Precursor and coda portions of short-period PcP waves (reflected P wave from the core-mantle boundary, CMB) recorded at J-array stations in Japan were analyzed in order to extract weak scattered signals originating from small-scale heterogeneities in the lowermost mantle beneath northeastern China. Two nuclear explosions at Lop Nor in China detonated on 21 May 1992 (Mb=6.5) and 8 June 1996 (Mb=5.9) were used for our analysis.Three-dimensional grids above the CMB were defined in the area around the PcP bounce points beneath northeastern China to calculate theoretical travel times of scattered waves which propagate from the sources to each grid point and arrive at each station based on the IASP91 model. Subsequently the waveforms were aligned with respect to the theoretical travel times and the semblance (an amplitude dependent measure of coherency) was calculated for each grid point. In order to obtain a more accurate travel time correction, we applied a cross correlation method to PcP waveforms in order to reduce picking error of the PcP onset time. A cross convolution method was also applied so that the two events could be analyzed simultaneously without using unstable deconvolutions.We could identify regions with relative high semblance values in semblance contour maps at about 200 and 375 km above the CMB. Stacking waveforms with respect to the theoretical travel times for the grid points with relative high semblance values indicate coherent wavelets originating at those grid points, that is, they correspond to scattered waves originating from small-scale heterogeneities in the lowermost mantle. Our results indicate the existence of small-scale scattering objects in the D″ layer, especially in the depth range of 200 and 375 km above the CMB beneath northeastern China. Considering recent tomographic images of high velocity anomalies in this area, these scattering objects could be fragments of old oceanic crusts which have subducted through the lower mantle and have accumulated in the D″ layer beneath northeastern China. 相似文献
The earlyP wave coda (5–15 sec after the first arrival) of underground explosions at the Nevada Test Site is studied in the time domain using 2082 teleseismic short-period recordings, with the intent of identifying near-source contributions to the signals in the frequency range 0.2–2.0 Hz. Smaller magnitude events tend to have relatively high coda levels in the 0.4–0.8 Hz frequency band for both Yucca Flat and Pahute Mesa explosions. Coda complexity in this low-frequency passband is negatively correlated with burial depth for Pahute Mesa events but is only weakly correlated with depth for Yucca Flat events. Enhanced excitation of relatively long-period scattered waves for smaller, less deeply buried events is required to explain this behavior. Coda complexity in the 0.8–1.1 Hz band is positively correlated with magnitude and depth for Pahute Mesa events, but has no such dependence for Yucca Flat events. This may result from systematic variations between the spectra of direct signals and coda arrivals caused bypP interference for the largest events, all of which were detonated at Pahute Mesa. Another possible explanation is a frequency-dependent propagation effect on the direct signals of the larger events, most of which were located in the center of the mesa overlying strong lateral velocity gradients in the crust and upper mantle. Event average complexity varies spatially for both test sites, particularly in the 0.8–1.1 Hz band, providing evidence for frequency-dependent focussing or scattering by near-source structure. Both the direct arrivals and the early coda have strong azimuthal amplitude patterns that are produced by defocussing by mantle heterogeneity. The direct arrivals have stronger coherent azimuthal patterns than the early coda for Pahute Mesa events, indicating more pronounced deep crustal and shallow mantle defocussing for the direct signals. However, for Yucca Flat events the direct arrivals have less coherent azimuthal patterns than the coda, suggesting that a highly variable component of near-source scattering preferentially affecting the downgoing energy is superimposed on a pattern produced by mantle heterogeneity that affects the entire signal. This complicated behavior of the direct arrivals may be the result of triplications and caustics produced by the complex basement structure known to underlie the Yucca Flat test site. The presence of strong azimuthal patterns in the early coda indicates that source strength estimates based on early coda are subject to biases similar to those affecting estimates based on direct arrivals. 相似文献
