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1.
A layeredP- andS-wave velocity model is obtained for the Friuli seismic area using the arrival time data ofP- andS-waves from local earthquakes. A damped least-squares method is applied in the inversion.The data used are 994P-wave arrival times for 177 events which have epicenters in the region covered by the Friuli seismic network operated by Osservatorio Geofisico sperimentale (OGS) di Trieste, which are jointly inverted for the earthquake hypocenters andP-wave velocity model. TheS-wave velocity model is estimated on the basis of 978S-wave arrival times and the hypocenters obtained from theP-wave arrival time inversion. We also applied an approach thatP- andS-wave arrival time data are jointly used in the inversion (Roecker, 1982). The results show thatS-wave velocity structures obtained from the two methods are quite consistent, butP-wave velocity structures have obvious differences. This is apparent becauseP-waves are more sensitive to the hypocentral location thanS-waves, and the reading errors ofS-wave arrival times, which are much larger than those ofP-waves, bring large location errors in the joint inversion ofP- andS-wave arrival time. The synthetic data tests indicated that when the reading errors ofS-wave arrivals are larger than four times that ofP-wave arrivals, the method proposed in this paper seems more valid thanP- andS-wave data joint inversion. Most of the relocated events occurred in the depth range between 7 and 11 km, just above the biggest jump in velocity. This jump might be related to the detachment line hypothesized byCarulli et al. (1982). From the invertedP- andS-wave velocities, we obtain an average value 1.82 forV p /V s in the first 16 km depth.  相似文献   

2.
We demonstrate how multiples, generated at the interfaces of plane parallel beds, modify the propagation characteristics of an originally coherent seismic wave. For waves propagating at an angle to the bedding plane we find that theSV andP-waves couple so that neither is a pure mode. TheSH-wave, while modified in its propagation characteristics by multiples, remains a pure mode. The coupling ofSV-multiples into the quasi-P-mode appears weaker than the coupling ofP-wave multiples into the quasi-SV mode; at least this is so for the two simple cases of (a) density fluctuations only and (b) correlatedV p andV s fluctuations which conserve Poisson's ratio.We also find that the coupling is sensitive to both the angle of propagation and frequency. In addition there is a cut-off angle forP-wave multiples influencing the quasi-SV mode. Propagation angles larger than the cut-off permit theP-multiples to modify the phase of the quasi-SV mode, but not its effective attenuation. No such cut-off effect is found for SV-multiples influencing the quasi-P mode, whose angle-dependent and frequency-dependent phase distortion and effective attenuation are influenced both byP-wave multiples andSV-multiples.In view of the mathematical complexity of the expressions describing the phase, and effective attenuation of modes when allowance is made forP-andS-wave multiples, we strongly advocate numerical coding of the major mathematical formulae. By so doing a systematic study can be undertaken of the frequency and offset dependence of seismic waves as a function of seismic source input and power spectral behavior of the fluctuations in density and elastic constants of beds. It is our opinion that the full mathematical expressions are too involved to permit an analytic, systematic investigation to be given of the phase and attenuation of seismic waves with any degree of sophistication or generality.  相似文献   

