Mode Rayleigh Wave Phase Velocities Research Articles

Long Period Rayleigh Wave Focal Spot Imaging Applied to USArray Data

AbstractWe demonstrate the effectiveness of seismic dense array surface wave focal spot imaging using USArray data from the western‐central United States. We study dispersion in the 60–310 s period range and assess the image quality of fundamental mode Rayleigh wave phase velocity maps. We apply isotropic spatial autocorrelation models to the time domain zero lag noise correlation wavefield data at distances of about one wavelength. Local estimates of the phase velocity, its uncertainty, and the regression quality imply overall better ZZ relative to ZR or RZ results. The extension of the depth resolution compared to passive surface wave tomography is demonstrated by the inversion of three clustered dispersion curves from different tectonic units. We observe anisotropic surface wave energy flux and the influence of body wave energy, but sensitivity tests at 60 s targeting the data range, correlation component, and processing choices show that the ZZ focal spots yield consistent high‐quality images compared to regional tomography results in the 60–150 s period range. In contrast, at 200–300 s the comparatively small scales of the imaged structures and the imperfect agreement with low‐resolution global tomography results highlight the persistent challenge to reconcile imaging results based on different data sources, theories, and techniques. Our study shows that surface wave focal spot imaging is an accurate, robust, local imaging approach. Better control over clean autocorrelation fields can further improve applications of this seismic imaging tool for increased resolution of the elastic structure below dense seismic arrays.

Lithospheric Structure of the South India Precambrian Terrains From Surface Wave Tomography

AbstractWe have developed a 3‐D shear velocity (Vsv) model of Precambrian terrains of South India with a lateral resolution of 55 km to a depth of 250 km by inversion of fundamental mode Rayleigh wave phase velocity dispersion data in the period of 30–140s, combined with a well‐constrained crustal velocity model from an earlier study. The dispersion data were computed from 748 earthquakes recorded at 85 homogeneously distributed seismographs. Our velocity model shows 150–200 km thick lithosphere in most of the Archean Dharwar craton where Vsv is >4.7 km/s in the depth of 50–100 km and progressively decreases to 4.6 km/s at 120–150 km followed by a constant velocity of 4.5 km/s beyond 150–200 km depth. It correlates well with the results of the petrological studies of kimberlite xenoliths. An extraordinary high shear velocity (up to 4.8 km/s) and thick lithosphere (150 km) are observed beneath the Proterozoic Carbonatite complex, located at the south‐eastern edge of the Dharwar craton. We infer compositional modification of lower lithosphere at the south‐western margin of the Dharwar craton and lithospheric erosion at the Granulite terrain both possibly due to interaction with the Marion mantle plume at ∼90 Ma. The region is underlain by a 2%–3% lower velocity channel at ∼180–220 km depth in the asthenosphere, uncorrelated with the overlying lithosphere, possibly due to relative motion between them. It may be attributed to deep Earth‐processes such as asthenospheric upwelling.

Seismic site characterization and site response study of Nirsa (India)

The Nirsa block, a coal mining area, and an industrial town lies in the southern part of Dhanbad district, at the border between the Indian states of Jharkhand and West Bengal. This area manifests the significant seismic risk posed by earthquakes, natural as well as induced, of high enough seismic intensity VI and VII from regional and local earthquakes. Hence, it is indispensable to seismically characterize the top surficial layers to reduce the risk associated with earthquakes in this region. We characterize these layers, using ambient noise acquired from multichannel simulation with one receiver and single station method to subsequently invert for shear wave velocities, via iterative global search technique of the guided Monte Carlo method by fitting fundamental mode Rayleigh wave phase velocity dispersion curves and horizontal-to-vertical spectral ratios (HVSR). We survey the area to record a sufficient amount of ambient noise within the frequency range of 1–30 Hz at seventy-eight observation points, with uniform spacing of 250 m, covering a total area of about 4 km2. Our results reveal that the bedrock depth lies in the range of 19.5–2.7 m; while Vs30 values vary in the range of 250–950 m/s in the Nirsa region. The variations in fundamental resonance frequencies, obtained from HVSR curves, are found to be more scattered in the range between 2 and 23 Hz. The maps of fundamental resonance frequency, peak HVSRs’ amplitude, bedrock depth and Vs30 derived from this research exhibit good correlation with each other. The Vs30 prediction equation based on the fundamental resonance frequency (fr) and HVSR amplitude (A0) will help future studies to determine Vs30 of unknown regional sites. The efficacy of our results is reliable as there is good concurrence between shear wave velocity profiles, HVSR profiles and litholog data sets. Our results may prove to be important assets to agencies such as National Disaster Management Authorities and Geological Survey of India for construction of seismic microzonation maps in Nirsa.

