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1.
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   

2.
In this paper, the "spectral amplitude ratio method" (SAR), "energy method" (EN) and "coda wave method" (CW) are used to calculate theQ value variations of gneiss in the preparing rupture process. The obtained results show that the variation state ofQ values by SAR features the shape of relative stability—gradual increment to the maximum—then decrement and final rupture. The variation state ofQ values by EN is just contrary to that by SAR, i. e. with the shape of stability—decrement—increment—and final rupture. The varation state ofQ values by CW is similar to that by EN, its main frequency features the shape of relatively high value—decrement to the minimum—increment—and final rupture. But to the high frequency (higher than the main frequency), the variation state ofQ values features the shape of the stable value-increment to the maximum-decrement-and final rupture. At the same time, the results by coda wave amplitude spectrum show that, when stress reaches 70% of rupture stress, the high frequency component of S wave rapidly reduces (Q c increasing); at the time of impending the main rupture, the main frequency component reduces with a large scale (Q c increasing again), this may be the reason which causes the different variation states of two codaQ values. The result of amplitude spectra of P, S (initial wave) waves also show that with the appearance of microcracks the frequency band of S wave turn to be narrow, the high frequency component is reduced quickly, i. e. the S wave spectra have different variation states with different frequency components. That is why theQ s obtained by different methods have different variation characteristics.  相似文献   

3.
Earlier attenuation studies for the South American continent indicate that for Sn and Lg waves there is low attenuation in the shield region east of the Andes, whereas in the west of South America, with some exceptions, there is high attenuation. Those studies, however, were nonquantitative. In this study theQ 0 (1-Hz values) for Lg waves for South America are presented, based on a scattering model ofAki (1969) as extended byHerrmann (1980) for the coda waves of shallow local and near-regional earthquakes. The results of the codaQ method are compared with those obtained byNuttli's method (1973). TheseQ 0 values are in good agreement with the apparentQ 0 of Lg waves obtained by the latter method. The data were obtained from over 100 local and regional earthquakes recorded by 12 WWSSN stations throughout continental South America. They provided a range of frequencies from 0.4 to 1.4 Hz, for which the frequency dependence ofQ was investigated by assumingQ=Q 0(f/f 0). The observed data indicate that the tectonic region of western South America is characterized by lowQ 0 and a large frequency-dependent factor , the values ranging from 150 to 350 and 0.4 to 0.7, respectively. TheQ 0 values increase in the shield region east of the Andes, but frequency dependence decreases. The average crustalQ 0 for north and central Argentina ranges from 420 to 580, and ranges from 0.2 to 0.3. TheQ 0 is larger in the Brazil region, ranging from 580 to 980, and varies from 0.0 to 0.2. In the lower-attenuation region of eastern South America higher values of attenuation correlate with greater thickness of the sedimentary layers.  相似文献   

4.
Three types of seismic data recorded near Coalinga, California were analyzed to study the behavior of scattered waves: 1) aftershocks of the May 2, 1983 earthquake, recorded on verticalcomponent seismometers deployed by the USGS; 2) regional refraction profiles using large explosive sources recorded on essentially the same arrays above; 3) three common-midpoint (CMP) reflection surveys recorded with vibrator sources over the same area. Records from each data set were bandpassed filtered into 5 Hz wide passbands (over the range of 1–25 Hz), corrected for geometric spreading, and fit with an exponential model of amplitude decay. Decay rates were expressed in terms of inverse codaQ (Q c –1 ).Q c –1 values for earthquake and refraction data are generally comparable and show a slight decrease with increasing frequency. Decay rates for different source types recorded on proximate receivers show similar results, with one notable exception. One set of aftershocks shows an increase ofQ c –1 with frequency.Where the amplitude decay rates of surface and buried sources are similar, the coda decay results are consistent with other studies suggesting the importance of upper crustal scattering in the formation of coda. Differences in the variation ofQ c –1 with frequency can be correlated with differences in geologic structure near the source region, as revealed by CMP-stacked reflection data. A more detailed assessment of effects such as the depth dependence of scattered contributions to the coda and the role of intrinsic attenuation requires precise control of source-receiver field geometry and the study of synthetic seismic data calculated for velocity models developed from CMP reflection data.  相似文献   