The application of standard array processing techniques to the study of coda presents difficulties due to the design criteria of these techniques. Typically the techniques are designed to analyze isolated, short arrivals with definite phase velocity and azimuth and have been useful in the frequency range around 1 Hz. Coda is long in time and may contain waves of different types, phase velocities and azimuths. Nonetheless, it has proved possible to use or adapt array methods to answer two questions: what types of waves are present in coda and where are they scattered? Most work has been carried out on teleseismicP coda; work on local coda has lagged due to lack of suitable data and the difficulties of dealing with high frequencies. The time domain methods of beamforming and Vespagram analysis have shown that there is coherent energy with a high phase velocity comparable toP orPP in teleseismicP coda. These methods can detect this “coherent” coda because it has a fairly definite phase velocity and the same, or close to, azimuth as firstP orPP. This component must consist ofP waves and is either scattered near the source, or reflected in the mantle path as apdpP or precursorPP reflection. The Fourier transform method of the frequency-wavenumber spectrum has been adapted by integrating around circles of constant phase velocity (constant total wavenumber) to produce the wavenumber spectrum, which shows power as a function of wavenumber, or phase velocity. For teleseismicP coda, wavenumber spectra demonstrate that there is a “diffuse” coda of shear,Lg or surface waves scattered from teleseismicP near the receiver. Wavenumber spectra also suggest that the coherent coda is produced by near-source scattering in the crust, not mantle reflection, since it is absent or weak for deep-focus events. Crustal earthquakes have a very strong coherent component of teleseismic coda, suggesting scattering from shear to teleseismicP near the source. Three-component analysis of single-station data has shown the presence of off-azimuth arrivals and may lead to the identification of waves scattered from a single scatterer. 相似文献
The hypothesis of scattering of PKP waves near the mantle-core boundary provides a comprehensive interpretation of the observed precursors to PKIKP, certain features of which have not been adequately accounted for by any alternative interpretation. These features include the variation with distance of the times and slownesses of precursor onsets, and the variations in amplitude, azimuth and slowness observed in precursor wavetrains.The observed times and slownesses of the earliest precursor onsets are in close agreement with the theoretical least time curve for singly scattered waves.Amplitudes and slownesses of scattered waves have been calculated for earth models which are spherically symmetrical except for random variations in density and elastic parameters in a layer 200 km thick at the base of the mantle. The calculations show that observed precursor amplitudes and slownesses can be accounted for by random variations of about one percent having a correlation distance of about 30 km in this layer. In particular, it is shown that scattering structures up to 900 km above the mantle-core boundary inferred by Doornbos and Vlaar (1973) are not required by their data. There is a suggestion that the main scattering may actually occur inside a layer much less than 200 km thick at the base of the mantle. 相似文献
Coda waves are sensitive to changes in the subsurface because the strong scattering that generates these waves causes them
to repeatedly sample a limited region of space. Coda wave interferometry is a technique that exploits this sensitivity to
estimate slight changes in the medium from a comparison of the coda waves before and after the perturbation. For spatially
localized changes in the velocity, or for changes in the source location, the travel-time perturbation may be different for
different scattering paths. The coda waves that arrive within a certain time window are therefore subject to a distribution
of travel-time perturbations. Here I present the general theory of coda wave interferometry, and show how the time-shifted
correlation coefficient can be used to estimate the mean and variance of the distribution of travel-time perturbations. I
show how this general theory can be used to estimate changes in the wave velocity, in the location of scatterer positions,
and in the source location. 相似文献
