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1.
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   

2.
The physical implication of coda amplitude ratio is discussed in term of energy ratio. The digitized data recorded at the station of Beijing Telemetered Seismograph Network between 1989 and 1990 are used to calculate amplitude ratios of coda to direct S wave, and energy ratios. The spectral energy ratios are used to estimate the coda Q and mean free path l in the Beijing area, as well as the two quality factors Q i and Q S separately due to intrinsic absorption and scattering attenuation. The decay of seismic waves in their propagation seems mainly resulted from the intrinsic absorption in Beijing region. The temporal variations of amplitude ratio and energy ratio at Changli station during the above two years are inspected; some of them largely depart from their mean value. It may reflect the seismogenic process, but using the data lasting longer time with more case histories needs further study. This study is sponsored by the Key Project of State Science and Technology of China, No. 96-918.  相似文献   

3.
In numerical simulations of ground motion, the constant quality factor Q of a viscoelastic medium can be determined using the time-domain constitutive approximation method of the generalized standard linear solid (GSLS) model. This study introduces a numerical seismic wavefield simulation method which combines the spectral element method with the constant-Q model. The method is used to simulate the seismic wavefield of the 1994 Northridge earthquake. The optimal attenuation coefficient for the simulated seismic waves in this study area is determined empirically based on a quantitative analysis of the deviation curve. Further, the effect of the quality factor on the simulated wavefield are analyzed according to the site characteristics of each seismic station. The quality factor shows a variable effect on the different frequency components of the simulated wavefield. The effect of the quality factor also varies with the characteristic parameters of each seismic station site, such as site velocity structure, fault distance, and azimuth angle.  相似文献   

4.
周浩  符力耘 《地球物理学报》2018,61(3):1083-1094
实际地球介质的弹性波衰减,无论是对于地球内部结构反演、含油气储层描述,还是对于地震岩石物理本身的基础研究而言,都十分重要.通过岩石物理实验测量岩石的弹性波衰减是相关研究的基本手段,而谱比法是一种最常用的测量方法.利用谱比法测量的弹性波衰减,简称谱比法衰减,由介质的本征吸收特性和非均质散射特性共同决定.然而,在谱比法理论中,没有将这两种衰减特性的作用区分对待.因此,实验室测量的谱比法衰减中是否含有散射特性,其散射特性是否可以忽略,常不明确.本文基于对岩石物理实验中样品波形特征的基本认识,通过能流理论描述了弹性波的本征吸收和非均质散射过程,以及对应的尾波形成机理.文章还应用能流理论测量岩石的弹性波衰减,即能流法衰减.无论是通过理论推导,还是通过实验验证,谱比法衰减与能流法衰减都显示出良好的一致性.这两种直达波衰减的一致性暗示了谱比法衰减中存在着两种衰减特性.文中还对比分析了谱比法衰减与喷射流本征衰减的实验结果,该结果同样展示了谱比法衰减中可能存在着散射特性.因此,通过能流理论,可以利用样品波形的尾波特征定性判断谱比法衰减中可能存在的散射特性.即异常明显的尾波振幅可能意味着强烈的直达波非均质散射,谱比法衰减中的散射特性因而不可忽略.  相似文献   

5.
Empirical scaling equations for Fourier amplitude spectra of strong ground motion are used to describe A0 and τ in the assumed (high-frequency) shape of strong motion amplitudes: FS(φ) = A0e-πτφ. The res of computed A0 and τ with other related estimates of spectral amplitudes; (2) smooth decay of strong motion spectral amplitudes up to φ = 25 Hz, without an abrupt low-pass filtering of high frequecies; and (3) good agreement with other estimates of the regionally specific attenuation of high-frequncy seismic waves.As the recorded strong earthquake shaking in the western United States typically samples only the shallow (10 km) and local (100km) characteristics of wave attenuation, the processed strong motion accelerograms can be used as the most direct means of describing the nature of the high-frequency attenuation of the entire strong motion signal for use in earthquake engineering applications. Seismological body wave, Lg and coda wave estimates of Q sample different volumes of the crust surrounding the station, and involve different paths of the waves. These differences must be carefully documented and understood before the results can be used in earthquake engineering characterization of strong motion amplitudes.  相似文献   