5.
Based on the single scattering model of coda power spectrum analysis, digital waveform data of 50 events recorded by the real-time processing system of the Chengdu telemetry network are analyzed to estimate the Q c values of earth medium beneath the Chengdu telemetry network for several specified frequencies. It is found that the Q c shows the frequency dependency in the form of Q c = Q 0 f n in the range of 1.0 to 20.0Hz. Estimated Q 0 ranges from 60.83 to 178.05, and n is found to be 0.713 to 1.159. The average value of Q 0 and n are 117 and 0.978 respectively. This result indicates the strong frequency dependency of the attenuation of coda waves beneath the Chengdu telemetry network. Comparing with the results obtained in other regions of the world, it is found that Q 0 −1 value and its change with frequency are similar to those in regions with strong tectonic activity. This subject is supported by the Ministry of Personnel, China for partly sponsoring.  相似文献   

6.
785 traces of vertical components from shallow earthquakes recorded by 10 CDSN (Chinese Digital Seismographic Network) stations and 5 GSN (Global Seismographic Network) stations were collected to study the attenuation characteristics ofL g coda in the Chinese continent and its adjacent regions. The records were processed first using the stack spectral ratio method to obtain the average values ofQ 0 (Q at 1Hz) and η, the frequency dependence, ofL g coda in the ellipses corresponding to the paths. The back-projection technique was then employed to obtain the tomographic maps ofQ 0 and η values, and the distribution of their errors. Results indicate that in the studied areaQ 0 varies between 200 and 500. The lowest value ofQ 0 exists in the Yun-nan-Tibetan region, while the highest value ofQ 0 occurs in the southern edge of Siberian platform. η varies between 0.3 and 0.8. For most part of the studied area η varies inversely withQ 0.  相似文献   

7.
We analyzed the local earthquakes waveform recorded on a broadband seismic network in the northwestern Himalayan Region to compute lapse time and frequency dependence of coda Q (Q c). The observed Q c values increase with increasing lapse time at all frequency bands. The increase in Q c values with lapse time is attributed to an increase in Q c with depth. This implies that attenuation decreases with increasing depth. The approximate radius of medium contributing to coda generation varies from 55 to 130 km. By comparing the Q c values with those from other regions of the world, we find that they are similar to those obtained from tectonically active regions. The estimated Q c values show a frequency-dependent relationship, Q c = Q 0 f n , where Q 0 is Q c at 1 Hz and n represents degree of frequency dependence. They represent the level of heterogeneity and tectonic activity in an area. Our results show that northwest Himalayas are highly heterogeneous and tectonically very active. Q 0 increases from 113 ± 7 to 243 ± 10 and n decreases from 1.01 ± 0.05 to 0.85 ± 0.03 when lapse time increases from 30 to 70 s. As larger time window sees the effect of deeper part of the Earth, it is concluded that Q 0 increases and n decreases with increasing depth; i.e., heterogeneity decreases with depth in the study area.  相似文献   

8.
The dependence of coda attenuationQ c on frequency and lapse time was studied. Data from small local earthquakes, recorded at three stations (VMR, VSI and VFI) of the VOLNET network operating in central Greece, were used.Q c was estimated by applying the single scattering model to bandpass-filtered seismograms, over a frequency range of 1 to 12 Hz. Analysis was performed every 10 s until the end on overlapping time windows.Q c is found to depend on frequencyf in Hz according to a power law,Q c =Q 0 f n . ObservedQ 0 ranges from 30 to 100 and the powern ranges from 0.90 to 0.70.Q 0 increases andn decreases with lapse time increasing. A strong dependence ofQ c on lapse time was also found. In the frequency range of 1 to 8 Hz and at a short lapse time,Q c values were found to be similar for all three stations. On the other hand, at the longest analyzed time window (50 s), the estimatedQ c values show a discrepancy which is more obvious at a higher frequency band. The scattering coefficient around the central station VSI is found to range from 0.029 to 0.0041 km–1.Q c from the single scattering model andQ s from the amplitude ratio of directS to coda waves for the VSI station are similar. We believe dependence of coda attenuationQ c on frequency and lapse time is caused by a combination of geotectonic features and depth variation asQ s .  相似文献   