A joint inversion of receiver function and Rayleigh wave phase velocity dispersion data to estimate crustal structure in West Antarctica

SUMMARY We determine crustal shear wave velocity structure and crustal thickness at recently deployed seismic stations across West Antarctica, using a joint inversion of receiver functions and fundamental mode Rayleigh wave phase velocity dispersion. The stations are from both the UK Antarctic Network (UKANET) and Polar Earth Observing Network/Antarctic Network (POLENET/ANET). The former include, for the first time, four stations along the spine of the Antarctic Peninsula, three in the Ellsworth Land and five stations in the vicinity of the Pine Island Rift. Within the West Antarctic Rift System (WARS) we model a crustal thickness range of 18–28 km, and show that the thinnest crust (∼18 km) is in the vicinity of the Byrd Subglacial Basin and Bentley Subglacial Trench. In these regions we also find the highest ratio of fast (Vs = 4.0–4.3 km s–1, likely mafic) lower crust to felsic/intermediate upper crust. The thickest mafic lower crust we model is in Ellsworth Land, a critical area for constraining the eastern limits of the WARS. Although we find thinner crust in this region (∼30 km) than in the neighbouring Antarctic Peninsula and Haag-Ellsworth Whitmore block (HEW), the Ellsworth Land crust has not undergone as much extension as the central WARS. This suggests that the WARS does not link with the Weddell Sea Rift System through Ellsworth Land, and instead has progressed during its formation towards the Bellingshausen and Amundsen Sea Embayments. We also find that the thin WARS crust extends towards the Pine Island Rift, suggesting that the boundary between the WARS and the Thurston Island block lies in this region, ∼200 km north of its previously accepted position. The thickest crust (38–40 km) we model in this study is in the Ellsworth Mountain section of the HEW block. We find thinner crust (30–33 km) in the Whitmore Mountains and Haag Nunatak sectors of the HEW, consistent with the composite nature of the block. In the Antarctic Peninsula we find a crustal thickness range of 30–38 km and a likely dominantly felsic/intermediate crustal composition. By forward modelling high frequency receiver functions we also assess if any thick, low velocity subglacial sediment accumulations are present, and find a 0.1–0.8-km-thick layer at 10 stations within the WARS, Thurston Island and Ellsworth Land. We suggest that these units of subglacial sediment could provide a source region for the soft basal till layers found beneath numerous outlet glaciers, and may act to accelerate ice flow.

The uppermost mantle seismic velocity structure of West Antarctica from Rayleigh wave tomography: Insights into tectonic structure and geothermal heat flow

We present a shear wave model of the West Antarctic upper mantle to ∼200km depth with enhanced regional resolution from the 2016-2018 UK Antarctic Seismic Network. The model is constructed from the combination of fundamental mode Rayleigh wave phase velocities extracted from ambient noise (periods 8-25s) and earthquake data by two-plane wave analysis (periods 20-143s). We seek to (i) image and interpret structures against the tectonic evolution of West Antarctica, and (ii) extract information from the seismic model that can serve as boundary conditions in ice sheet and glacial isostatic adjustment modelling efforts. The distribution of low velocity anomalies in the uppermost mantle suggests that recent tectonism in the West Antarctic Rift System (WARS) is mainly concentrated beneath the rift margins and largely confined to the uppermost mantle (<180km). On the northern margin of the WARS, a pronounced low velocity anomaly extends eastward from beneath the Marie Byrd Land dome toward Pine Island Bay, underlying Thwaites Glacier, but not Pine Island Glacier. If of plume-related thermal origin, the velocity contrast of ∼5% between this anomaly and the inner WARS translates to a temperature difference of ∼125-200C∘. However, the strike of the anomaly parallels the paleo-Pacific convergent margin of Gondwana, so it may reflect subduction-related melt and volatiles rather than anomalously elevated temperatures, or a combination thereof. Motivated by xenolith analyses, we speculate that high velocity zones imaged south of the Marie Byrd Land dome and in the eastern Ross Sea Embayment might reflect the compositional signature of ancient continental fragments. A pronounced low velocity anomaly underlying the southern Transantarctic Mountains (TAM) is consistent with a published lithospheric foundering hypothesis. Taken together with a magnetotelluric study advocating flexural support of the central TAM by thick, stable lithosphere, this points to along-strike variation in the tectonic history of the TAM. A high velocity anomaly located in the southern Weddell Sea Rift System might reflect depleted mantle lithosphere following the extraction of voluminous melt related to Gondwana fragmentation. Lithospheric thickness estimates extracted from 1D shear wave velocity profiles representative of tectonic domains in West Antarctica indicate an average lithospheric thickness of ∼85km for the WARS, Marie Byrd Land, and Thurston Island block. This increases to ∼96km in the Ellsworth Mountains. A surface heat flow of ∼60mW/m2 and attendant geotherm best explains lithospheric mantle shear wave velocities in the central WARS and in the Thurston Island block adjacent to Pine Island Glacier; a ∼50mW/m2 geotherm best explains the velocities in the Ellsworth Mountains, and a ∼60mW/m2 geotherm best explains a less well-constrained velocity profile on the southern Antarctic Peninsula. We emphasise that these are regional average (many hundreds of km) heat flow estimates constrained by seismic data with limited sensitivity to upper crustal composition.