6.
At the CO2CRC Otway geosequestration site, the abundance of borehole seismic and logging data provides a unique opportunity to compare techniques of Q (measure of attenuation) estimation and validate their reliability. Specifically, we test conventional time-domain amplitude decay and spectral-domain centroid frequency shift methods versus the 1D waveform inversion constrained by well logs on a set of zero-offset vertical seismic profiles. The amplitude decay and centroid frequency shift methods of Q estimation assume that a seismic pulse propagates in a homogeneous medium and ignore the interference of the propagating wave with short-period multiples. The waveform inversion explicitly models multiple scattering and interference on a stack of thin layers using high-resolution data from sonic and density logs. This allows for stable Q estimation in small depth windows (in this study, 150 m), and separation of the frequency-dependent layer-induced scattering from intrinsic absorption. Besides, the inversion takes into account band-limited nature of seismic data, and thus, it is less dependent on the operating frequency bandwidth than on the other methods. However, all considered methods of Q estimation are unreliable in the intervals where subsurface significantly deviates from 1D geometry. At the Otway site, the attenuation estimates are distorted by sub-vertical faults close to the boreholes. Analysis of repeated vertical seismic profiles reveals that 15 kt injection of the CO2-rich fluid into a thin saline aquifer at 1.5 km depth does not induce detectable absorption of P-waves at generated frequencies 5–150 Hz, most likely because the CO2 plume in the monitoring well is thin, <15 m. At the Otway research site, strong attenuation Q ≈ 30–50 is observed only in shaly formations (Skull Creek Mudstone, Belfast Mudstone). Layer-induced scattering attenuation is negligible except for a few intervals, namely 500–650 m from the surface, and near the injection interval, at around 1400–1550 m, where Qscat ≈ 50–65.  相似文献   

7.
We exploit S-wave spectral amplitudes from 112 aftershocks (3.0 ≤ ML ≤ 5.3) of the L’Aquila 2009 seismic sequence recorded at 23 temporary stations in the epicentral area to estimate the source parameters of these events, the seismic attenuation characteristics and the site amplification effects at the recording sites. The spectral attenuation curves exhibit a very fast decay in the first few kilometers that could be attributed to the large attenuation of waves traveling trough the highly heterogeneous and fractured crust in the fault zone of the L’Aquila mainshock. The S-waves total attenuation in the first 30 km can be parameterized by a quality factor QS(f) = 23f 0.58 obtained by fixing the geometrical spreading to 1/R. The source spectra can be satisfactorily modeled using the omega-square model that provides stress drops between 0.3 and 60 MPa with a mean value of 3.3±2.8 MPa. The site responses show a large variability over the study area and significant amplification peaks are visible in the frequency range from 1 to more than 10 Hz. Finally, the vertical component of the motion is amplified at a number of sites where, as a consequence, the horizontal-to-vertical spectral ratios (HVSR) method fails in detecting the amplitude levels and in few cases the resonance frequencies.  相似文献   

8.
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   

9.
—Measurements of seismic attenuation (Q ?1) can vary considerably when made from different parts of seismograms or using different techniques, particularly at high frequencies. These discrepancies may be methodological, or may reflect earth processes. To investigate this problem, we compare body wave with coda Q ?1 results utilizing three common techniques i) parametric fit to spectral decay, ii) coda normalization of S waves, and iii) coda amplitude decay with lapse time. Q ?1 is measured from both body and coda waves beneath two mountain ranges and one platform, from recordings made at seismic arrays in the Caucasus and Kopet Dagh over paths ≤ 4° long. If Q is assumed frequency independent, spectral decay fits show Q s and Q coda near 700–800 for both mountain paths and near 2100–2200 for platform paths. Similar values are determined with the coda normalization technique. However, frequency-dependent parameterizations fit the data significantly better, with Q s ?(1 Hz) and Q coda?(1 Hz) near 200–300 for mountain paths and near 500–600 for platform paths. Lapse decay measurements are close to the frequency-dependent values, showing that both spectral and lapse decay methods can give similar results when Q has comparable parameterizations. Above 6 Hz, coda measurements suggest some enrichment relative to body waves, perhaps due to scattering, but intrinsic absorption appears to dominate at lower frequencies. All approaches show sharp path differences between the Eurasian platform and adjacent mountains, and all are capable of resolving spatial variations in Q.  相似文献   