9.
21 earthquakes recorded by a temporary seismic network in the Changbaishan Tianchi volcanic area in Northeast China operated during the summer of 2002 and 2003 were analyzed to estimate the S coda attenuation. The attenuation quality factor Qc was estimated using the single scattering attenuation model of Sato (1977) in the frequency band from 4 to 24 Hz. All the events studied in this paper occurred at depths from 2 to 6 km with ML of 1.4–2.8. The epicentral distances are less than 25 km. For all events which occurred near the Tianchi Lake (caldera), the Qc patterns obtained at the stations near the lake are similar, and the Qc values are relatively small. At the stations located about 15 km east of the Tianchi Lake, however, the average Qc is significantly higher. For an event which occurred 25km from the lake to the west, Qc patterns derived at the stations near the lake are quite similar to the above mentioned Qc for stations located in the east. Further study shows that Qc value in the north and central areas of the volcano is relatively lower than that in the surrounding area. Compared to other volcanic areas in the world, the average Qc of the Changbaishan Tianchi volcanic area is obviously lower. The deep seismic sounding and teleseismic receiver function studies indicated more than one lower velocity layer in the crust. The MT studies suggested the presence of high conductive bodies beneath the area. We interpret the strong attenuation of coda waves near the Changbaishan Tianchi volcano as being possibly related to high temperature medium caused by shallow magma chambers.  相似文献   

10.
QC-estimates of Kachchh Basin in western India have been obtained in a high frequency range from 1.5 to 24.0 Hz using the aftershock data of Bhuj earthquake of January 26, 2001 recorded within an epicentral distance of 80 km. The decay of coda waves of 30 sec window from 186 seismograms has been analysed in four lapse time windows, adopting the single backscattering model. The study shows that Qc is a function of frequency and increases as frequency increases. The frequency dependent Qc relations obtained for four lapse-time windows are: Qc=82 f1.17 (20–50 sec), Qc=106 f1.11 (30–60 sec), Qc=126f1.03 (40–70 sec) and Qc=122f1.02 (50–80 sec). These empirical relations represent the average attenuation properties of a zone covering the surface area of about 11,000, 20,000, 28,000 and 38,000 square km and a depth extent of about 60, 80, 95, 110 km, respectively. With increasing window length, the degree of frequency dependence, n, decreases marginally from 1.17 to 1.02, whereas Q0 increases significantly from 82 to 122. At lower frequencies up to 6 Hz, Qc−1 of Kachchh Basin is in agreement with other regions of the world, whereas at higher frequencies from 12 to 24 Hz it is found to be low.  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.
In the light of the single scattering model of coda originating from local earthquakes, and based on the aftershock coda registered respectively at the 4 short period stations installed near the foci shortly after theM7.6 Lancang andM7.2 Gengma earthquakes, this paper has tentatively calculated the rate of amplitude attenuation and theQ c-value of the coda in the Lancang and Gengma areas using a newly-founded synthetic determination method. Result of the study shows the rate of coda amplitude attenuation demonstrates remarkable regional differences respectively in the southern and northern areas. The southern area presents a faster attenuation (Q c=114), whereas the northern area shows a slower attenuation (Q c=231). The paper also discusses the reasons causing such differences. Result of the study also suggests a fairly good linear relation between the coda source factorA o(f) and the seismic moment and the magnitude. Using the earthquake scaling law, the following formulas can be derived: lgM 0=lgA 0(f)+17.6,M D=0.67lgA 0(f)+1.21 and logM 0=1.5M D+15.79. In addition, the rates of amplitude attenuationβ s andβ m are respectively calculated using the single scattering and multiple scattering models, and the ratioβ s/βm=1.20−1.50 is found for the results respectively from the two models. Finally, the mean free pathL of the S-wave scattering in the southern and northern areas are determined to be 54 km and 122 km respectively by the relations which can distinguish between the inherentQ i and scatteringQ s, testify to this areas having lowQ-values correspond to stronger scatterings. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, 71–82, 1992. This study is partly supported by the Seismological Science Foundation of the State Seismological Bureau of China, and the present English version of the paper is translated from its Chinese original by Wenyi Xia, Seismological Bureau of Yunnan Province.  相似文献   

