Higher-mode Surface Waves Research Articles

The thermal and anisotropic structure of the top 300 km of the mantle

From our analyses of fundamental and higher-mode surface waves from ∼1.2×107 seismograms, we constructed tomographic models of Vsv and Vsh. At depths shallower than 300 km, models of the temperature of cooling oceanic lithosphere, pressure and temperature estimates from garnet peridotite nodules, and the estimated average potential temperature of the convecting upper mantle from the thickness of the oceanic crust are combined with laboratory measurements of shear modulus and attenuation to parameterize the dependence of Vsv on temperature and pressure. The resulting parameters agree with those from independent laboratory measurements, which extend over a range of nine orders of magnitude of the dimensionless period. The fitting parameters were then used to produce global estimates of the temperature at depths shallower than 300 km and of the lithospheric thickness. The resulting map is similar to those of previous studies, though the use of such a large number of seismograms has considerably improved the resolution. Extensive regions of the continents are underlain by lithosphere whose thickness is less than that of old oceanic lithosphere. In contrast, the lithospheric thickness beneath most cratons is between 180 and 220 km. Where rifted continental margins formed when Pangea fragmented and cut across the cratonic lithosphere, the present continental margins coincide with the margins of the cratons.

Observation of Higher-Mode Rayleigh Waves from Ambient Noise in the Tarim Basin, China

Abstract Higher mode surface waves, which can provide additional constraints on subsurface structures in addition to fundamental modes in surface-wave tomography, have been observed from ambient noise cross-correlation functions (CCFs) in sedimentary basins in oceans or near coastlines. However, few studies show that higher mode surface waves can be observed and extracted directly from ambient noise CCFs in inland basins. In this study, we report observations of high signal-to-noise ratio fundamental and the first higher mode Rayleigh waves at a period range of 0.2–1.90 s and 0.2–1.35 s, respectively, from ambient noise CCFs in the southeastern margin of the Tarim basin, the biggest inland basin in China. We confirm the credibility of the first higher mode surface waves by showing that the observed first higher mode dispersion curves are matched with predicted ones calculated from S velocity models solely constrained by fundamental-mode dispersion curves. After the verification of the credibility of the first higher mode surface waves, we demonstrate that the inclusion of the first higher mode dispersion curves helps image deeper structures with an increase of average depths from ∼0.73 to ∼1.24 km, which will be beneficial to future explorations of deep oil and gas resources in the Tarim basin.

Forest trees as a natural metamaterial for surface wave attenuation in stratified soils

Forest trees have been shown to act as a natural metamaterial to attenuate elastic waves. However, the existing studies assumed that forest trees are located on homogeneous soils and did not consider the soil stratification that widely exists in natural strata. This study investigates the forest trees as a natural metamaterial to manipulate the propagation of elastic waves in stratified soils. A semi-analytical method is proposed to calculate the dynamic responses of a layered half-space coupled with arbitrary number of trees. A close-form Green’s function for a harmonic line load on a layered half-space is derived by the thin layer method, and subsequently used to determine the multiple scattered wavefield induced by forest trees as surface oscillators. The attenuation performance of forest trees on stratified soils is systematically investigated. Trees with a graded array of heights can provide a wide attenuation zone at frequencies below 80 Hz. Since the higher surface wave modes in a layered half-space can propagate forward without being affected by trees, the attenuation efficiency of trees on stratified soils is significantly lower than that of trees on corresponding homogeneous soils. The performance uncertainty induced by the randomness of the height, sizes, and spacing of trees is subsequently investigated through a Monte Carlo simulation. The forest trees can still obtain a significant attenuation efficiency at frequencies below 80 Hz even the randomness of tree properties being considered. The randomness in height and radius leads to significant uncertainty in attenuation performance, while the randomness in spacing results in reasonable uncertainty.