Upper mantle shear velocity structure beneath the equatorial East Pacific Rise from array-based teleseismic surface-wave dispersion analysis

Summary The Discovery/Gofar transform faults system is associated with a fast-spreading center on the equatorial East Pacific Rise. Most previous studies focus on its regular seismic cycle and crustal fault zone structure, but the characteristics of the upper mantle structure beneath this mid-ocean ridge system are not well known. Here we invert upper mantle shear velocity structure in this region using both teleseismic surface waves and ambient seismic noise from 24 ocean bottom seismometers (OBSs) deployed in this region in 2008. We develop an array analysis method with multi-dimensional stacking and tracing to determine the average fundamental mode Rayleigh wave phase-velocity dispersion curve (period band 20–100 s) for 94 teleseismic events distributed along the E-W array direction. Then, we combine with the previously measured Rayleigh wave phase-velocity dispersion (period band 2–25 s) from ambient seismic noise to obtain the average fundamental mode (period band 2–100 s) and the first-higher mode (period band 3–7 s) Rayleigh wave phase-velocity dispersion. The average dispersion data are inverted for the 1-D average shear wave velocity (Vs) structure from crust to 200-km depth in the upper mantle beneath our study region. The average Vs between the Moho and 200-km depth of the final model is about 4.18 km/s. There exists an ∼5-km thickness high-velocity lid (LID) beneath the Moho with the maximum Vs of 4.37 km/s. Below the LID, the Vs of a pronounced low-velocity zone (LVZ) in the uppermost mantle (15–60 km depth) is 4.03–4.23 km/s (∼10 per cent lower than the global average). This pronounced LVZ is thinner and shallower than the LVZs beneath other oceanic areas with older lithospheric ages. We infer that partial melting (0.5–5 per cent) mainly occurs in the shallow upper mantle zone beneath this young (0–2 Myr) oceanic region. In the deeper portion (60–200 km depth), the Vs of a weak LVZ is 4.15–4.27 km/s (∼5 per cent lower than the global average). Furthermore, the inferred lithosphere-asthenosphere boundary (LAB) with ∼15-km thickness can fit well with the conductive cooling model. These results are useful for understanding the depth distribution and melting characteristics of the upper mantle lithosphere and asthenosphere in this active ridge-transform fault region.

Transdimensional ambient noise tomography of Bass Strait, southeast Australia, reveals the sedimentary basin and deep crustal structure beneath a failed continental rift