13.
Digital recordings of three component microearthquake codas from shallow seismic events in the volcanic region of Campi Flegrei — Southern Italy — were used with an automatic technique to calculate the attenuation factorQ c (codaQ) in the hypothesis of singleS toS backscattering.Results show the same value ofQ for each of the three components. This result is interpreted as due to isotropicS wave radiation pattern.A check of the coda method was performed using a single station method based on simple assumptions on the direct SH wave spectrum. Single stationQ was averaged over the stations and over the earthquakes. Results show that the two methods lead to comparable results.A frequency dependence quite different from that evaluated in active tectonic regions was found for coda attenuation, comparable to other volcanic areas throughout the world. This is interpreted as due to the presence of magma that affects anelasticity and scattering.  相似文献   

14.
We present the first systematic study of attenuation derived from the S-wave coda in the frequency range 1-32 Hz for the southern part of the Netherlands and its surroundings. For this we used two methods, the codaQ (Q c) method and the Multiple Lapse Time Window (MLTW) method. In the interpretation of the results both single and multiple scattering in a half space are considered. Our aim is to validate these interpretations in our region and to try to identify theeffects of attenuation due to intrinsic absoprtion (Q i)and scattering attenuation (Q s). For this we analyzedmore than 100 3-component high-quality digital seismograms from 43 crustalevents and 23 different stations in the Netherlands, Germany and Belgium.Coda Q results show smaller Q c (=Q 0fn) values for epicentral distances shorter than 25 km (Q 0=90) compared to larger epicentral distances (Q 0=190), but similar frequency dependence (f-0.9). Interpretation of MLTW results provided a seismic albedo smaller then 0.5, suggesting that the intrinsic absorption dominates over scattering in this region. Both Q i and Q s show similar frequency dependences as Q c. These results are comparable to those obtained in other areas, but we also show that more sophisticated models are required to remove ambiguities in the interpretation. For short lapse times and shortevent-station distances we find for the simple half space model a correspondinginterpretation of both methodologies, where Q c correspondsto Q t, suggesting that a model with single scattering in ahalf space is appropriate. For long lapse times and long event station distances, however, we find that the S-wave coda is, most probably, too much influenced by crust-mantel heterogenities and more sophisticated Qinversion models using larger data sets are required for more reliable attenuation estimates.  相似文献   

15.
Coda wave quality factor (Q c ) was investigated by using digital data (100 sample sec–1) recorded by a vertical component short-period station installed for this study. The station was located in the greater Fairbanks area in central Alaska. From several hundred earthquakes recorded by this station in about a year, 27 earthquakes were selected for the above study; 7 of these selected earthquakes were located along the Alaska Wadati-Benioff zone (Pacific plate). The other 20 earthquakes were located in the area of intraplate seismicity (North American plate). The data was filtered using 9 pass-bands with center frequency varying from 1.5 Hz to 16 Hz with octave bandwidth. The values ofQ c obtained from the coda amplitude decay rates measured on the filtered data after corrections due to the recording instrument and source-receiver separation show appreciable frequency dependence. The value ofQ c varies in the range of 253 and 1190 corresponding to the frequency interval from 1.5 Hz to 16 Hz for the study area. This variation is close to that reported by others for the Kanto region of Japan. Moreover, the characteristics ofQ c obtained in the present case seem to be independent of epicentral distance and hypocentral depth.  相似文献   

16.
For short-period near-earthquake records in eastern China, from the empirical attenuation formula of coda ground motion amplitudeA with timeτ: lgA=G−2. 235 lgτ, using the single scattering theory modified with epicentral distance, we obtain the curve family of corrected coda amplitudeA c(r,t), andω/2Q c values for each time interval of coda. From this,Q c(f,h) values, which correspond to each observational average frequency and sampling depth, are calculated. The results substantially agree with those observationalQ c values in Yunnan, Beijing and central Asia.  相似文献   