10.
王芳  李丽  王宝善 《地震学报》2017,39(6):831-847
基于2011年建立的云南普洱大寨深井台站,开展了噪声压制及附近波场特征研究.通过计算该台站的噪声功率谱概率密度函数,显示该井下台站对1 Hz以上的高频噪声具有明显的压制效果, 最高能降低40 dB,其降噪能力优于其它井下台阵,推断与该台站附近的场地条件有关.基于地表与井下地震记录的差异,应用正则化反卷积干涉方法进一步研究该台站附近的波场特征.以地表记录为参考,对井下记录进行反卷积,获取两台站之间的格林函数,直接识别出了原始记录上无法区分的上行入射波与下行地表反射波,然后利用两震相的到时差建立了一个浅层地震波速度模型,与理论模拟的结果一致.研究结果表明,相对于地表观测,井下台站在压制噪声和近地表地震波传播特征研究等方面具有很大的优势,同时该研究对其它地区开展深井观测具有参考意义.   相似文献   

11.
在阿波罗月震记录中普遍存在着强烈持久的尾波信号,这样的波形特征无法用均匀分层月球模型解释.一个普遍被接受的解释是月震尾波由月球浅层结构对月震波的散射引起.我们采用基于交错网格的伪谱和有限差分混合方法模拟研究非均匀上月壳对月震波的散射效应,在此基础上解释月震尾波的形成机制,并估计出上月壳速度扰动的强度.我们发现,在均匀分层模型基础上,进一步考虑上月壳中的非均匀结构对月震波的散射效应,能有效地解释月震信号中强烈持久的尾波.我们认为月震尾波可能是由上月壳中的低波速、低衰减和散射这三个因素的共同作用所引起.采用不同的扰动标准差模拟上月壳的非均匀性,并比较模拟波形与真实月震图的相似程度,我们发现上月壳中速度扰动的标准差应该在3%到5%之间,很可能接近于3%.  相似文献   

12.
We investigated the seismic attenuation of compressional (P‐) and converted shear (S‐) waves through stacked basalt flows using short‐offset vertical seismic profile (VSP) recordings from the Brugdan (6104/21–1) and William (6005/13–1A) wells in the Faroe‐Shetland Trough. The seismic quality factors (Q) were evaluated with the classical spectral ratio method and a root‐mean‐square time‐domain amplitude technique. We found the latter method showed more robust results when analysing signals within the basalt sequence. For the Brugdan well we calculated effective Q estimates of 22–26 and 13–17 for P‐ and S‐waves, respectively, and 25–33 for P‐waves in the William well. An effective QS/QP ratio of 0.50–0.77 was found from a depth interval in the basalt flow sequence where we expect fully saturated rocks. P‐wave quality factor estimates are consistent with results from other VSP experiments in the North Atlantic Margin, while the S‐wave quality factor is one of the first estimates from a stacked basalt formation using VSP data. Synthetic modelling demonstrates that seismic attenuation for P‐ and S‐waves in the stacked basalt flow sequence is mainly caused by one‐dimensional scattering, while intrinsic absorption is small.  相似文献   

13.
Coherent noise in land seismic data primarily consists in source‐generated surface‐wave modes. The component that is traditionally considered most relevant is the so‐called ground roll, consisting in surface‐wave modes propagating directly from sources to receivers. In many geological situations, near?surface heterogeneities and discontinuities, as well as topography irregularities, diffract the surface waves and generate secondary events, which can heavily contaminate records. The diffracted and converted surface waves are often called scattered noise and can be a severe problem particularly in areas with shallow or outcropping hard lithological formations. Conventional noise attenuation techniques are not effective with scattering: they can usually address the tails but not the apices of the scattered events. Large source and receiver arrays can attenuate scattering but only in exchange for a compromise to signal fidelity and resolution. We present a model?based technique for the scattering attenuation, based on the estimation of surface‐wave properties and on the prediction of surface waves with a complex path involving diffractions. The properties are estimated first, to produce surface?consistent volumes of the propagation properties. Then, for all gathers to filter, we integrate the contributions of all possible diffractors, building a scattering model. The estimated scattered wavefield is then subtracted from the data. The method can work in different domains and copes with aliased surface waves. The benefits of the method are demonstrated with synthetic and real data.  相似文献   