This paper reviews applications of the finite-difference and finite-element methods to the study of seismic wave scattering in both simple and complex velocity models. These numerical simulations have improved our understanding of seismic scattering in portions of the earth where there is significant lateral heterogeneity, such as the crust. The methods propagate complete seismic wavefields through highly complex media and include multiply scattered waves and converted phases (e.g.,P toSV, SV toP, body wave to surface wave). The numerical methods have been especially useful in cases of moderate and strong scattering in complex media where multiple scattering becomes important. Progress has been made with numerical methods in understanding how near-surface, low-velocity basin structures scatter surface waves and vertically-incident body waves. The numerical methods have proven useful in evaluating scattering of surface waves and body waves from topography of both the free surface and interfaces buried at depth. Numerical studies have demonstrated the importance of conversions from body waves to surface waves (andvice versa) when lateral heterogeneities and topographic relief are present in the uppermost crust. Recently, several investigations have applied numerical methods to study seismic wave propagation in velocity models which vary randomly in space. This stochastic approach seeks to understand the effects of small-scale complexity in the earth which cannot be resolved deterministically. These experiments have quantified the relationships between the statistical properties of the random heterogeneity and the measurable properties of high-frequency (1 Hz) seismograms. These simulations have been applied to the study of many features observed in actual high-frequency seismic waves, including: the amplitude and time decay of seismic coda, the apparent attenuation from scattering, the dispersion of waveforms, and the travel time and waveform variations across arrays of receivers. 相似文献
The analysis and interpretation of coda waves have received increasing attention since the early seventies. In the past few years interest in this subject has spread worldwide, and the study of high-frequency seismic coda waves has become a very important seismological topic. As a conclusion of the studies accomplished in this time, coda waves are considered the result of scattering processes caused by heterogeneities acting on seismic waves.P andS waves play a particularly important role in this interaction. The process introduces an attenuation which, added to the intrinsic absorption, gives the observed apparent attenuation. Therefore, coda waves constitute a thumbprint left by the heterogeneities on the seismograms. Coda waves offer decisive information about the mechanism of how scattering and attenuation take place. This review describes coda waves in detail, and summarizes the work done in this subject to 1986. The relation between coda waves and attenuation in the context of research on seismic scattering problems is stressed. Particular attention has been given to the application of coda waves to estimate source and medium parameters. The state-of-the-art of the temporal variations of coda wave shape, and the possible use of these variations as an earthquake precursor also are presented. Care has been taken to introduce the statistical models used to deal with the heterogeneities responsible for scattering. 相似文献
One of the many important contributions that Aki has made to seismology pertains to the origin of coda waves (Aki, 1969; Aki
and Chouet, 1975). In this paper, I revisit Aki's original idea of the role of scattered surface waves in the seismic coda.
Based on the radiative transfer theory, I developed a new set of scattered wave energy equations by including scattered surface
waves and body wave to surface wave scattering conversions. The work is an extended study of Zeng et al. (1991), Zeng (1993) and Sato (1994a) on multiple isotropic-scattering, and may shed new insight into the seismic coda wave
interpretation. The scattering equations are solved numerically by first discretizing the model at regular grids and then
solving the linear integral equations iteratively. The results show that scattered wave energy can be well approximated by
body-wave to body wave scattering at earlier arrival times and short distances. At long distances from the source, scattered
surface waves dominate scattered body waves at surface stations. Since surface waves are 2-D propagating waves, their scattered
energies should in theory follow a common decay curve. The observed common decay trends on seismic coda of local earthquake
recordings particular at long lapse times suggest that perhaps later seismic codas are dominated by scattered surface waves.
