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1.
Michael A. H. Hedlin Peter M. Shearer 《Physics of the Earth and Planetary Interiors》2002,130(3-4):195-208
We investigate the utility of PKP coda waves for studying weak scattering from small-scale heterogeneity in the mid-mantle. Coda waves are potentially a useful probe of heterogeneity in the mid-mantle because they are not preferentially scattered near the CMB, as PKP precursors are, but are sensitive to scattering at all depths. PKP coda waves have not been used for this purpose historically because of interference with other late-arriving energy due to near-surface resonance and scattering. Any study of deep mantle scattering using coda waves requires the removal of near-surface effects from the data. We have analyzed 3624 recordings of PKP precursors and coda made by stations in the Incorporated Research Institutions for Seismology (IRIS) Global Seismographic Network (GSN). To study the range and time dependence of the scattered waves, we binned and stacked envelopes of the recordings. We have considered precursors that arrive within a 20 s window before PKP and coda waves in a 60 s window after PKP. The PKP scattered waves increase in amplitude rapidly with range as predicted by scattering theory. At ranges below 125°, we predict and observe essentially no scattered energy preceding PKP. Coda amplitudes at these ranges are independent of range and provide an estimate of energy due to near-surface effects that we can expect at all ranges. We use the average coda amplitude at ranges from 120 to 125° to correct coda amplitudes at other ranges. PKP coda waves show a strong dependence on time and range and are clearly influenced by scattering in the lower mantle. PKP coda waves, however, do not provide a tighter constraint on the vertical distribution of mantle heterogeneity than is provided by precursors. This is due, in part, to relatively large scatter in coda amplitudes as revealed by a resampling analysis. Modeling using Rayleigh–Born scattering theory and an exponential autocorrelation function shows that PKP coda amplitudes are not highly sensitive to the vertical distribution of heterogeneity in the mantle. To illustrate this we consider single-scattering in two extreme models of mantle heterogeneity. One allows heterogeneity just at the CMB; the other includes heterogeneity throughout the mantle. The amplitudes of precursors are tightly constrained by our stack and support our earlier conclusion that small-scale heterogeneity is uniformly distributed throughout the lower mantle. The best-fit model includes 8 km scale length heterogeneity with an rms velocity contrast throughout the mantle of 1%. 相似文献
2.
在实验室内利用超声尾波观测大尺度(1.5 m)岩石断层的黏滑过程.利用基于尾波干涉的观测方法,我们获得了高达10~(-6)的相对波速变化的观测精度,这相当于~10 kPa的应力变化.利用高精度的测量,我们获取两种不同加载速率下(1μm·s~(-1),10μm·s~(-1))黏滑过程三个阶段(恢复、加载和滑动)基于波速变化的特征量.我们更进一步获取了断层失稳阶段波速变化的时空演化过程.最后讨论了该观测方法需要改进的地方.以上研究结果表明作为一种对现有实验观测手段的有益补充,利用超声尾波观测实验室大尺度岩石断层的动力学过程是可行的. 相似文献
3.
根据地方震尾波散射理论，利用云南武定MS6.5主震后近场田心和云龙两个流动数字化台站记录的余震序列地震资料，以不同中心频率（1.5～20Ｈｚ）取样分频研究了近场尾波振幅衰减率（f ）的空间分布特征．结果表明，两个台站给出的尾波振幅衰减率（f ）存在明显的小区域范围非均匀性变化．位于主发震断层两侧的云龙台（东侧）和田心台（西侧）测得的尾波振幅衰减率（f ）值，当 f ＜6.0Ｈｚ时基本一致．相应的Qc（f ）值为66～120的低值；但在高频范围（f 6.0Ｈｚ），由于穿过主断层的近场尾波遭到断层破碎带强烈的吸收，因而云龙台测得的尾波振幅衰减率（f ）值，比不穿过主断层的田心台给出的（f ）降低30％，并讨论了产生近场尾波振幅衰减空间分布差异的原因．本文的研究结果还表明，波源因子A0（f）不仅与震源强度相关，也与频率犳相关，用1.5～20Ｈｚ求出的Ｓ波平均自由程犔在主断层东侧为30ｋｍ、西侧为40ｋｍ ，由此证实了地震波穿过主断层破碎带时有更强的散射． 相似文献
4.
