Period Rayleigh Waves Research Articles

Seismic Azimuthal Anisotropy Within the Juan de Fuca ‐ Gorda Plate System

AbstractWe estimate seismic azimuthal anisotropy for the Juan de Fuca ‐ Gorda plates from inversion of a new 10–80 s period Rayleigh wave dataset, resulting in a two‐layer model to 80 km depth. In the lithosphere, most anisotropy patterns reflect the kinematics of plate formation, as approximated from seafloor‐age‐based paleo‐spreading, except for regions close to propagator wakes and near plate boundaries. In the asthenosphere, the fast propagation orientations align with convective shear as inferred from the NUVEL1A plate motion model, which is indicative of a ∼3 Myr average, rather than with the more recent, ∼0.8 Myr, motions inferred from MORVEL. Regional anisotropy of this young plate system thus records convection like older plates such as the Pacific. On smaller scales, anisotropy imaging provides insights into dynamics of plate generation and can further elucidate plate reorganizations and changes in boundary loading.

Automatic determination of focal depth with the optimal period of Rayleigh wave amplitude spectra at local distances

SUMMARY Focal depth of earthquakes is essential for studies of seismogenic processes and seismic hazards. In regions with dense seismic networks, focal depth can be resolved precisely based on the traveltime of P and S, which is less feasible in case of sparse networks. Instead, surface waves are usually the strongest seismic phases at local and regional distances, and its excitation is sensitive to source depth, thus theoretically important for estimating focal depth even with a limited number of seismic stations. In this study, short-period (0.5–20 s) Rayleigh waves are explored to constrain focal depths. We observe that the optimal period (the period corresponding to the maximum amplitude) of Rayleigh waves at local distances (≤200 km) shows an almost linear correlation with focal depth. Based on this finding, we propose an automated method for resolving the focal depth of local earthquakes using the linear regression relation between the optimal period of Rayleigh wave amplitude spectra and focal depth. Synthetic tests indicate the robustness of this method against source parameters (focal mechanism, source duration and non-double-couple component) and crustal velocity structure. Although the attenuation (Q factor) of shallow crust can introduce complexities in determining focal depth, it can be simultaneously estimated if a sufficient number of stations are available. The proposed method is applied to tens of small-to-moderate earthquakes (Mw 3.5–5.0) in diverse tectonic settings, including locations in the United States (Oklahoma, South Carolina, California, Utah, etc.) and China (Sichuan, Shandong). Results demonstrate that reliable focal depth, with uncertainty of 1–2 km, can be determined even with one or a few seismic stations. This highlights the applicability of the method in scenarios characterized by sparse network coverage or historical events.

Monitoring the 2021 Mw 8.2 Alaska Earthquake by an Offshore Seismic and Fluid Pressure Observation Network and Implications for Ocean‐Crust Dynamic Coupling

AbstractGround shaking caused by earthquakes is accompanied by seafloor and sub‐seafloor formation fluid pressure variations in offshore areas, but there have been few collocated observations of these signals. In this work, we report seismic and high‐sampling‐rate fluid pressure records of the 2021 Mw 8.2 Alaska earthquake by the Ocean Networks Canada (ONC) NEPTUNE observatory at an epicentral distance of ∼2,200 km in the northeast Pacific Ocean. The system comprises observatory nodes in various tectonic environments, with each node including buried broadband seismometers, seafloor pressure sensors, and, at two nodes, borehole pressure sensors. Seismic and tsunami waveforms of the Mw 8.2 earthquake were documented in detail. Seismic seafloor pressure variations (Psf) were dominated by Rayleigh waves of periods between 5 and 50 s, with peak amplitudes of 3–4 kPa at most sites. Waveform similarity and the linear scaling between Psf and vertical ground acceleration indicate forced acceleration of the water column being dominant in governing Psf during long‐period surface‐wave arrivals, with an additional component of elastic oscillation occurring at higher frequencies (>0.1 Hz) causing extra pressure signals. Analysis of formation pressure variations due to various types of ocean loading of distinctly different frequencies (e.g., tides, tsunami, and infragravity waves) shows stable one‐dimensional vertical loading efficiencies that depend on lithology at each borehole site, with loading response being strongly influenced by the presence of free gas at shallow depths within the Cascadia accretionary prism. Inter‐site comparisons of seismic and seafloor pressure waveforms demonstrate a key role of sediment thickness in the amplification of surface wave amplitudes.

