SH Velocity Research Articles

Lithospheric Velocity Structure Beneath the Northern Margin of the South China Sea

AbstractImaging the lithospheric structure beneath the South China Sea (SCS) not only provides crucial constraints on the tectonic history of southeastern Asia, but also provides an important sample for understanding the evolution of the oceanic lithosphere. However, due to the lack of seismic sampling, the lithospheric structures beneath the SCS are not well resolved. Here, with China National Seismic Network, we observe the “Lid signal” generated by earthquakes below the lithosphere, which propagates through the fast mantle Lid and acts as the first arrival. The Lid signal provides robust constraints on the structure of the lithosphere. Through modeling the Lid signals of events occurred at different depths, we find that the thickness of the lithosphere beneath the northern margin of the SCS is ∼65 km. Across the lithosphere‐asthenosphere boundary, the SH velocity decreases by 6% ∼ 8% within 60 km. Such a large velocity contrast may be due to the presence of melt in the asthenosphere. We also detect another velocity jump at a depth of ∼250 km in both SH and P wave data, which correlates with the depth of the X‐discontinuity. A comparison between the lithospheric structure beneath the SCS basin and other oceanic plates further suggests a hot asthenosphere beneath the SCS, which could be related to the existence of the Hainan Plume nearby.

Velocity structure and radial anisotropy of the lithosphere in southern Madagascar from surface wave dispersion

SUMMARYWe investigate the upper mantle seismic structure beneath southern Madagascar and infer the imprint of geodynamic events since Madagascar’s break-up from Africa and India and earlier rifting episodes. Rayleigh and Love wave phase velocities along a profile across southern Madagascar were determined by application of the two-station method to teleseismic earthquake data. For shorter periods (<20 s), these data were supplemented by previously published dispersion curves determined from ambient noise correlation. First, tomographic models of the phase velocities were determined. In a second step, 1-D models of SV and SH wave velocities were inverted based on the dispersion curves extracted from the tomographic models. As the lithospheric mantle is represented by high velocities we identify the lithosphere–asthenosphere boundary by the strongest negative velocity gradient. Finally, the radial anisotropy (RA) is derived from the difference between the SV and SH velocity models. An additional constraint on the lithospheric thickness is provided by the presence of a negative conversion seen in S receiver functions, which results in comparable estimates under most of Madagascar. We infer a lithospheric thickness of 110−150 km beneath southern Madagascar, significantly thinner than beneath the mobile belts in East Africa (150−200 km), where the crust is of comparable age and which were located close to Madagascar in Gondwanaland. The lithospheric thickness is correlated with the geological domains. The thinnest lithosphere (∼110 km) is found beneath the Morondava basin. The pre-breakup Karoo failed rifting, the rifting and breakup of Gondwanaland have likely thinned the lithosphere there. The thickness of the lithosphere in the Proterozoic terranes (Androyen and Anosyen domains) ranges from 125 to 140 km, which is still ∼30 km thinner than in the Mozambique belt in Tanzania. The lithosphere is the thickest beneath Ikalamavony domain (Proterozoic) and the west part of the Antananarivo domain (Archean) with a thickness of ∼150 km. Below the eastern part of Archean domain the lithosphere thickness reduces to ∼130 km. The lithosphere below the entire profile is characterized by positive RA. The strongest RA is observed in the uppermost mantle beneath the Morondava basin (maximum value of ∼9 per cent), which is understandable from the strong stretching that the basin was exposed to during the Karoo and subsequent rifting episode. Anisotropy is still significantly positive below the Proterozoic (maximum value of ∼5 per cent) and Archean (maximum value of ∼6 per cent) domains, which may result from lithospheric extension during the Mesozoic and/or thereafter. In the asthenosphere, a positive RA is observed beneath the eastern part Morondava sedimentary basin and the Proterozoic domain, indicating a horizontal asthenospheric flow pattern. Negative RA is found beneath the Archean in the east, suggesting a small-scale asthenospheric upwelling, consistent with previous studies. Alternatively, the relatively high shear wave velocity in the asthenosphere in this region indicate that the negative RA could be associated to the Réunion mantle plume, at least beneath the volcanic formation, along the eastern coast.

