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1.
Based on the long period surface wave data recorded by the China Digital Seismograph Network (CDSN), theQ R of fundamental mode Rayleigh wave with periods from 10 s to 146 s is determined for the eastern Sino-Korean paraplatform in this paper. TheQ β models of the crust and upper mantle are respectively obtained for the 4 paths, with the aid of stochastic inverse method. It shows that in the eastern Sino-Korean paraplatform, the average crustalQ β is about 200, and that there exists a weak attenuation layer in the middle crust (about 10–20 km deep) which is possibly related to earthquake-prone layer. A strong attenuation layer (lowQ) of 70 km thick extensively exists in the uppermost mantle, with the buried depth about 80 km. The averageQ R of fundamental mode Rayleigh wave is between the value of stable tectonic region and that of active tectonic region, and much close to the latter.  相似文献   

2.
IntroductionVelocityanditsattenuationinformationiscloselylinkedwiththeoreticalstudiesonthegroundmovementsduringearthquake.Therecentstudy(Malagnini,1996)showedthatthevelocitystructureofshearwaveinshallowsoilabove30mplaysanimpoftantroletoestimatestfonggroundmotionofsite.However,itishardtopreciselymeasurethesoilstructuresanddynamiccharacteristics.First,theloosesoilabsorbstheseismicwaveswithhighfrequencies;Second,theeffectsoffocusinganddispersioncausedbylocallyinhomogeneoussitecannotbeneglectedin…  相似文献   

3.
On the basis of data of long period Rayleigh surface wave, we select 43 two-station paths which cover the eastern China thoroughly. By using the improved method of multi-filtration, we obtain the group velocity and amplitude spectrum, and then get attenuation factor for each paths. We employ Talentola inversion method to get local attenuation factor, and further invert the three-dimension Q β image under the crust and upper mantle in the eastern Chinese continent. The Q β image shows the following basic characters. There is correlation between the seismic activity and Q β structure under the crust and upper mantle in North China region. The Yangtze block begins to collide with and subduct to the North China block from the southern border of the Qinling in the southern Shaanxi. In the large part of Yangtze quasi-platform appear an obvious high Q β area at 88 km deep. In the east of Sichuan depression platform, the juncture of Sichun and Guizhou, and the Jiangnan block near the juncture of Guizhou and Hunan, the lateral variation of Q β in the crust is little, and there is a high-Q β layer no thinner than 40 km in the top mantle. In the Dian-Qian fold and fracture region between Yunnan and Guizhou, the vertical variation of Q β at the region of the crust and upper mantle is little, there is a low-Q β layer in the top mantle, about 40 km thick, low-Q β layer of the upper mantle begins to appear at about 95 km deep. In the east of Yangtze quasi-platform and the central and eastern part of the South China fold system, the Moho is smooth, the lateral variation of Q β in the crust is also little, low-Q β layer of the upper mantle begins to appear at about 85 km deep.  相似文献   

4.
Fundamental-mode Rayleigh wave attenuation data for stable and tectonically active regions of North America, South America, and India are inverted to obtain several frequency-independent and frequency-dependentQ models. Because of trade-offs between the effect of depth distribution and frequency-dependence ofQ on surface wave attenuation there are many diverse models which will satisfy the fundamental-mode data. Higher-mode data, such as 1-Hz Lg can, however, constrain the range of possible models, at least in the upper crust. By using synthetic Lg seismograms to compute expected Lg attenuation coefficients for various models we obtained frequency-dependentQ models for three stable and three tectonically active regions, after making assumptions concerning the nature of the variation ofQ with frequency.In stable regions, ifQ varies as , where is a constant, models in which =0.5, 0.5, and 0.75 satisfy fundamental-mode Rayleigh and 1-Hz Lg data for eastern North America, eastern South America, and the Indian Shield, respectively. IfQ is assumed to be independent of frequency (=0.0) for periods of 3 s and greater, and is allowed to increase from 0.0 at 3 s to a maximum value at 1 s, then that maximum value for is about 0.7, 0.6, and 0.9, respectively, for eastern North America, eastern South America, and the Indian Shield. TheQ models obtained under each of the above-mentioned two assumptions differ substantially from one another for each region, a result which indicates the importance of obtaining high-quality higher-mode attenuation data over a broad range of periods.Tectonically active regions require a much lower degree of frequency dependence to explain both observed fundamental-mode and observed Lg data. Optimum values of for western North America and western South America are 0.0 if is constant (Q is independent of frequency), but uncertainty in the Lg attenuation data allows to be as high as about 0.3 for western North America and 0.2 for western South America. In the Himalaya, the optimum value of is about 0.2, but it could range between 0.0 and 0.5. Frequency-independent models (=0.0) for these regions yield minimumQ values in the upper mantle of about 40, 70, and 40 for western North America, western South America, and the Himalaya, respectively.In order to be compatible with the frequency dependence ofQ observed in body-wave studies,Q in stable regions must be frequency-dependent to much greater depths than those which can be studied using the surface wave data available for this study, andQ in tectonically active regions must become frequency-dependent at upper mantle or lower crustal depths.On leave from the Department of Geophysics, Yunnan University, Kunming Yunnan, People's Republic of China  相似文献   