Modern inversion workflow of the multimodal surface wave dispersion curves: staging strategy and Pattern search with embedded Kuhn–Munkres algorithm

SUMMARY The dispersive information of the higher surface wave modes is beneficial for improving the resolution of the inverted S-wave velocity structure, increasing the penetration depth and enhancing the robustness of the inversion. The recently proposed frequency-Bessel (F-J) transformation can achieve the relatively stable measurement of the higher-mode surface waves. However, sometimes the phenomena of the mode losses and aliases would appear in the dispersion energy image of the seismic records, which may lead to the mode misidentification. And the wrong mode identification is likely to cause the negative impacts on the dispersion inversion and geological interpretation about the survey site. In view of this, we recommend a staging strategy for the inversion of multimodal surface wave dispersion curves. The pattern search (PS) is used to invert the reliable segment of the fundamental-mode surface wave phase velocities for the first stage. For the second stage, the inverted result of the first stage is set as the initial model, the PS with embedded Kuhn–Munkres (PSEKM) algorithm is adopted for inverting the observed phase velocities of all modes. And for each frequency, a weighted bipartite graph is established between the observed values with no-explicitly specified-mode-order (NESMO) and predicted values of the model m during the inversion, then the maximum match is determined by the KM algorithm for calculating the minimum distance between the observed and predicted data sets. The mode-order information of the observed phase velocities with NESMO would be dynamically evaluated for each model m occurred in the inversion process. The synthetic reconstruction tests have confirmed the effectiveness of the novel workflow. Also, the performance of the preconditioned steepest-descent algorithm of local optimization methods and influence of the mode misidentification on the inversion result are also clarified in the synthesis tests. The comparison results show that the proposed workflow can realize the nice data fitting and model reconstruction without the time-consuming manual mode-identification for the higher-mode surface waves. Then, the new workflow is applied to the analysis of the actual surface wave data sets collected on two roadbeds, which is still satisfactory. Finally, we discuss the role of the staging strategy of the workflow.

Denoising Surface Waves Extracted From Ambient Noise Recorded by 1‐D Linear Array Using Three‐Station Interferometry of Direct Waves

AbstractWe develop an automatic workflow for enhancing surface wave signals in ambient noise cross correlations (ANCs) calculated for a one‐dimensional (1‐D) linear array. The proposed array‐based method is applied to a 1.6 km‐long dense linear nodal array crossing surface traces of the San Jacinto fault near Anza, California. Fundamental and higher modes of surface waves are observed in ANCs of the nodal array. After attenuating the surface wave overtones by applying a frequency‐dependent tapering window to the ANCs, signals dominated by the fundamental mode surface wave are then enhanced through a denoising process based on three‐station interferometry of direct waves. The signal‐to‐noise ratio is significantly increased at high frequencies (>2 Hz) after denoising. Phase travel times are extracted reliably in the frequency domain for the period ranges of 0.3–1.2 s and 0.3–1.6 s for Rayleigh and Love waves, respectively. The corresponding period‐dependent phase velocity profiles derived from the eikonal equation reveal high‐resolution details of fault zone internal structures beneath the array. A broad (500–1,000 m) low‐velocity zone that narrows with increasing period is observed, illuminating a flower‐shaped structure of the San Jacinto fault damage zone.

Advances in surface-wave tomography for near-surface applications

Surface-wave (SW) tomography is a technique that has been widely used in the field of seismology. It can provide higher resolution relative to the classical multichannel SW processing and inversion schemes that are usually adopted for near-surface applications. Nevertheless, the method is rarely used in this context, mainly due to the long processing times needed to pick the dispersion curves as well as the inability of the two-station processing to discriminate between higher SW modes. To make it efficient and to retrieve pseudo-2D/3D S-wave velocity (VS) and P-wave velocity (VP) models in a fast and convenient way, we develop a fully data-driven two-station dispersion curve estimation, which achieves dense spatial coverage without the involvement of an operator. To handle higher SW modes, we apply a dedicated time-windowing algorithm to isolate and pick the different modes. A multimodal tomographic inversion is applied to estimate a VS model. The VS model is then converted to a VP model with the Poisson's ratio estimated through the wavelength-depth method. We apply the method to a 2D seismic exploration data set acquired at a mining site, where strong lateral heterogeneity is expected, and to a 3D pilot data set, recorded with state-of-the-art acquisition technology. We compare the results with the ones retrieved from classical multichannel analysis.