IntroductionSeismic wave is an important information source to analyze and study seismic source and internal structure of the earth. Studies of wave on different frequency ranges can provide many features of seismic source and internal structure of the earth. In recent years in quantitative study of seismic wave seismologists at home and abroad have developed a wide study field on the coda. Especially seismologists have done a lot of work in coda geneses, mechanism and its attenuation to explo… 相似文献
5.
Aki (1969) first modeled coda waves of a local earthquake as a superposition of scattered surface waves. This paper attempts
to clarify the constituents of surface-wave coda at long periods at very long lapse times. For a large earthquake of magnitude
7 or larger, vertical component oscillation in periods from 90 to 180 s persists for more than 20 hours from the earthquake
origin time. Although the early portion of the coda envelope is successfully modeled by assuming incoherent scattered Rayleigh
waves by heterogeneities distributed all over the Earth, the later potion of the observed coda envelope (roughly later than
35,000 s) has systematically larger amplitude than theoretical prediction. To clarify the cause of this discrepancy, we studied
the constituents of vertical-component seismograms of three large earthquakes recorded by the F-net in Japan using the f-k power spectral analysis. We found that the direct and scattered fundamental-mode Rayleigh waves of velocity about 3.7 km/s
are dominant in the earlier part of each envelope. It justifies the use of a scattering model of the fundamental Rayleigh
waves for synthesizing the envelope. At lapse times later than 20,000 s–35,000 s, higher modes with phase velocities around
20 km/s become dominant. The transition time to the dominance of higher modes is found to become earlier for a deeper focus
earthquake. The small coda attenuation factor from (1.90±0.23) × 10−3 to (2.38±0.32) × 10−3 estimated from later coda envelopes recorded at IRIS stations distributed worldwide also agrees with the attenuation factor
of spheroidal modes according to PREM. We may interpret that higher mode waves are uniformly distributed at large lapse time
due to large velocity dispersion and/or scattering and they dominate over the fundamental mode waves because of smaller attenuation
in the lower mantle. The coda attenuation measurement proposed by Aki is found to be useful even for long periods and at very
large lapse times. 相似文献
6.
Anomalous earthquakes such as creep events, tsunami earthquakes and silent earthquakes have been reported in the recent literature. In this paper we discuss an anomalous “slow earthquake” that occurred on June 6, 1960 in southern Chile. Although the surface-wave magnitude of this event is only 6.9, it excited anomalously large long-period multiple surface waves with a seismic moment of 5.6 · 1027 dyn cm. The Benioff long-period seismogram of this earthquake recorded at Pasadena shows an extremely long, about 1.5–2 h coda of Rayleigh waves, with a period of 10–25 s. The coda length for other events with a comparable magnitude which occurred in the same region is about 10 min. This observation suggests that the long coda length is due to a long source rupture process which lasted at least 1 h. Although at least 15 distinct events can be identified in the coda, no short-period body waves were recorded corresponding to these, except for the first one. These results suggest that a relatively small (Ms ? 6.9) earthquake triggered a series of slow events; the duration of the whole sequence being longer than 1 h. This event probably occurred on a transform fault on the extension of the Chile Rise and provides important information regarding the nature of the transform fault. 相似文献
7.
