ScS Waves Research Articles

ScS shear-wave splitting in the lowermost mantle: Practical challenges and new global measurements

Many regions of the Earth's mantle are seismically anisotropic, including portions of the lowermost mantle, which may indicate deformation due to convective flow. The splitting of ScS phases, which reflect once off the core-mantle boundary (CMB), is commonly measured to identify lowermost mantle anisotropy, although some challenges exist. Here, we use global wavefield simulations to evaluate commonly used approaches to inferring a lowermost mantle contribution to ScS splitting. We show that due to effects of the CMB reflection, only the epicentral distance range between 60° and 70° is appropriate for ScS splitting measurements. For this distance range, splitting is diagnostic of deep mantle anisotropy if no upper mantle anisotropy is present; however, if ScS is also split due to upper mantle anisotropy, the reliable diagnosis of deep mantle anisotropy is challenging. Moreover, even in the case of a homogeneously anisotropic deep mantle region sampled from a single azimuth by multiple ScS waves with different source polarizations (in absence of upper mantle anisotropy), different apparent fast directions are produced. We suggest that ScS splitting should only be measured at "null" stations and conduct such an analysis worldwide. Our results indicate that seismic anisotropy is globally widespread in the deep mantle.

A high-resolution map of Hawaiian ULVZ morphology from ScS phases

We use core reflected ScS waves sensitive to a broad region of the core-mantle boundary beneath Hawaii to create the first high-resolution map of the Hawaiian ultralow-velocity zone (ULVZ). Positive ScS-S differential times are used to identify regions of strong slow velocity anomalies in the lowermost mantle, and the presence of pre/post-cursors around the main ScS phase confirms the sharp top of a basal ULVZ layer. Pre/post-cursor arrivals are mapped into a volume across their region of sensitivity to produce a detailed image of ULVZ morphology. The variability observed across the ULVZ can be interpreted in terms of varying height or velocity reduction, but the large range of velocity variations required to explain observations suggests that most variability reflects varying layer thickness.The Hawaiian ULVZ is observed to be a large-scale regional feature of varying height (∼5-25 km) extending across a wide area along the edge of the Pacific large low-velocity province (LLVP). Variability in previous models of the Hawaiian ULVZ can be explained by studies imaging different parts of this strongly variable, large-scale feature. Maximum ULVZ thicknesses (no taller than 30 km) are found in a flat-topped, steep-sided region on the order of 1000 km in diameter located west of Hawaii. This feature is coincident with the previously identified Hawaiian mega-ULVZ, and is interpreted to represent the root of the Hawaiian plume, which is offset from the volcanic surface expression. A tall, asymmetric ridge of ULVZ material is observed to the east of Hawaii. Both regions of maximum ULVZ heights are bounded by the Pacific LLVP to the south, and fast seismic velocity anomalies interpreted as slab remnants to the north. This points to a potential geodynamical scenario where dense ULVZ material is pushed by subducted slab remnants into thicker piles against LLVP margins.

STRUKTUR KECEPATAN GELOMBANG S DI DAERAH DEPAN BIDANG SUBDUKSI ALASKA DENGAN MENGANALISA SEISMOGRAM GEMPA C022801L DI STASIUN WHY

In this research the measured seismogram of C022801L, Washington U.S.A. is compared to the synthetic seismogram, which is recorded at WHY observation station in three dimensions, where the wave path resides in front area of Alaska subduction zone. The synthetic seismogram is calculated with the GEMINI Program, whose input is in the form of an earth model, which is radial symmetry and transversal isotropic, and the CMT solution of the quake.Simulation and seismogram comparison can only be conducted till a frequency to 15 mHz, because big discrepancies are found at surface wave and the depth wave phase ScS. The surface wave propagates along earth surface till a depth which is equivalent to depth of upper mantle; so that the fitting can be obtained by altering speed structure till base of upper mantle, where the corrections are conducted at bh speed gradient. Correction at upper mantle structure does not bring repair at wave phase ScS. Corrective is further executed at speed structure of shear wave S till CMB, until the good fitting at ScS wave is obtained. S velocity structure in front area of Alaska subduction zone has in reality a strong positive anomaly, and to get fitting at ScS wave, the positive anomaly continues at earth layers below upper mantle till CMB