Debate is ongoing as to which tectonic model is most consistent with the known geology of southeast Australia, formerly part of the eastern margin of Gondwana. In particular, numerous tectonic models have been proposed to explain the enigmatic geological relationship between Tasmania and the mainland, which is separated by Bass Strait. One of the primary reasons for the lack of certainty is the limited exposure of basement rocks, which are masked by the sea and thick Mesozoic–Cenozoic sedimentary and volcanic cover sequences. We use ambient noise tomography recorded across Bass Strait to generate a new shear wave velocity model in order to investigate crustal structure. Fundamental mode Rayleigh wave phase velocity dispersion data extracted from long-term cross-correlation of ambient noise data are inverted using a transdimensional, hierarchical, Bayesian inversion scheme to produce phase velocity maps in the period range 2–30 s. Subsequent inversion for depth-dependent shear wave velocity structure across a dense grid of points allows a composite 3-D shear wave velocity model to be produced. Benefits of the transdimensional scheme include a data-driven parametrization that allows the number and distribution of velocity unknowns to vary, and the data noise to also be treated as an unknown in the inversion. The new shear wave velocity model clearly reveals the primary sedimentary basins in Bass Strait as slow shear velocity zones which extend down to 14 km in depth. These failed rift basins, which formed during the early stages of Australia–Antarctica break-up, appear to be overlying thinned crust, where high velocities of 3.8–4.0 km s−1 occur at depths greater than 20 km. Along the northern margin of Bass Strait, our new model is consistent with major tectonic boundaries mapped at the surface. In particular, we identify an east dipping velocity transition zone in the vicinity of the Moyston Fault, a major tectonic boundary between the Lachlan and Delamerian orogens, which are part of the Phanerozoic accretionary terrane that makes up eastern Australia. A pronounced lineament of high shear wave velocities (∼3.7–3.8 km s−1) in the lower crust of our new model may represent the signature of relict intrusive magmatism from failed rifting in the early stages of Australia–Antarctica break-up along a crustal scale discontinuity in the Selwyn Block microcontinent which joins Tasmania and Victoria.

A Bayesian method to quantify azimuthal anisotropy model uncertainties: application to global azimuthal anisotropy in the upper mantle and transition zone

Seismic anisotropy is a powerful tool to constrain mantle deformation, but its existence in the deep upper mantle and topmost lower mantle is still uncertain. Recent results from higher mode Rayleigh waves have, however, revealed the presence of 1% azimuthal anisotropy between 300 km and 800 km depth, and changes in azimuthal anisotropy across the mantle transition zone boundaries. This has important consequences for our understanding of mantle convection patterns and deformation of deep mantle material. Here, we propose a Bayesian method to model depth variations in azimuthal anisotropy and to obtain quantitative uncertainties on the fast seismic direction and anisotropy amplitude from phase velocity dispersion maps. We applied this new method to existing global fundamental and higher mode Rayleigh wave phase velocity maps to assess the likelihood of azimuthal anisotropy in the deep upper mantle and to determine whether previously detected changes in anisotropy at the transition zone boundaries are robustly constrained by those data. Our results confirm that deep upper mantle azimuthal anisotropy is favored and well-constrained by the higher mode data employed. The fast seismic directions are in agreement with our previously published model. The data favor a model characterized, on average, by changes in azimuthal anisotropy at the top and bottom of the transition zone. However, this change in fast axes is not a global feature as there are regions of the model where the azimuthal anisotropy direction is unlikely to change across depths in the deep upper mantle. We were, however, unable to detect any clear pattern or connection with surface tectonics. Future studies will be needed to further improve the lateral resolution of this type of model at transition zone depths

Lithospheric structure of the southeastern margin of the Tibetan Plateau from Rayleigh wave tomography

AbstractLithospheric shear wave velocity beneath the southeastern margin of the Tibetan Plateau is obtained from Rayleigh wave tomography using earthquake data recorded by the temporary ChinArray and permanent China Digital Seismic Array. Fundamental mode Rayleigh wave phase velocities at periods of 20–100 s are determined and used to construct the 3‐D shear wave velocity model. Low‐velocity anomalies appear along or close to the major faults in the middle crust and become a broad zone in the lower crust, suggesting block extrusion in the shallow crust and diffuse deformation in the lower crust, both of which play important roles in accommodating the collision between the Indian and Eurasian plates. A vertical low‐velocity column beneath the Tengchong Volcano is observed, which could be caused by upwelling of warm mantle due to the lithosphere extension in the Thailand rift basin to the south or by fluid‐induced partial melting due to the subduction of the Burma slab. The western Yangtze Craton is characterized by low velocity in the crust and uppermost mantle above the fast mantle lithosphere, indicating possible thermal erosion at the western craton edge resulted from the extrusion of the Tibetan Plateau. A low‐velocity zone is imaged at the depths of 70–150 km beneath the eastern part of the Yangtze Craton, which could be caused by small‐scale mantle convection associated with the subduction of the Burma microplate and/or the opening of the South China Sea.