Multimodal surface-wave tomography to obtain S- and P-wave velocities applied to the recordings of unmanned aerial vehicle deployed sensors

Exploration seismic surveys in hard-to-access areas such as foothills and forests are extremely challenging. The Multiphysics Exploration Technologies Integrated System (METIS) research project was initiated to design an exploration system, facilitating the acquisition in these areas by delivering the receivers from the sky using unmanned aerial vehicles. Air dropping of the sensors in vegetated areas results in an irregular geometry for the acquisition. This irregularity can limit the application of conventional surface wave methods. We have developed a surface wave workflow for estimating the S-wave velocity ([Formula: see text]) and P-wave velocity ([Formula: see text]) models and that supports the irregular geometry of the deployed sources and receivers. The method consists of a multimodal surface-wave tomography (SWT) technique to compute the [Formula: see text] model and a data transform method (the wavelength/depth [W/D] method) to determine the Poisson’s ratio and [Formula: see text] model. We applied the method to the METIS’s first pilot records, which were acquired in the forest of Papua New Guinea. Application of SWT to the data resulted in the first 90 m of the [Formula: see text] model. The W/D method provided the Poisson’s ratio averaged over the area and the [Formula: see text] model between 10 and 70 m from the surface. The impact of the acquisition scale and layout on the resolution of the estimated model and the advantages of including the higher modes of surface waves in the tomographic inversion are assessed in detail. The presence of shots from diverse site locations significantly improves the resolution of the obtained model. Including the higher modes enhances the data coverage and increases the investigation depth.

High-Gain Millimeter-Wave Patch Array Antenna for Unmanned Aerial Vehicle Application.

A high-gain millimeter-wave patch array antenna is presented for unmanned aerial vehicles (UAVs). For the large-scale patch array antenna, microstrip lines and higher-mode surface wave radiations contribute enormously to the antenna loss, especially at the millimeter-wave band. Here, the element of a large patch array antenna is implemented with a substrate integrated waveguide (SIW) cavity-backed patch fed by the aperture-coupled feeding (ACF) structure. However, in this case, a large coupling aperture is used to create strongly bound waves, which maximizes the coupling level between the patch and the feedline. This approach helps to improve antenna gain, but at the same time leads to a significant level of back radiation due to the microstrip feedline and unwanted surface-wave radiation, especially for the large patch arrays. Using the SIW cavity-backed patch and stripline feedline of the ACF in the element design, therefore, provides a solution to this problem. Thus, a full-corporate feed 32 × 32 array antenna achieves realized gain of 30.71–32.8 dBi with radiation efficiency above 52% within the operational band of 25.43–26.91 GHz. The fabricated antenna also retains being lightweight, which is desirable for UAVs, because it has no metal plate at the backside to support the antenna.

Observation of Higher-Mode Surface Waves from an Active Source in the Hutubi Basin, Xinjiang, China

ABSTRACTBasins with thick sediments can amplify and prolong the incoming seismic waves, which may cause serious damage to surface facilities. The amplification of seismic energy depends on the shear-wave velocity of the uppermost layers, which is generally estimated through surface wave analysis. Surface waves may propagate in different modes, and the mechanism of the mode development is not well understood. Exploiting a recently deployed permanent airgun source in the Hutubi basin, Xinjiang, northwest China, we conducted a field experiment to investigate the development of multimode surface waves. We observed surface waves at the frequency of 0.3–5.0 Hz with apparent group velocities of 200–900 m/s, and identified five modes of surface waves (three Rayleigh-wave modes and two Love-wave modes) through time–frequency and particle-motion analyses. We then measured 125 group velocity dispersion curves of the fundamental- and higher-mode surface waves, and further inverted the 1D S-wave velocity structure of the Hutubi basin. The S-wave velocity increases abruptly from 238 m/s at the surface to 643 m/s at 300 m depth. Synthetic seismograms with the inverted velocity structure capture the main features of the surface waves of the different modes. Synthetic tests suggest that the low velocity, high velocity gradient, and shallow source depth are likely the dominant contributing factors in the development of higher-mode surface waves.