Mantle Seismic Anisotropy beneath the Amur Plate According to the Data of ScS Waves from Deep-Focus Earthquakes

Mantle seismic anisotropy beneath the Amur Plate is studied using the data of ScS waves reflected from the outer core of the Earth from local deep earthquakes in the area of five stations located in Primorye and Priamurye regions. The results of measuring the parameters of the split ScS waves near the stations show the dominance of the polarization azimuths of the fast wave along the eastern directions and are consistent with azimuthal anisotropy and the direction of motion of the Pacific Plate (300°) and Amur Plate (~120°) depending on the epicenter–station direction. In Primorye, in the vicinity of the TEY station, the polarization azimuths of the fast ScS wave dominate in the interval of NE–E directions orthogonally to the lines of the mantle flow along the complex 3D surface of the subsiding Pacific Plate. It is revealed that the delay time of ScS waves increases to 2 s and 3.4 s in the upper mantle and in the mantle transition zone, respectively, as the depth of events increases. The highest degree of anisotropy manifests itself in the upper mantle. In the case of the vertical propagation of waves under conditions of horizontal mantle flow, the difference in the arrival times of the ScS waves is the lowest. The anisotropy in the upper zone of the transition mantle can be related to the wadsleyite texture with the polarization of the fast ScS wave parallel or orthogonally to the motion of the stagnant plate. The anisotropy in the upper part of the lower mantle is associated with the texture of perovskite and periclase with the orientation of the symmetry axis and the polarization of the ScS waves in parallel to the plate subsidence and the subduction direction.

The limits of ray theory when measuring shear wave splitting in the lowermost mantle withScSwaves

Observations of shear wave splitting provide unambiguous evidence of the presence of anisotropy in the Earth’s lowermost mantle, a region known as D′ ′. Much recent work has attempted to use these observations to place constraints on strain above the core–mantle boundary (CMB), as this may help map flow throughout the mantle. Previously, this interpretation has relied on the assumption that waves can be modelled as infinite-frequency rays, or that the Earth is radially symmetric. Due to computational constraints it has not been possible to test these approximations until now. We use fully three-dimensional, generally-anisotropic simulations of ScS waves at the frequencies of the observations to show that ray methods are sometimes inadequate to interpret the signals seen. We test simple, uniform models, and for a D′′ layer as thin as 50 km, significant splitting may be produced, and we find that recovered fast orientations usually reflect the imposed fast orientation above the CMB. Ray theory in these cases provides useful results, though there are occasional, notable differences between forward methods. Isotropic models do not generate apparent splitting. We also test more complex models, including ones based on our current understanding of mineral plasticity and elasticity in D′′. The results show that variations of anisotropy over even several hundred kilometres cause the ray-theoretical and finite-frequency calculations to differ greatly. Importantly, models with extreme mineral alignment in D′′ yield splitting times not dissimilar to observations (δt ≤ 3 s), suggesting that anisotropy in the lowermost mantle is probably much stronger than previously thought—potentially ∼10 % shear wave anisotropy or more. We show that if the base of the mantle is as complicated as we believe, future studies of lowermost mantle anisotropy will have to incorporate finite-frequency effects to fully interpret observations of shear wave splitting.

