Teleseismic P-wave Tomography Research Articles

P-wave tomography of potential convective downwellings and their source regions, Sierra Nevada, California

Teleseismic P-wave tomography using the Sierra Nevada Earthscope Project (SNEP) deployment, older temporary deployments in the Sierra, and broadband stations from permanent and USArray Transportable Array (TA) stations was derived from starting models either lacking lateral variation (one dimensional [1-D]) or created from three-dimensional (3-D) surface-wave models. The use of multiple starting models permits examination of the robustness of different features while limiting the inherent ambiguities of teleseismic body-wave tomography. Our results confirm that mafic residuum of the Mesozoic Sierran batholith has been removed from the eastern Sierra north to at least 39°N. Low-wavespeed material near the Moho under the eastern Sierra is probably silicic lower crust and warm and possible melt-laden upper mantle. If the residuum remains in the upper mantle, there are three possible locations for it: a high-wavespeed (+∼5%) body extending down to ∼250 km near 36°N, 119.3°W termed the Isabella anomaly, an unusually high-wavespeed (to +10%) and shallow (top ∼40–50 km) anomaly at the south end of the Gorda slab near 40.5°N, 122.25°W termed the Redding anomaly, and a more ill-defined region of high wavespeeds in the crust to uppermost mantle (

Deep structure of the central uplift belt in the Qiangtang terrane Tibetan Plateaufrom teleseismic P-wave tomography

We have determined a detailed three-dimensional P-wave velocity structure of the crust and upper mantle down to a depth of 200 km beneath the central uplift belt of Qiangtang by applying Fast Marching Teleseismic Tomography (FMTT) to 9532 high-quality P-wave arrival times. The travel time data were determined by careful interpretations of original seismograms of 506 teleseismic events recorded by a network of 54 broadband stations deployed in the Qiangtang terrane on the Tibetan Plateau during September 2008 and November 2010. Our tomographic imaging beneath the central uplift belt of Qiangtang shows a deep boundary rupture structure for which the lithospheric velocity structure is significantly different on the northern and southern sides. Tomographic images along 33癗 and 33.5癗 show that lithospheric structures beneath the sides of the central uplift belt have different scales. The crust at the sides of the central uplift belt exhibits E-W trending high-velocity anomalies. However very low velocities are found within the crust beneath the northern Qiangtang basin overlying and high-velocity anomalies in the lithospheric mantle. Imaging along north-south profiles (88.5癊 and 89.5癊) shows that the central uplift of Qiangtang is a significant tectonic boundary fault zone.

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.

Segmented African lithosphere beneath the Anatolian region inferred from teleseismic P-wave tomography

SUMMARY Lithospheric deformation throughout Anatolia, a part of the Alpine–Himalayan orogenic belt, is controlled mainly by collision-related tectonic escape of the Anatolian Plate and subduction roll-back along the Aegean Subduction Zone. We study the deeper lithosphere and mantle structure of Anatolia using teleseismic, finite-frequency, P-wave traveltime tomography. We use data from several temporary and permanent seismic networks deployed in the region. Approximately 34 000 P-wave relative traveltime residuals, measured in multiple frequency bands, are inverted using approximate finite-frequency sensitivity kernels. Our tomograms reveal segmented fast seismic anomalies beneath Anatolia that corresponds to the subducted portion of the African lithosphere along the Cyprean and the Aegean trenches. We identify these anomalies as the subducted Aegean and the Cyprus slabs that are separated from each other by a gap as wide as 300 km beneath Western Anatolia. This gap is occupied by slow velocity perturbations that we interpret as hot upwelling asthenosphere. The eastern termination of the subducting African lithosphere is located near the transition from central Anatolia to the Eastern Anatolian Plateau or Arabian–Eurasian collision front that is underlain by large volumes of hot, underplating asthenosphere marked by slow velocity perturbations. Our tomograms also show fast velocity perturbations at shallow depths beneath northwestern Anatolia that sharply terminates towards the south at the North Anatolian Fault Zone (NAFZ). The associated velocity contrast across the NAFZ persists down to a depth of 100–150 km. Hence, our study is the first to investigate and interpret the vertical extent of deformation along this nascent transform plate boundary. Overall, the resolved upper-mantle structure of Anatolia is directly related with the geology and tectonic features observed at the surface of the Anatolian Plate and suggest that the segmented nature of the subducted African lithosphere plays an important role in the evolution of Anatolia and distribution of its tectonic provinces.

