Strong Low Velocity Research Articles

The upper mantle beneath the South Atlantic Ocean, South America and Africa from waveform tomography with massive data sets

SUMMARYWe present a tomographic model of the crust, upper mantle and transition zone beneath the South Atlantic, South America and Africa. Taking advantage of the recent growth in broadband data sampling, we compute the model using waveform fits of over 1.2 million vertical-component seismograms, obtained with the automated multimode inversion of surface, S and multiple S waves. Each waveform provides a set of linear equations constraining perturbations with respect to a 3-D reference model within an approximate sensitivity volume. We then combine all equations into a large linear system and solve it for a 3-D model of S- and P-wave speeds and azimuthal anisotropy within the crust, upper mantle and uppermost lower mantle. In South America and Africa, our new model SA2019 reveals detailed structure of the lithosphere, with structure of the cratons within the continents much more complex than seen previously. In South America, lower seismic velocities underneath the transbrasilian lineament (TBL) separate the high-velocity anomalies beneath the Amazon Craton from those beneath the São Francisco and Paraná Cratons. We image the buried portions of the Amazon Craton, the thick cratonic lithosphere of the Paraná and Parnaíba Basins and an apparently cratonic block wedged between western Guyana and the slab to the west of it, unexposed at the surface. Thick cratonic lithosphere is absent under the Archean crust of the São Luis, Luis Álves and Rio de La Plata Cratons, next to the continental margin. The Guyana Highlands are underlain by low velocities, indicating hot asthenosphere. In the transition zone, we map the subduction of the Nazca Plate and the Chile Rise under Patagonia. Cratonic lithosphere beneath Africa is more fragmented than seen previously, with separate cratonic units observed within the West African and Congo Cratons, and with cratonic lithosphere absent beneath large portions of Archean crust. We image the lateral extent of the Niassa Craton, hypothesized previously and identify a new unit, the Cubango Craton, near the southeast boundary of the grater Congo Craton, with both of these smaller cratons unexposed at the surface. In the South Atlantic, the model reveals the patterns of interaction between the Mid-Atlantic Ridge (MAR) and the nearby hotspots. Low-velocity anomalies beneath major hotspots extend substantially deeper than those beneath the MAR. The Vema Hotspot, in particular, displays a pronounced low-velocity anomaly under the thick, high-velocity lithosphere of the Cape Basin. A strong low velocity anomaly also underlies the Cameroon Volcanic Line and its offshore extension, between Africa and the MAR. Subtracting the global, age-dependent VS averages from those in the South Atlantic Basins, we observe areas where the cooling lithosphere is locally hotter than average, corresponding to the location of the Tristan da Cunha, Vema and Trindade hotspots. Beneath the anomalously deep Argentine Basin, we image unusually thick, high-velocity lithosphere, which suggests that its anomalously great depth can be explained, at least to a large extent, by isostatic, negative lithospheric buoyancy.

Global Mid-Ocean Ridges Mantle Tomography Profiles

We have studied mantle tomography profiles of global mid-ocean ridges to investigate their depth of origin and other characteristics. The Mid-Atlantic and the South West Indian Ridges are deep rooted ridges that extend as far down in the mantle to 250-300 km. The Central Indian Ridge, South East Indian Ridge, Antarctica Nazca, Pacific Antarctica, Pacific Nazca and Juan de Fuca are shallow rooted ridges that extend down in the mantle only to 100 km. The deep rooted ridges appear to be characterized by a weak low velocity ridge anomaly while the shallow rooted ridges are characterized by strong low velocity ridge anomaly. This may be due to the variations of the geochemistry of the ridges due to the fractionation process during magma ascent. Furthermore, despite the three ridges that make the Indian Rodriguez Triple Junction having different characteristics, all the three sections of the ridges that make the Triple Junction are characterized by strong low velocity ridge anomaly and extended down to only 100 km. This is particularly typical for the length of nine to ten degrees from the Triple Junction which makes it appear as an independent system.