Seismic anisotropy of the mantle beneath Eastern Asia based on ScS and S waves from deep-focus earthquakes

The seismic anisotropy of the mantle is studied based on the data of S and ScS waves from earthquakes occurred in the mantle transition zone over the period of 2007–2013 and recorded by seismic stations in the continental margin of Asia, on Sakhalin Island, and in the southern part of the Kamchatka Peninsula. The measurements of the azimuths of polarization of the fast S and ScS waves in the continental margin of Asia show that they are predominantly oriented in the E–SE directions. Based on the distribution of the shear wave splitting parameters, the symmetry of the medium can be described in terms of a transversely isotropic model with a horizontal symmetry axis and may correspond to horizontal flow in the upper mantle beneath the Amur Plate. The fast azimuths of polarization of ScS wave, which were determined to be of N–NE directions in the northern area of Sakhalin Island and in the continental part of Asia, may correspond to an inclined flow under the conditions of oblique subduction and complex geometry of the downgoing Pacific Plate. In the south of the Kamchatka Peninsula, the S- and ScS-wave azimuths of polarization from the M 8.4 Sea of Okhotsk earthquake are determined to be oriented along the direction of the Pacific Plate motion. The fast-S-wave azimuths of polarization from the aftershocks of the Sea of Okhotsk earthquake and from other large events of 2008–2009 are determined to be nearly parallel to the motion trend of the Pacific Plate, but orthogonal to it for the events of 2008–2009. On the basis of the distribution of azimuths of polarization of the fast S waves, the symmetry of the medium can be described in terms of a transversely isotropic model with the symmetry axis inclined orthogonally to the plane of downgoing plate and oriented westward orthogonally to the trench strike.

Seismic shear waves as Foucault pendulum

AbstractEarth's rotation causes splitting of normal modes. Wave fronts and rays are, however, not affected by Earth's rotation, as we show theoretically and with observations made with USArray. We derive that the Coriolis force causes a small transverse component for P waves and a small longitudinal component for S waves. More importantly, Earth's rotation leads to a slow rotation of the transverse polarization of S waves; during the propagation of S waves the particle motion behaves just like a Foucault pendulum. The polarization plane of shear waves counteracts Earth's rotation and rotates clockwise in the Northern Hemisphere. The rotation rate is independent of the wave frequency and is purely geometric, like the Berry phase. Using the polarization of ScS and ScS2 waves, we show that the Foucault‐like rotation of the S wave polarization can be observed. This can affect the determination of source mechanisms and the interpretation of observed SKS splitting.

Splitting of ScS and S waves from the M w 8.4 Okhotsk deep-focus earthquake (May 24, 2013) and its strong aftershocks

The parameters of split shear ScS and S waves from the strongest (Mw = 8.4) Okhotsk earthquake and its strong aftershocks that occurred in the mantle’s transitional zone in the backarc part of the Kamchatka subduction zone are measured. The measurement results show domination of the fast ScS wave’s east azimuths (110°–149°), which are orthogonal to the trench orientation, and the time difference between the arrivals of the ScS waves (δt) in the range of 0.9–1.6 s. The fast S wave from the Okhotsk earthquake is east-directed (89°), and the time difference between its arrivals is up to 2.5 s. As for aftershocks, the azimuths of the fast S waves are oriented in parallel to the trench, and δt are estimated at 1.2–1.3 s. The orientations of the azimuths for the ScS and S waves are consistent with the model of the transverse-isotropic symmetry of the medium with the symmetry axis tilted along the plate dip and/or along the trench strike.

Evaluating post-perovskite as a cause of D′′ anisotropy in regions of palaeosubduction

Seismic anisotropy in the Earth's lowermost mantle (D′′) is often attributed to the alignment of MgSiO3 post-perovskite (ppv) by the movement of dislocations in response to mantle flow. However, ppv's plastic yield surface is not known; nor do we know if this is the main deformation mechanism. We make use of a heterogeneous, generally anisotropic model of elasticity in D′′ derived from a 3-D model of mantle flow, which is obtained by inversion of geophysical observables. Unlike previous approaches, completely general, 3-D flow and full anisotropy are permitted, yielding more information to compare with observations than has been possible before. We model observations of anisotropy in D′′ by calculating the shear wave splitting predicted in ScS waves for a series of models of ppv plasticity. We find that observations in regions of the lowermost mantle beneath subduction zones are best fit by a model which accommodates slip on (010). Our results show that, within one standard deviation, slip on (010)—or a mechanism giving the same style of anisotropy—explains D′′ anisotropy beneath these regions.