On the anisotropy of seismic waves in the Carpathian region

The anisotropy of seismic waves in the continental regions still belongs to the category of controversial issues, since its estimates in different areas show a different sign of the anisotropy coefficient. In contrast to studies of oceanic regions, where SH velocities always prevail over SV velocities, in the continental regions the relations between the velocities are very different. The explanation for this, first of all, is the difference in structure. The structure of the crust and upper mantle under the oceans is much more homogeneous in comparison with the structure of the continental regions. There are several approaches to the estimation of anisotropy. The most traditional method is to use the maximum amount of data separately for Love and Rayleigh waves to study the lateral distributions of SH- and SV-wave velocity, despite the fact that the density of the coverage by paths, and, consequently, the regions of best resolution can be of different shapes and sizes. It was decided to use this method as the first approximation in creating an anisotropic portrait of the Carpathian region. The Carpathian region was chosen as the object of study, since it contains interesting contrasting features: (1) the Pannonian Basin, which is characterized by a thin crust, a thinned lithosphere, and anomalously high values of the heat flux; (2) the Tornquist-Teisseyre zone, which is parallel to the strike of the Eastern Carpathians, and represents the contact zone of the Precambrian lithosphere of the EEP and the relatively young lithosphere of Western Europe. (3) The third feature is the Vrancea zone, one of the most active seismic zones in Europe. It is located in the junction of young tectonic structures: the Southern and Eastern Carpathians, the Transylvanian Depression and the Pre-Carpathian Depression. The results of the study confirm that the Tornquist-Teisseyre Zone divides the structures of the ancient East European Platform and orogenic zones of Western Europe: the upper mantle throughout EEP is characterized by high velocities, whereas velocities throughout WE are markedly lower. Low velocity anomalies prevail under Pannonian Basin which is characterized by anomalously high heat flow values. The distribution of the anisotropy coefficient demonstrates an extended layer of low values of the anisotropy coefficient at depths of 150-200 km. Above this layer, velocity distributions reveal the block structure of the lithosphere. The earthquake sources in the Vrancea zone fall into the transition zone from the highvelocity mantle under the EEP to the low-velocity mantle under the WE. Earthquakes do not occur below the revealed asthenospheric layer.

Mantle Transition Zone Structure Beneath Northeast Asia From 2‐D Triplicated Waveform Modeling: Implication for a Segmented Stagnant Slab

AbstractThe structure of the mantle transition zone (MTZ) in subduction zones is essential for understanding subduction dynamics in the deep mantle and its surface responses. We constructed the P (Vp) and SH velocity (Vs) structure images of the MTZ beneath Northeast Asia based on two‐dimensional (2‐D) triplicated waveform modeling. In the upper MTZ, a normal Vp but 2.5% low Vs layer compared with IASP91 are required by the triplication data. In the lower MTZ, our results show a relatively higher‐velocity layer (+2% Vp and −0.5% Vs compared to IASP91) with a thickness of ~140 km and length of ~1,200 km atop the 660‐km discontinuity. Taking this anomaly as the stagnant slab and considering the plate convergence rate of 7–10 cm/year in the western Pacific region during the late Cenozoic, we deduced that the stagnant slab has a subduction age of less than 30 Ma. This suggests that the observed stagnancy of the slab in the MTZ beneath Northeast Asia may have occurred no earlier than the Early Oligocene. From the constraints derived individually on Vp and Vs structures, high Vp/Vs ratios are obtained for the entire MTZ beneath Northeast Asia, which may imply a water‐rich and/or carbonated environment. Within the overall higher‐velocity stagnant slab, a low‐velocity anomaly was further detected, with a width of ~150 km, Vp and Vs reductions of 1% and 3% relative to IASP91. Such a gap may have provided a passage for hot deep mantle materials to penetrate through the thick slab and feed the Changbaishan volcano.

SH wave structure of the crust and upper mantle in southeastern margin of the Tibetan Plateau from teleseismic Love wave tomography

The deep structure of southeastern Tibet is important for determining lateral plateau expansion mechanisms, such as movement of rigid crustal blocks along large strike-slip faults, continuous deformation or the eastward crustal channel flow. We invert for 3-D isotropic SH wave velocity model of the crust and upper mantle to the depth of 110 km from Love wave phase velocity data using a best fitting average model as the starting model. The 3-D SH velocity model presented here is the first SH wave velocity structure in the study area. In the model, the Tibetan Plateau is characterized by prominent slow SH wave velocity with channel-like geometry along strike-slip faults in the upper crust and as broad zones in the lower crust, indicating block-like and distributed deformation at different depth. Positive radial anisotropy (VSH > VSV) is suggested by a high SH wave and low SV wave anomaly at the depths of 70–110 km beneath the northern Indochina block. This positive radial anisotropy could result from the horizontal alignment of anisotropic minerals caused by lithospheric extensional deformation due to the slab rollback of the Australian plate beneath the Sumatra trench.

