Determination of Site-Dependent Scattering Q From P-Wave Coda Analysis With an Energy-Flux Model     

Michael Korn 《Geophysical Journal International》1993,113(1):54-72

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The complexity of the crust from refraction/wide-angle reflection data   总被引：3，自引：0，他引：3  

R. F. Mereu 《Pure and Applied Geophysics》1990,132(1-2):269-288

ThePmP wide-angle reflected signal from the Moho shows a wide variation in its characteristics when cornpared from region to region. If the earth's crust is simple and homogeneous and the Moho is a sharp discontinuity, supercriticalPmP wavelets are large and isolated in the 100 km to 300 km distance range when compared to the preceding signals on the seismic traces. If the crust itself has numerous short reflectors and the Moho transition zone is thick or badly disrupted from past tectonic disturbances, thenPmP is often poorly defined and difficult to identify in the coda of earlier arriving signals. These signals are poorly correlated because the reflectors themselves are in general not only discontinuous but tend to be distributed at various depths within the crust. The effect of the vertical velocity gradient in the crust is to make the reflective field for downgoing waves much greater than for upgoing waves. The large variations in reflection coefficients wiht angle of incidence has the effect of making the coda generated by a reflective machanism distance dependent. The reflected signals are also contaminated with scattered signals from smaller scale heterogeneities which may be distributed more uniformly within the crust.In conventional seismic inversion methods applied to crustal refraction experiments, emphasis is usually placed on obtaining a velocity model of the subsurface structure. These models are relatively simple and are limited in the amount of complexity which can be uniquely inferred from the data. The main problem arises because of the difficulties in the identification of the origin of the signals which are themselves often incoherent from trace to trace.In this paper it is shown how the conventional record section may be complemented with a normal moveout corrected intensity section which emphasizes areas of large signal complexities. Data from this intensity section is then used as input to obtain a quantitative measurement of a complexity parameter. These measurements may be used to infer or compare differences in crustal heterogeneity from one region to another. The discussion is illustrated with both numerical modeling data as well as data from recent crustal experiments which were conducted over the Canadian Shield.  相似文献   

Separation of scattering and intrinsic attenuation in southern Spain and western Anatolia (Turkey)   总被引：2，自引：0，他引：2  

A. Akinci  E. Del  Pezzo J. M. Ibáñez 《Geophysical Journal International》1995,121(2):337-353

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Vertical profile of turbidity and coda Q     

Alexander A. Gusev 《Geophysical Journal International》1995,123(3):665-672

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Changes in the early part of the seismic coda due to localized scatterers: The estimation ofQ in a stope environment     

A. Cichowicz  R. W. E. Green 《Pure and Applied Geophysics》1989,129(3-4):497-511

A single scattering model was used to analyse the temporary changes in the mean density of scattered waves in a discrete random medium. The model of the mean energy density, originally proposed bySato (1977) for spherical radiation and isotropic scattering, has been modified and applied to a medium in which the scatterers are confined to a specified volume. The time variation of the early part of the mean energy density function for the different source durations was investigated. The dominant effect on the theoretical mean energy density is caused by the specified volume containing scatterers. The duration of the source pulse influences the early part of the coda fort/t ₀<1.2, wheret is the lapse time measured from the source origin time, andt ₀is arrival time of the body wave.The analysis of the coda signal of micro-events occurring immediately in front of the face enables us to estimate the size of the fracture zone induced by the stope. The model of the mean energy density of coda for a medium containing scatterers close to the seismic source was used to analyse a large number of events recorded close to an advancing mine face in a deep level gold mine in South Africa. The coda decay rate has two trends: the first, with a steep decay of coda, is produced by a larger deviation of rock parameters and/or larger size of the scatterers; the second trend, which decays more slowly, has the corresponding mean-free path ranging from 20 m to 200 m. The analysis indicates that the rock mass about 15–20 m from the stope contains a large proportion of fractured and blocked rock, which is the source of scattering. The scattering of theS-wave was much stronger and more stable, with the mean-free path varying from 11 m to 45 m. This is due to the shorter wavelength of theS wave in comparison with theP wave. The quality factor for theP coda wave varies from 30 to 100 in the fracture zone of stope and outside this zone it has a value of 300. The quality factor of theS wave varies from 20 to 78 in the equivalent volume. For rock surrounding the stope the ratioQ _sp ^–1 /Q _ss ^–1 varied from 0.31 to 0.69. This suggests that the radii of scatterers are smaller than 3.5 m.  相似文献   

Spatial distribution of scatterers in the crust by inversion analysis of coda envelopes: a case study of Gauribidanur seismic array (Southern India)     