Shear‐Wave Velocity Structure of the Southern African Upper Mantle: Implications for Craton Structure and Plateau Uplift

AbstractWe present a 3D shear‐wave velocity model of the southern African upper mantle developed using 30–200 s period Rayleigh waves recorded on regional seismic networks spanning the subcontinent. The model shows high velocities (∼4.7–4.8 km/s) at depths of 50–250 km beneath the Archean nucleus and several surrounding Paleoproterozoic and Mesoproterozoic terranes, placing the margin of the greater Kalahari Craton along the southern boundary of the Damara Belt and the eastern boundaries of the Gariep and Namaqua‐Natal belts. At depths ≥250 km, there is little difference in velocities beneath the craton and off‐craton regions, suggesting that the cratonic lithosphere extends to depths of about 200–250 km. Upper mantle velocities beneath uplifted areas of southern Africa are higher than the global average and significantly higher than beneath eastern Africa, indicating there that is little thermal modification of the upper mantle present today beneath the Southern African Plateau.

Crustal Heating and Lithospheric Alteration and Erosion Associated With Asthenospheric Upwelling Beneath Southern New England (USA)

AbstractThe Northern Appalachian Anomaly (NAA), a region of exceptionally low seismic velocities in the asthenosphere beneath southern New England and easternmost New York State, has been interpreted as a site of mantle upwelling. We synthesize a combination of new and previously published data that indicates the following: (1) The upwelling has eroded or delaminated the lithosphere in a localized region centered in southern Vermont that we call the “Green Mountains Anomaly.” Forty‐second period Rayleigh wave phase velocities, which have peak sensitivity at lithospheric depths, are slow in this region, and S wave receiver functions are dominated by shallow (60‐km depth) mantle structures indicating reduced velocities. Thermal springs and sites with anomalously high concentrations of mantle‐derived helium‐3 are concentrated at the borders of the Green Mountains anomaly, perhaps due to stress concentrations produced by geologically recent, uncompensated delamination of the lower lithosphere. (2) S wave receiver functions indicate intense (>10%), shallow (60‐km depth) short wavelength structures along the southern and western edges of the NAA, indicating that the lithosphere there is being intensely altered, possibly by a combination of shearing and introduction of volatiles. And (3) Notwithstanding the localized thinning and alteration, the NAA lithosphere as a whole does not appear to have experienced pervasive heating, for the compressional wave quality factor of QP = 870 inferred from the decay rate of Po waves is very significantly above the QP ≈ 60 value previously reported for the NAA asthenosphere.

Anelasticity beneath the Aegean inferred from Rayleigh wave attenuation

Anelasticity of the Earth crucially affects the propagation of seismic waves especially, in the long period range. However, even though the elastic properties of the Aegean deep lithosphère and upper mantle have been thoroughly investigated, their quantitative anelastic properties that influence the long period wavefield are still largely unknown. This work is towards contributing to the better knowledge of the deep structure of the Aegean by introducing experimental anelastic parameters via the study of long period Rayleigh waves attenuation. For this scope, fundamental mode attenuation coefficients (γ%) have been obtained for different two-station great-circle paths across the Aegean. The data used were provided by a broadband array installed in the area for 6 months in 1997. More than 1100 seismograms were analyzed in the 10-100 s range to obtain 17 sets of path average γR(T) functions. The attenuation coefficients are in the range 2.5*10~3 — 0.15 x 10' km' and correlate sufficiently with both experimental measurements in active tectonic regions elsewhere and synthetics generated with the use of an attenuation reference model inferred from other sources. By applying a stochastic uncoupled causal inversion method an average joint Qß'1 and shear velocity model representative of the under study area was obtained. Furthermore, path average JR(T) functions were combined in a continuous regionalization tomographic scheme to obtain local yR(T) and tomograms were constructed in the range 10-60 s. The most prominent feature in the tomograms is a high attenuation region in the central and north Aegean. This region is located south of the North Anatolian Trough and correlates well with a low shear velocity zone inferred from surface wave phase velocities. Moreover, it is associated with observed intense extensional deformation rates, mantle olivine anisotropy, recent volcanism and high heat flow.