基于Aki等（1975）的地方震尾波单次散射模型,利用文山地震台记录到2005年8月13日在云南文山县发生的5.3级地震余震序列的数字化波形观测资料,测量了震源区尾波Q（f）值。测量结果表明,当中心频率为1．5Hz时,文山地区的尾波Q值在52～155之间,平均值为91,尾波的振幅衰减率β（f）在0.013～0．039之间,平均值为0．024;测量得到该区尾波Qc值与频率f的关系为Qc（f）=62f^“0.87;尾波波源因子A0与震级ML成正比关系,满足关系lgA0=1．02ML-0．73。此次地震的发震构造为文山断裂带,地震波受到破碎带强烈的非弹性吸收,尾波Qc值明显降低。 相似文献
8.
地震波的散射与盆地的场地效应 总被引：3，自引：0，他引：3
根据射线理论对二维盆地型试场地与Ｐ波波列特征的关系，波列中震相的性质及其产生的原因等进行了研究，对决定震相振幅大小的散射系数，几何扩散和波阻抗３个因子等的研究表明，因界面弯曲引起的广角反射和聚焦分别Ｐ波波阵中出现大能量团的第一和第二位的重要因素，在层内纵，横波波速比和层间波速比大范围变动的条件下，对界面散射系数的计算表明，介质内界面上Ｐ到Ｓ的反射转换波是指示界面两侧速度反差强弱以及入射波所在介质层 相似文献
9.
Michel Campillo 《Pure and Applied Geophysics》1990,132(1-2):1-19
TheLg wave consists of the superposition ofS waves supercritically reflected, and thus trapped, in the crust. This mode of propagation explains the strong amplitude of this phase and the large distance range in which it is observed. The numerical simulation leads to successful comparison between observed seismograms in stable continental areas and synthetics computed for simple standard crustal models. In regions with strong lateral variations, the influence of large-scale heterogeneities on theLg amplitude is not yet clearly established in terms of the geometrical characteristics of the crustal structure.The analysis of the decay of amplitude ofLg with epicentral distance allows the evaluation of the quality factor ofS waves in the crust. The results obtained show the same trends as codaQ: a clear correlation with the tectonic activity of the region considered, both for the value ofQ at 1 Hz and for its frequency dependence, suggesting that scattering plays a prominent part among the processes that cause the attenuation.The coda ofLg is made up of scatteredS waves. The study of the spatial attenuation of the coda indicated that a large part of the arrivals that compose the coda propagate asLg. The relative amplitude of the coda is larger at sites located on sediments because, in these conditions, a part ofLg energy can be converted locally into lower order surface modes. 相似文献
10.
Ultrasonic coda waves are widely used to study high-frequency scattering. However, ultrasonic coda waves are strongly affected by interference from by boundary-reflected waves. To understand the effect of boundary-reflected waves, we performed ultrasonic experiments using aluminum and shale samples, and the rotating staggered-mesh finite-difference method to simulate the wavefield. We analyzed the wavefield characteristics at the different receiving points and the interference characteristics of the boundary-reflected waves with the ultrasonic coda wave, and the effect of sample geometry on the ultrasonic coda waves. The increase in the aspect ratio of the samples delays the interference effect of the laterally reflected waves and reduces the effect on the ultrasonic coda waves. The main waves interfering with the ultrasonic coda waves are laterally reflected PP-, PS-, PPP-, and PPS-waves. The scattering and attenuation of the high-frequency energy in actual rocks can weaken the interference of laterally reflected waves with the ultrasonic coda waves. 相似文献
11.
利用围绕澜沧江水利水电枢纽工程地区的8个地震台记录到的数字化波形资料，根据尾波散射理论，以不同的中心频率研究了该地区的尾波衰减。结果显示：研究区尾波的Q0值在46～80之间，平均值为63；田值在0．66～0．94之间，平均值为0．79。与云南其它地区相比，研究区的Q0值比构造活动强烈地区的要高，属于中等构造活动地区；与世界上其它地区相比，研究区的尾波衰减不同于构造地震活动相对较弱的地区，而与构造地震活动频繁、强烈的地区较为接近，因此判断澜沧江流域及周边地区未来有发生中强地震的可能。另外，地震波衰减与断层的破碎程度有明显的依赖关系，地震波经过断层破碎带时，被强烈散射和吸收，Q0值明显降低，而且，不但近场的地震波衰减具有这种性质，远场的地震波衰减也同样具有这种特性。 相似文献
12.