14.
—Northeastern Venezuela has been studied in terms of coda wave attenuation using seismograms from local earthquakes recorded by a temporary short-period seismic network. The studied area has been separated into two subregions in order to investigate lateral variations in the attenuation parameters. Coda-Q ?1 (Q c ?1) has been obtained using the single-scattering theory. The contribution of the intrinsic absorption (Q i ?1) and scattering (Q s ?1) to total attenuation (Q t ?1) has been estimated by means of a multiple lapse time window method, based on the hypothesis of multiple isotropic scattering with uniform distribution of scatterers. Results show significant spatial variations of attenuation the estimates for intermediate depth events and for shallow events present major differences. This fact may be related to different tectonic characteristics that may be due to the presence of the Lesser Antilles subduction zone, because the intermediate depth seismic zone may be coincident with the southern continuation of the subducting slab under the arc.  相似文献   

15.
双相介质中地震波衰减的物理机制   总被引：1，自引：0，他引：1  
High-frequency seismic attenuation is conventionally attributed to anelastic absorption. In this paper, I present three studies on high-frequency seismic attenuation and propose that the physical mechanism results from the interference of elastic microscopic multiple scattering waves. First, I propose a new theory on wave propagation in a two-phase medium which is based on the concept that the basic unit for wave propagation is a nano- mass point. As a result of the elasticity variations of pore fluid and rock framework, micro multiple scattering waves would emerge at the wavelength of the seismic waves passing through the two-phase medium and their interference and overlap would generate high- frequency seismic attenuation. Second, I present a study of the frequency response of seismic transmitted waves by modeling thin-layers with thicknesses no larger than pore diameters. Results indicate that high-frequency seismic waves attenuate slightly in a near-surface water zone but decay significantly in a near-surface gas zone. Third, I analyze the seismic attenuation characteristics in near-surface water and gas zones using dual-well shots in the Songliao Basin, and demonstrate that the high-frequency seismic waves attenuate slightly in water zones but in gas zones the 160-1600 Hz propagating waves decay significantly. The seismic attenuation characteristics from field observations coincide with the modeling results. Conclusions drawn from these studies theoretically support seismic attenuation recovery.  相似文献   

16.
—?We have used micro-earthquake recordings (M= 1.8–4.1) of local events in the distance range of 5–60?km in order to quantify the attenuation and site effects in the vicinity of the Bursa city, Marmara region, Turkey. The data set consists of 120 three-component recorded accelograms from 69 earthquakes, recorded at six stations. Each station is deployed on different geologic units, such as massive limestone, slope deposit and Quaternary young sediments, in the framework of the Marmara Poly-Project.¶In this study a nonparametric inversion method was applied to acceleration records from the Bursa region to estimate source, site and path effects using a two-step inversion. At the first step, we determined attenuation functions by analyzing the distance dependence of the spectral amplitudes and retrieved values of Q s (f) = 46.59f 0.67. At the second step, the corrected S-waves spectral records for the attenuation function, including the geometrical spreading effect, were inverted to separate source and site response for 21 different frequencies selected between 0.5 and ～25?Hz. The near-surface attenuation, κ value, was also estimated by using the model proposed by Anderson and Hough (1984) at each site. We observed that κ0 is smaller for stations located on rock site (I?dιr, SIGD, κ0～0.004) compared to the one that is located on Neogene sediment (Çukurca, SCKR, κ0～0.018).¶Site amplifications from inversion showed that the station located within the Bursa basin, Çukurca (SCKR), is the most important site with about 4.0 amplification value at 1.8?Hz. Demirta? (SDEM) amplifies the spectral amplitudes about 3.0 times at 2.0?Hz, SHMK about 3.5 times between 2.5 and 3.5?Hz and SHMT nearly reaching 3.5 times between 1.5 and 4.0?Hz. However, stations located on the Uluda? Mountain Massif (SKAY and SIGD), which correspond to a deep limestone geological unit, have the smallest amplification, that values between 0.6 and 1.4.  相似文献   