Uniqueness of modeling results from teleseismic P-Wave tomography in Project Tor

Within Project Tor, which is about Teleseismic Tomography across the Tornquist Zone in Germany–Denmark–Sweden, we have confirmed very significant deep lithosphere differences. And modeling is substantiated via completely independent methods. In 1996–1997 our 130 seismographs constituted the largest seismic antenna ever in Europe. The Tor area was chosen along a well studied crustal profile of an earlier project, and the modeling efforts were concentrated on the deep lithosphere and asthenosphere differences to depths around 300 km. The Tor data have been subjected to P-wave travel time tomography, surface wave and receiver function analysis as well as anisotropy and scattering measurements. An important goal of the project was to make several independent inversions of the tomography data, and compare the results in an attempt to evaluate uniqueness, resolution and accuracy of these inversions. The comparisons of this paper involve more diversity in methods than any previous comparison. The geological outcome is a substantiation of earlier statements that: “The transition is interpreted to be sharp and steep in two places. It goes all through the lithosphere at the northern rim of the Tornquist Zone near the border between Sweden and Denmark, and here the lithosphere difference is large to depths more than 200 km. The other lithosphere difference, of smaller scale, is found near the southern edge of the Ringkøbing-Fyn High near the border between Denmark and Germany. Also this transition is sharp and steep, and goes all through the lithosphere to depths around 120 km. These two sharp transitions divide the Tor region into 3 different lithosphere structures distinguishable in P-wave travel time tomography, surface wave dispersion, P- and S-wave anisotropy and partly in P-wave scattering”. The mentioned broad-scale features are judged to be unambiguously determined, with well-described resolution and accuracy. Unfortunately a detail like the slope of the subcrustal lithosphere transition right under the Tornquist Zone cannot be constrained even if this is where the resolution is best, and the curiosity largest.

An upper-mantle upwelling underneath Ireland revealed from non-linear tomography

SUMMARY We present results of a non-linear teleseismic P-wave traveltime tomography in Ireland. Relative traveltime residuals are calculated from the data set of the Irish Seismological Lithospheric Experiment (ISLE 2002/3) and permanent stations onshore Ireland. At each of the 28 station sites, a local crustal 1-D P-wave velocity (vP) model is determined because the 3-D crustal vP structure of Ireland is well known. These 1-D models are used for a traveltime correction for crustal effects. The corrected residuals indicate two large-scale traveltime anomalies of low amplitude: a negative traveltime anomaly in the western part of Ireland and a positive anomaly in the eastern part of the country. The subsequent traveltime tomography is calculated using an iterative non-linear inversion using a variable parametrization and 3-D ray tracing. Our inversion result contains a low P-wave velocity (vP) zone underneath central Ireland, at 31–120 km depth. The amplitudes of the low velocity zone reach up to −2.5 per cent vP at 31–60 km depth, but decrease to −0.5 to −1 per cent vP at 120–150 km depth. Reconstruction tests confirm that large anomalies in our model are well resolved, although the resolution decreases towards the edges of the model. We infer that the low velocity zone underneath central Ireland is related to lithospheric thinning towards the north, which was determined by a previous S-receiver function study. This anomaly is interpreted as presumably ancient or even recent mantle upwelling, related to the spreading of the Iceland plume head.