3.
Two large shallow earthquakes occurred in 1942 along the South American subduction zone inclose proximity to subducting oceanic ridges: The 14 May event occurred near the subducting Carnegie ridge off the coast of Ecuador, and the 24 August event occurred off the coast of southwestern Peru near the southern flank of the subducting Nazca ridge. Source parameters for these for these two historic events have been determined using long-periodP waveforms,P-wave first motions, intensities and local tsunami data.We have analyzed theP waves for these two earthquakes to constrain the focal mechanism, depth, source complexity and seismic moment. Modeling of theP waveform for both events yields a range of acceptable focal mechanisms and depths, all of which are consistent with underthrusting of the Nazca plate beneath the South American plate. The source time function for the 1942 Ecuador event has one simple pulse of moment release with a duration of 22 suconds, suggesting that most of the moment release occurred near the epicenter. The seismic moment determined from theP waves is 6–8×1020N·m, corresponding ot a moment magnitude of 7.8–7.9. The reported location of the maximum intensities (IX) for this event is south of the main shock epicenter. The relocated aftershcks are in an area that is approximately 200 km by 90 km (elongated parallel to the trench) with the majority of aftershocks north of the epicenter. In contrast, the 1942 Peru event has a much longer duration and higher degree of complexity than the Ecuador earthquake, suggesting a heterogeneous rupture. Seismic moment is released in three distinct pulses over approximately 74 seconds; the largest moment release occurs 32 seconds after rupture initiation. the seismic moment as determined from theP waves for the 1942 Peru event is 10–25×1020N·m, corresponding to a moment magnitude of 7.9–8.2. Aftershock locations reported by the ISS occur over a broad area surrounding the main shock. The reported locations of the maximum intensities (IX) are concentrated south of the epicenter, suggesting that at least part of the rupture was to the south.We have also examined great historic earthquakes along the Colombia-Ecuador and Peru segments of the South American subduction zone. We find that the size and rupture length of the underthrusting earthquakes vary between successive earthquake cycles. This suggests that the segmentation of the plate boundary as defined by earthquakes this century is not constant.  相似文献   

4.
The source parameters, moment, stress drop and source dimension are estimated for 61 events from the January 1975 Brawley earthquake swarm. Earthquakes studied range in local magnitude from 1.0 to 4.7. Stress drops range from 1 to 636 bars and increase with source depth. It is estimated that the sedimentary structure of the Imperial Valley amplifies shear waves by a factor of 2 to 3 in addition to the free surface amplification of 2. Estimates of moment from 10 sec surface waves are 4 to 6 times larger than the moment estimated from the relatively flat part of the local body wave spectrum at 1 sec. This may be due to after-slip on the fault, a long thin fault, or partial stress drop. It is shown that the experimentally determined ratio of stress drop to apparent stress should be approximately 4.0 when spectrum integration is used to obtainS-wave energy and theP-wave energy is 1/3 theS-wave energy.  相似文献   

5.
Interpretation ofP/S-wave successions is used in induced or passive microseismicity. It makes the location of microseismic events possible when the triangulation technique cannot be used. To improve the reliability of the method, we propose a technique that identifies theP/S-wave successions among recorded wave successions. A polarization software is used to verify the orthogonality between theP andS polarization axes. The polarization parameters are computed all along the 3-component acoustic signal. Then the algorithm detects time windows within which the signal polarization axis is perpendicular to the polarization axis of the wave in the reference time window (representative of theP wave). The technique is demonstrated for a synthetic event, and three application cases are presented. The first one corresponds to a calibration shot within which the arrivals of perpendicularly polarized waves are correctly detected in spite of their moderate amplitude. The second example presents a microseismic event recorded during gas withdrawal from an underground gas storage reservoir. The last example is chosen as a counter-example, concerning a microseismic event recorded during a hydraulic fracturing job. The detection algorithm reveals that, in this case, the wave succession does not correspond to aP/S one. This implies that such an event must not be located by the method based on the interpretation of aP/S-wave succession as no such a succession is confirmed.  相似文献   