Ambient noise tomography across Mount St. Helens using a dense seismic array

AbstractWe investigated upper crustal structure with data from a dense seismic array deployed around Mount St. Helens for 2 weeks in the summer of 2014. Interstation cross correlations of ambient seismic noise data from the array were obtained, and clear fundamental mode Rayleigh waves were observed between 2.5 and 5 s periods. In addition, higher‐mode signals were observed around 2 s period. Frequency‐time analysis was applied to measure fundamental mode Rayleigh wave phase velocities, which were used to invert for 2‐D phase velocity maps. An azimuth‐dependent traveltime correction was implemented to mitigate potential biases introduced due to an inhomogeneous noise source distribution. Reliable phase velocity maps were only obtained between 3 and 4 s periods due to limitations imposed by the array aperture and higher‐mode contamination. The phase velocity tomography results, which are sensitive to structure shallower than 6 km depth, reveal an ~10–15% low‐velocity anomaly centered beneath the volcanic edifice and peripheral high‐velocity anomalies that likely correspond to cooled igneous intrusions. We suggest that the low‐velocity anomaly reflects the high‐porosity mixture of lava and ash deposits near the surface of the edifice, a highly fractured magmatic conduit and hydrothermal system beneath the volcano, and possibly a small contribution from silicate melt.

Underplating along the northern portion of the Zagros suture zone, Iran

A 2-D absolute shear wave velocity model has been resolved beneath a seismic profile across the northeastern margin of the Arabian Plate—Central Iran by simultaneously inverting data from P receiver functions and fundamental mode Rayleigh wave phase velocity. The data were gathered by a linear seismic array crossing the Zagros fold and thrust belt, Urmia–Dokhtar magmatic arc and Central Iran block assemblage as three major structural components of the Arabia–Eurasia collision. Our model shows a low-velocity tongue protruding from upper to lower crust which, north of the Zagros suture, indicates the signature of an intracontinent low-strength shear zone between the underthrusting and overriding continents. The velocity model confirms the presence of a significant crustal root as well as a thick high-velocity lithosphere in footwall of the suture, continuing northwards beneath the overriding continent for at least 200 km. These features are interpreted as underthrusting of Arabia beneath Central Iran. Time to depth migration of P receiver functions reveals an intracrustal flat interface at ∼17 km depth south of the suture; we interpret it as a significant decoupling within the upper crust. All these crustal scale structural features coherently explain different styles and kinematics of deformation in northern Zagros (Lorestan zone) with respect to its southern part (Fars zone).

Seismic anisotropy changes across upper mantle phase transitions

The mantle transition zone is believed to play an important role in the thermochemical evolution of our planet and in its deep water cycle. Constraining mantle flow at these depths can help elucidate its nature and better understand mantle dynamics and the history of plate tectonics. Seismic anisotropy, i.e., the directional dependence of seismic wave velocity, provides us with the most direct constraints on mantle deformation. Its detection below ∼250km depth is challenging, and it is often assumed that the deep upper mantle is seismically isotropic due to a change in mantle deformation mechanism. Here, we present a global model of azimuthal anisotropy for the top 1000km of the mantle. We used a dataset composed of fundamental and higher mode Rayleigh wave phase velocity maps, which provides resolution of azimuthal anisotropy to much greater depths than in previous studies. Our model unravels the presence of significant anisotropy in the transition zone, challenging common views of mantle deformation mechanisms, and reveals a striking correlation between changes in anisotropy amplitudes and in the fast direction of wave propagation where upper mantle phase transitions occur. The newly found relation between anisotropy changes and phase transformations gives new insight on the nature of the MTZ and suggests that the anisotropy originates from the lattice preferred orientation of anisotropic material. While the interpretation of our results in terms of mantle deformation is not straightforward due to many possible scenarios, possible explanations for our findings include recrystallization during phase transformations, or changes in the slip system across the MTZ boundaries, which in turn could be explained by changes in the water content of mantle material, consistent with the idea that the transition zone acts as a water filter.