17.
In this paper, the "spectral amplitude ratio method" (SAR), "energy method" (EN) and "coda wave method" (CW) are used to calculate theQ value variations of gneiss in the preparing rupture process. The obtained results show that the variation state ofQ values by SAR features the shape of relative stability—gradual increment to the maximum—then decrement and final rupture. The variation state ofQ values by EN is just contrary to that by SAR, i. e. with the shape of stability—decrement—increment—and final rupture. The varation state ofQ values by CW is similar to that by EN, its main frequency features the shape of relatively high value—decrement to the minimum—increment—and final rupture. But to the high frequency (higher than the main frequency), the variation state ofQ values features the shape of the stable value-increment to the maximum-decrement-and final rupture. At the same time, the results by coda wave amplitude spectrum show that, when stress reaches 70% of rupture stress, the high frequency component of S wave rapidly reduces (Q c increasing); at the time of impending the main rupture, the main frequency component reduces with a large scale (Q c increasing again), this may be the reason which causes the different variation states of two codaQ values. The result of amplitude spectra of P, S (initial wave) waves also show that with the appearance of microcracks the frequency band of S wave turn to be narrow, the high frequency component is reduced quickly, i. e. the S wave spectra have different variation states with different frequency components. That is why theQ s obtained by different methods have different variation characteristics.  相似文献   

18.
Attenuation of seismic waves is very essential for the study of earthquake source parameters and also for ground-motion simulations, and this is important for the seismic hazard estimation of a region. The digital data acquired by 16 short-period seismic stations of the Delhi Telemetric Network for 55 earthquakes of magnitude 1.5 to 4.2, which occurred within an epicentral distance of 100 km in an area around Delhi, have been used to estimate the coda attenuation Qc. Using the Single Backscattering Model, the seismograms have been analyzed at 10 central frequencies. The frequency dependence average attenuation relationship Qc = 142f 1.04 has been attained. Four Lapse-Time windows from 20 to 50 seconds duration with a difference of 10 seconds have been analyzed to study the lapse time dependence of Qc. The Qc values show that frequency dependence (exponent n) remains similar at all the lapse time window lengths. While the change in Q0 values is significant, change in Q0 with larger lapsetime reflects the rate of homogeneity at the depth. The variation of Qc indicates a definitive trend from west to east in accordance with the geology of the region.  相似文献   

19.
This article summarizes work on multiple scattering based on models of media with randomly distributed scatterers. The scatterers are isotropic and statistically uniform. Measuring distance in terms of mean-free pathL s and time in terms of the mean-free timesL s/V, whereV is the velocity of scattered waves, we have more convenient dimensionless distance and time. It can be shown that after the dimensionless time equals 0.65 energy contributed from multiple scattering becomes predominant. Thus the later coda reflects the effect of multiple scattering rather than single scattering. Treating the seismic record, including starting and tail parts, as a whole, the diffusion theory predicts that at a dense distribution of scatterers and a small distance between source and receiver, codas reflect mainly intrinsicQ i. Of course, this conclusion is coincident with the presumption of the diffusion theory,Q s>Q i. However, from a new integral equation of multiple scattering, which deals with the scattered waves and primary waves separately, the conclusion is similar but clearer. This article quotes the new expression for coda energy in two-dimensional space. It shows that if the receiver is close to the source, the coda decay reflects only intrinsicQ i, then as the distance increases, effects of scatteringQ s, are involved in the decay feature. The theoretical plots of coda decay show that it seems in most cases in the earthQ i should not be smaller than one tenth ofQ s.Project Sponsored by the Joint Earthquake Science Foundation of China.  相似文献   

20.
Coda of local earthquakes that occurred during 2006–2007 are used to study the attenuation characteristics of the Garhwal–Kumaun Himalayas. The coda attenuation characteristics are represented in terms of coda Q or Q c . It is observed that Q c increases with frequency. Q c also varies with increase in lapse time of coda waves. Q c increases up to an 85-s average lapse time. This is similar to observations around the world reported by many workers who have interpreted this as a manifestation of the fact that heterogeneity decreases with depth. However, around a 90-s average lapse time Q c is lower than its values for lower and higher average lapse times. This is interpreted as an indication of possible presence of a fluid-filled medium or a medium having partial melts at around a 160-km depth. Q 0, i.e., Q c at 1 Hz, increases, and frequency parameter n decreases with increasing lapse time, barring around a 90-s lapse time. This again shows that in general, heterogeneity decreases with increasing depth. The Q 0 and n values for smaller lapse times are similar to those for tectonically active areas. By comparing Q c values obtained in this study with those obtained by us using the 1999 Chamoli earthquake aftershocks, it is concluded that the crust is turbid and the mantle is more transparent. However, whether the variation in Q c values between 1999 and 2006–2007 is temporal or not cannot be definitely established from the available data set.  相似文献   

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