Evidence of stretching of the lithosphere under Denmark

The structure of the lithosphere under Denmark has been investigated in relation to adjacent regions of Sweden and Germany. The most interesting result of the study is that the 120 km thick lithosphere under Denmark appears to be a stretched version of the Swedish lithosphere, which is more than twice as thick. During the international project Teleseismic Tomography across the Tornquist Zone (Tor), field work and international interpretation were carried out between 1996 and 2002. Following the field work period, model velocity computations were undertaken based on observations of distant earthquakes (e.g. Arlitt 1999; Shomali et al. 2002; Voss et al. 2006), and recently an evaluation of the Tor results was completed (Nielsen 2007). The Tor project investigates deeper parts of the Earth than previous projects, and in particular the depth interval 50–300 km, which is below the crystalline crust. The investigations have included many geophysical features such as teleseismic P-wave tomography, Rayleigh wave velocities, shear wave splitting and wave scattering. We have distinguished between relatively high- and low-velocity zones, which also show variations in anisotropy and scatter characteristics. Generalised high-velocity zones correspond to the lithosphere, while generalised relatively low-velocity zones are equivalent to the asthenosphere. The main outcome of the combined studies is that the deep lithosphere can be divided into three blocks separated approximately along the national boundaries between Sweden and Denmark and between Denmark and Germany. The boundaries between the blocks are steep, almost vertical. The Denmark block has lithosphere properties between those to the north and south. Based on previous crustal studies and the Tor results, we suggest that the Denmark block has evolved by stretching. The details in the new evaluation are derived from teleseismic tomography. Here we present a synthesis of the many derived models in the light of the new evaluation.

Teleseismic P‐Wave Tomography Evidence for the Indian Lithospheric Mantle Subducting Northward Beneath the Qiangtang Terrane

AbstractWe collected all the available arrival time data recorded by the temporary seismic stations from the projects of PASSCAL, INDEPTH‐II, INDEPTH‐III, HIMNT, WKL and TIBET 973, and phase reports from the International Seismological Center from January 1990 to February 2004 in Tibet and its surrounding areas. In the tomographic inversion, we used 139021 P‐wave arrival times from 9649 teleseismic events recorded by 305 seismic stations. Our study area covers the southern Ganges plain in India to the northern Tarim basin and Qaidam basin in China. Our tomographic images show that the subducting angles of Indian lithospheric mantle are different under different areas, but their front locations are all beneath the Qiangtang terrane. The tomographic image along 88°E shows that the Indian lithospheric mantle is underthrusting northward with a dip angle of about 22° beneath the center of Qiangtang terrane at about 34°N latitude, and its frontier has reached to the deep part of the upper mantle. The tomographic image along a northeasterly profile shows that the Indian mantle underthrust nearly horizontally under Tibet from the Ganges plain to 33°N. Then, the Indian mantle broke off down to the asthenosphere and caused the asthenophere upwelling. A consequence of the asthenophere upwelling was to form a big low‐velocity zone beneath the Qiangtang terrane.

Teleseismic P-wave tomography and the upper mantle structure of the central Tien Shan orogenic belt

In this study, an updated version of the teleseismic tomographic method is applied to 7176 high-quality first P-wave arrivals from four combined seismic networks to determine the detailed deep structure under the central Tien Shan. Our high-resolution tomographic model not only displays the general features contained in the previous models, but also reveals some new features. A much narrower tilted columnar high-velocity anomaly with a diameter of about 60 km below 150–200 km depth under the Tien Shan extends down to the mantle transition zone, suggesting the breaking- and dropping-off of the collided portions of the southward underthrusting Kazakh Shield and the northward underthrusting Tarim Basin. Prominent low-velocity (low-V) anomalies above 150–200 km depth beneath the Tien Shan may be related to two branches of ascending hot anomalies. One rises upward from the lower mantle beneath the Tarim Basin, while the other rises up from the top of the mantle transition zone beneath the western Kazakh Shield. These results suggest that the underthrusting of the Tarim Basin and Kazakh Shield and the upwelling of hot anomalies indeed play an important role in the mountain building of the central Tien Shan. These dynamic processes might be related to the collision of the Indian plate with Eurasian plate.