6.
We show that the multiple scattering by small fractures of seismic waves with wavelengths long compared to the fracture size and fracture spacing is indistinguishable from multiple-scattering effects produced by regular porosity, except for an orientation factor due to fracture alignment. The fractures reduce theP-wave andS-wave velocities and produce an effective attenuation of the coherent component of the seismic waves. The attenuation corresponds to 1000/Q of about unity for a Gaussian spectrum of fractures, and it varies with frequencyf asf 3. For a Kolmogorov spectrum of fractures of spectral index the attenuation is an order of magnitude or so larger and varies with frequency asf 3-v The precise degree of attenuation depends upon the matrix properties, the fracture porosity, the degree of fracture anisotropy, the type of fluid filling the fractures, and the incidence angle of the wave.For fracture porosities less than about 15% theP-wave andS-wave velocities are decreased by the order of 5–10% with a lesser dependence on the type of fluid filling the fractures (gas, oil, or brine) and with a dependence on both the degree of anisotropy and the incident angle made by the wave. The tendency of fractures to occur perpendicularly to bedding suggests that the best way to measure seismically fractured rock behavior in situ is by using the travel-time delay and reflection amplitude. As both the offset and the azimuth of receivers vary from a shot, the travel-time delay and reflection amplitude should both show an elliptical pattern of behavior—the travel-time delay in response to the varying seismic speed, and the reflection amplitude in response to angular variations in the multiple scattering. Observations of attenuation at several frequencies should permit (a) determination of the spectrum of fractures (Gaussian versus Kolmogorovian) and (b) determination of the contribution of viscous damping to the effective attenuation.  相似文献   

7.
Over the past 20 years the study ofP- andS-wave velocities in the upper mantle of the Mediterranean area and continental Europe has been the subject of intensive research work. We present a summary of results based on the inversion of available surface-wave dispersion data andP-wave trave time observations. For areas characterized by different tectonic settings and very large lateral variations, a discussion is made about structural models based on seismological, geothermal and electrical conductivity data.  相似文献   

8.
The attenuation of amplitude is seen in seismic waves which pass through the central region of the Aso caldera, in Kyushu, Japan. It is also recognized from spectral analysis of seismic waves that the higher frequencies of the P-wave are reduced in the waves which pass through the central region of the caldera. It is shown that the relative attenuation increases remarkably for the frequency range of 5 to 10 Hz. The specific attenuation factor Q of the P-wave train is about 100. From the surface projection of the ray paths with low Q values through the Aso caldera to each station, the attenuating region is located beneath the center of the caldera, extending to the north of the central cones. In conjunction with the low Q value of the P-wave and the decreases of S-wave amplitudes, the relative P-wave residual times have comparatively large values for seismic waves passing through the central region beneath the caldera. In order to attempt to provide additional information on the depth configuration of the attenuating material, the ray paths of P-wave's first arrivals are located in three-dimensional space. It indicates that the low-velocity material is located beneath the center of the caldera at depths of about 6 to 9 km. However, lowvelocity anomalies above the depth of 6 km and below the depth of 15 km were not able to be detected, because most of the available seismic ray paths had crossed the caldera at depths of about 6 to 15 km. Furthermore, the relative residual times have numerous errors resulting from incorrect hypocenter locations, origin times, inhomogeneities in the structure and uncertainty of the velocity structure. At shallow depths in the Aso caldera, refraction or reflection studies are required for an accurate estimate of the structure and more detailed properties of the attenuating material.  相似文献   

9.
Recent seismic events for which macroseismic intensities and accelerometric records are simultaneously available are investigated in order to derive empirical relationships between intensities and ground accelerations. 20 events with local magnitudes 3.0 to 5.4 are selected in a single country (France), in order to have homogeneous intensity data. Records are obtained in about 50 stations. Relationships are first established between intensities, magnitudes and distances on one side, between S-wave horizontal peak ground accelerations (PGA), magnitudes and distances R on the other side. They show that the PGA decays with distance roughly as R −2, in agreement with previous studies, and that PGA and intensities lead to different attenuation models. An intensity-acceleration relationship is established from direct observations, and from a combination of the previous relationships. It reveals that the intensity felt depends not only on the PGAs, but also on the distance. This may be explained by the frequency dependent attenuation of the waves, and by a different sensitivity of humans to the different frequencies. The influence of frequency on the felt intensity is then investigated, and a relation between intensity, PGA and frequency is established. It shows that the acceleration needed to be felt with a given intensity is larger at high frequency than at low frequency.Finally, as sound also contributes to earthquake perception, the P-wave displacement is analysed in an attempt to find in which conditions a perceptible sound is generated. The perturbation in air pressure induced by the P-wave is compared to the threshold of hearing in two frequency ranges, 20–40 Hz and 40–60 Hz. The maximum distance of perceptibility as a function of magnitude deduced from the P-wave displacement alone is found to be below the experimental distances of perception reported in the macroseismic enquiries.  相似文献   

