Estimates of coda Q attenuation in the João Câmara area (Northeastern Brazil)     

André Pereira Dias  Jorge Luis de Souza 《Journal of Seismology》2004,8(2):235-246

S coda wave of seventy-four local earthquakes recorded in a network of ten seismic stations were used to calculate coda Q attenuation (Q_c) in the João Câmara area (northeastern Brazil). The estimates show Q_c as a strong function of frequency in the range from 6.0 to 20.0 Hz. We found out that Q_c in João Câmara has a functional form given by Q_c= Q₀ f, where Q₀= 151 ± 99 and = 0.98 ± 0.05. If the standard deviations are taken into account,we conclude that there are no relevant changes in both Q₀ and values from one station to another. The estimated Q₀ values at the different stations suggest that the Samambaia fault is a boundary between two different seismic attenuation zones. In one side of the fault (left), where stations were installed in Pre-Cambrian terrain and thick sedimentary layer, the seismic attenuation is stronger than in the other side (stations installed in thin sedimentary layer and limestone outcrop).The anomalous Q₀ values in the left side of the Samambaia fault can be explained due to the presence of a shallow conductive layer in the upper crust( 10 km), such as proposed by Padilha et al. (1992). According to our results, if there is a conductive layer in the area, it probably spreads over João Câmara city and surrounding regions.However, more detailed investigation either with seismic methods (seismic attenuation,3D tomography with P and/or S wave velocities) or with other geophysical methods is needed to interpret the observed differences in Q₀ values between the two sides of the Samambaia fault.  相似文献   

Relation between mainshock rupture process and Omori's law for aftershock moment release rate   总被引：5，自引：0，他引：5  

Yan Y. Kagan  Heidi Houston 《Geophysical Journal International》2005,163(3):1039-1048

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Mapping random lithospheric heterogeneities in northern and central Europe   总被引：1，自引：0，他引：1  

S. Hock  M. Korn  J. R. R. Ritter  E. Rothert † 《Geophysical Journal International》2004,157(1):251-264

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Scattering of waves in elastic media containing passive and active cracks     

Masatoshi Miyazawa  Ichiro Nakanishi 《Geophysical Journal International》2006,165(2):658-667

Scattering of wavefields in a 3-D medium that includes passive and/or active structures, is numerically solved by using the boundary integral equation method (BIEM). The passive structures are velocity anomalies that generate scattered waves upon incidence, and the active structures contain endogenous fracture sources, which are dynamically triggered by the dynamic load due to the incident waves. Simple models are adopted to represent these structures: passive cracks act as scatterers and active cracks as fracture sources. We form cracks using circular boundaries, which consist of many boundary elements. Scattering of elastic waves by the boundaries of passive cracks is treated as an exterior problem in BIEM. In the case of active cracks, both the exterior and interior problems need to be solved, because elastic waves are generated by fracturing with stress drop, and the growing crack boundaries scatter the incident waves from the outside of the cracks. The passive cracks and/or active cracks are randomly distributed in an infinite homogeneous elastic medium. Calculations of the complete waveform considering a single scatter show that the active crack has weak influence on the attenuation of first arrivals but strong influence on the amplitudes of coda waves, as compared with those due to the passive crack. In the active structures, multiple scattering between cracks and the waves triggered by fracturing strongly affect the amplitudes of first arrivals and coda waves. Compared to the case of the passive structures, the attenuation of initial phase is weak and the coda amplitudes decrease slowly.  相似文献   

The attenuation mechanism of seismic waves in northwestern Himalayas   总被引：3，自引：0，他引：3  

S. Mukhopadhyay  C. Tyagi  S. S. Rai 《Geophysical Journal International》2006,167(1):354-360

We analysed local earthquake waveforms recorded on a broad-band seismic network in northwestern Himalayas to compute the intrinsic and scattered attenuation parameters from coda waves. Similar to other tectonically active and heterogeneous regions, attenuation-frequency relation for western Himalaya is   Q ⁻¹ _c = (113 ± 7)  f ^(1.01±0.05)  where   Q_c   is the coda Q parameter. Intrinsic  ( Q ⁻¹ _i )  and scattering  ( Q ⁻¹ _s )  attenuations was separated using   Q_c   and direct S -wave Q data  ( Q_d )  . It is observed that estimated   Q ⁻¹ _c   is close to   Q ⁻¹ _i   and both of them are much larger than   Q ⁻¹ _s   suggesting that coda decay is predominantly caused by intrinsic attenuation. At higher frequencies, both the attenuation parameters   Q_c   and,   Q_d   are similar indicating that coda is predominantly composed of back-scattered S waves at these frequencies.  相似文献   

The Theory of Coda Wave Interferometry   总被引：7，自引：0，他引：7  

Roel Snieder 《Pure and Applied Geophysics》2006,163(2-3):455-473

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   

Estimation of frequency dependent coda wave attenuation structure at the vicinity of Cairo Metropolitan Area   总被引：1，自引：0，他引：1  

Salah El-Hadidy  M. E. Mohamed Adel  Ahmed Deif  Ahmed Sayed Abu El-Ata  S. R. Moustafa Sayed 《Acta Geophysica》2006,54(2):177-186