安徽"霍山窗"中小地震的 地震波衰减特征研究 总被引：1，自引：0，他引：1
利用安徽合肥台、 六安台和豹子崖台记录到的2001—2010年安徽“霍山窗”发生的ML≥2.0地震的波形资料, 计算了其振幅衰减系数K值和卓越频率衰减系数a值, 分析研究了K值和a值的时空变化特征, 并讨论了其与华东地区中强地震的对应关系及与Qc值的相关性. 结果表明, 3个台站均记录到了明显的变化特征: ① “霍山窗”ML4.0地震、 江西九江MS5.7地震及河南太康ML5.0地震前6个月左右时间内, K值和a值均呈现持续下降的异常低值状态, 平均下降幅度K值为20%—35%, a值为45%—51%, 并于上升的过程中发震, 震后趋于正常; ② K值的平面分布特征显示, K值异常地震主要位于断裂带交汇区附近, 并沿断裂带方向分布; ③ K值、 a值与Qc值基本呈负相关. 上述特征为进一步了解该区域的地壳介质状态和研究中强震前后Qc的变化特征提供了一定的依据. 该研究结果可能对中长期地震预报和震后趋势判断有一定的指示意义. 相似文献
13.
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
14.
Phase and Correlation in `Random' Seismic Fields and the Reconstruction of the Green Function 总被引：3，自引：0，他引：3
Michel Campillo 《Pure and Applied Geophysics》2006,163(2-3):475-502
We first present a summary of recent results on coda interpretation. We emphasize the observation of the stabilization of
P to S energy ratio indicating the modal equipartition of the wavefield. This property clearly shows that the coda waves are
in the regime of multiple scattering. Numerical solutions of the elastic radiative transfer equation are used to illustrate
the evolution of the wave-field towards P-to-S energy stabilization, and asymptotically to complete isotropy. The energy properties
of the coda have been widely studied but the phase properties have often been neglected. The recently observed coherent backscattering
enhancement, an expression of the so-called `weak localization', demonstrates that interference effects still persist for
multiple diffracted waves. Another manifestation of the persistence of the phase is the possibility to reconstruct the Green
function between two stations by averaging the cross correlation of coda waves produced by distant earthquakes and recorded
at those two stations. This reconstruction is directly related to the properties of reciprocity and time reversal of any wavefield.
Using broadband seismic coda waves, we show that the dominant phases of the Green function in the band 2 s–10 s, namely fundamental
mode Rayleigh and Love waves, are reconstructed. We analyze the time symmetry of the cross correlation and show how the level
of symmetry evolves with the isotropization of the diffuse field with lapse time. Similarly we investigate the correlation
in continuous ambient noise records. Whereas the randomness of the coda results from multiple scattering by randomly distributed
scatterers, we assume that the seismic noise is random mostly because of the distribution of sources at the surface of the
Earth. Surface waves can be extracted from long time series. The dispersion curves of Rayleigh waves are deduced from the
correlations. On paths where measurements from earthquake data are also available, we show that they are in good agreement
with those deduced from noise correlation. The measurement of velocities from correlation of noise along paths crossing different
crustal structures opens the way for a `passive imaging' of the Earth's structure. 相似文献
15.