Triggering of tremors and slow slip event in Guerrero, Mexico, by the 2010 Mw 8.8 Maule, Chile, earthquake

We investigate the triggering of seismic tremor and slow slip event in Guerrero (Mexico) by the February 27, 2010 Maule earthquake (Mw 8.8). Triggered tremors start with the arrival of S wave generated by the Maule earthquake, and keep occurring during the passing of ScS, SS, Love and Rayleigh waves. The Rayleigh wave dispersion curve footprints the high frequency energy envelope of the triggered tremor, indicating a strong modulation of the source of tremors by the passing surface wave. This correlation and modulation by the passing waves is progressively lost with time over a few hours. The tremor activity continues during the weeks/months after the earthquake. GPS time series suggest that the second sub‐event of the 2009–2010 SSE in Guerrero is actually triggered by the Maule earthquake. The southward displacement of the GPS stations starts coincidently with the earthquake and tremors. The long duration of tremors indicate a continuing deformation process at depth, which we propose to be the second sub‐event of the 2009–2010 SSE. We show a quasi‐systematic correlation between surface displacement rate measured by GPS and tremor activity, suggesting that the NVT are controlled by the variations in the slip history of the SSE. This study shows that two types of tremors emerge: (1) Those directly triggered by the passing waves and (2) those triggered by the stress variations associated with slow slip. This indicates the prominent role of aseismic creep in the Mexican subduction zone response to a large teleseismic earthquake, possibly leading to large‐scale stress redistribution.

An iterative algorithm for separation ofSandScSwaves of great earthquakes

Teleseismic SH waves are essential for imaging the rupture processes of large earthquakes. However, for great earthquakes (M8+) such as the 2004 Sumatra earthquake, the 2008 Wenchuan earthquake and the recent Tohoku-Oki earthquake, the source duration is very long (>100 s). Thus the direct SH waves are overlapped with ScS waves for epicentral distances larger than 60°, leaving contaminated S waves for source processes modelling. Therefore artefacts in finite fault models of large earthquake could be produced with such contaminated body waves. We propose an iterative algorithm based on the slowness information of S and ScS waves and stacking technique, to separate S and ScS waves with records from a regional seismic network. Tests on various synthetic data sets show that the algorithm is effective in retrieving teleseismic SH waveforms from complicated wave trains containing both S and ScS. Separation of waveforms for the 2008 Wenchuan earthquake with our algorithm clearly demonstrates the influence of ScS energy, suggesting necessity of recovering S waves.

Seismic attenuation in the eastern Australian and Antarctic plates, from multiple ScS waves