3D upper mantle structure beneath Japan and its surrounding region from inter-station dispersion measurements of surface waves

Three-dimensional shear wave speed structure of the upper mantle beneath the Japanese Islands and the surrounding regions is constructed from the measurements of inter-station phase speeds of surface waves. We employ the dense broad-band seismic network (F-net), deployed throughout Japan, as well as permanent stations of global seismic network in East Asia. A temporary broad-band seismic network in Far-East Russia, which has been constructed as a part of the Stagnant Slab Project since 2004, is also used in combination with the permanent stations. Using seismic events from 2005 to 2007 with moment magnitude greater than 6.0 and depth shallower than 100km, we measure the phase speeds of the fundamental-mode Rayleigh and Love waves, working with the conventional two-station technique. Phase speed maps of the Japanese Islands and the Sea of Japan are obtained from the inversions of inter-station phase speeds that are measured for a large number of station-pairs. We have collected over 5300 paths for Rayleigh waves and 3800 paths for Love waves. The phase speed maps of Rayleigh waves, in the period range from 25 to 140s, are inverted to obtain an isotropic 3D shear wave speed model in the depth range from 40 to 200km. The isotropic S wave model represents prominent fast wave speed anomalies in northeastern Japan, associated with the subducting Pacific Plate. Fast anomalies of the lithosphere beneath the Sea of Japan are also imaged, implying that the average thickness of lithosphere beneath the Yamato Rise and Japan Basin is about 60±10km. Immediately below this oceanic lithosphere, slow anomalies are found in the asthenosphere beneath the entire region of the Sea of Japan in the depth down to about 200km. In southwestern Japan, the low-angle subduction of the Philippine Sea Plate is mapped clearly, indicating that the northern end of the plate extends beyond the northern coast of the Chugoku region, reaching beneath the Oki Islands in the Sea of Japan. A strong low velocity anomaly off the west coast of Kyushu is found in the depth down to about 130km, which supports the existence of an upwelling flow beneath this area. We also invert the phase speed maps of Rayleigh and Love waves in the period range from 30 to 80s simultaneously, to construct the radially anisotropic shear wave speed model. A remarkable anisotropy with SH>SV is found in the mantle wedge beneath northeastern Japan, while only weak anisotropy is seen beneath the Sea of Japan. Such anisotropic properties, which cannot readily be obtained from body wave studies only, will be an important key to understanding the dynamic processes in this tectonically actively region.

S wave tomography of the crust and uppermost mantle in China

We use 280,000 local and regional arrival times from 13,105 local earthquakes recorded by 220 seismic stations to determine a three‐dimensional S wave velocity structure of the crust and uppermost mantle under China and surrounding regions. We use a traveltime tomography code that accommodates a heterogeneous crust and varying Moho depth. The grid for tomographic inversion is 1° × 1° in the horizontal direction and 10 km in depth. Our results show that large velocity variations of more than 6% exist in the crust and upper mantle in the China region. The velocity image of the upper crust correlates with surface geological features. Crustal heterogeneity is clearly observed, and velocity changes are visible across some of the large fault zones. In Tibet, low velocities exist in the thick crust, and high Sn velocities exist in the southern and eastern parts. Low‐velocity zones beneath volcanic sites and the rifts are clearly observed in our tomographic results. Under the Tengchong, Hainan, and Changbai volcanic areas, strong low velocity zones are visible down to the upper mantle, indicating the existence of magma chambers beneath the volcanoes.

Whole mantle SH velocity model constrained by waveform inversion based on three-dimensional Born kernels

SUMMARY A whole mantle SH velocity model is obtained by using a unique data set and techniques. Body and surface waveforms including major and multi-orbit phases are used as a data set and are inverted by using 3-D Born kernels. The resultant model, SH18CE, reveals the different natures of the two major upwelling systems: the strong low velocity anomalies beneath Africa extend for more than 1000 km from the core–mantle boundary (CMB), whereas those beneath the Pacific are restricted to 300–400 km from the CMB. The results also show the variable natures of stagnant slabs on the 670 discontinuity around Japan: the depths of the strongest high velocity anomalies within the stagnant slabs are different region by region, which is consistent with the detailed delay time tomography model in this area.

