Global finite-frequency tomography of the 220-km discontinuity

Stacked images combining over 5 years of long‐period Global Digital Seismograph Network data reveal many phases associated with reflections and conversions from upper mantle discontinuities. These images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking. Results obtained for P, S, SS, and PP reference arrivals resolve numerous phases from discontinuities near 410 and 660 km, while some of the stacks also show evidence for a weaker discontinuity near 520 km. Phases of particular interest include P and SH multiples resulting from topside reflections, precursors to SS from underside reflections, and P‐to‐SV converted phases. These phases can be clearly seen both in the waveform stacks and in cross‐correlation analysis of individual seismograms. Travel times for these arrivals are converted to discontinuity depths relative to velocities in the Preliminary Reference Earth Model, and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle. Average apparent depths to the discontinuities for the different phases agree to within ±3 km for the 410‐km discontinuity, to within ±4 km for the 520‐km discontinuity, and to within ±8 km for the 660‐km discontinuity. The best global averages are obtained from the SS precursor data which indicate discontinuities at 415, 519, and 659 km. Discontinuity depths obtained from the P‐to‐SV converted phases at over 35 individual seismic stations exhibit variations of less than ±20 km. Apparent depths to the 660‐km discontinuity consistently show greater variability than depths to the 410‐km discontinuity, supporting recent laboratory results which indicate that the Clapeyron slope for the 660‐km discontinuity is significantly larger in magnitude than the slope for the 410‐km discontinuity. Precursors to SS (seen between 110° and 180°) are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point Apparent discontinuity depths computed from SS precursors for different tectonic regions agree to within about ±5 km. The SS precursors have especially good coverage near the subducting slabs in the northwest Pacific. Analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000‐ to 1500‐km‐wide region near the slab in which the 660‐km discontinuity is depressed by about 20 km. Measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking. Relative amplitude analysis suggests that the P and S wave impedance changes at 410 km are about 0.8–1.1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0.3 and 0.6 of the changes at 410 km.

The asthenosphere is a weak layer in the upper mantle where geotherm may exceed mantle solidus and partial melt occurs. Although it has been suggested that an increase in seismic wavespeed at about 220 km depth represents the base of the asthenosphere, seismic studies to‐date have not been able to provide evidence for the existence of such a global interface in the oceanic regions. In this study, we report observations of SS precursors reflected at this boundary throughout the global oceans. The average depth of the discontinuity is approximately 250 km, with a velocity jump of about 7% across the interface. Finite‐frequency tomography of SS precursor traveltimes reveals large depth variations of the discontinuity over short spatial distances, which explains the absence of this discontinuity in previous global stacks. The depth perturbations are characterized by alternating linear bands of shallow and deep anomalies that roughly follow seafloor age contours, indicating a fundamental connection between seafloor spreading and asthenosphere convection. The base of the asthenosphere is smoother under seafloors formed at slow‐spreading centers and becomes much rougher under seafloors formed at fast‐spreading centers with a spreading rate greater than mm/yr. This observation suggests that different geophysical processes at slow and fast spreading centers generate lithospheric plates with different chemical compositions and physical properties, which in turn influences the convection in the oceanic asthenosphere.