Origin of intraplate volcanism in northeast China from Love wave constraints

AbstractThe tectonic source for widespread volcanism in northeast China has not been completely understood. We develop a 3‐D SH velocity model in NE China that provides new constraints to the origin of the volcanism. The 3‐D model is constructed from fundamental mode Love waves at the periods of 20–125 s recorded at 269 broadband seismic stations. The Changbai Mountain is characterized by a significant low velocity in the lower crust and uppermost mantle, which probably results from mantle upwelling due to the subduction of the Pacific plate. A fast and thin mantle lid of ~75 km is present beneath the Songliao Basin, indicating lithosphere extension from back‐arc rifting. The slow velocity in the middle and fast velocities in the south and north at 75–115 km depths in the Songliao Basin suggest complex mantle flow with upwelling and downwelling. Unlike the other volcanic fields (Changbaishan volcano, Jingpohu volcano, and Abaga volcano), the Halaha volcano has high velocity in the lower crust and upper mantle, implying a limited melt supply from mantle source recently. The subduction‐induced upwelling leads to complicated small‐scale mantle convection, which is responsible for the intraplate magmatism in northeast China.

Joint inversion for global isotropic and radially anisotropic mantle structure including crustal thickness perturbations

AbstractWe present a new global whole‐mantle model of isotropic and radially anisotropic S velocity structure (SGLOBE‐rani) based on ~43,000,000 surface wave and ~420,000 body wave travel time measurements, which is expanded in spherical harmonic basis functions up to degree 35. We incorporate crustal thickness perturbations as model parameters in the inversions to properly consider crustal effects and suppress the leakage of crustal structure into mantle structure. This is possible since we utilize short‐period group‐velocity data with a period range down to 16 s, which are strongly sensitive to the crust. The isotropic S velocity model shares common features with previous global S velocity models and shows excellent consistency with several high‐resolution upper mantle models. Our anisotropic model also agrees well with previous regional studies. Anomalous features in our anisotropic model are faster SV velocity anomalies along subduction zones at transition zone depths and faster SH velocity beneath slabs in the lower mantle. The derived crustal thickness perturbations also bring potentially important information about the crustal thickness beneath oceanic crusts, which has been difficult to constrain due to poor access compared with continental crusts.

In situ seismic measurements in claystone at Tournemire (France)

Compressional and shear wave seismic measurements were performed in an old railway tunnel and in galleries excavated in a 250-m-thick Toarcian claystone formation in the Tournemire experimental station (France). Three component (3C) geophones and three orthogonal orientations of the vibroseismic force source were used. Additionally, vertical seismic profiling (VSP) measurements were recorded with a 3C borehole geophone, a hydrophone and a microphone in a 159 m deep borehole (ID180) in the tunnel. The seismic data show that Toarcian claystone has strong transverse isotropy (TI) with a vertical symmetry axis. The qP, SH and qSV wave propagation velocities in horizontal directions—the plane of isotropy of the TI medium—are measured as 3550, 1850 and 1290 m s−1, respectively. The zero-offset VSP reveals that only one shear wave propagates in the vertical (depth) direction and the P- and S-wave velocities are 3100 and 1375 m s−1, respectively. Four elastic moduli of the TI medium are determined from the seismic velocities and from the bulk density of 2.53 g cm−3: c11 = 31.9 GPa, c33 = 24.3 GPa, c44 = 4.5 GPa and c66 = 8.7 GPa. A walkaway VSP with the borehole geophone at 50 m depth in borehole ID180 and shot points in the galleries leads to oblique seismic ray paths which allow us to determine the fifth elastic modulus of the TI medium to c13 = 16 GPa. The tube wave recorded by a hydrophone in the water filled lower part of the borehole propagates with 1350 m s−1, which confirms the estimate of the elastic constant c66. The analysis of body wave and surface wave data from a seismic experiment in Galerie Est shows reflections from several fracture zones in the gallery floor. The thickness of the excavation damaged zone (EDZ) in the floor of Galerie Est is estimated to 0.7 m.