Basin-effects observed during the 2012 Emilia earthquake sequence in Northern Italy

During the 2012 Emilia earthquake sequence, prolonged shaking associated with long-period motions were observed in the Po Plain basin in Northern Italy. Such anomalous characteristics of ground-motion were unnoticed at the rocky sites outside the Po Plain basin. To explain these phenomena, a series of detailed analyses were carried out using the strong motion records from May 20 to May 29 main events. The observed amplification, calculated using the spectral ratio method indicates a fundamental resonance period at 5s. Well-dispersed surface Rayleigh waves of periods between 3 and 10s were noticed and the contribution of surface waves to the total motion was significant despite the source being located beneath the Plain. The late arriving long-period surface waves significantly increased the duration of ground shaking. The envelope delay spectrum shows that the duration lengthening of ground motion could be well correlated with the dispersion of surface waves. The greatest lengthening of the records was observed around the fundamental period of the basin. Large peak ground-motions were observed in the near-field region, especially in the vertical component which is attributed to source effects (predominantly vertical movement of the causative fault) while the prolonged duration of motions seems caused by surface waves.

The 12 February 2013 North Korean Underground Nuclear Test

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Shear wave velocity structure of the Bushveld Complex, South Africa

The structure of the crust in the environs of the Bushveld Complex has been investigated by jointly inverting high-frequency teleseismic receiver functions and 2–60s period Rayleigh wave group velocities for 16 broadband seismic stations located across the Bushveld Complex. Group velocities for 2–15s periods were obtained from surface wave tomography using local and regional events, while group velocities for 20–60s periods were taken from a published model. 1-D shear wave velocity models obtained for each station show the presence of thickened crust in the center of the Bushveld Complex and a region at the base of the crust where shear wave velocities exceed 4.0km/s. The shear wave velocity models also suggest that velocities in some upper crustal layers may be as high as 3.7–3.8km/s, consistent with the presence of mafic lithologies. These results favor a continuous-sheet model for the Bushveld Complex in which the outcropping mafic layers of the western and eastern limbs are continuous at depth beneath the center of the complex. However, detailed modeling of receiver functions at one station within the center of the complex indicates that the mafic layering may be locally disrupted due to thermal diapirism triggered by the emplacement of the Bushveld Complex or thermal and tectonic reactivation at a later time.

Velocity Anisotropy in the Crust and Upper Mantle of North China

AbstractThis paper presents the result of a surface wave study on azimuthal anisotropy in the crust and upper mantle of North China, and makes a preliminary comparison with the result of S‐wave splitting in the region. The anisotropy pattern of Rayleigh waves of different periods exhibits obvious lateral variation, which is closely related with the tectonic divisions and vertical layering of the crust and upper mantle of North China. In the stable blocks of Ordos and Alxa significant anisotropy is rather uniform in the lithospheric mantle down to 160 km; while in the eastern part of the North China Craton, where lithosphere thinning occurred in Meso‐Cenozoic time, no azimuthal anisotropy is detected in the depth range of 80~150 km, which may indicate that no significant horizontal tectonic movement occurred during the process of lithospheric thinning. The anisotropy in the study region is characterized by obvious layering, as evidenced by the inversion result from surface waves. On the other hand the remarkable scatter of apparent splitting parameters may also be attributed to multi‐layering and/or slanting symmetry axis of the anisotropy. Assuming a multi‐layer anisotropy model, the differences between surface wave and S‐wave splitting in most cases can be qualitatively explained. In the future study the detecting depth and resolving power of surface wave should be increased and more splitting measurements are needed in order to establish a quantitative or semi‐quantitative 3D anisotropy model.