The lithosphere‐asthenosphere boundary (LAB) separating the rigid lid from the underlying weaker, convecting asthenosphere is a fundamental interface in mantle dynamics and plate tectonics. However, the exact depth and defining mechanism of the LAB interface remain poorly understood. The ocean plates are ideal for testing hypotheses regarding the nature of a plate since they make up 70% of Earth's surface area and have a relatively simple geological history. Seismically imaging the oceanic LAB at high resolution has proved challenging. Yet, several studies have recently increased resolution with provocative results. We summarize recent imaging of discontinuity structure beneath much of the Pacific using receiver functions from ocean floor borehole seismometers and land stations located at ocean‐continent margins, SS precursors, and waveform modeling of multiple phases including multiple bounce S waves, ScS reverberations, and surface waves. Overall, there is much agreement among these different approaches about the reported depth of a negative discontinuity that occurs near the expected depth of the LAB. Some of the apparent discrepancies in depth are explained by the variation in sensitivity of seismic waves that sample structure at different wavelengths. Yet, when the results are considered together, no single age‐depth relationship is illuminated. There are also puzzling discrepancies in where the discontinuity is detected, which again suggests greater complexity. Here we test the possibility that discrepant detection of a strong sharp discontinuity is caused by anisotropic structure. We stack SS waveforms with bounce points in the central Pacific into azimuthal bins. We use two methods, one that inverts for discontinuity structure based on subtle variations in the character of the SS waveform, and another that considers SS at higher frequency. We find azimuthal variation in the amplitude of the waveform, including a polarity reversal. We suggest that anisotropy is an important factor in imaging and constraining discontinuity structure of the oceanic plate, and must be carefully considered to constrain the age‐depth dependence and defining mechanism of the oceanic lithosphere.

Estimates of tau functions for a tectonically regionalized earth are obtained from over 1.25 million seismic ray paths of ISC Bulletin data to study the correlation of potential lateral variations in mantle P-wave velocities with recognized surface heterogeneity. The use of regionalized travel-time data relies on statistical regularity criteria to check the consistency of the global regionalization. Estimates of tau perturbations attributed to velocity structure at the seismic source and receiver regions are derived by a simple algebraic formulation. Contributions from near-surface heterogeneity are thereby removed and permit the assessment of lateral velocity variations at depth. Estimates of ‘single region’ equivalent tau functions are constructed and inverted to obtain velocity-depth functions and extremal bounds at the 99.9 per cent confidence level for seven different tectonic regions. Deviations from a regionally weighted reference mean velocity function which agrees well with PREM (Dziewonski & Anderson) indicate significant differences, particularly between oceanic and continental tectonic regions, extending to a depth of 700 km. P-wave velocity anomalies are typically less than ± 1 per cent of the reference velocity. The age-dependence of the shallow oceanic mantle is observed by increased velocities from young (> 25 Myr) to old (< 100 Myr) regions, and similarly from orogenic zones and magmatic belts to stable continental regions. Unlike for young and old oceanic regions, stable continental regions do not show a pronounced gradient in the mean velocity residuals above 250 km. Continental platforms and particularly shields, however, show a compensation in the sign of the mean velocity residual at depths between about 350 and 700 km. Evidence for a velocity anomaly between 700 and 950 km is indicated. Significant negative residuals are observed centred at a depth of about 780 km below oceanic ridges and about 880 km below active continental regions. This anomaly may be related to a boundary layer between the upper and lower mantle. The level of velocity variations decreases below 950 km. Lateral variations are also suggested within 250 km of the core boundary.

We image lithospheric interfaces using variations in the character of SS waveform stacks, a method we term SS Lithospheric Interface Profiling (SSLIP). The variations are caused by reflected phases, that is, underside reflections (SS precursors) and topside multiples (SS reverberations), created at velocity discontinuities near the midpoint of the SS ray path. Stacks from continental versus oceanic bounce point regions produce distinctly different SS waveforms, consistent with the large continent/ocean difference in crustal thickness. To investigate the potential of SS waveform stacks to constrain Moho depths under continents, we develop a method to fit continental bounce point stacks with a reference SS waveform convolved with a crustal operator. The SSLIP inferred Moho depths agree with the CRUST 2.0 model in Asia for those regions where the SS bounce point density is the highest. The SSLIP depths are correlated (correlation coefficient 0.82) with the CRUST 2.0 values averaged over sample bins of 10° radius. The SSLIP method has broad lateral resolution in comparison to most other methods for resolving crustal thickness, but has the potential to sample regions where station coverage may be sparse.