The attenuation of seismic shear waves in the mantle beneath the eastern Australian and Antarctic plates is analyzed using a large data set of multiple ScSn waves, reflected n times at the core-mantle boundary and (n-1) times at the surface. The data are the transverse components of deep earthquakes from the subduction zones north and east of Australia, recorded at stations in Antarctica, Australia, Indonesia, New Caledonia and New Zealand. The data are filtered with narrow band-pass filters at 5 frequencies in the range 0.013-0.040 Hz. The ScSn+1/ScSn amplitude ratios of successive ScS phases are compared to the ratios computed for synthetic seismograms for the same paths and same focal mechanisms, in order to eliminate the effects of source radiation and geometric attenuation. The synthetic seismograms are computed from a summation of toroidal modes for the 1-D reference model PREM (Dziewonski & Anderson 1981). The observed to computed spectral ratios appear consistent for similar paths. They reveal that the attenuation is not frequency dependent, that the contribution of scattering to attenuation is low, and that the PREM model is a valuable reference model for the study region at the considered frequencies. An inversion of the data at 0.026 Hz is performed to retrieve the quality factor Q in the upper mantle, in regions defined using a priori constraints inferred from seismic shear velocities. Q-values close to those of PREM are found beneath the Australian and Antarctic cratons, lower values beneath the Eastern Australian Phanerozoic margin, and very low values beneath the oceanic region between Australia and Antarctica, where ridges and a triple junction are present. The Australian-Antarctic Discordance along the South-Indian ridge appears as an exception with a Q-value close to those of stable continents. The highest Q-values are found beneath the subduction zones, a feature which is not apparent in global attenuation models possibly because of its narrow lateral extension, and because it extends at depths larger than those sampled by surface waves. Despite limitations due to the uneven distribution of the ScSn bounce points at the surface and to the difficulty of collecting a large number of high quality data, our approach appears very promising. It is complementary to the more widely used determination of seismic attenuation using surface waves because it provides increased depth coverage, and a broader spectral coverage. It therefore has a considerable potential in future investigations of mantle structure and dynamics.

Passive seismic experiment and ScS wave splitting in the southwestern subbasin of South China Sea

The seismic experiment of 3D array of OBS in the southwestern sub-basin of the South China Sea (SCS) is briefly introduced in this paper. The data analysis of broadband OBS shows that totally 93 earthquakes with magnitude of Ms 6.0-6.9 and 10 earthquakes with magnitude above Ms 7.0 were recorded in high quality during this experiment, especially the catastrophic earthquake Ms 9.0 occurred in the east sea area of Japan on March 11, 2011. The anisotropy parameters inversion of ScS wave of four events above Ms 7.0 indicates that the fast direction of shear wave is N58A degrees E parallel to the ceased spreading ridge of the southwestern sub-basin of SCS (the slow direction is S35A degrees E perpendicular to the spreading ridge), which means the spreading ridge is under compressing stress at present and the cessation of seafloor spreading is related to such stress field as well.

Seismic signature of small melt fraction atop the transition zone

This article explores the combined effect of thermal, chemical, and melting anomalies of seismic velocities above the transition zone. While thermal and chemical effects influence the seismic velocities at subsolidus temperatures, the velocity structures are greatly modified in the presence of partial melting. We model the impedance contrast atop a low velocity layer at a depth of 350 km beneath the south Pacific, based on seismic observations of ScS wave reflectivity in the region. A compositionally distinct layer with varying basalt fraction fails to produce the observed average shear impedance contrast of -2.7% , for a range of potential temperatures between 1500 and 1700 K. A partially molten layer containing approximately 1 vol.% melt, explains the observed shear impedance. The melt fraction necessary to explain the observed shear impedance also trades off with the dihedral angle of the aggregate. For dihedral angles between 25 o and 30 o, between 1 to 1.1 vol.% melting is necessary to explain the observed impedance contrast. For such small volume fraction of melting, a near neutrally-buoyant melt can be redistributed by surface tension over the observed layer thickness of ∼ 70 km. Due to the high frictional resistance and strong surface tensions prevalent at such small melt fractions, density-driven melt drainage will likely be inefficient.

Observed travel times for Multiply-reflected ScS waves from a deep-focus earthquake

Tlie deep-focus Argentinean earthquake of December 81962, generated multiply reflected ScS phases which were recorded veryclearly at stations of the IGY and the TJSC&GS standardized worldwidenetworks and at Canadian stations. The data gathered from this earthquakefor the multiply-reflected ScS and sScS were used to construct thetravel times and to extend them to shorter epicentral distances. Thesenew data brought to light an error in published travel times for the 2(ScS)phase.