Tomographic low‐velocity anomalies in the uppermost mantle around the northeastern edge of Okinawa Trough, the backarc of Kyushu

A simultaneous inversion for three‐dimensional velocity structure and hypocentral parameters is used to determine the tomographic images of the backarc of Kyushu, southwestern Japan. The data used in the imaging are the first arrival times of 2736 local earthquakes recorded at 305 stations in a wide region from western Japan to eastern China. Additional data are provided by including arrival times of 357 teleseismic earthquakes. We got the high resolution of the uppermost mantle images for P waves at least in the depth range to 150 km, the latitude range of 30°–34°N and the longitude range of 128°–132°E. Results of inversion reveal a major zone of strong low velocity under west off Kyushu Island in the depth from 40 km to 100 km. This feature may result from partially melting in relation to the upwelling of hot mantle around the northeastern edge of the extending Okinawa trough.

The Diffuse Interstellar Clouds toward 23 Orionis

Spectra obtained with the Hubble Space Telescope Goddard High Resolution Spectrograph are combined with high-resolution optical spectra and UV spectra from Copernicus to study the abundances and physical conditions in the diffuse interstellar clouds seen along the line of sight to the star 23 Ori. Multiple absorption components are present for each of several distinct types of gas, which are characterized by different relative abundance and depletion patterns and physical conditions.?????Strong low-velocity (SLV) absorption, due to cool, moderately dense neutral gas and representing about 92% of the total N(H I), is seen for various neutral and singly ionized species at +20 km s-1 v? +26 km s-1. Most typically severely depleted species are less depleted by factors of 2-4, compared to the cold, dense cloud pattern found, for example, in the main components toward ? Oph.For the two strongest SLV components, T ~ 100 K and the thermal pressure log (nHT) ~ 3.1 cm-3 K; we thus have nH ~ 10-15 cm-3 and a total thickness of 12-16 pc. The adopted average SLV electron density, ne = 0.15 ? 0.05 cm-3, implies a relatively large ne/nH ~ 0.01 and thus some ionization of hydrogen in these predominantly neutral components.?????Weaker low-velocity (WLV) absorption, probably largely due to warmer neutral gas, is seen primarily for various singly ionized species at 0 km s-1 v? +30 km s-1. The depletions in the WLV gas are typically less severe by a factor of 2-3 than in the SLV gas and are somewhat similar to the warm cloud pattern seen in lines of sight with low reddening, low mean density, and/or low molecular fraction. If T ~ 3000 K for the WLV components, then we have log(nHT) ~ 4.7-4.8 cm-3 K, nH ~ 15-20 cm-3, ne ~ 0.2 cm-3, ne/nH ~ 0.01, and a total thickness of 0.7-0.9 pc.?????Absorption from a number of singly and doubly ionized species, perhaps due to a radiative shock, is seen at -108 km s-1 v? -83 km s-1. While the depletions in these ionized components are uncertain owing to unobserved ionization stages, aluminum (typically severely depleted) is probably depleted there by only a factor ~3, even at cloud velocities in excess of 100 km s-1. The individual high-velocity components typically have T ~ 8000 ? 2000 K, ne = nH ~ 0.4-0.5 cm-3, thermal pressure log(2neT) ~ 3.7-4.0 cm-3 K, and thicknesses of order 0.1 pc.?????Weak absorption components from ionized (H II) gas are seen in C II, Mg II, and Si III at intermediate velocities (-43 km s-1 v? -4 km s-1). Broad, weak absorption from the higher ions S III, C IV, Si IV, and N V is centered at -5 km s-1 v? +6 km s-1. No obvious absorption is discerned from a circumstellar H II region around 23 Ori itself.?????The large range in ne (from 0.04 to 0.95 cm-3) derived independently from nine pairs of neutral and singly ionized species in the SLV gas suggests that additional processes besides simple photoionization and radiative recombination affect the ionization balance. Charge exchange with protons may reduce the abundances of S I, Mn I, and Fe I; dissociative recombination of CH+ may help to enhance C I. The large apparent fractional ionization in the SLV and WLV gas may be due to an enhanced flux of X-rays in the Orion region, to mixing of neutral and ionized gas at the boundary of the Orion-Eridanus bubble, or perhaps (in part) to charge exchange between singly ionized atomic species and large molecules (in which case the true ne would be somewhat smaller). Comparisons of the SLV depletions and nH with those found for the strong component B (v? ~ -14 km s-1) blend toward ? Oph hint at a possible relationship between depletion and local density for relatively cold interstellar clouds. Calcium appears to be more severely depleted in warm, low density gas than has generally been assumed.?????An appendix summarizes the most reliable oscillator strengths currently available for a number of the interstellar absorption lines analyzed in this work.