Joint geodynamical and seismic modelling of the Eifel plume

SUMMARY Teleseismic P-wave tomography has revealed a columnar low-velocity anomaly in the upper mantle below the volcanic Eifel region in western Germany extending to at least 400 km depth. Here we explore whether a geodynamically consistent model of a mantle plume can explain the observed traveltime residuals. We use a 3-D mantle convection code with temperature and pressure-dependent viscosity to generate a suite of model plumes that rise from the transition zone and spread below a stationary or drifting lithospheric plate. We use ray tracing to calculate synthetic travel times and vary the plume location, radius, temperature and the rate and direction of plate motion in order to fit the observed travel times. Our results show a fair correlation between synthetic and observed traveltime residuals. The presence of additional structures in the lithosphere and upper mantle of the Eifel region that are not covered by a simple plume model prevents a perfect fit of the observed seismological data and may bias to some degree the derived plume parameters. The traveltime anomalies are mainly caused by the plume stem with smaller contributions from the plume head. Models with and without an axisymetric plume head below the lithosphere fit the data almost equally well and we conclude that the absence of a plume head in tomographic images does not rule out its existence. In the best-fitting model the plume stem has a radius of 60 km and rises about 50 km to the SW of the quaternary volcanic field below a lithosphere that this slowly moving in the NNE direction. The temperature of the plume and its flux cannot be constrained tightly from our model results, but combining them with other constraints we estimate an excess temperature of ∼200 ◦ C and a buoyancy flux of 500–1000 kg s −1 .

Seismic studies of the Brasília fold belt at the western border of the São Francisco Craton, Central Brazil, using receiver function, surface-wave dispersion and teleseismic tomography

The Tocantins Province in Central Brazil is composed of a series of SSW–NNE trending terranes of mainly Proterozoic ages, which stabilized in the Neoproterozoic in the final collision between the Amazon and São Francisco cratons. No previous information on crustal seismic properties was available for this region. Several broadband stations were used to study the regional patterns of crustal and upper mantle structure, extending the results of a recent E–W seismic refraction profile. Receiver functions and surface wave dispersion showed a thin crust (33–37 km) in the Neoproterozoic Magmatic Arc terrane. High average crustal Vp/Vs ratios (1.74–1.76) were consistently observed in this unit. The foreland domain of the Brasília foldbelt, on the other hand, is characterized by thicker crust (42–43 km). Low Vp/Vs ratios (1.70–1.72) were observed in the low-grade foreland fold and thrust zone of the Brasília belt adjacent to the São Francisco craton. Teleseismic P-wave tomography shows that the lithospheric upper mantle has lower velocities beneath the Magmatic Arc and Goiás Massif compared with the foreland zone of the belt and São Francisco craton. The variations in crustal thickness and upper mantle velocities observed with the broadband stations correlate well with the measurements along the seismic refraction profile. The integration of all seismic observations and gravity data indicates a strong lithospheric contrast between the Goiás Massif and the foreland domain of the Brasília belt, whereas little variation was found across the foldbelt/craton surface boundary. These results support the hypothesis that the Brasília foreland domain and the São Francisco craton were part of a larger São Francisco-Congo continental plate in the final collision with the Amazon plate.