Group velocity tomography of the Indo‐Eurasian collision zone

We present results of a Rayleigh and Love wave group velocity dispersion study of the Indo‐Eurasian collision zone. Group velocity dispersion curves are measured and combined to produce dispersion maps for 10–70 s period Rayleigh waves from 4054 paths and for 15–70 s Love waves from 1946 paths. Group velocity maps benefit from the inclusion of data recorded at a large number of stations within India, an advantage over previous global studies. This has the largest impact at short periods as a result of the improved path length distribution. Synthetic tests are used to estimate resolution, which ranges from 3° to 5° on the continents for Rayleigh wave maps and from 5° to 7.5° for Love wave maps. Group velocities correspond well with known geological and tectonic features and show good correlation with sediment thickness at short periods. The cratons of the Indian Shield can be distinguished in the short‐period and midperiod group velocities. Group velocities are slow across Tibet until 70 s whereas the cratonic cores of the Indian Shield appear as a high velocity anomaly at 70 s. Dispersion curves extracted from the Rayleigh wave group velocity maps are inverted for shear wave velocity as a function of depth for profiles across India and Tibet. The relationship between shear velocity contours and the Moho indicated by receiver function studies has been used to obtain a first‐order estimate of crustal thickness across the collision zone. Results suggest a slow Tibetan midcrust and low sub‐Moho velocities beneath the central and northeastern Tibetan Plateau.

Shear-wave velocity structure at Mt. Etna from inversion of Rayleigh-wave dispersion patterns (2 s < T < 20 s)

In the present study, we investigated the dispersion characteristics of medium-to-long period Rayleigh waves (2 s < T < 20 s) using both singlestation techniques (multiple-filter analysis, and phase-match filter) and multichannel techniques (horizontal slowness [p] and angular frequency [~] stack, and cross-correlation) to determine the velocity structure for the Mt. Etna volcano. We applied these techniques to a dataset of teleseisms, as regional and local earthquakes recorded by two broad-band seismic arrays installed at Mt. Etna in 2002 and 2005, during two seismic surveys organized by the Istituto Nazionale di Geofisica e Vulcanologia (INGV), sezione di Napoli. The dispersion curves obtained showed phase velocities ranging from 1.5 km/s to 4.0 km/s in the frequency band 0.05 Hz to 0.45 Hz. We inverted the average phase velocity dispersion curves using a non-linear approach, to obtain a set of shear-wave velocity models with maximum resolution depths of 25 km to 30 km. Moreover, the presence of lateral velocity contrasts was checked by dividing the whole array into seven triangular sub-arrays and inverting the dispersion curves relative to each triangle.

Numerical assessment of the effects of topography and crustal thickness on martian seismograms using a coupled modal solution–spectral element method

Abstract The past 4 decades of Mars exploration have provided much information about the Mars surface, when its interior structure remains relatively poorly constrained. Today available data are compatible with a large range of model parameters. Seismology is able to provide valuable additional data but the number of seismographs will likely be quite limited, specially in the early-stage of future Mars seismic networks. It is thus of importance to be able to correctly isolate effects induced by the crust structure. Mars topography is characterized by spectacular reliefs like the Tharsis bulge or the Hellas basin and by the so-called “Mars dichotomy”: the north hemisphere is made up of low-altitude plains above a relatively thin crust when the south hemisphere is characterized by a thick crust sustaining high reliefs. The aim of this paper is to study the effects induced on seismograms by the topography of the surface and crust–mantle discontinuities. Synthetic seismograms were computed using the coupled spectral element–modal solution method, which reduces the numerical cost by limiting the use of the spectral element method to the regions where lateral variations, like the presence of a topography, are considered. Due to numerical cost, this study is limited to long period and thus focuses on surface waves, mainly on long period Rayleigh waves. We show that reliefs like the Tharsis bulge or the Hellas basin can induce an apparent velocity anomaly up to 0.5% when only the surface topography is introduced. Apparent anomalies can raise up to 1.0% when the surface topography is fully compensated by a mirror-image topography of the crust–mantle discontinuity. Travel-time of surface wave are systematically increased for seismometers in the north hemisphere of Mars and decreased in the south hemisphere. When comparing effects on seismograms by the Earth and Mars topography, we found them to be larger for the Earth. It is due to the fact that we work with a seismic velocity model of Mars with a mean crust thickness of 110 km when the crust thickness has a mean value of 50 km for the Earth. When changing the Mars model for a thinner crust with a mean thickness of 50 km, effects by the topography on Mars seismograms becomes of the same order when not larger than what is observed on the Earth.