17.
Records of small local Kamchatka earthquakes were processed using Multiple Lapse Time Window Analysis (MLTWA). The method makes use of normalized integrals of 3D seismic energy density in several windows applied to an earthquake record that has been put through a bandpass filter. The intrinsic attenuation and scattering properties of the earth were estimated by choosing parameters that provide the best agreement between experimental and theoretical integrals as functions of hypocentral distance. The theoretical normalized integrals as functions of distance were computed using an analytical solution to the equation of seismic energy transport based on a simple model of isotropic scattering for scalar seismic waves excited by an impulse point source in an earth that is uniform as to scattering and intrinsic attenuation. The resulting estimates of attenuation and scattering parameters are similar to those derived in other studies of Kamchatka, as well as of other continents: America (California, US), Europe (the Canary Is., Spain, and Italy), and Asia (Japan).  相似文献   

18.
In this study, the attenuation properties of the crust and the quality factor of S wave in eastern Anatolia (Turkey) were determined by local earthquakes for two different areas, Oltu and Erzurum. Seismic wave attenuation can be changed with high pressure or structural effects. Therefore, we argued that the estimation of attenuation coefficient in seismic active zones in Eastern Anatolia is a very useful tool to determine seismic activities. It uses regional waveform data set from two stations, OLT and ERZ, for 95 events that occurred in these regions between 2001 and 2005. The attenuation has been determined using the Chobra–Alexeev model based on the epicenter distance–amplitude relations. This model allows for investigation of the effects of variations in attenuation properties for different areas. We introduced a new magnitude formula for these areas using the amplitude normalization methods for reference values ML=4, so as to correct effects of the magnitudes. We also determined velocity of seismic waves. The average attenuation coefficient (α), average quality factor (Qs) and P and S waves velocities were obtained with normalized amplitude values for Erzurum (ERZ) and Oltu (OLT) as 0.0135 km−1, 37, 6.20 km/s and 3.38 km/s and 0.0151, 34, 6.13 and 3.48.  相似文献   

19.
The analysis and interpretation of coda waves have received increasing attention since the early seventies. In the past few years interest in this subject has spread worldwide, and the study of high-frequency seismic coda waves has become a very important seismological topic. As a conclusion of the studies accomplished in this time, coda waves are considered the result of scattering processes caused by heterogeneities acting on seismic waves.P andS waves play a particularly important role in this interaction. The process introduces an attenuation which, added to the intrinsic absorption, gives the observed apparent attenuation. Therefore, coda waves constitute a thumbprint left by the heterogeneities on the seismograms. Coda waves offer decisive information about the mechanism of how scattering and attenuation take place. This review describes coda waves in detail, and summarizes the work done in this subject to 1986. The relation between coda waves and attenuation in the context of research on seismic scattering problems is stressed. Particular attention has been given to the application of coda waves to estimate source and medium parameters. The state-of-the-art of the temporal variations of coda wave shape, and the possible use of these variations as an earthquake precursor also are presented. Care has been taken to introduce the statistical models used to deal with the heterogeneities responsible for scattering.  相似文献   

20.
地球深部圈层及沉积盆地是一种分区非均匀介质系统,其中不规则地层边界(含起伏地表)对地震波的主要特征有显著影响,而地层的随机非均匀性则主要影响地震波的散射和衰减特征.为了精确刻画不规则地层边界对地震波的反射、透射效应以及非均质体散射引起的地震波衰减效应,全局广义R/T递推传播矩阵法(GGRTM)被提出并逐步发展成为继有限元和有限差分方法之后的另一种复杂介质高精度地震波传播半解析求解方法.在已有的此类方法中,不规则边界均匀地层GGRTM法的优势在于对不规则地层边界的反射和透射效应的准确模拟,而非均质地层薄板化GGRTM法则能准确描述非均质体散射对地震波衰减的影响.本文吸收这两种已有方法的优势,提出了一种考虑非均匀介质、不规则边界的全局广义R/T递推传播矩阵混合方法,并将其用于对边界不规则、层内非均质的复杂模型的二维SH波场模拟.随后在本文方法与边界元法对比研究的基础上讨论了方法的模拟精度.研究结果表明本文提出的混合法是一种解决复杂模型高精度地震模拟的有效方法.  相似文献   

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