P and SH velocity structure in the upper mantle beneath Northeast China: Evidence for a stagnant slab in hydrous mantle transition zone

Using high-dense regional body waves for three deep earthquakes that occurred around Russia–China border, we investigate both S and P wave velocity structures in the mantle transition zone beneath Northeast China and northern part of North China Craton, where the northwestern Pacific plate is imaged to subhorizontally lie above the 660-km discontinuity. We observe an increasing trend of S–P travel time residuals along the epicentral distance within a distance range of 11–16.5°, indicating a velocity anomaly in MTZ. We seek the simplest model that explains the observed broadband waveforms and relative travel times of triplication for a confined azimuth sector. Both SH and P data suggest a ∼140±20km high velocity layer lying above a slightly depressed and broad 660-km discontinuity. Shear velocity reduction of ∼2.5% in the deeper part of the transition zone is required to compensate for the significantly large relative time between AB and CD triplicate branches and the increased trending of S–P travel time residuals as well. The MTZ, as a whole, is featured by low shear velocity and high Vp/Vs ratio. A water-rich mantle transition zone with 0.2–0.4wt% of H2O may account for the discrepancy between the observed Vp and Vs velocity structures. Our result supports the scenario of a viscosity-dominated stagnant slab with an increased thickness of ∼140km, which was caused by the large viscosity contrast between the lower and upper mantles. The addition of water and eastward trench retreat might facilitate stagnation of the subducting Pacific slab beneath Northeast China.

Lateral variations in SH velocity structure of the transition zone beneath Korea and adjacent regions

Using seismic profiles of triplicated waveforms, we show significant lateral variations in the SH velocity (Vs) structure of the transition zone (TZ) beneath Korea and adjacent regions. Beneath Sakhalin, we detected a high Vsanomaly (∼2%) limited to middle regions of the TZ (mid‐TZ), and a largeVsjump across the 660‐km discontinuity. A similar jump inVs also occurs beneath the northern portion of the North China Craton (NCC). Beneath Korea, a high Vs anomaly (∼2%) in the lower TZ is inferred, accompanied by a relatively small Vsjump across the 660‐km discontinuity, which is depressed by about ∼15–20 km. The deep structure under the eastern part of northeast China (NEC) also includes a slightVsanomaly (∼1%) in the lower TZ but does not exhibit significant depression of the 660‐km discontinuity. Compared with previous study, our observations reveal strong regional variations of the TZ structure on a relatively short scale. These variations most likely reflect the geometrical distribution of the subducting northwest Pacific plate. Our results suggest that the subducting slab dips across the mid‐TZ under Sakhalin, and becomes flattened atop of the 660‐km discontinuity beneath Korea, while only the tip of the slab reaches the lower TZ beneath the NEC. The TZ beneath the NCC does not show evidence of the slab stagnation.

A teleseismic finite-fault rupture model for the August 17, 1999, İzmit earthquake (Mw = 7.6): implications for the seismic nucleation phase

<p>A linear finite-fault inversion procedure is applied to teleseismically recorded broadband P and SH velocity waveforms of the August 17, 1999, İzmit earthquake, to derive spatial and temporal distributions of the co-seismic slip over the representative three-segment model fault. The model fault is longer than the mapped surface rupture, and it extends offshore for 25 km in the west, to define the western end of the earthquake rupture. The teleseismically derived slip model suggests a bilateral rupture with a total seismic moment release of 2.6 ×10<sup>20</sup> Nm, and that the rupture was dominated by failure of two major asperities with peak slip amplitudes reaching 7 m. The hypocentral area was represented by the relatively low displacement that separated the large asperities. In the west, the rupture reached the eastern entrance of the Çınarcık basin beneath the Sea of Marmara, with an average slip of ca. 2 m. This indicates that the rupture propagated offshore for ca. 20 km after crossing Hersek Peninsula. The analysis also reveals that the total rupture process time was 32 s, while the main seismic moment release, which corresponded to the rupture of the two large asperities, occurred between 4 s and 16 s after rupture initiation. The strong wave energy arrivals from the failure of the large asperities were preceded by weak wave arrivals in the initial section of the teleseismic waveforms used in this study. Along with some observations from previous studies, this emergent onset of the wave arrivals prompts us to discuss the possibility of a seismic nucleation process for this earthquake.</p>