Trans-Pacific whole mantle structure

Recent reports on modeling USArray data reveal mostly vertical microplates with little resemblance to preliminary reference Earth model (PREM). Such complexity at plate boundaries makes it difficult to form reliable images of ocean basins using global paths. Here, we report on modeling stacked seismograms obtained from the first broadband array (TriNet) situated on the edge of the Pacific Plate, southern California, with no major subduction zone blocking its view. Extended records, including multi-S and ScS waves up to four bounces from 18 Tonga-Fiji deep events (140 to 620 km) are analyzed to check the validity of existing models and derive the whole mantle shear velocity structure along this corridor. Synthetics generated from 3-D tomographic models do not fit the upper mantle triplication data or the mantle reverberations associated with the ScS multiples as well as the 1-D model PAC06. We construct a hybrid model (HPAC), which remains one dimensional down to 800 km (PAC06). The lower portion of HPAC is essentially the tomography model S20RTS with velocity variation inflated by a factor of 2 for the lowermost 600 km. Thus, the mid-Pacific large low shear velocity province (LLSVP) has a lower shear velocity of about 2% relative to PREM and extends into the midmantle, similar to that beneath South Africa. Moreover, rapid changes in the differential (ScS-S) and (ScS2-S) times as a function of distance suggest ultra low velocity zones near the eastern edge and under the LLSVP, again similar to that found beneath Africa.

SCARDEC: a new technique for the rapid determination of seismic moment magnitude, focal mechanism and source time functions for large earthquakes using body-wave deconvolution

Accurate and fast magnitude determination for large, shallow earthquakes is of key importance for post-seismic response and tsumami alert purposes. When no local real-time data are available, which is today the case for most subduction earthquakes, the first information comes from teleseismic body waves. Standard body-wave methods give accurate magnitudes for earthquakes up to Mw= 7-7.5. For larger earthquakes, the analysis is more complex, because of the non-validity of the point-source approximation and of the interaction between direct and surface-reflected phases. The latter effect acts as a strong high-pass filter, which complicates the magnitude determination. We here propose an automated deconvolutive approach, which does not impose any simplifying assumptions about the rupture process, thus being well adapted to large earthquakes. We first determine the source duration based on the length of the high frequency (1-3 Hz) signal content. The deconvolution of synthetic double-couple point source signals--depending on the four earthquake parameters strike, dip, rake and depth--from the windowed real data body-wave signals (including P, PcP, PP, SH and ScS waves) gives the apparent source time function (STF). We search the optimal combination of these four parameters that respects the physical features of any STF: causality, positivity and stability of the seismic moment at all stations. Once this combination is retrieved, the integration of the STFs gives directly the moment magnitude. We apply this new approach, referred as the SCARDEC method, to most of the major subduction earthquakes in the period 1990-2010. Magnitude differences between the Global Centroid Moment Tensor (CMT) and the SCARDEC method may reach 0.2, but values are found consistent if we take into account that the Global CMT solutions for large, shallow earthquakes suffer from a known trade-off between dip and seismic moment. We show by modelling long-period surface waves of these events that the source parameters retrieved using the SCARDEC method explain the observed surface waves as well as the Global CMT parameters, thus confirming the existing trade-off. For some well-instrumented earthquakes, our results are also supported by independent studies based on local geodetic or strong motion data. This study is mainly focused on moment determination. However, the SCARDEC method also informs us about the focal mechanism and source depth, and can be a starting point to study systematically the complexity of the STF.

A Large Mw 6.0 Aftershock of the 2008 Mw 7.9 Wenchuan Earthquake Triggered by Shear Waves Reflected from the Earth's Core