When efficient body wave to surface wave conversion mechanisms are present in the shallow crustal layers, such as soft sediment
layers, the scattered surface waves dominate the seismic coda at even early arrival times for shallow sources and at later
arrival times for deeper events. 相似文献
Aki (1969) first modeled coda waves of a local earthquake as a superposition of scattered surface waves. This paper attempts
to clarify the constituents of surface-wave coda at long periods at very long lapse times. For a large earthquake of magnitude
7 or larger, vertical component oscillation in periods from 90 to 180 s persists for more than 20 hours from the earthquake
origin time. Although the early portion of the coda envelope is successfully modeled by assuming incoherent scattered Rayleigh
waves by heterogeneities distributed all over the Earth, the later potion of the observed coda envelope (roughly later than
35,000 s) has systematically larger amplitude than theoretical prediction. To clarify the cause of this discrepancy, we studied
the constituents of vertical-component seismograms of three large earthquakes recorded by the F-net in Japan using the f-k power spectral analysis. We found that the direct and scattered fundamental-mode Rayleigh waves of velocity about 3.7 km/s
are dominant in the earlier part of each envelope. It justifies the use of a scattering model of the fundamental Rayleigh
waves for synthesizing the envelope. At lapse times later than 20,000 s–35,000 s, higher modes with phase velocities around
20 km/s become dominant. The transition time to the dominance of higher modes is found to become earlier for a deeper focus
earthquake. The small coda attenuation factor from (1.90±0.23) × 10−3 to (2.38±0.32) × 10−3 estimated from later coda envelopes recorded at IRIS stations distributed worldwide also agrees with the attenuation factor
of spheroidal modes according to PREM. We may interpret that higher mode waves are uniformly distributed at large lapse time
due to large velocity dispersion and/or scattering and they dominate over the fundamental mode waves because of smaller attenuation
in the lower mantle. The coda attenuation measurement proposed by Aki is found to be useful even for long periods and at very
large lapse times. 相似文献
Earthquake events from the Indonesian subduction zone recorded in northern Australia show a long and high-frequency coda associated with both P and S waves. Regional events recorded by Warramunga array in northern Australia can separate out wave propagation through the mantle by focusing on the coherent signal across the medium-aperture array. Most of the incoherent wave components result from structures in the vicinity of the array with small-scale lengths of 1–2 km or smaller. The coherent phases with relatively rapid changes in waveforms are associated with the scattering of seismic waves by crustal and mantle heterogeneity, but in some case can be related to structural effects near the source. As the depth of the source increases, the coherent portion of the seismic wavefield tends to become much simpler, which suggests that the heterogeneity tends to weaken at depth with larger-scale length. We compare the coherent signal features of earthquakes from the Indonesian subduction zone that have occurred in recent years with those in the early 1980s, first studied by Kennett (Phys Earth Planet Inter 47: 319–332, 1987). The general characteristics of the coherent signal variation with depth in recent years are the same as those observed in 1980s, but the variations are larger. This change suggests a stringer variation in heterogeneity with depth than before, which may bear important information about the dynamic processes and evolution of the crust and upper mantle. 相似文献
We use a total of 839,369 PcP, PKPab, PKPbc, PKPdf, PKKPab, and PKKPbc residual travel times from [Bull. Seism. Soc. Am. 88 (1998) 722] grouped in 29,837 summary rays to constrain lateral variation in the depth to the core-mantle boundary (CMB). We assumed a homogeneous outer core, and the data were corrected for mantle structure and inner-core anisotropy. Inversions of separate data sets yield amplitude variations of up to 5 km for PcP, PKPab, PKPbc, and PKKP and 13 km for PKPdf. This is larger than the CMB undulations inferred in geodetic studies and, moreover, the PcP results are not readily consistent with the inferences from PKP and PKKP. Although the source-receiver ambiguity for the core-refracted phases can explain some of it, this discrepancy suggest that the travel-time residuals cannot be explained by topography alone. The wavespeed perturbations in the tomographic model used for the mantle corrections might be too small to fully account for the trade off between volumetric heterogeneity and CMB topography. In a second experiment we therefore re-applied corrections for mantle structure outside a basal 290 km-thick layer and inverted all data jointly for both CMB topography and volumetric heterogeneity within this layer. The resultant CMB model can explain PcP, PKP, and PKKP residuals and has approximately 0.2 km excess core ellipticity, which is in good agreement with inferences from free core nutation observations. Joint inversion yields a peak-to-peak amplitude of CMB topography of about 3 km, and the inversion yields velocity variations of ±5% in the basal layer. The latter suggests a strong trade-off between topography and volumetric heterogeneity, but uncertainty analyses suggest that the variation in core radius can be resolved. The spherical averages of all inverted topographic models suggest that the data are best fit if the actual CMB radius is 1.5 km less than in the Earth reference model used (i.e. the average outer core radius would be 3478 km). 相似文献
Scattering by a slightly-rough core-mantle boundary (CMB) with small-scale radial variations of up to a few hundred metres, has been an attractive (though non-unique) interpretation of at least part of the precursors to PKIKP. Here it is shown that a slightly-rough CMB has an observable effect on PKKP as well, if the signal-to-noise ratio is sufficiently high. The effect may be observed as precursive arrivals and is due to back-scattering at CMB. This work was prompted by observations by Chang and Cleary at LASA of “PKKP” and precursors from the Novaya Zemlya explosions. NORSAR data from several source regions are presented here; small-scale radial variations of 100–200 metres are inferred from these data, although in some regions the CMB appears to be much smoother. On the other hand, the LASA data are anomalous and suggest much larger topography in the sampled region of the CMB. Both large- and small-scale topography must be dynamically produced, if current estimates of the viscosity of the lower mantle (～1022 Poise) are correct. 相似文献
The quality factors of coda and shear waves have been estimated for the SE Sabalan Mountain, geothermal region in northwestern
Iran. We have analyzed 65 local earthquakes with magnitude of 2.8 to 6.1 and 2.8 to 5 for shear and coda wave quality factor
estimation, respectively. These events were recorded on five stations installed by Building and Housing Research Center Network.