Estimation of seismic wave attenuation in the shallow crust in terms of coda wave Q structure previously investigated in the vicinity of Cairo Metropolitan Area was improved using seismograms of local earthquakes recorded by the Egyptian National Seismic Network. The seismic wave attenuation was measured from the time decay of coda wave amplitudes on narrow bandpass filtered seismograms based on the single scattering theory. The frequency bands of interest are from 1.5 to 18 Hz. In general, the values obtained for various events recorded at El-Fayoum and Wadi Hagul stations are very similar for all frequency bands. A regional attenuation law Q _c = 85.66 f ^0.79 was obtained.  相似文献   

Phase and Correlation in `Random' Seismic Fields and the Reconstruction of the Green Function   总被引：3，自引：0，他引：3  

Michel Campillo 《Pure and Applied Geophysics》2006,163(2-3):475-502

We first present a summary of recent results on coda interpretation. We emphasize the observation of the stabilization of P to S energy ratio indicating the modal equipartition of the wavefield. This property clearly shows that the coda waves are in the regime of multiple scattering. Numerical solutions of the elastic radiative transfer equation are used to illustrate the evolution of the wave-field towards P-to-S energy stabilization, and asymptotically to complete isotropy. The energy properties of the coda have been widely studied but the phase properties have often been neglected. The recently observed coherent backscattering enhancement, an expression of the so-called `weak localization', demonstrates that interference effects still persist for multiple diffracted waves. Another manifestation of the persistence of the phase is the possibility to reconstruct the Green function between two stations by averaging the cross correlation of coda waves produced by distant earthquakes and recorded at those two stations. This reconstruction is directly related to the properties of reciprocity and time reversal of any wavefield. Using broadband seismic coda waves, we show that the dominant phases of the Green function in the band 2 s–10 s, namely fundamental mode Rayleigh and Love waves, are reconstructed. We analyze the time symmetry of the cross correlation and show how the level of symmetry evolves with the isotropization of the diffuse field with lapse time. Similarly we investigate the correlation in continuous ambient noise records. Whereas the randomness of the coda results from multiple scattering by randomly distributed scatterers, we assume that the seismic noise is random mostly because of the distribution of sources at the surface of the Earth. Surface waves can be extracted from long time series. The dispersion curves of Rayleigh waves are deduced from the correlations. On paths where measurements from earthquake data are also available, we show that they are in good agreement with those deduced from noise correlation. The measurement of velocities from correlation of noise along paths crossing different crustal structures opens the way for a `passive imaging' of the Earth's structure.  相似文献   

Lg Coda Q Variations across South America and their Relation to Crustal Evolution   总被引：5，自引：0，他引：5  

J. L. de Souza  B. J. Mitchell 《Pure and Applied Geophysics》1998,153(2-4):587-612

Nine broadband seismograph stations in South America have provided 389 recordings of Lg coda with paths that cover most of the continent. Lg coda Q (Q0v) and frequency dependence <(eta)> values at 1 Hz, obtained from these records, were inverted using back-projection tomography to obtain regionalized maps of Q0 and <eta>. The entire western margin of the continent (the active Andean mountain belt) is typified by low Q0 (250–450), whereas broad regions of high Q0 (700–1100) span the central Brazilian shield and contiguous regions to the north and south. Intermediate Q0 (450–700) characterizes the northern Patagonia platform and most of the Atlantic shield. Reduced Q0 in the Atlantic shield may be related to tectonic or igneous activity that occurred during the breakup of Gondwanaland during the Jurassic period. This Q distribution is generally consistent with earlier studies where Q0 was found to be directly proportional to the time that has elapsed since the most recent episode of major tectonic or orogenic activity in any region. Reduced Q0 in the Patagonian platform may, however, be due to young sediments there. Q0 is slightly higher in two portions of the Andean belt (between latitudes 2.0°N and 10.0°S, and between latitudes 24.0°S and 34.0°S) than in other portions of the belt. These variations are consistent with results of earlier studies of body-wave attenuation and heat flow in the Andean mountain belt. Spatial variations of <eta> generally vary inversely with Q0v, being low (0.0–0.2) throughout a broad region centered in the central Brazil shield and extending to the northeastern coast. All surrounding regions except that to the northeast exhibit intermediate to high (0.4–0.8 and possibly higher) <eta> values. Possible biasing of Lg coda Q measurements by proximity to the transition between the South American and Pacific plates was examined using records from a station near that boundary and was found to be small.  相似文献   

Temporal change in scattering and attenuation associated with the earthquake occurrence—A review of recent studies on coda waves     

Haruo Sato 《Pure and Applied Geophysics》1988,126(2-4):465-497

The study of coda waves has recently attracted increasing attention from seismologists. This is due to the fact that it is viewed as a new means by which the stress accumulation stage preceding a large earthquake can be measured, since the scattering paths nearly uniformly cover a fairly large region around the focus and observation stations, compared with the direct ray paths. To date, we have had many reports on the temporal variation of the relation between coda duration and amplitude magnitude, and that of the coda attenuationQ _c ^–1 which is estimated from coda amplitude decay. Some of these have shown a precursor-like behavior; however, others seem to have shown a coseismic change. We have critically reviewed these reports, and discussed what these observational facts tell us about the change in the heterogeneous crust. We found significant temporal variations, not only in the mean but also in the scatter ofQ _c ^–1 , associated with the mainshock occurrence. The formation of new cracks, the reopening and growing of existing cracks, the interaction of these cracks, and the pore water movement through these cracks might correspond to such variations. In addition, we may expect an inhomogeneous distribution of crack clusters in a fairly large region, compared with the aftershock region. The gradual appearance of such crack clusters seems to be the most plausible mechanism by which coda decay gradients are caused to largely scatter in the stress accumulation stage.  相似文献