Regional Crustal Imaging by Inversion of Multimode Rayleigh Wave Dispersion Curves Measured From Seismic Noise: Application to the Basque‐Cantabrian Zone (N Spain)

AbstractSeismic‐noise tomography is routinely applied for imaging geological structures at different spatial scales. The frequently used time‐domain approach presents two limitations. First, extracting surface‐wave group velocities from time‐domain cross‐correlations requires interstation distances of at least three wavelengths, which may be problematic when working at local or regional scales. Second, the presence of higher modes of surface waves in the cross‐correlation functions is often disregarded, which may cause loss of valuable information about the shear‐wave velocity structure. In this work, we present a complete inversion scheme that avoids these limitations and use it to obtain a 3D shear‐wave velocity model of the Basque‐Cantabrian Zone (N Spain), a structurally complex area affected by multiple tectonic events. The resulting model agrees with the existing geological and geophysical knowledge and significantly extends the area for which high‐resolution information is available.

Validation of a fast semi-analytic method for surface-wave propagation in layered media

SUMMARYGreen’s functions provide an efficient way to model surface-wave propagation and estimate physical quantities for near-surface processes. Several surface-wave Green’s function approximations (far-field, no mode conversions and no higher mode surface waves) have been employed for numerous applications such as estimating sediment flux in rivers, determining the properties of landslides, identifying the seismic signature of debris flows or to study seismic noise through cross-correlations. Based on those approximations, simple empirical scalings exist to derive phase velocities and amplitudes for pure power-law velocity structures providing an exact relationship between the velocity model and the Green’s functions. However, no quantitative estimates of the accuracy of these simple scalings have been reported for impulsive sources in complex velocity structures. In this paper, we address this gap by comparing the theoretical predictions to high-order numerical solutions for the vertical component of the wavefield. The Green’s functions computation shows that attenuation-induced dispersion of phase and group velocity plays an important role and should be carefully taken into account to correctly describe how surface-wave amplitudes decay with distance. The comparisons confirm the general reliability of the semi-analytic model for power-law and realistic shear velocity structures to describe fundamental-mode Rayleigh waves in terms of characteristic frequencies, amplitudes and envelopes. At short distances from the source, and for large near-surface velocity gradients or high Q values, the low-frequency energy can be dominated by higher mode surface waves that can be captured by introducing additional higher mode Rayleigh-wave power-law scalings. We also find that the energy spectral density for realistic shear-velocity models close to piecewise power-law models can be accurately modelled using the same non-dimensional scalings. The frequency range of validity of each power-law scaling can be derived from the corresponding phase velocities. Finally, highly discontinuous near-surface velocity profiles can also be approximated by a combination of power-law scalings. Analytical Green’s functions derived from the non-dimensionalization provide a good estimate of the amplitude and variations of the energy distribution, although the predictions are quite poor around the frequency bounds of each power-law scaling.

Physical Properties and Structure of the Near Subsurface in the Săcel Area, Romania, from Shallow Seismic Measurements

Results are presented for shallow seismic reflection measurements performed southwest of Săcel village in Romania for the purpose of obtaining information about the geological structure in the near subsurface. The P-wave and S-wave velocity distributions were also obtained below the soil surface. The measurements were performed along a nearly linear profile on the top of an elongated hill. Most of the shot gathers were characterized by a good signal-to-noise ratio. A depth-converted migrated section was obtained after the processing of shot gathers, on which an image of sedimentary deposits with various thicknesses, separated by shallow faults until a depth of about 80 m, were observed. The P-wave and S-wave velocity-depth models for two segments were of considerable interest for a geotechnical study proposed for the construction of a windmill park. The two- and three-layered P-wave velocity-depth models were comparable until depths of about 10 m after first-arrival traveltime inversions. The lateral variations in the subsurface geological structure and lithology reflected the variations in the P-wave velocity values from both models. The S-wave velocity-depth models for comparable depth intervals were similar to those from the P-wave velocity-depth models. Reliable S-wave velocity distributions were obtained after inversion of fundamental-mode and higher-mode surface waves.