10.
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   

11.
The relative contribution of scattering (Q s –1 ) and intrinsic (Q i –1 ) attenuation to the totalS-wave attenuation for the frequencies of 1.5, 3.0, 6.0 and 12.0 Hz has been studied by applying the radiative energy transfer theory, Data of local earthquakes which occurred in northern Greece and were recorded by the permanent telementered network of the Geophysical Laboratory of the University of Thessaloniki have been used. The results show that in this area the scattering attenuation is dominant over all frequencies while intrinsic attenuation is significantly lower. The estimatedQ s –1 andQ i –1 values have frequency dependences off –0.72 andf –0.45, respectively. The frequency dependence ofQ s –1 is the same as that of the codaQ c –1 , obtained by applying the single scattering model, which probably implies that the frequency dependence of the coda wave attenuation is attributed to the frequency dependence of the scattering attenuation.Q c –1 values are very close to scattering attenuation for short lapse times, (10–20 sec), and intermediate between scattering and intrinsic attenuation for the longer lapse times, (50–100 sec). This difference is explained as the result of the depth-dependent attenuation properties and the multiple scattering effects.  相似文献   

12.
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   

13.
T phases of three earthquakes from the Indian Ocean region, recorded by a short-period vertical-component seismic station network located in the vicinity of Kanyakumari on the southernmost tip of India, are studied. Two of these earthquakes are located west of 90°E ridge and one in the Nicobar Island region. However, seven other earthquakes which occurred 150–200 km south of Kanyakumari in the ocean did not produceT phases. An analysis ofT-waves (tertiary waves) travel time reveals the zone ofP-wave toT-wave conversion (i.e.,PT phase) region to coincide with the western continental slope of Srilanka. Further, it is observed that the disposition of the bathymetry between Srilanka and southern India strongly favours the downslope propagation mechanism ofT-wave travel to the southern coast of India through SOFAR channel. These observations are reported for the first time from India.  相似文献   

14.
A series of Hokkaido events, recorded by the FBV Broadband Seismograph System at the KHC Seismic Station, is used to study the structure of the earthquake magnitude scale on the basis of maximum velocity amplitudesA vmax of teleseismicPwaves in different period bands. Amplitude-periodband (APB) diagrams are constructed for each event. According to the shape of the APB diagrams the events investigated can be divided into three types: (a) events with largestA vmax values in the intermediate period range (periods ofA vmax from 2.2 to 23 sec), (b) events with largestA vmax values in the short-period range (periods ofA vmax from 1 to 2 sec), (c) events exhibiting anomalous APB diagrams. Type (a) events seem to represent the process of wave generation that prevails for shallow earthquakes. Type (b) events approach to explosive-like generation of seismic waves. The nature of the exceptionally occurring type (c) events must be clarified in further investigations. The influence of the type of earthquake on the magnitude values estimated on the basis of standard class A and B (short-period and intermediate-band) seismograms is demonstrated. It appears that for estimating correct values of earthquake magnitudes complementary information on the process of seismic wave generation in the focus is necessary. At teleseismic distances this information can be obtained from either APB diagrams or amplitude spectra ofP waves recorded, e.g., by broadband velocity sensing instruments.  相似文献   