Coda normalization and Spectral decay methods have been used to estimate the frequency dependence attenuation relation for
shear wave, and single back-scattering method for coda waves. We have observed that the coda normalization method has supplied
significantly higher QS values as compared to the spectral method. The results show that, in general, Q values are significantly smaller for the entire frequency range as compared to tectonically active areas and are close to
the values for volcanic areas. 相似文献
The phenomenon of the seismic coda, which is composed of seismic energy delayed by scattering, is seen on both the Earth and the Moon. On the Moon the scattered coda is very large relative to body wave arrivals with a delay of the time of maximum energy, whereas on Earth scattered codas are relatively small and show no delay of the energy maximum. In both cases the form of the coda is controlled by three distance scales, the mean free path L, which is the average distance seismic energy travels before it is scattered, the attenuation distance , which is the average distance seismic energy travels before it is attenuated, and the source-receiver distance R. Two coda models are discussed based on these parameters; a strong scattering (diffusion) model, and a weak scattering (single scattering) model. A discussion of the diffusion scattering model indicates that if , diffusion scattering is an appropriate model, but if , single scattering is the appropriate model, within the appropriate range of R. A survey of the literature indicates that for the frequency range 0.5–10 Hz, diffusion scattering is important in lunar codas, but for the frequency range 1–25 Hz single scattering is important in terrestrial codas. Another important effect of attenuation is the elimination of scattering paths much longer than . On the Moon, this means that seismic energy in the coda can only propagate directly in the near-surface strong scattering zone between surface sources and the seismometer for source-seismometer separations of the order of ; otherwise, scattering is limited to regions near the source and the receiver. On Earth, this effect probably prevents multiple scattering. 相似文献
Numerical modelling ofSH wave seismograms in media whose material properties are prescribed by a random distribution of many perfectly elastic cavities and by intrinsic absorption of seismic energy (anelasticity) demonstrates that the main characteristics of the coda waves, namely amplitude decay and duration, are well described by singly scattered waves in anelastic media rather than by multiply scattered waves in either elastic or anelastic media. We use the Boundary Integral scheme developed byBenites
et al. (1992) to compute the complete wave field and measure the values of the direct waveQ and coda wavesQ in a wide range of frequencies, determining the spatial decay of the direct wave log-amplitude relation and the temporal decay of the coda envelope, respectively. The effects of both intrinsic absorption and pure scattering on the overall attenuation can be quantified separately by computing theQ values for corresponding models with (anelastic) and without (elastic) absorption. For the models considered in this study, the values of codaQ–1 in anelastic media are in good agreement with the sum of the corresponding scatteringQ–1 and intrinsicQ–1 values, as established by the single-scattering model ofAki andChouet (1975). Also, for the same random model with intrinsic absorption it appears that the singly scattered waves propagate without significant loss of energy as compared with the multiply scattered waves, which are strongly affected by absorption, suggesting its dominant role in the attenuation of coda waves. 相似文献
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