specific regions beneath the High Himalaya and Nepal. This study focuses on these specific regions to estimate the detailed velocity structure of the lower crust and upper mantle. We selected 202 earthquakes from the Himalaya Nepal Tibet Seismic Experiment (HIMNT) and relocated these earthquakes accurately by applying a genetic algorithm locator, GA-MHYPO (Kim et al., 2006). The detailed onedimensional P-velocity structure is estimated based on travel-time inversion using hypocentral parameters determined by GA-MHYPO. Computational results show two velocity anomalies exhibiting upper-mantle velocities at depth of about 45–50 and 60 km, respectively, beneath the High Himalayan region, whereas a single Moho exists at about 55 km depth beneath Nepal. To validate the stability of these results, a bootstrapping method was used for the inversion.

Slab-induced waveform effects as revealed by the TAIGER seismic array: Evidence of slab beneath central Taiwan

Here we tackle a tectonically important question – the upper mantle velocity structure beneath central Taiwan – with seismically interesting observations – receiver-side slab waveform effects. We use teleseismic P waveforms of the NS broadband array deployed by the TAIGER project to examine patterns of variation in arrival time, pulse width, and amplitude – measuring the first two by Gaussian fitting – and contrast measurements of earthquakes to the southeast (SE earthquakes) with those of one Sumatra earthquake in order to focus on upper mantle heterogeneities. Overall variation patterns as a function of earthquake are compatible with ray-tracing predictions. Relative reduced arrival times and amplitudes at central Taiwan stations suggest the existence of a deep aseismic slab below. From simulations of 2-D wave propagation, we conclude that lateral heterogeneity of crust and uppermost mantle primarily contributes to variations in arrival time and only secondarily to variations in amplitude and pulse width. Furthermore, discrepancies between source-side and receiver-side waveform effects, where the latter are not always amplitude-reduced, are explained by constructive interference between the fast and slow phase. Thus, the use of full waveform information can provide independent constraints to complement results of previous studies. A future extension will be to incorporate S waves and apply waveform inversion to yield quantitative constraints.

Peeling linear inversion of upper mantle velocity structure with receiver functions

A peeling linear inversion method is presented to study the upper mantle (from Moho to 800 km depth) velocity structures with receiver functions. The influences of the crustal and upper mantle velocity ratio error on the inversion results are analyzed, and three valid measures are taken for its reduction. This method is tested with the IASP91 and the PREM models, and the upper mantle structures beneath the stations GTA, LZH, and AXX in northwestern China are then inverted. The results indicate that this inversion method is feasible to quantify upper mantle discontinuities, besides the discontinuities between 3hM (hM denotes the depth of Moho) and 5hM due to the interference of multiples from Moho. Smoothing is used to overcome possible false discontinuities from the multiples and ensure the stability of the inversion results, but the detailed information on the depth range between 3hM and 5hM is sacrificed.