15.
A Seismic Model of Casing Failure in Oil Fields   总被引：1，自引：0，他引：1  
—We develop a seismic model that characterises the sudden tensional failure of oil-well casings. The energy released by the rupture of a well casing is transformed into heat and seismic energy. The upper bound of the seismic efficiency of this process is estimated at about 3%. The static situation at the completion of a casing failure episode is modelled by calculating the static displacement field generated by two opposing forces separated by an arm. The azimuthal patterns of these displacements and the change in the strain and stress fields caused by the force couple are described. The dynamics of the failure episode are modelled as a dipole with a seismic moment equivalent to the product of the average drop in shear stress, the failure surface, and an arm. The radiated P and S waves have mean-square radiation pattern coefficients of 1/5 for P waves and 2/15 for S waves. The displacement field as a function of time during rupture and the spectral properties in the far field are derived. The most promising seismic parameters that can be used for distinguishing between casing failure events and other possible events are polarisation properties of S waves and S/P amplitude ratios. S-wave polarisation distinguishes between shear events and casing failure events. S/P amplitude ratios distin guish between tensile events and casing failure events.  相似文献   

16.
Source parameters of the earthquakes of the Baikal rift system   总被引：1，自引：0，他引：1  
The dynamic parameters of the earthquake source—the seismic moment, the moment magnitude, the source radius, the stress drop, and the amplitude of displacement—are determined by the amplitude Fourier spectra of the body shear waves (S-waves) for 62 earthquakes of the Baikal rift system with the energy class of K P = 9.1–15.7. In the calculations I used the classical Brune model. The seismic moment of the earthquakes being investigated changes from 3.65 × 1011 N m to 1.35 × 1018 N m, and the radii of earthquake sources vary from 390 m to 1.84 km. The values of the drop in stress Δσ grow with an increase in the seismic moment up to 1.7 × 108 Pa. For the group of weak earthquakes (M w = 1.7–3.3), extremely low values of the drop in stress 103–104 Pa are observed. The maximum amplitude of displacement in the source amounts to 5.95 m. The empirical equations between the seismic moment and the other dynamic parameters of the source are determined. The regional dependence of the seismic moment and energy class is obtained: log M 0 ± 0.60 = 1.03K P + 3.17. The character of the relationship between the seismic moment and the corner frequency indicates that the classical scaling law of the seismic spectrum for the earthquakes in question is not fulfilled. The obtained estimates of the dynamic parameters are in satisfactory agreement with the published data concerning the analogous parameters of the other rift zones, which reflects the general regular patterns of the destruction of the lithosphere and the seismicity in the extension zones of the lithosphere.  相似文献   

17.
Summary About 360 seismic events from almost all directions recorded at 13 seismological observatories in Sweden and Finland have been investigated. The depths of these events vary from the surface to 650 km and the epicentral distances from 9° to 119° with fairly even coverage. The two most separated stations in this array are about 15° apart (Karlskrona in Sweden and Kevo in Finland). Comparison of observed travel-time curves and their slope with those ofJeffreys-Bullen andHerrin are made. Generally, the observed travel times are earlier than theJeffreys-Bullen times and later than theHerrin ones. Path and depth effects on residuals with respect to the two given tables are studied, and station corrections and source corrections are estimated. Global and regional travel-time tables of theP-wave have been constructed for this network of stations.On leave from Geophysical Institute, Tehran University, Tehran, Iran  相似文献   

18.
—We determined the response to P- and S-wave incidence of the permanent stations of the seismic network of Baja California (RESNOM) using two independent methods. We selected 65 events with magnitudes between 2.2 and 4.8 and hypocentral distances ranging between 5 and 330 km. The site response of the ten stations analyzed was first estimated using average spectral ratios between the horizontal and the vertical components of motion (H/V ratios). As a second approach we performed a simultaneous inversion for source and site. In order to invert the spectral records to determine the site response, we made an independent estimate of the attenuation for two different source-station path regions. Then we corrected the spectral records for the attenuation effect before we made the inversion. Although the average H/V ratio of many sites is inside the error bars of the site response estimated with the spectral inversion, the spectral inversion tends to give higher values. For the S wave some sites show similar frequency of predominant peak when comparing the responses obtained with both methods. In contrast, for the P waves the H/V ratios disagree with the results of the inversion. In general, the site response of the stations is strongly frequency dependent for both P and S waves. We also found that the natural frequency of resonance of the sites is near 0.5 Hz for P and near 0.8 Hz for the S waves.  相似文献   

19.
    