3D shear-wave velocity structure of the Eifel plume, Germany

The Quaternary Eifel volcanic fields, situated on the Rhenish Massif in Germany, are the focus of a major interdisciplinary project. The aim is a detailed study of the crustal and mantle structure of the intraplate volcanic fields and their deep origin. Recent results from a teleseismic P-wave tomography study reveal a deep low-velocity structure which we infer to be a plume in the upper mantle underneath the volcanic area [J.R.R. Ritter et al., Earth Planet. Sci. Lett. 186 (2001) 7–14]. Here we present a travel-time investigation of 5038 teleseismic shear-wave arrivals in the same region. First, the transverse (T) and radial (R) component travel-time residuals are treated separately to identify possible effects of seismic anisotropy. A comparison of 2044 T- and 2994 R-component residuals demonstrates that anisotropy does not cause any first-order travel-time effects. The data sets reveal a deep-seated low-velocity anomaly beneath the volcanic region, causing a delay for teleseismic shear waves of about 3 s. Using 3773 combined R- and T-component residuals, an isotropic non-linear inversion is calculated. The tomographic images reveal a prominent S-wave velocity reduction in the upper mantle underneath the Eifel region. The anomaly extends down to at least 400 km depth. The velocity contrast to the surrounding mantle is depth-dependent (from −5% at 31–100 km depth to at least −1% at 400 km depth). At about 170–240 km depth the anomaly is nearly absent. The resolution of the data is sufficient to recover the described features, however the anomaly in the lower asthenosphere is underestimated due to smearing and damping. The main anomaly is similar to the P-wave model except the latter lacks the ‘hole’ near 200 km depth, and both are consistent with an upper mantle plume structure. For plausible anhydrous plume material in the uppermost 100 km of the mantle, an excess temperature as great as 200–300 K is estimated from the seismic anomaly. However, 1% partial melt reduces the required temperature anomaly to about 100 K. The temperature anomaly associated with the deeper part of the plume (250 to about 450 km depth) is at least 70 K. However, this estimate is quite uncertain, because the amplitude of the shear-wave anomaly may be larger than the modelled one. Another possibility is water in the upwelling material. The gap at 170–240 km depth could arise from an increase of the shear modulus caused by dehydration processes which would not affect P-wave velocities as much. An interaction of temperature and compositional variations, including melt and possibly water, makes it difficult to differentiate quantitatively between the causes of the deep-seated low-velocity anomaly.

Crustal and upper mantle structure across the Dead Sea rift and Israel from teleseismic P-wave tomography and gravity data

New findings of the structure of the crust and the upper mantle across the Dead Sea rift and Israel were obtained by applying P and PKP wave relative travel time residuals inversion of 612 teleseisms, recorded by the seismic networks in Israel and Jordan. Independently of the relative teleseismic travel time residuals, the Bouguer gravity anomalies were used as an additional aid to examine the structure of the crust and upper mantle. An examination of the teleseismic travel time residuals in Israel suggests the existence of two prominent provinces of distinct crustal structure. The northward increase of the teleseismic travel time residuals is in good agreement with a gradual thickening of the sedimentary sequence and a simultaneous thinning of the crust. Examination of the inverted velocity anomalies indicates that the upper and lower crusts are sub-divided into a number of structures. We identify several velocity anomalies that are associated with prominent geological structures. The southern Dead Sea basin is characterized with the largest decrease of velocity, in both upper and lower crustal layers, due to the infilling of light material relative to the surrounding material. The velocity anomaly in the Carmel structure is probably associated with variations in the upper part of the upper mantle. Examination of a cross-section that traverses the Dead Sea transform supports the existence of lighter mass density material on the eastern side relative to that on the western side.

Litho—asthenospheric structures of northern Italy as inferred from teleseismic P-wave tomography

Regional three-dimensional inversions of teleseismic P-wave travel time residuals recorded by high-frequency regional and local seismic networks operating along the Western Alps and surrounding regions were carried out and lithosphere and upper mantle P-wave velocity models down to 300 km were obtained. Residuals of more than 500 teleseismic events, recorded by 98 fixed and temporary seismic stations, have been inverted. The comparison between real residuals and the ones obtained from tomographic model indicates that the method is able to solve the feature of the regional heterogeneities. Where the resolution is good, coherent lithospheric and upper mantle structures are imaged. In the shallower layers, high- and low-velocity anomalies follow the structural behaviour of the Alpine-Apenninic chains showing the existence of very strong velocity contrasts. In the deepest layers, velocity contrast decreases however two deep-seated high-velocity structures are observed. The most extended in depth and approximately trending NE-SW has been interpreted as a wreck of the oldest subduction responsible of the Alpine orogenesis. The second one, connected to the northwestern sector of the Apenninic chain, appears to vanish at depths greater than 180 km and is probably due to still active Apenninic roots. Cross-sections depict the spatial trend of perturbations and in particular outline the sub-vertical character of the Alpine and Apenninic anomalies. Under the Ligurian Sea, the 3-D inversion confirms the uplift of the asthenosphere in agreement with the tectonic evolution of the basin.

New recognition of NNE-strike belt of negative aeromagnetic anomaly in Tibetan plateau

DOI = 10.3126/hjs.v5i7.1310 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.123-124