Juan de Fuca subduction zone from a mixture of tomography and waveform modeling

Seismic tomography images of the upper mantle structures beneath the Pacific Northwestern United States display a maze of high‐velocity anomalies, many of which produce distorted waveforms evident in the USArray observations indicative of the Juan de Fuca (JdF) slab. The inferred location of the slab agrees quite well with existing contour lines defining the slab's upper interface. Synthetic waveforms generated from a recent tomography image fit teleseismic travel times quite well and also some of the waveform distortions. Regional earthquake data, however, require substantial changes to the tomographic velocities. By modeling regional waveforms of the 2008 Nevada earthquake, we find that the uppermost mantle of the 1D reference model AK135, the reference velocity model used for most tomographic studies, is too fast for the western United States. Here, we replace AK135 with mT7, a modification of an older Basin‐and‐Range model T7. We present two hybrid velocity structures satisfying the waveform data based on modified tomographic images and conventional slab wisdom. We derive P and SH velocity structures down to 660 km along two cross sections through the JdF slab. Our results indicate that the JdF slab is subducted to a depth of 250 km beneath the Seattle region, and terminates at a shallower depth beneath Portland region of Oregon to the south. The slab is about 60 km thick and has a P velocity increase of 5% with respect to mT7. In order to fit waveform complexities of teleseismic Gulf of Mexico and South American events, a slab‐like high‐velocity anomaly with velocity increases of 3% for P and 7% for SH is inferred just above the 660 discontinuity beneath Nevada.

Detection of ridge-like structures in the Pacific Large Low-Shear-Velocity Province

Waveform tomography is conducted for SH velocity structures of the entire mantle using approximately 3.5 times the data used for obtaining the previous model, SH18CE. The resultant new model, SH18CEX, exhibits a cluster of ridge-like low-velocity anomalies in the western part of the Pacific Large Low-Shear-Velocity Province (LLSVP). The location of the ridge-like anomalies is in good agreement with the location of the abrupt change in the topography of the D″ discontinuity. These results suggest that the LLSVP is associated with a cluster of ridge-like-piles, rather than a single large pile spread over the entire region. The piles probably consist of intrinsically dense material; however, either their volume or density contrast may not be sufficiently large to develop large-scale domes.

Crustal Structure of the Northwest Deccan Volcanic Province, India,and the Adjoining Continental Shelf from Observed Surface-Wave Dispersion

Abstract The existence of elastic anisotropy in the crust of the continental shelf adjoining the northwest Deccan Volcanic Province (DVP) is obtained through inversion of group velocities of Rayleigh and Love waves at periods of 5–80 s. An isotropic crustal model fairly explains the adjoining onshore surface-wave velocity data, but a radially anisotropic crust is necessary to explain similar data across the offshore region (continental shelf). The mean of SH and SV velocities in the anisotropic crust of the offshore region is close to the S -wave velocity of the isotropic crust of the adjoining onshore region. This shows that the elastic properties of the crustal layers were the same in the offshore and the adjoining onshore region, and the properties changed to radial anisotropy due to extensional tectonic deformation in the offshore region, which resulted in lattice-preferred orientation (LPO) of crystallographic axes of elastic anisotropic minerals. Results also indicate that the western trend of increasing trap thickness in DVP continues to the continental shelf.

Structure of North American mantle constrained by simultaneous inversion of multiple-frequencySH,SS, and Love waves

[1] We simultaneously invert for the velocity and attenuation structure of the North American mantle from a mixed data set: SH wave traveltime and amplitude anomalies, SS wave differential traveltime anomalies, and Love wave fundamental mode phase delays. All data are measured for multiple frequency bands, and finite frequency sensitivity kernels are used to explain the observations. In the resulting SH velocity model, a lower mantle plume is observed to originate at about 1500 km depth beneath the Yellowstone area, tilting about 40° from vertical. The plume rises up through a gap in the subducting Farallon slab. The SH velocity model confirms high-level segmentation of the Farallon slab, which was observed in the recent P velocity model. Attenuation structure is resolvable in the upper mantle and transition zone; in estimating it, we correct for focusing. High-correlation coefficients between δlnVS and δlnQS under the central and eastern United States suggest one main physical source, most likely temperature. The smaller correlation coefficients and larger slopes of the δlnQS − δlnVS relationship under the western United States suggest an influence of nonthermal factors such as the existence of water and partial melt. Finally, we analyze the influence of the different components of our data set. The addition of Love wave phase delays helps to improve the resolution of both velocity and attenuation, and the effect is noticeable even in the lower mantle.