Global seismic data reveal little water in the mantle transition zone

SUMMARY Long-period underside SS wave reflections have been widely used to furnish global constraints on the presence and depth of mantle discontinuities and to document evidence for their origins, for example, mineral phase-transformations in the transition zone, compositional changes in the mid-mantle and dehydration-induced melting above and below the transition zone. For higher-resolution imaging, it is necessary to separate the signature of the source wavelet (SS arrival) from that of the distortion caused by the mantle reflectivity (SS precursors). Classical solutions to the general deconvolution problem include frequency-domain or time-domain deconvolution. However, these algorithms do not easily generalize when (1) the reflectivity series is of a much shorter period compared to the source wavelet, (2) the bounce point sampling is sparse or (3) the source wavelet is noisy or hard to estimate. To address these problems, we propose a new technique called SHARP-SS: Sparse High-Resolution Algorithm for Reflection Profiling with SS waves. SHARP-SS is a Bayesian deconvolution algorithm that makes minimal a-priori assumptions on the noise model, source signature and reflectivity structure. We test SHARP-SS using real data examples beneath the NoMelt Pacific Ocean region. We recover a low-velocity discontinuity at a depth of $\sim 69 \pm 4$ km which marks the base of the oceanic lithosphere, consistent with previous work derived from surface waves, body wave conversions, and ScS reverberations. We anticipate high-resolution fine mantle stratification imaging using SHARP-SS at locations where seismic stations are sparsely distributed.

We stack the teleseismic depth phases sS, sP, and pP produced by deep focus earthquakes to image precursory arrivals that result from near‐source, underside reflections off the 410‐km seismic velocity discontinuity (hereinafter referred to as the 410) and use differential time measurements between these phases and their precursors to compute discontinuity depths near seven subduction zones around the Pacific Ocean margin. We begin by selecting seismograms with high‐quality depth phase arrivals recorded by several long‐period networks between the years 1976 and 1996. Filtering the waveforms and stacking them along theoretical travel‐time curves reveals clear precursors which vary in shape and timing. We compute confidence levels to evaluate the reliability of the observed precursory features using a bootstrap method that randomly resamples the seismograms prior to stacking. We measure the differential travel time between the reference pulse and the precursor using a cross‐correlation technique and convert this time to an apparent discontinuity depth using the isotropic Preliminary Reference Earth Model (PREM) at 25‐s period, corrected to an oceanic crustal thickness. The lateral resolution of our long‐period stacks for 410 topography is limited compared to that sometimes achieved in short‐period analyses but is much higher than that obtained from global SS precursor studies. For most subduction zones the results indicate little change in the average depth to the 410‐km discontinuity in the local areas sampled by the precursor bounce points compared to broad regional depths inferred from SS precursor results. This implies that any large variations in depth to the 410‐km discontinuity near subduction zones are limited to a narrow zone within the slab itself where they may be difficult to resolve with long‐period data. Coverage for the Tonga and Peru‐Chile subduction zones is sufficiently dense that we can observe lateral variations in 410 depths. In Tonga the results suggest depth variations perpendicular to the slab of up to 33 km, after correcting for probable lateral heterogeneity in velocity above 400 km depth, and variations parallel to the slab orientation as large as 13 km. The cross‐slab variation is consistent with the elevation of olivine phase transformations in cold regions; the variation along strike suggests a more complex thermal heterogeneity that may be related to the subduction history of this region. We see evidence for additional reflectors above the 410 in some of the waveform stacks, but the inconsistency and weak amplitude of these features preclude definitive interpretations.