Shear wave velocity and attenuation structure for the shallow crust of the southern Korean peninsula from short period Rayleigh waves

We analyzed the short period Rayleigh waves from the first crustal-scale seismic refraction experiment in the Korean peninsula, KCRUST2002, to determine the shear wave velocity and attenuation structure of the uppermost 1 km of the crust in different tectonic zones of the Korean peninsula and to examine if this can be related to the surface geology of the study area. The experiment was conducted with two large explosive sources along a 300-km long profile in 2002. The seismic traces, recorded on 170 vertical-component, 2-Hz portable seismometers, show distinct Rayleigh waves in the period range between 0.2 s and 1.2 s, which are easily recognizable up to 30–60 km from the sources. The seismic profiles, which traverse three tectonic regions (Gyeonggi massif, Okcheon fold belt and Yeongnam massif), were divided into five subsections based on tectonic boundaries as well as lithology. Group and phase velocities for the five subsections obtained by a continuous wavelet transform method and a slant stack method, respectively, were inverted for the shear wave models. We obtained shear wave velocity models up to a depth of 1.0 km. Overall, the shear wave velocity of the Okcheon fold belt is lower than that of the Gyeonggi and Yeongnam massifs by ∼ 0.4 km/s in the shallowmost 0.2 km and by 0.2 km/s at depths below 0.2 km. Attenuation coefficients, determined from the decay of the fundamental mode Rayleigh waves, were used to obtain the shear wave attenuation structures for three subsections (one for each of the three different tectonic regions). We obtained an average value of Q β − 1 in the upper 0.5 km for each subsection. Q β − 1 for the Okcheon fold belt (∼ 0.026) is approximately three times larger than Q β − 1 for the massif areas (∼ 0.008). The low shear wave velocity in the Okcheon fold belt is consistent with the high attenuation in this region.

A Probability of Detection Method for Reducing Short-Period mb-Ms False Alarm Rates

The 20-sec m b− M s discriminant is one of the most reliable and best understood methods for identifying underground nuclear explosions. To extend the discriminant to lower magnitude thresholds at regional distances it is necessary to perform the M s measurement at shorter periods. Although short-period Rayleigh- wave measurements have lower detection thresholds than those at 20 sec, they are more influenced by the effects of earthquake depth that cause overlap of earthquake and explosion populations. We present a technique using a probability of detection model (pxd) to estimate the probability that a surface-wave detection came from an underground explosion. The key to the method is the development of a simple analytic model to predict the maximum expected amplitude probability distribution (upper tail) from an underground explosion of a given body-wave magnitude recorded at a specified distance. The model assumes full coupling and accounts for material effects, attenuation, and amplification from tectonic release. The detection classifier is applicable when there is a signal detection above a specified signal-to-noise cutoff and the detection is of greater amplitude than the maximum expected explosion amplitude. We assume that in general, earthquakes generate larger 6- to 12-sec Rayleigh-wave amplitudes than explosions for a given body-wave magnitude. For a given sensor we define the probability of detection given that the source was an explosion. Using Bayes’ Rule we determine the probability that the signal detection originated from an explosion. The surface-wave probability of detection curve for a given period and the prior probability that an explosion occurred can also be included in the formulation. We compute the conditional probability (represented as a p value) that the detected signal originated from an explosion. No assumptions regarding the non-explosion source are necessary other than the fact that the maximum amplitudes are expected to be greater than those from the explosion under full coupling conditions. Under the specified conditions, the p value will always be a small value indicating a low probability that the detected signal originated from an explosion. The p value can also be thought of as a random variable that can be combined with other discriminants in a multivariate setting. We show results of the signal detection formulation using short- period Rayleigh waves from earthquakes and explosions in Eurasia and compare to the traditional m b− M s at different periods. For a set of earthquakes and explosions recorded at wmq measured at a 6- to 12-sec period, false alarm rates are reduced from 28.3% for m b− M s to 18.6% using the probability of detection model. Using pxd by itself as an earthquake identifier, 78% of all events are assigned a p value resulting in a false alarm rate of 22% that is better than m b− M s alone for this dataset.