Development of Novel LOVE Wave Biosensor for Simultaneous Detection of Multi-Analyte

In this paper, a novel 208MHz LOVE wave biosensor with one reference element and three sensing elements was developed in a single sensor. PMMA was chosen for the waveguide layer because it can be easily produced using a spin-coating technique. In addition, PMMA has a relatively low density (1.17g/cm3), low SH velocity (1105 m/s), a good elastic property, and high stiffness modulus (1.7GPa). The developed sensor shows high sensitivity, and detects three different biomolecules and improves the reliability of results using reference element. The evaluated sensitivity of the sensor was 7kHz/ mga•ml-1. Long-term stabilities of the developed sensor in liquid environment were also observed. From these results, we confirmed that the developed Love wave biosensor is very promising for simultaneous detections of multi-bioanalyte in a single sensor and for improving the reliabilities of the sensors.

Ambient noise Love wave tomography in the eastern margin of the Tibetan plateau

We collected continuous waveform data from June to December 2007 recorded by 49 broadband seismic stations in western Sichuan and eastern Tibet, and applied an ambient noise technique to obtain a high-resolution tomography model of the crust beneath the eastern margin of the Tibetan plateau. Cross-correlations of ambient noise data are calculated in one-hour segments and stacked over 7 months. Then Love wave group dispersion curves along approximately 700-station pairs were determined using the multiple-filter analysis method. The study region was divided into a grid of 0.2° × 0.2°, and group velocity distribution maps between 8 and 40 s were obtained. The Love wave group velocity maps show good correlations with surface features. From the group velocity maps we inverted a 3-D model for SH wave velocity. The inferred SH velocity heterogeneities generally agree with variations in SV derived from Rayleigh wave dispersion, discrepancies between them are examined in view of non-uniqueness of the models and radial anisotropy. Both the Rayleigh and Love wave inversions show the significant difference of the crustal structure and reveal a prominent low-velocity zone within the mid-to-lower crust beneath the eastern margin of the Tibetan plateau.

10.1007/s11486-008-1008-z

The deep structure of the upper mantle is determined from data on phase velocities of Love and Rayleigh waves measured by a differential method on traces between two stations in central Western Europe. One-dimensional velocity structures are first constructed from data of each pair of stations, after which two-dimensional distributions of SH and SV velocities are calculated by the method of two-dimensional tomography from S wave velocities at fixed depths. The results are presented in the form of 2-D vertical structures of the average S wave velocity (S = (SV + SH)/2) constructed along profiles crossing the region in directions of the best resolution. The main structural features are a higher velocity zone at depths of 60–80 km in the area (48°–50°N, 9°–11°E) and a lower velocity zone in the western part of the region at depths of 100–150 km, probably extending farther beyond the studied area.