Surface-Wave Extraction Based on Morphological Diversity of Seismic Events

It is essential to extract high-fidelity surface waves in surface-wave surveys. Because reflections usually interfere with surface waves on X components in multicomponent seismic exploration, it is difficult to extract dispersion curves of surface waves. To make matters worse, the frequencies and velocities of higher-mode surface waves are close to those of PS-waves. A method for surface-wave extraction is proposed based on the morphological differences between surface waves and reflections. Frequency-domain high-resolution linear Radon transform (LRT) and time-domain high-resolution hyperbolic Radon transform (HRT) are used to represent surface waves and reflections, respectively. Then, a sparse representation problem based on morphological component analysis (MCA) is built and optimally solved to obtain high-fidelity surface waves. An advantage of our method is its ability to extract surface waves when their frequencies and velocities are close to those of reflections. Furthermore, the results of synthetic and field examples confirm that the proposed method can attenuate the distortion of surface-wave dispersive energy caused by reflections, which contributes to extraction of accurate dispersion curves.

Three-dimensional simulations of strong ground motion in the Shidian basin based upon the spectral-element method

The strong motion of a small long and narrow basin caused by a moderate scenario earthquake is simulated by using the spectral-element method and the parallel computing technique. A total of five different geometrical profiles within the basin are used to analyze the generation and propagation of surface waves and their relation to the basin structures in both the time and frequency domain. The amplification effects are analyzed by the distribution of peak ground velocity (PGV) and cumulative kinetic energy (Ek) in the basin. The results show that in the 3D basin, the excitation of the fundamental and higher surface wave modes are similar to that of the 2D model. Small bowls in the basin have great influence on the amplification and distribution of strong ground motion, due to their lateral resonances when the wavelengths of the lateral surface waves are comparable to the size of the bowls. Obvious basin edge effects can be seen at the basin edge closer to the source for constructive interference between direct body waves and the basin-induced surface waves. The Ek distribution maps show very large values in small bowls and some corners in the basin due to the interference of waves propagating in different directions. A high impedance contrast model can excite more surface wave modes, resulting in longer shaking durations as well as more complex seismograms and PGV and Ek distributions.

Surface wave retrieval in layered media using seismic interferometry by multidimensional deconvolution

Virtual-source surface wave responses can be retrieved using the crosscorrelation (CC) of wavefields observed at two receivers. Higher mode surface waves cannot be properly retrieved when there is a lack of subsurface sources that excite these wavefields, as is often the case. In this paper, we present a multidimensional-deconvolution (MDD) scheme that is based on an approximate convolution theorem. The scheme introduces an additional processing step in which the CC result is deconvolved by a so-called point-spread tensor. The involved point-spread functions capture the imprint of the lack of subsurface sources and possible anelastic effects, and quantify the associated spatial and temporal smearing of the virtualsource components that leads to the poor surfacewave retrieval. The functions can be calculated from the same wavefields as used in the CC method. For a 2-D example that is representative of the envisaged applications, we show that the deconvolution partially corrects for the smearing. The retrieved virtual-source response only has some amplitude error in the ideal situation of having the depth of the required vertical array equal to the depth penetration of the surface waves. The error is due to ignored cross-mode terms in the approximate convolution theorem. Shorter arrays are also possible. In the limit case of only a single surface receiver, the retrieved virtual-source response is still more accurate than the CC result. The MDD scheme is valid for horizontally layered media that are laterally invariant, and includes exclusively multicomponent point-force responses (rather than their spatial derivatives) and multicomponent observations. The improved retrieval of multimode surface waves can facilitate dispersion analyses in shallow-subsurface inversion problems and monitoring, and surface wave removal algorithms.