Upper mantle velocity structure beneath Italy from direct and secondary P-wave teleseismic tomography

High-quality teleseismic data digitally recorded by the National Seismic Network during 1988-1995 have been analysed to tomographically reconstruct the aspherical velocity structure of the upper mantle beneath the Italian region. To improve the quality and the reliability of the tomographic images, both direct (P, PKPdf) and secondary (pP,sP,PcP,PP,PKPbc,PKPab) travel-time data were used in the inversion. Over 7000 relative residuals were computed with respect to the IASP91 Earth velocity model and inverted using a modified version of the ACH technique. Incorporation of data of secondary phases resulted in a significant improvement of the sampling of the target volume and of the spatial resolution of the heterogeneous zones. The tomographic images show that most of the lateral variations in the velocity field are confined in the first ~250 km of depth. Strong low velocity anomalies are found beneath the Po plain, Tuscany and Eastern Sicily in the depth range between 35 and 85 km. High velocity anomalies dominate the upper mantle beneath the Central-Western Alps, Northern-Central Apennines and Southern Tyrrhenian sea at lithospheric depths between 85 and 150 km. At greater depth, positive anomalies are still observed below the northernmost part of the Apenninic chain and Southern Tyrrhenian sea. Deeper anomalies present in the 3D velocity model computed by inverting only the first arrivals dataset, generally appear less pronounced in the new tomographic reconstructions. We interpret this as the result of the ray sampling improvement on the reduction of the vertical smearing effects.

Formal inversion of ISC arrival times for mantle P-velocity structure

SUMMARY Seismic traveltime data are used to determine and assess a weighted least-squares model of the 3-D P-velocity structure of the Earth's mantle. A total of 682090 summary rays constructed from over three million ISC traveltime observations are used to constrain velocity perturbations in 124966 by 6 cells. An iterative block LU decomposition procedure, applied on the massively parallel Connection Machine 2 (CM-2), is used to compute a formal weighted least-squares model of P-velocity variations as well as the full resolution and covariance matrices. It is found that the shallow mantle beneath the continents of the northern hemisphere is well resolved. However, the majority of mid-oceanic features, such as spreading ridges and hot spots, are not well constrained. As a function of depth, the resolution is highest in the upper to mid-mantle, due to variations in ray coverage. Primarily because of ray geometry, single blocks are not resolved below 1870km. The estimates of spatial resolution, constructed from the complete resolution matrix, are useful in judging whether subducting lithosphere extends into the lower mantle. The results indicate that possible extensions of subduction zones in the northern hemisphere are imaged reliably down to at least 1470 km. However, areas beneath most subduction zones in the southern hemisphere are averaged over scale lengths of 800km or more below a depth of 1070km. Velocity estimates for the mid to 'lower mantle beneath Hawaii are the result of averaging over 1900 km or more. Generally, vertical and lateral averaging of 600 to lo00 km is occurring in the depth range 2270 to 2670km. P-wave velocity values in the region just above the Core-Mantle Boundary (CMB) are large-scale spatial averages (1500 km or more) and individual cells are poorly resolved. The model parameter standard error remains moderate throughout the lower mantle due to smaller traveltime errors associated with rays that bottom in this region. These standard errors reach no more than 0.3 per cent of the Jeffreys-Bullen average velocity. The greatest standard errors, 0.9 per cent of the average velocity, are found in the upper mantle underlying the Pacific basin. These large parameter errors are due primarily to the poor ray coverage in the Pacific coupled with the large arrival-time uncertainties for P recordings in the epicentral distance range 0 to 20. The upper 200 km of the weighted least-squares velocity model correlates strongly with known tectonic features. A circum-Pacific ring of low velocity, with a maximum of 2 per cent, dominates the surface anomalies. The most likely origin of this feature are the high temperatures associated with back-arc magmatism. The most conspicuous mid-ocean velocity anomaly is Iceland, which appears as a strong low velocity region extending over many cells. Continental cratons appear as positive velocity (1-2 per cent) anomalies extending to depths of 200 to 400km. Low velocities underly the continents of Eurasia and North America at a depth range of 400 to 670 km. In the mid-mantle, 670 to 1270 km deep, a 1-2 per cent fast velocity anomaly encircles the region below the Pacific basin. A fast anomaly also underlies