Because the Earth’s outer core is liquid, shear waves generated by a large earthquake that travel to the core ( ScS ) are totally reflected with strong seismic energy. It is interesting to note that one large aftershock ( M w 6.0) of the 2008 Wenchuan earthquake ( M w 7.9) in China occurred when the ScS phase returned around 15.5 m after the mainshock. Estimated peak dynamic stress (∼3.3 kPa) of the ScS amplitude reaches the critical level of the lower boundary capable of triggering earthquakes. Both the temporal correlation between aftershock occurrence and ScS arrival, as well as the dynamic stress capable of triggering an earthquake, suggest that the large aftershock might well have been triggered dynamically by ScS waves. A similar feature of another large aftershock ( M b 5.3) occurring around the time of ScS arrival has been also found from the 2003 Hokkaido earthquake ( M w 8.3). Further statistical examinations of 25 major earthquakes ( M w>8.0) between 1994 and 2008 shows some large aftershocks ( M w>5.0) also occurring around the time of ScS -phase arrival. It is reasonable to conclude that these observations suggest aftershocks being triggered dynamically by ScS -phase arrivals, and thus a temporal warning for potentially destructive aftershocks might be considered for future great earthquakes.

TOMOGRAPHY S VELOCITY STRUCTURE BETWEEN WASHINGTON’S EARTHQUAKE C022801L AND OBSERVATIONAL STATION TUC THROUGH SEISMOGRAM ANALYSIS

In this research the S speed structure is investigated by seismogram analysis of Washington's earthquake, C022801L using data of TUC station, Tucson, Arizona, U.S.A. The seismogram comparison between the observed and the synthetic seismogram is conducted in time domain and three components simultaneously. The initially input for the calculation of synthetic seismogram is earth model of PREMAN and CMT solution from the earthquake. A low-pass Butterworth filter with corner frequency of 20 mHz is convolved to observed and synthetic seismogram. Waveform comparison shows a real deviation when travel time and waveform of some wave phase are compared, namely on S wave, surface wave of Love and Rayleigh and wave ScS and ScS-2. This research shows, how sensitive the waveform is to the earth model, better than the method of travel time or the dispersion analysis. Research hereinafter is addressed to finish the found discrepancies at S wave, surface wave of Love and Rayleigh and ScS and ScS-2 wave, in observation station TUC. To obtain the seismogram fitting, correction for S speed structure in earth model is needed, that are changes of earth crust thickness, the speed model ofb in upper mantle covering the speed gradient ofb h and value of zeroeth order coefficient for theb h andb v, for accomplishing the discrepancies at surface wave of Love and Rayleigh. Further correction on S speed is conducted to accomplish the deviation at S wave at earth layering systems from Upper Mantle up to a 630 km depth. Mean while for the ScS and ScS-2 wave phase the correction is carried out on S speed in the earth layers up to CMB. Fitting Seismogram is obtained at waveform of various wave phases that is S wave, surface wave of Love and Rayleigh and ScS, ScS-2 wave, either on travel time or especially also at oscillation number in Love wave. This result indicates that the anisotropy is occurred not only in upper mantle but till deeper earth layers, till CMB.

Frequency-dependent effects on global S-wave traveltimes: wavefront-healing, scattering and attenuation

We present a globally distributed data set of ∼400 000 frequency-dependent SH-wave traveltimes. An automated technique is used to measure teleseismic S, ScS and SS traveltimes at several periods ranging from 10 to 51 s. The targeted seismic phases are first extracted from the observed and synthetic seismograms using an automated time window algorithm. Traveltimes are then measured at several periods, by cross-correlation between the selected observed and synthetic filtered waveforms. Frequency-dependent effects due to crustal reverberations beneath each receiver are handled by incorporating crustal phases into WKBJ synthetic waveforms. After correction for physical dispersion due to intrinsic anelastic processes, we observe a residual traveltime dispersion on the order of 1–2 s in the period range of analysis. This dispersion occurs differently for S, ScS and SS, which is presumably related to their differing paths through the Earth. We find that: (1) Wavefront-healing phenomenon is observed for S and to a lesser extent SS waves having passed through very low velocity anomalies. (2) A preferred sampling of high velocity scatterers located at the CMB may explain our observation that ScS waves travel faster at low-frequency than at high-frequency. (3) A frequency-dependent attenuation q(ω) ∝q0×ω−α, with α∼ 0.2, is compatible with the globally averaged dispersion observed for S waves.