410km and 660km discontinuities are very clear and very easily identified discontinuities other than the Moho layer. This research utilizes SS precursor data, bouncepoints in the northern part of Sumatra. The data used is the depth of the epicenter &lt;70 km, earthquake magnitude 5.5 and the distance between the epicenter and earthquake recording station more than 100 0 . This study is a preliminary study to determine changes in the depth of discontinuity in the study area. The SS phase is very well observed in the transversal component seismogram which is the result of the rotation of two horizontal components NS and EW, to obtain a good seismogram a 0.03Hz low pass filter is performed. In this study used 38 data transversal component seismograms, from 76 horizontal component seismograms. The most important thing in this study is the determination of the SS phase used as a reference (point 0), the SS phase is determined using the AK135 table guide, then the SS precursors are determined which can be seen at 450 seconds, 300 seconds, 90 seconds and 50 seconds before the SS . SS prekursors that are very clear at 450 seconds, are strongly suspected as a 660 km discontinuity. SS prekursors can be seen clearly after the stacking process.

We evaluate the performance of Galileo broadcast NeQuick model by comparing it with GPS broadcast Klobuchar and the original NeQuick2 models. The broadcast coefficients of Galileo NeQuick model are computed from 23 globally distributed tracking stations of the International GNSS Service (IGS), by ingesting the Global Positioning System (GPS)-derived ionospheric total electron content (TEC) into the original NeQuick2 model. The accuracy of the three ionospheric models is evaluated over both the continental and oceanic regions for the year 2013. In continental regions, ionospheric TEC derived from 34 IGS stations is used as references for comparison. In oceanic regions, where the IGS stations are sparse, high-quality vertical TEC sources provided by JASON-1&2 altimeters are used as references. The evaluation results show that in continental regions, GPS broadcast Klobuchar and the original and broadcast NeQuick can mitigate the ionospheric delay by 56.8, 63.3 and 72.4 %, respectively. In oceanic regions, the three models can correct for 51.1, 61.2 and 68.6 % of the ionospheric delay. Galileo broadcast NeQuick model outperforms Klobuchar by 15.6 and 17.5 % over the continental and oceanic regions, respectively, for the test period. The broadcast NeQuick model can provide accurate ionospheric error corrections when Galileo begins full operational capability.

&amp;lt;p&amp;gt;Lateral variations in the depths of the transition-zone discontinuities are generally attributed to variations in temperature, causing local changes in the depth of the dominant phase transition. At moderate temperatures the dominant phase transitions are those of olivine, characterized by a positive Clapeyron slope (dP/dT) at 410 km depth and a negative Clapeyron slope at 660 km depth. An anticorrelation between topography on the 410 and 660-km discontinuities is therefore expected in the absence of variations in chemical composition, as an increase in temperature would lower the 410-km discontinuity and elevate the 660-km discontinuity. Simultaneously, this temperature increase would result in a decrease in seismic velocity and density of the mantle material. Comparing models of transition-zone topography, seismic velocity and density therefore gives valuable insight into the nature of transition-zone discontinuities. Existing global models of transition-zone topography have been created using SS and PP precursor measurements, which need to be corrected for mantle velocity structure using an independent velocity model before the discontinuity depths can be calculated. Here, we present new global models of transition-zone topography and whole-mantle S-wave velocity, P-wave velocity and density that have been simultaneously inferred from a different type of seismic data: Earth&amp;amp;#8217;s normal modes. Normal modes are whole-Earth oscillations induced by large earthquakes (M&amp;lt;sub&amp;gt;w&amp;lt;/sub&amp;gt;&amp;amp;#8805;7.5). We use our models, which can be readily compared to one another, to analyze the nature of the transition-zone discontinuities. We also discuss the trade-offs between the different model parameters and the model uncertainties, the latter of which is additional information provided by the Hamiltonian Monte Carlo method used for our inversion. Finally, we compare our models to transition-zone topography obtained from SS precursor data.&amp;lt;/p&amp;gt;

During the operation of the waterflooding technique, it is necessary to identify the waterflooding areas to enhance oil displacement efficiency. A casing-surface electrical resistivity tomography (CSERT) system using a well casing as a long electrode is able to detect a wide lateral scope, but its vertical resolution and ability to identify deep anomalies in the reservoir are limited, particularly for reservoirs with high-conductivity anomalies in the shallow subsurface, which disturb the response from the water floods at depth. In this study, we first simplified this kind of reservoir into a dual layered anomaly model. Then the log–inject–log method with time-lapse inversion was proposed and evaluated regarding its ability to improve the imaging of the deep waterflooding areas and the shallow anomaly. The results were compared with the commonly used static measurement with static inversion. In the static inversion results, the shallow anomaly was imaged well but the deep anomaly was unobservable. The results of the proposed log–inject–log method with time-lapse inversion showed that it is able to identify the shallow and deep anomalies better under various conditions, thus validating its ability to improve the vertical resolution of the CSERT system.