The Effect of Anelasticity on Periods of the Earth's Free Oscillations (Toroidal Modes)

It is known that the anelastic properties of the Earth characterized by a ‘Q’ structure will affect the periods of free oscillation. It is generally considered that the effect is negligible compared to the other perturbing effects due to rotation, ellipticity, and lateral inhomogeneities. Nevertheless, it is of some interest to investigate the precise magnitude of this effect for the observed free oscillation modes since it could provide us with another constraint in the determination of the Q structure of the Earth. An application of perturbation theory provides us with a good estimate of the magnitude of the changes in the periods of an elastic model due to inclusion of anelastic effects. Calculations based on currently accepted mean elastic and anelastic models for the Earth show that the shift in period due to anelasticity is at most 0·023 per cent for the toroidal modes from _0T_2 to _0T_(99), the maximum occurring near _0T_(60). For more extreme Q models, which may be locally applicable, period shifts of the order 0·1 per cent occur, with the maximum again near _0T_(60), corresponding to a period of approximately 150 s. Observational accuracy for the toroidal oscillations is around 0·1 per cent so that anelastic shifts in toroidal oscillation periods are at the present limit of observational accuracy. Viewed in terms of propagating surface waves, the dispersion due to anelasticity results in at most 0·005-0·01 km s^(−1) variations in the phase and group velocities. Such shifts are within the observational resolution of surface dispersion measurements using narrow band filtering techniques. Compared to other perturbing effects, anelasticity is significant for the toroidal oscillation only in the 50- to 300-s period range. In this range, lateral variations in structure generally cause larger perturbations. However, when viewed in terms of propagating surface waves in selected homogeneous regions, anelasticity becomes the dominating effect. Further, the frequency shift due to anelasticity is scaled by (1/Q)^2, so that the anelastic effect can be well within observational accuracy and comparable to any perturbing effect for more extreme, yet acceptable, Q models. In particular, when applied to surface waves propagating across a tectonic region with a strong low velocity zone in the upper mantle, the anelasticity induced dispersion on frequency shift can be significant and measurable. In such cases a joint inversion of elastic and anelastic properties is appropriate.

Some Practical Aspects of Interference Attenuation in Routine Seismic Exploration

Abstract An economic and a rather simplified scientific approach to the solution ofmore proper spacing of pattern holes and geophone arrays in the field isdiscussed. A method of recording and calculating the surface or apparent wavelength of interference waves is outlined. Several examples of actual fieldseismograms are included to illustrate the degree of interference attenuationthat can be obtained with a minimum amount of recording experimentation. This paper is designed primarily to point out an economic and rathersimplified scientific approach to the solution of more proper spacing ofpattern holes and multiple geophone arrays in the field. The proceduresoutlined are not proposed as a "cure-all," but merely are intended to depict alogical sequence of steps which, in many instances, will yield at least apartial solution to the enhancement of the quality of seismic reflectionsignals. Initially a few of the fundamental concepts of wave propagation should bereviewed. The word interference is defined as any unwanted signal generated atthe shot hole whereas noise is defined as any unwanted signal generated by someextraneous source such as wind, power lines, well pump, etc. There are numerous types of interference such as Rayleigh waves, Love waves, ground roll, reflected refractions. Both the primary and the harmonic form ofthese various waves are frequently detected on or are suspected to be on thefield seismogram. This paper is primarily concerned with the attenuation of thetype of interference wave that travels from the shot through the weathered andsub-weathered near surface materials at velocities of 1000 to 8000/second andpossesses frequencies from 10 cps to 40 cps. The primary or fundamental lowfrequency wave can usually be cancelled by electrical filtering or, as in thecase of a strong low velocity surface wave, the employment of an in-line offsetspread removed sufficiently in distance to allow all reflection data to berecorded before the interference wave is recorded. However, in areas where thenear surface material possesses exceptional acoustic transmission properties(i.e. the ability to readily transmit most types of sound waves), the first, and even second harmonics of the low frequency interference waves aretransmitted through the near surface and are sufficiently strong in amplitudeto mask, or at least in part distort, the desired reflection signals.