An earthquake ofM S=6.9 occurred at the Gonghe, Qinghai Province, China on April 26, 1990. Three larger aftershocks took place at the same region,M S=5.5 on May 7, 1990,M S=6.0 on Jan. 3, 1994 andM S=5.7 on Feb. 16, 1994. The long-period recordings of the main shock from China Digital Seismograph Network (CD-SN) are deconvolved for the source time functions by the correspondent recordings of the three aftershocks as empirical Green’s functions (EGFs). No matter which aftershock is taken as EGF, the relative source time functions (RSTFs) obtained are nearly identical. The RSTFs suggest theM S=6.9 event consists of at least two subevents with approximately equal size whose occurrence times are about 30 s apart, the first one has a duration of 12 s and a rise time of about 5 s, and the second one has a duration of 17 s and a rise time of about 8 s. Comparing the RSTFs obtained from P- and SH-phases respectively, we notice that those from SH-phases are a slightly more complex than those from P-phases, implying other finer subevents exist during the process of the main shock. It is interesting that the results from the EGF deconvolution of long-period wavform data are in good agreement with the results from the moment tensor inversion and from the EGF deconvolution of broadband waveform data. Additionally, the two larger aftershocks are deconvolved for their RSTFs. The deconvolution results show that the processes of theM S=6.0 event on Jan. 3, 1994 and theM S=5.7 event on Feb. 16, 1994 are quite simple, both RSTFs are single impulses. The RSTFs of theM S=6.9 main shock obtained from different stations are noticed to be azimuthally dependent, whose shapes are a slightly different with different stations. However, the RSTFs of the two smaller aftershocks are not azimuthally dependent. The integrations of RSTFs over the processes are quite close to each other, i. e., the scalar seismic moments estimated from different stations are in good agreement. Finally the scalar seismic moments of the three aftershocks are compared. The relative scalar seismic moment of the three aftershocks deduced from the relative scalar seismic moments of theM S=6.9 main shock are very close to those inverted directly from the EGF deconvolution. The relative scalar seismic moment of theM S=6.9 main shock calculated using the three aftershocks as EGF are 22 (theM S=6.0 aftershock being EGF), 26 (theM S=5.7 aftershock being EGF) and 66 (theM S=5.5 aftershock being EGF), respectively. Deducing from those results, the relative scalar sesimic moments of theM S=6.0 to theM S=5.7 events, theM S=6.0 to theM S=5.5 events and theM S=5.7 to theM S=5.5 events are 1.18, 3.00 and 2.54, respectively. The correspondent relative scalar seismic moments calculated directly from the waveform recordings are 1.15, 3.43, and 3.05. Contribution No. 96B0007, Institute of Geophysics, SSB, China.  相似文献   

20.
Velocity as well as attenuation factorQ –1 ofP-wave in a dry granitic rock sample under uniaxial compressions were measured in the range of frequency between 100 kHz and 710 kHz by using the pulse transmission technique. Above the stress of 0.5 f , where f is the fracture stress, theP-wave velocity decreases with increasing axial stress, whereasQ –1 increases. Particularly, the change ofQ –1 is greater for high frequency than for low frequency. At a given stress level, the higher the frequency, the higher theP-wave velocity and the largerQ –1. This result means that the velocity decrease with increasing stress is smaller for higher frequency. Because of this frequency-dependence of velocity decrease, theP-wave in the rock under dilatant state shows dispersion. The body wave dispersion is more remarkable at higher stress, and is not found in a homogeneous material with no cracks. Thus the disperison is attributed to the generation of cracks. When the frequency-dependence ofQ –1 is approximated asf n in the present frequency range, the exponentn takes a value from 0.63 to 0.77.  相似文献   

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