Tectonically regionalized tau estimates are used to obtain seismic P-wave travel-time corrections for lateral variations in the Earth's crust and shallow-mantle velocity structure. The corrections, in the form of estimates of tau perturbations, are functions of slowness and are assessed for both the source and receiver regions. The functional form is derived analytically and allows interpretations of causative velocity anomalies. Corrections for both ray path endpoints constrain effects typically assigned solely to the receiver, thereby helping assess systematic errors in hypocentral parameters. Over 1.25 million ISC Bulletin P-wave ray paths are used to estimate tau perturbation functions for seven types of tectonic regions. Particular attention is given to the problem of uniformly sampling all tectonic regions at all propagation depths in the mantle.Estimates of source and receiver tau perturbations are consistently less than 1 sec and show a definite and systematic difference between the travel-time correction functions for oceans and continents. Source tau perturbations are indicative of negative velocity anomalies in the shallow mantle beneath oceanic regions and positive anomalies beneath continental regions and oceanic trenches. Heterogeneity confined to the upper 250 km of the mantle suggests velocity variations within ±5 per cent of the lateral mean over a depth interval of approximately 100 km. Differences between the receiver and source perturbations for a common tectonic region are attributed to variations in the characteristic crustal structure of the region, constrained by the receiver perturbation, and errors in origin time and/or focal depth, determined from the source perturbation. The estimates of perturbation functions therefore suggest systematic regional biases in crustal structure and hypocentral parameters. A less than average crustal thickness in oceanic regions and greater thickness in stable continental regions is indicated. Furthermore, it appears that the origin times of sources in oceanic regions are systematically determined too late and/or focal depths located too shallow, whereas origin times of sources in continental regions are systematically determined too early and/or focal depths located too deep.

Thick, rigid continents move over the weaker underlying mantle, although geophysical and geochemical constraints on the exact thickness and defining mechanism of the continental plates are widely discrepant. Xenoliths suggest a chemical continental lithosphere ~175 kilometers thick, whereas seismic tomography supports a much thicker root (>250 kilometers) and a gradual lithosphere-asthenosphere transition, consistent with a thermal definition. We modeled SS precursor waveforms from continental interiors and found a 7 to 9% velocity drop at depths of 130 to 190 kilometers. The discontinuity depth is well correlated with the origin depths of diamond-bearing xenoliths and corresponds to the transition from coarse to deformed xenoliths. At this depth, the xenolith-derived geotherm also intersects the carbonate-silicate solidus, suggesting that partial melt defines the plate boundaries beneath the continental interior.

The association of lightning flashes with mean cloud ice size over continental and oceanic region in the tropical areas has been analyzed using the observations from various satellite platforms (MODIS, TRMM, and LIS) for the period 2000–2011. We found that frequency of lightning in general is higher over the continental region compared to oceanic region, whereas larger size of cloud ice is observed over the oceanic regions compared to the continental regions. Relationship between lighting and cloud ice size shows similar features over both continental and oceanic regions. For the first time, we show that total lighting increases with increase in the cloud ice size; attends maximum at certain cloud ice size and then decreases with increase in cloud ice size. Maximum lightning occurred for the mean cloud ice size of around 23–25 µm over the continental region and mean cloud ice size of around 24–28 µm over the oceanic region. Based on our observation we argue that the relation between lightning and mean cloud ice size follow the curve linear pattern, and not linear.