Rayleigh Waves Generated by Mining Explosions and Upper Crustal Structure around the Powder River Basin, Wyoming

Fundamental-mode, intermediate period Rayleigh waves generated by mining explosions are utilized to constrain the crustal structure of Wyoming. Broadband seismic stations recorded data during two regional seismic experiments conducted in Wyoming in 1996 and 1997. The stations were deployed in a ring surrounding the Powder River Basin (PRB) at ranges 100-360 km from the Black Thunder Coal Mine located in the northeast corner of Wyoming. Signals generated by four explosions with total explosive weight from 1.1 kiloton to 2.7 kiloton are analyzed with the goal of resolving the crustal structure in northeastern Wyoming. Data from the experiments show evidence that kiloton-sized millisecond-delay-fired cast blasts produce strong 1- to 15-sec period surface waves, some extending to 20-sec period. Group velocities of fundamental-mode Rayleigh waves were estimated using the Multiple Filter Analysis (MFA) technique and refined with Phase Matched Filtering (PMF). The surface waves are dependent on propagation direction and source orientation. The northwest-southeast trend of the PRB means that some propagation paths are parallel to the basin orientation and others perpendicular. Group velocities exhibit normal dispersion and range from 0.97 (1 sec) to 3.1 (20 sec) km/sec. A least square inversion technique was used to invert group velocity dispersion curves for the shallow shear-wave velocity structure. The average model consists of nine layers in the upper 20 km of the crust. The two top layers are thin (0.35 and 0.45 km) with slow velocities (1.0 to 1.6 km/sec) as a result of the basin sediments. At depths below 15 km, the shear velocity is nearly the same for paths parallel and perpendicular to the PRB axis. A possible low-velocity zone is found along the PRB axis with a velocity contrast of 0.7 km/sec at depth of 9.8 km to 14.8 km. These detailed models give a much better fit to dispersion curves than the global CRUST2.0, particularly for periods below 5 sec, which indicates the need for path-specific models when analyzing mid-period surface waves. Comparison of theoretical seismograms using the empirically derived structure with the observed seismograms provides a basis for constraining the source processes of these mining explosions. The generation of the intermediate-period surface waves is directly related to the total source time duration that is on the order of several seconds for these cast blasts. Observational data suggest that source orientation is important as well, resulting from some combination of mine free face orientation, blasting direction, and material casting. Manuscript received 24 July 2003.

Surface wave tomography for southeastern Asia using IRIS-FARM and JISNET data

Previous seismic studies for southeastern Asia around Indonesia have used global seismic data and thus included few regional broadband seismic stations. To better understand seismic structure of this area, we temporarily deployed Japan–Indonesia Seismic NETwork (JISNET) which was composed of 23 broadband seismic stations in central to western Indonesia since 1998 under the Super Plume Project of Japan. In this study we analyse regional JISNET data and global Incorporated Research Institute of Seismology (IRIS)-Fast Archive Recovery Method (FARM) data, in order to obtain detailed images of 3D S-wave velocity (Vs) of this area. We use vertical component LP seismograms of FARM data recorded from 1989 to 1993 (13471 seismograms) and vertical component of broadband JISNET data recorded from 1998 to 2000 (1782 seismograms). We select seismic events with body wave magnitudes greater than 5.5, and take hypocenters and centroid moment tensor solutions from the Harvard CMT catalogue. Phase velocity anomalies of fundamental mode Rayleigh waves (period from 34 to 197 s) relative to Preliminary Reference Earth Model (PREM) are determined by measuring phase differences between the synthetic waveforms and the observations. Global distribution of Rayleigh wave phase velocities are determined by block inversions using damped least squares with a finer block size of 5°×5° for the Indonesia region than the surrounding area. Firstly, we compare the results obtained from the inversion using the FARM data alone (called “FARM inversion”) and those from the inversion using both the FARM and the JISNET data (called “JISNET + FARM inversion”). For the fundamental Rayleigh waves of short periods, phase velocities obtained from the JISNET + FARM inversion show greater degrees of negative anomaly throughout Indonesia than those from the FARM inversion. The clearest difference is seen for the region around Kalimantan and Java: slower phase velocities of less than 1% relative to PREM at periods of about 70 s are obtained by the FARM inversion, whereas including JISNET data requires lower phase velocity anomalies of 2.5%. Secondly, we compare the resolution kernels of both data set. Around Java, Sumatra, Kalimantan, and Sulawesi the resolution of the JISNET + FARM inversion is substantially better than that of the FARM data alone, with the peaks of resolution kernels up to five times as high as those for the FARM inversion. The best resolution improvement is obtained for west Kalimantan to Java Sea. Thirdly, we invert the phase velocity perturbations for the modes to obtain the upper mantle 3D S-wave velocity (Vs) structure of the study area. The JISNET + FARM inversion tends to give larger (up to four times) negative Vs anomalies when compared with the FARM inversion. The Vs heterogeneities concentrate in the uppermost mantle shallower than 200 km throughout Indonesia. Beneath Sumatra, Java, and Kalimantan, the uppermost 100 km of the mantle is slower than PREM by about 2–3%. A prominent high Vs anomaly (2–4%) around the depths from 100 to 200 km extends from northern Australia, through the Banda Sea, to east Sulawesi and Halmahera. The subducted Indian plate is visible as a nearly 1% high Vs anomaly.