5.
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   

6.
The seismic attenuation in the Vrancea region (Romania) is investigated from teleseismic recordings of P and pP waves during the four major, intermediate-depth Romanian events that occurred since the onset of digital instrumentation. Most stations are located in Canada and in the United States, being equipped with a variety of sensors, especially short-period ones. The amplitude spectral ratio method is used, assuming no frequency dependence of the Q P factor in the range 0.2–2 Hz. No apparent correlation between the derived attenuation value and the type of recording sensor is observed. Lateral variations of the attenuation are obtained, with a very low Q P area (values down to 33) located in the northwestern part of the Vrancea seismogenic volume. For the stations with different azimuth angles in relation to the epicentral area, Q P values routinely exceed 200. Most likely, the low attenuation values are related to an upwelling mantle material located immediately beneath the crust, but limited in depth to at least 100 km.  相似文献   

7.
The seismic quality factor (Q c) and the attenuation coefficient (δ) in the earth’s crust in southwest (SW) Anatolia are estimated by using the coda wave method based on the decrease of coda wave amplitude by time on the seismogram. The quality factor Q o, the value of Q c at 1 Hz, and its frequency dependency η are determined from this method depending on the attenuation properties of scattered coda waves. δ is determined from the observations of amplitude variations of seismic waves. In applying the coda wave method, firstly, a type curve representing the average pattern of the individual coda decay curves for 0.75, 1.5, 3.0, 6.0, 12.0, and 24.0 Hz values was estimated. Secondly, lateral variation of coda Q and the attenuation coefficients for three main tectonic patterns are estimated. The shape of the type curve is controlled by the scattering and attenuation in the crustal volume sampled by the coda waves. The Q o and η values vary from 30 to 180 and from 0.55 to 1.25, respectively for SW Anatolia. In SW Anatolia, coda Q–f relation is described by and δ = 0.008 km−1. These results are expected to help in understanding the degree of tectonic complexity of the crust in SW Anatolia.  相似文献   

8.
The fundamental mode Love and Rayleigh waves generated by earthquakes occurring in Kashmir, Nepal Himalaya, northeast India and Burma and recorded at Hyderabad, New Delhi and Kodaikanal seismic stations are analysed. Love and Rayleigh wave attenuation coefficients are obtained at time periods of 15–100 seconds, using the spectral amplitude of these waves for 23 different paths along northern (across Burma to New Delhi) and central (across Kashmir, Nepal Himalaya and northeast India to Hyderabad and Kodaikanal) India. Love wave attenuation coefficients are found to vary from 0.0003 to 0.0022 km–1 for northern India and 0.00003 km–1 to 0.00016 km–1 for central India. Similarly, Rayleigh wave attenuation coefficients vary from 0.0002 km–1 to 0.0016 km–1 for northern India and 0.00001 km–1 to 0.0009 km–1 for central India. Backus and Gilbert inversion theory is applied to these surface wave attenuation data to obtainQ –1 models for the crust and uppermost mantle beneath northern and central India. Inversion of Love and Rayleigh wave attenuation data shows a highly attenuating zone centred at a depth of 20–80 km with lowQ for northern India. Similarly, inversion of Love and Rayleigh wave attenuation data shows a high attenuation zone below a depth of 100 km. The inferred lowQ value at mid-crustal depth (high attenuating zone) in the model for northern India can be by underthrusting of the Indian plate beneath the Eurasian plate which has caused a low velocity zone at this shallow depth. The gradual increase ofQ –1 from shallow to deeper depth shows that the lithosphere-asthenosphere boundary is not sharply defined beneath central India, but rather it represents a gradual transformation, which starts beneath the uppermost mantle. The lithospheric thickness is 100 km beneath central India and below that the asthenosphere shows higher attenuation, a factor of about two greater than that in the lithosphere. The very lowQ can be explained by changes in the chemical constitution taking place in the uppermost mantle.  相似文献   

9.
The digital seimograph network set up by China and France in Zhangye of China had been operated in 1988. The Zhangye network is situated in the middle segment of Hexi corridor and Qilina mountain, which was regarded as a monitoring earthquake area. Using the records of Zhangye digital network theQ-values in and around Minle basin have been measured. The results of this study showed that theQp-values range from 500 to 780, andQs-values range from 230 to 460. TheQ-values of inside of Minle basin are higher than that around the basin. The greater parts of moderate and strong earthquakes occurred along tectonic belts around the Minle basin. Moreover, TheQ-values increased with the depth of penctration of wave ray. The attenuation of S wave is stronger than P wave in shallow layer of crust. Some problems ofQ-value change versus time before and after Sunan eathquake (M s=5.7) have been also disscussed. These results can be applied to study and to monitor seismic danger of the Minle monitoring area. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,15, 296–302, 1993.  相似文献   

10.
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   

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