Seismic waveform modelling in a 3-D Earth using the Born approximation: potential shortcomings and a remedy

S U M M A R Y Although the use of the first-order Born approximation for the computation of seismic observ-ables and sensitivity kernels in 3-D earth models shows promise for improving tomographic modelling, more work is necessary to systematically determine how well such methods forward model realistic seismic data compared with more standard asymptotic methods. Most work so far has been focused on the analysis of secondary data, such as phase velocity, rather than time domain waveforms. We here compare synthetic waveforms obtained for simple models using standard asymptotic approximations that collapse the sensitivity to 3-D structure on the great circle plane and those obtained using the 3-D linear Born approximation, with accurate numerical 3-D synthetics. We find, not surprisingly, that 3-D Born more accurately models the perturbation effects of velocity anomalies that are comparable in wavelength to or are smaller than the first Fresnel zone. However, larger wavelength and amplitude anomalies can easily produce large phase delays that cause the first-order (linear) Born approximation to break down, whereas asymptotic methods that incorporate the effect of heterogeneity in the phase rather than in the amplitude of the waveform are more robust. Including a path average phase delay to the Born calculated waveforms significantly improves their accuracy in the case of long-wavelength structure, while still retaining the ability to correctly model the effect of shorter-wavelength structure. Tests in random models with structural wavelengths consistent with existing global seismic models indicate that the linear Born approximation frequently breaks down in realistic earth models, with worse misfit for first and second orbit Rayleigh and higher mode surface waveforms than the great-circle based approximations at all distances tested (>20 •). For fundamental modes, the average misfit for the waveforms calculated with the linear Born formalism is quite poor, particularly for distances larger than 60 •. The modified Born formalism consistently improves the fit relative to the linear Born waveforms, but only outperforms the great-circle based approximations for the higher mode surface waveforms. We note, however, that phase delay kernels for multitaper measurements of fundamental mode Rayleigh wave phase velocities developed from the Born approximation do not demonstrate the problems associated with the linear waveform kernels. There is general agreement with measurements and moderate improvement relative to phase delays predicted by the path-average approximation.

Surface wave tomography of the Gulf of California

We measured interstation fundamental mode Rayleigh wave phase velocities using data from the NARS‐Baja seismic network located around the Gulf of California. A region‐average, shear velocity model and a set of azimuthally anisotropic phase velocity maps are obtained from these data. The average shear velocity structure shows a strong low‐velocity zone underlying a thin lid and the data are suggestive of low velocities down into the transition zone. The phase velocity maps display signatures of sedimentary layers, crustal thickness variations, upwelling under the plate boundary, and the presence of the subducted Farallon microplate remnants beneath the Gulf. The upper mantle features inferred from this study provide new seismic evidence on the tectonic evolution of the region.

Shear-Wave Structure of the South Indian Lithosphere from Rayleigh Wave Phase-Velocity Measurements

We investigate the upper mantle shear-wave speed structure beneath the south Indian shield by measuring and modeling fundamental mode Rayleigh wave phase-velocity dispersion. Observed phase velocities for the south Indian shield closely match those observed for the Canadian shield. We constrain the south Indian crust using published receiver function results and invert the dispersion data for upper mantle shear-wave structure. The ~155-km-thick seismic lithosphere of the south Indian shield is composed of a 35 km-thick, two-layer crust and a ~120-km-thick, high-velocity upper mantle lid. Beneath the Moho the average Sn wave speed is ~4.7 km sec–1. Both Sn travel times data and the dispersion data suggest a positive sub-Moho shear-wave speed gradient. Beneath the seismic lithosphere there is a low- velocity layer where the shear-wave speed drops to ~4.4 km sec–1.

New seismic model of the upper mantle beneath Africa

We present a seismic model of the upper 400 km of the mantle beneath Africa and surrounding regions. This model is constructed by inverse modeling of fundamental mode Rayleigh wave phase velocities (40–200 s) for about 8000 propagation paths. Among the most pronounced anomalies are high shear velocity structures (as much as 6% higher than in the Preliminary Reference Earth model) beneath the West African, Congo, and Kalahari cratons that extend to about 250 km depth. These structures have near-vertical margins across which the shear velocity changes by as much as 6% over 500 km distance. Anomalous low shear velocities (3%–4% lower than in the Preliminary Reference Earth Model) structures are observed beneath the East African, Red Sea, and Gulf of Aden rifts, and beneath the northwestern Indian Ocean. These structures extend to a depth of at least 250 km. Our model cannot be reconciled with models that invoke a large number of plumes that have impinged on the base of the lithosphere, nor does our seismic model indicate that high-temperature, low-density material beneath the lithosphere is responsible for the uplift of southern Africa.