Multimodal direct fitting of SPAC coefficients using amplitude response

We usually analyse microtremors without any considerations of higher modes of surface waves. However, recent studies have demonstrated that the use of higher modes would improve estimated velocity models. In this study, we apply multi-mode analysis with amplitude response to the direct fitting method in SPAC method. The proposed method has the advantage that we don?t need to identify observed modes, which can avoid mode misidentification. The proposed method was applied to synthesized microtremors. Estimated SPAC coefficients were good agreed with theoretical ones proposed in this study, even if in the frequency where two modes have about the same power fractions. The S-wave velocity was then successfully inverted by the direct fitting of SPAC coefficients. The proposed method also works well in the analysis of field data acquired in Tsukuba City, Japan. In the inversion performed in this study, we do not use prior information about velocity structure. It is concluded that the multi-mode analysis proposed here is very robust in the multimodal analysis in SPAC method.

Noise Reduction Technique Applied to the Multichannel Analysis of Surface Waves

AbstractThis paper shows the presence of noises and technique to reduce these noises during the surface wave analysis. The frequency‐dependent properties of Rayleigh‐type surface waves can be used for imaging and characterizing the shallow subsurface. Interference by coherent source‐generated noise inhibits the reliability of shear‐wave velocities determined through inversion of the phase velocities of Rayleigh waves. Among these interferences by non‐planar, non‐fundamental mode Rayleigh waves (noise) are body waves, scattered and non‐source‐generated surface waves, and higher‐mode surface waves. For the reduction of noise, the filtering technique is implemented in this paper for the multichannel analysis of surface wave method (MASW). With the de‐noising technique during the MASW method, more robust and reliable outcome is achieved. The significance of this paper is to obtain pre‐awareness about noises during surface wave analysis and take better outcomes with de‐noising performance in near surface soil investigations.

Multimode inversion with amplitude response of surface waves in the spatial autocorrelation method

SUMMARY Microtremors are usually analysed without any consideration of the higher modes of surface waves. However, recent studies have demonstrated that higher modes contain useful information for improving the inverted S-wave velocity model. In this study, we propose two inversion methods that consider higher modes by using the amplitude response of each mode, which can avoid mode misidentification in the spatial autocorrelation (SPAC) method. One method is to compare the observed phase velocities by the extended spatial autocorrelation (ESPAC) method with the effective phase velocities calculated from theoretical dispersion curves and the amplitude responses of each mode. In the other method, SPAC coefficients are fit directly by comparing theoretical SPAC coefficients determined from dispersion curves and amplitude responses with the observed ones. The latter, direct-fitting approach is much simpler than the method using effective phase velocities. To investigate the effectiveness of these methods, a simulation study was conducted. Simulated microtremors that included higher modes were successfully inverted by the proposed multimode methods. The observed phase velocities and SPAC coefficients determined from field data were also consistent with theoretical ones constructed by the proposed methods except at low frequencies. The inversion using effective phase velocities required prior information about an infinite half-space to obtain a better S-wave velocity model whereas the direct-fitting inversion worked well without prior information, suggesting the direct-fitting method is more robust than the method using effective phase velocities. We conclude that our proposed inversion methods are effective for estimating the S-wave velocity structure even if higher modes of surface waves are predominant in observed microtremors.

On the use of the Norwegian Geotechnical Institute's prototype seabed-coupled shear wave vibrator for shallow soil characterization - I. Acquisition and processing of multimodal surface waves