The lithosphere and asthenosphere of the Iceland hotspot from surface waves

SUMMARY 1-D models were calculated for the velocity of shear waves, polarized vertically (SV ) and horizontally (SH) from dispersed Rayleigh and Love surface waves. These had been recorded in Iceland by the ICEMELT broad-band seismic network, with about half of the waves coming from near-distance earthquakes (≤1000 km). The analysis included unusually short periods, as brief as 5.0 s, and periods ranging up to 93 s. The Icelandic crust was revealed to have two basic layers: first, the upper and middle crust, which were largely detected as one layer, and second the layer of the lower crust. The half of Iceland surveyed had a weighted average crustal thickness of 25–26 km, less than previously estimated. It is under East and East Central Iceland that the crust is thickest, averaging 29–32 ± 3 km, and under the western margin of the West Fjords, 29 ± 2 km. The thinnest parts of the crust lie in West Central Iceland, 19 ± 1 km, and in the West Volcanic (or Rift) Zone, 19[+6/−1] km. This study examined how thicker crust away from the rift zone can be fitted with dynamic crust formation models. Possible explanations for different thicknesses include both crustal squeezing flow and imbalances between widths of the volcanic accretion and extensional stretching zones. The crust has highly anisotropic zones, with differences of up to 20 per cent between SV and SH velocities. Under rift zones, the lower crust is characterized by low velocities and, at depths of 8–18 km, by a channel with yet lower velocities. The lowest shear velocity in this channel is 5–9 per cent less than in the standard Icelandic velocity model. The thinnest lithosphere, 20 ± 2 km, lies under the East Central and North Volcanic Zones, where it extends up into the crust, while the thickest lithosphere is under East Iceland and the east shelf, nowhere less than 100 ± 20 km. This substantial contrast in lithosphere thickness of some 80 km occurs within a lateral distance of 100–150 km, implying an age unconformity at depth of several tens of millions of years. The thick East Iceland lithosphere may reduce or obstruct any eastward flow of the plume head. On the opposite side of the plume head, in Northwest Iceland and the West Fjords, the lithosphere is estimated to be 60 ± 10 km thick. Excepting the West Fjords and East Iceland, shear wave velocities are low in the island’s subcrustal mantle, up to 7–9 per cent below the world average according to the PREM model. This indicates a warm, partially molten mantle under much of Central Iceland and the active rift zones. There is a lateral difference of 10– 12 per cent in shear velocity between the shallowest mantle asthenosphere under Central Iceland and under the mantle lid to each side, that is, under the West Fjords and East Iceland. In the shallowest Central Iceland mantle, Vp/Vs-ratios suggest near solidus temperatures and a partial melt of 2–3 per cent. This paper describes structural variations in the asthenosphere down to 75–200 km. The low-velocity zone found 100–125 km below Central Iceland and the major part of western Iceland is interpreted as the onset of mantle plume melting. Mantle anisotropy is pronounced beneath Iceland, with SH and SV velocities differing by up to 10 per cent. The anisotropy structure is 3-D and normally reaches higher values in the asthenosphere than in the mantle lid. The main factor determining the asthenosphere’s generally azimuthal anisotropy may be the lattice-preferred orientation (LPO) induced by flow. Based on this interpretation and the observed anisotropy, it follows that the plume head is flowing westwards at a depth of 60–110 km. The deeper, more pervasive North Atlantic flow is towards

Distinct velocity variations around the base of the upper mantle beneath northeast Asia

Both the global and regional P wave tomographic studies have revealed significant deep structural heterogeneities in subduction zone regions. In particular, low-velocity anomalies have been observed beneath the descending high-velocity slabs in a number of subduction zones. The limited resolution at large depths and possible trade-off between the high and low velocities, however, make it difficult to substantiate this feature and evaluate the vertical extent of the low-velocity structure. From broadband waveform modeling of triplicated phases near the 660-km discontinuity for three deep events, we constrained both the P and SH wave velocity structures around the base of the upper mantle in northeast Asia. For the two events beneath the Southern Kurile, the rays traveled through the lowermost transition zone and uppermost lower mantle under the descending Pacific slab. Our preferred models consistently suggest normal-to-lower P and significantly low SH velocities above and below the 660-km discontinuity extending to about 760-km depth compared with the global IASP91 model, corroborating previous observations for a slow structure underneath the slab. In contrast, both high P and SH velocity anomalies are shown in our preferred models for the Japan subduction zone region, likely reflecting the structural feature of a slab stagnant above the 660-km discontinuity. The velocity jumps across the 660-km discontinuity were found to be on average 4.5% and 7% for P and S waves under the south Kurile, and 3% and 6% under the Japan subduction zone. The respective velocity contrasts in the two regions are consistent with mineralogical models for colder slab interior and hotter under-slab areas. Based on mineral physics data, the depth-averaged ∼1.5% P and ∼2.5% SH velocity differences in the depth range of 560–760 km between the two regions could be primarily explained by a 350–450 K temperature variation, although the presence of about 0.5–1 wt.% water might also contribute to the subtle velocity variations near the base of the transition zone in the Southern Kurile. From our modeling results, we speculate that the slow structure in the Southern Kurile may be correlated to the low-velocity zone observed previously around the 410-km discontinuity under Northern Honshu. If this is the case, both may be associated with a thermal anomaly rooted in the lower mantle beneath the subduction zone in northeast Asia.