An alternative parameterisation for surface waves in a transverse isotropic medium

The elastic properties of the Earth can only be retrieved by seismic waves when assumptions about the seismic anisotropy are made. One such an assumption is to assume that the Earth is transverse isotropic. In a transverse isotropic Earth Love waves are sensitive to two independent elastic parameters and Rayleigh waves to four parameters. In addition these waves are sensitive to density. However, resolving four elastic parameters together with density as function of depth from Rayleigh waves phase velocity curves is difficult as the large number of parameters can make the inverse problem easily under-determined. We show that the partial derivatives of fundamental and higher mode Rayleigh wave phase velocities with respect to the parameters of transverse isotropy for periods 20< T<200 s are very similar to each other. This means that the inversion of Rayleigh phase velocity data has intrinsic trade-offs. We show that Rayleigh wave data can only resolve three combinations of parameters of the transverse isotropic medium: the S-velocity β V, the sum of the horizontal and vertical P-velocity α +=1/2[ α H+ α V] and their difference α −=1/2[ α H− α V]. The influence of α − is limited to only the upper 100 km Earth. To take full advantage of this parameter set the data-set should consist of both fundamental and higher mode data with periods between 20< T<200 s. For this parameter set the influence of the density is small and we think it cannot be resolved when realistic variations in all parameters are considered. For Love waves the only relevant parameter is β H as β V and ρ cannot be resolved. We support our conclusions by a synthetic experiment where the bias in the inversion of Rayleigh wave phase velocity is investigated when the data is inverted for the reduced alternative parameter set.

Velocity structure of the continental upper mantle: evidence from southern Africa

The velocity model for southern Africa of Qiu et al. [Qiu, X., Priestley, K., McKenzie, D., 1996. Average lithospheric structure of southern Africa. Geophys. J. Int. 127, 563–587] is revised so as to satisfy both the regional seismic waveform data and the fundamental mode Rayleigh wave phase velocity data for the region. The revised S-wave model is similar to the original model of Qiu et al. except that the high velocity, upper mantle lid extends to 160 km depth in the revised model rather than to 120 km in the original model. Sensitivity tests of the regional seismic data show that the minimum velocity in the S-wave low velocity zone can be as high as 4.45 km s −1 compared to 4.32 km s −1 in the Qiu et al. model. The vertical S-wave travel time for the revised south African model is compared with the vertical S-wave travel times for the global tomographic models S12WM13 and S16B30, and they are found to be similar.

Shear‐wave structure of the lithosphere above the Hawaiian Hot Spot from two‐station Rayleigh wave phase velocity measurements

We have measured fundamental mode Rayleigh wave phase velocity dispersion from seismograms of five earthquakes recorded at stations on the islands of Hawaii and Oahu and inverted these data for upper mantle velocity structure. The seismic lithosphere of the velocity model is 88±7 km thick, which is similar to that of 80–90 Myr oceanic lithosphere, indicating that no significant lithospheric thinning takes place above the mantle plume. Below the lithosphere the shear wave velocity decreases to ∼4.0 km s−1. The seismic model is consistent with the structure of the Hawaiian plume model of Watson and McKenzie [1991] which has a 72‐km thick mechanical boundary layer with the shallowest melting occurring at 82 km depth. The difference between the dispersion measured on the Hawaii‐Oahu path and that observed by Woods and Okal [1996] on the Oahu‐Midway path can be explained by the cooling of the injected hot plume material.

Crust and upper mantle tomography in Tibet using surface waves

We have obtained maps of lateral variations of structure in the crust and lithosphere in Tibet and surrounding regions by inversion of single station measurements of fundamental mode Rayleigh wave phase velocities in the period range 25 ‐ 100 sec, for earthquakes located in central Asia and observed at teleseismic digital stations in Eurasia. Phase velocity maps are obtained using a tomographic method without a priori constraints on region boundaries. The lateral variations of S velocity reveal a symmetric pattern on each side of crustal low velocity zone centered on the Chang Tang region (north central Tibet). This zone is bounded by rather steep velocity gradients to the south and east, about 250 and 400 km north and west of the frontal thrusts of the Himalayas and Lungmen Shan, respectively. The northern boundary of the zone crosses the northern limit of the Tibet plateau (Altyn Tagh and Kunlun) extending into the southwest Tarim and Qaidam basins. The low crustal velocities are consistent with a thick, hot crust in the central part of Tibet, consistent with Quaternary volcanism in Chang Tang. The steep gradients towards higher velocities in the south and in the north are compatible with the documented continental subduction in the south, and suggest that a similar situation might exist in the north.