A. Ugalde  E. Carcolé  J. N. Tripathi 《Geophysical Journal International》2006,166(2):782-794

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Temporal Changes of Shallow Seismic Velocity Around the Karadere-Düzce Branch of the North Anatolian Fault and Strong Ground Motion     

Zhigang Peng  Yehuda Ben-Zion 《Pure and Applied Geophysics》2006,163(2-3):567-600

We analyze temporal variations of seismic velocity along the Karadere-Düzce branch of the north Anatolian fault using seismograms generated by repeating earthquake clusters in the aftershock zones of the 1999 M_w7.4 İzmit and M_w7.1 Düzce earthquakes. The analysis employs 36 sets of highly repeating earthquakes, each containing 4–18 events. The events in each cluster are relocated by detailed multi-step analysis and are likely to rupture approximately the same fault patch at different times. The decay rates of the repeating events in individual clusters are compatible with the Omori's law for the decay rate of regional aftershocks. A sliding window waveform cross-correlation technique is used to measure travel time differences and evolving decorrelation in waveforms generated by each set of the repeating events. We find clear step-like delays in the direct S and early S-coda waves (sharp seismic velocity reduction) immediately after the Düzce main shock, followed by gradual logarithmic-type recoveries. A gradual increase of seismic velocities is also observed before the Düzce main shock, probably reflecting post-seismic recovery from the earlier İzmit main shock. The temporal behavior is similar at each station for clusters at various source locations, indicating that the temporal changes of material properties occur in the top most portion of the crust. The effects are most prominent at stations situated in the immediate vicinity of the recently ruptured fault zones, and generally decrease with normal distance from the fault. A strong correlation between the co-seismic delays and intensities of the strong ground motion generated by the Düzce main shock implies that the radiated seismic waves produced the velocity reductions in the shallow material.  相似文献   

Scattered Surface Wave Energy in the Seismic Coda     

Yuehua Zeng 《Pure and Applied Geophysics》2006,163(2-3):533-548

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   

Lg coda Q0 value and its relation with the tectonics in Chinese mainland and adjacent regions     

苏伟吴建平  明跃红王椿镛 《地震学报(英文版)》2006,19(2):136-144

We have collected 432 vertical component records from 45 stations of new CENC （China Earthquake Network Center） in Chinese mainland and adjacent regions. These records were used to calculate Q0 （Q at 1Hz） and η values of Lg coda from each station by the stack spectral ratio （SSR） method. Then the tomography method was applied to obtaining lateral variation of Q0 and η values in Chinese mainland and adjacent regions. The result indicates that Q0 value varies between 150 and 600 in the studied areas. Yunnan, southwest Sichuan, and northwest Myanmar show the lowest Q0 value （Q0〈240） and the crust of these regions is characterized by complicated crack and strong hydrothermal activity. The highest Q0 value （Qo〉510） exists in the border of southern Mongolia, Alxa and Ordos block. The η value varies between 0.45 and 0.75 in Chinese mainland and its adjacent regions.  相似文献   

Size and duration of the high-frequency radiator in the source of the 2004 December 26 Sumatra earthquake     

A. A. Gusev  E. M. Guseva  G. F. Panza 《Geophysical Journal International》2007,170(3):1119-1128

We recover the gross space–time characteristics of high-frequency (HF) radiator of the great Sumatra-Andaman islands earthquake of 2004 December 26 ( M _w= 9.1–9.3) using the time histories of the power of radiated HF P waves. To determine these time histories we process teleseismic P waves at 36 BB stations, using, in sequence: (1) bandpass filtering (four bands: 0.4–1.2, 1.2–2, 2–3 and 3–4 Hz); (2) squaring wave amplitudes, making 'power signals' for each band and (3) stripping the propagation-related distortion ( P coda, etc.) from the power signal and thus recovering source time function for HF power. In step (3) we employ an inverse filter constructed from an empirical Green's function, which is estimated as the power signal from an aftershock. For each ray we thus obtain signals with relatively well-defined end and no coda. From these signals we extract: total duration (joint estimate for all four bands) and temporal centroid of signal power for each band. Through linear inversion, the set of duration values for a set of rays delivers estimates of the rupture stopping point and stopping time. Similarly, the set of temporal centroids can be inverted to obtain the position of the space–time centroid of HF energy radiator. The quality of inversion for centroid is acceptable for lower-frequency bands but deteriorates for higher-frequency bands where only a fraction of stations provide useful data. For the source length and duration the following joint estimates were obtained: 1241 ± 224 km, 550 ± 10 s. The estimated stopping point position corresponds to the northern extremity of the aftershock zone. Spatial HF radiation centroids are located at distances 350–700 km from the epicentre, in a systematic way: the higher is the frequency, the farther is the centroid from the epicentre. Average rupture propagation velocity is estimated as 2.25 km s^–1.  相似文献