ANOMALOUS RAYLEIGH-WAVE PROPAGATION ALONG OCEANIC TRENCH

A clear later phase of amplitude larger than the direct surface wave packet was observed at stations in Hokkaido, Japan, for several events of the December 1991 off-Urup earthquake swarm in the Kuril Islands region. From its particle motion, this phase is likely to be a fundamental Rayleigh wave packet that arrived with an azimuth largely deviated from each great-circle direction. As its origin, Nakanishi (1992) proposed that the sea-trench topography in this area as deep as 10 km may produce a narrow zone of low velocity for Rayleigh waves of periods around 15 sec. Following this idea, we compute ray paths and estimate how Rayleigh waves would propagate if we assume that lateral velocity variations are caused only by seafloor topography. We confirm that thick sea water in the trench indeed produces the phase velocity of Rayleigh waves to be smaller than in a surrounding area by the degree over 100%. Such a low-velocity zone appears only in a period range from 12 to 20 sec. Although this strong low-velocity zone disturbs the direction of Rayleigh wave propagation from its great circle, the overall ray paths are not so affected as far as an epicentre is outside this low-velocity zone, that is, off the trench axis. In contrast, the majority of rays are severely distorted for an event within the low-velocity zone or, in other words, in the neighborhood of the trench axis. For such an event, a part of wave energy appears to be trapped in this zone and eventually propagates outwards due to the curvature or bend of trench geometry, resulting in very late arriving waves of large amplitude with an incident direction clearly different from great circles. This phenomenon is observed only at a very limited period range around 16 sec. These theoretical results are consistent with the above mentioned observation of Nakanishi (1992).

Surface wave tomography of the crust and upper mantle of Chinese mainland and its neighboring region

The three dimensional S wave velocity structure of the crust and upper mantle of Chinese mainland and its neighboring region is obtained by genetic algorithm of surface wave tomography, with smoothness constraint, based on 25 wave group velocities for the periods from 10 s to 92 s, measured from long period Rayleigh waves recorded by 11 stations of CDSN and 12 digital seismometers surrounding China. The S wave velocity image is shown on two latitudinal sections along 30°N and 38°N, two longitudinal sections along 90°E and 120°E, and four horizontal slices at the different depths.

The upper and middle crustal velocity structure of the northern part of Hebei plain inferred from short period surface wave dispersion

Based on short period Rayleigh wave data recorded by Beijing Seismic Telemetered Network, the dispersion curves of Raleigh wave phase velocity, with period from 2 s to 18 s, are calculated by means of two-station method, for 5 paths across the earthquake zone located in the Beijing graben and the Hebei plain. According to the dispersion features, the upper and middle crustal S wave velocity structures are respectively obtained for the northern segment of Beijing graben and the northern part of Hebei plain. The results show that there is an obvious interface at the depth of 9 km in the Beijing graben, the velocity varies little with depth in the middle crust, and there is a low-velocity-zone, with a thickness of 5 km and a buried depth of 14.6 km, in the middle crust of the Hebei plain.