SUMMARY Pure shear wave data are only very rarely acquired for offshore site investigations and exploration. Here, we present details of a novel, seabed-coupled, shear wave vibrator and field data recorded by a densely populated, multicomponent ocean-bottom cable, to improve shallow soil characterization. The prototype shear wave vibrator uses vibroseis technology adopted for marine environments through its instalment on top of a suction anchor, assuring seabed coupling in combination with self-weight penetration. The prototype is depth rated to 1500 m water depth, and can be rotated while installed in the seabed. The philosophy is to acquire fully complementary seismic data to conventional P- and P-to-S-converted waves, in particular for 2-D profiling, VSP (vertical seismic profiling) or monitoring purposes, thereby exploiting advantages of shear waves over compressional waves for determining, for example, anisotropy, small-strain shear modulus and excess pore pressures/effective stress. The source was primarily designed for reservoir depths. However, significant energy is emitted as surface waves, which provide detailedgeotechnicalinformationthroughmappingofshearwavevelocitiesinpotentiallyhigh resolution of the upper soil units. To fully utilize pure shear wave content, a proper analysis of surface waves is paramount, due to the proximity of surface wave propagation speed with shear wave velocities. The experiment was carried out in the northern North Sea in 364 m water depth. Cable dragging was necessary to obtain close receiver spacing (2.5 m effective spacing), with total line length of 600 m. Frequency–waveform transforms reveal both Scholte and Love waves. Up to six surface wave modes are identified, that is, fundamental mode and several higher surface wave modes. The occurrence of these two different dispersive surface wave types with well-resolved higher modes allows for a unique analysis and inversion scheme for highresolution mapping of physical properties in the shallow subsurface as well as anisotropy, which is discussed in an accompanying paper. The data presented in this paper are thus a unique (long and densely populated receiver array allows for multimodal Love and Scholte surface waves from the marine environment) but challenging (marine operations) marine data set.

Relative excitation of the seismic shear waves Sn and Lg as a function of source depth and their propagation from Melanesia and Banda arcs to Australia

SUMMARY. - Seismic activity associated with the collision of the continentalpart of the Australian plate with the oceanic Melanesian arcs along Papua NewGuinea and the Banda arc provides an unusual opportunity to study the relativeexcitation of the seismic shear waves Sn and Lg. These waves are produced byearthquakes located along the arcs in the upper 200 km of the earth and arerecorded by the Australian WWSSN Stations at Charters Towers (CTA) and AliceSprings (ASP). The paths to these stations are predominantly continental. The dataclearly show that for events located at crustal depths, Lg is the predominant phaseon the records and Sn is either absent or very weak. For events deeper than about50-70 km, Sn becomes the predominant phase on the records. These observationsarc in qualitative agreement with the explanations of Sn and Lg as highermodes of surface waves, for the particle displacement amplitudes are maximumwithin the crust for Lg and maximum within the lid of the lithospheric mantlefor Sn. The data suggest that either the crustal wave guide for Lg is moreefficient than that for Sn, or that Lg is more easily excited than Sn. No clearLg is observed from shallow earthquakes when the length of the segment of thepath crossing oceanic structure is greater than about 200 km. Also, widespreadQuaternary volcanism within the « stable » area of central Papua New Guineato the south of the mobile belt does not seem to affect the efficient transmissionof high-frequency (1 Hz) shear energy.The paths from events located along the New Hebrides, Solomon, and NewBritain arcs to Australia traverse oceanic structure, and no Lg is observed fromthese paths. The inefficient propagation of Sn along these paths from bothshallow and intermediate-depth events can be explained as follows: 1) Forthe New Hebrides case, the inefficiency of Sn propagation for paths exceedingabout 20° distance is related to the relatively young age (Lower to MiddleEocene) of the suboceanic lithosphere of the Coral Sea. As proposed by CHINN,ISACKS and BARAZANGI (1979), such relatively thin lithosphere is probably not anefficient wave guide for shear energy of about 1 Hz 2). The inefficiency of Snpropagation from events located along the northern Solomon and the New Britainarcs is probably due to anomalous attenuation in the uppermost mantle beneaththe Woodlark basin and beneath southeastern Papua New Guinea. 3) Theinefficiency of Sn propagation from events deeper than about 150 km locatedin the westernmost part of the northerly-dipping Benioff zone of the NewBritain arc could be due either to structural isolation of the nearly verticaldescending segment of the plate in which the events occur, or could be due tostructural complexity of the plates in the region which interrupts the waveguide for Sn to the Australian stations. This structural feature could be theresult of the collision of the Australian plate and the New Britain arc.