—?In order to clarify the effects of contact geometry of faults on transmission waves, we have performed a series of experiments in which P and S waves with known wavelength were transmitted through an artificial fault. A pair of piezo-electric transducers (PZT) with various resonant frequency were used for the transmitter and the receiver. Parallel grooves were cut on disk surfaces and two disks were placed face to face with the grooves on one disk being perpendicular to those on the other disk. This yields evenly spaced square contacts on the fault. We regard the square contacts as asperity contacts, the size and the height of which were controlled by changing the width and the depth of the grooves. We found that the transmissivity of the waves is solely determined by the ratio of the groove depth/width to wavelength. The shallower and the narrower the groove depth and width are, the larger the amplitude of first arrival is for both P and S waves. When the groove depth is shallower than a quarter of wavelength, the effect of groove depth is negligible; deeper grooves significantly reduce the amplitude. We have made a mathematical model based on the stiffness of fault. By comparing the model calculations with the observation we found that the model has a limit at which the prediction by the model deviates from the data. The deviation occurs when the ratio of the groove depth/width to wavelength becomes 0.25. We refer to the wavelength as the critical wavelength. When the wavelength is larger than the critical wavelength, the observed data can be well explained by the model. Above this threshold, the model no longer fits the data. In this range, the amplitude of transmitted waves is found to be proportional to the real contact area. Although it is a kind of paradox that the amplitude, not the energy, is proportional to the real contact area, it is possibly explained by taking a non-uniform distribution of stress on the surface of the receiver PZT into account. 相似文献
16.
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
17.
Alemayehu Lakew Jemberie 《Pure and Applied Geophysics》2008,165(9-10):1835-1860
Strong and weak motion data from the Mississippi Embayment Seismic Excitation Experiment (ESEE) were analyzed for signatures of nonlinear site responses. This experiment was performed jointly by the University of Memphis and U. S. Geological Survey in October 2002, by detonating two explosions of 2500 and 5000 lbs. Intrinsic and scattering Q estimates (QI and QS) from the coda of the strong motion data were found to be very low compared to previously determined Q values of P- and Rayleigh waves of weak motion data from the same explosions. The QI estimates from P-wave late coda of the strong motion data are less by more than 100 at 3 Hz and by more than 200 at 10 Hz, compared to the P-wave Q values determined from the weak motion data by Langston et al (2005). Also, QI determined from the late coda of strong motion Rayleigh-wave data is less by more than 200 at 0.5 Hz and by more than 50 at 3.0 Hz, compared to Q values determined from Rayleigh-wave weak motion data. A resonance peak spectral amplitude of the early part of a strong motion seismogram is shifted to lower frequencies compared to that from a later part of the same seismogram. Spectral amplitude ratios between transverse and vertical components of the strong motion data are degraded between frequencies 2 and 10 Hz for P waves, and less than 4 Hz for Rayleigh waves compared to the weak motion transverse to vertical spectral ratio. All these are signatures of nonlinear site responses during strong ground motion. This study proves the non-transportability of weak motion attenuation results to estimate ground motion from a future large earthquake that may take place in areas like the New Madrid Seismic zone. 相似文献
18.
非线性高阶谐波和尾波波速变化均能够反映水泥材料内部微结构的应力变化。利用高阶谐波和尾波干涉实验测量系统，对引入高温作用后的3类不同粒径共6块水泥试样进行单轴加载的损伤演化实验，并与无高温作用的完整试样的实验结果进行对比。结果表明，从初始状态到25%抗压强度的过程中，高温作用后的试样的谐波幅值和尾波波速变化出现明显增强的现象（谐波幅值最大增幅约20%），而无高温作用的完整试样的谐波幅值和尾波波速变化较为平稳（谐波幅值最大增幅约5%）；在达到65%抗压强度的过程中，高温作用后的试样的谐波幅值和尾波波速变化急剧增大（谐波幅值最大增幅约100%），且粒径较大的试样的增幅高于粒径较小的试样，而无高温作用的完整试样的谐波幅值和尾波波速变化的增幅较小（谐波幅值最大增幅约10%）；当抗压强度超过75%以后，高温作用后的试样的谐波幅值和尾波波速变化急剧衰减（谐波幅值最大衰减幅度约140%），而无高温作用的完整试样的谐波幅值和尾波波速的最大衰减幅值在40%以内。基于以上观测结果对高温作用后水泥制品损伤演化的物理机制以及这两类监测方法的适用性进行了讨论。 相似文献
19.
Haruo Sato 《Pure and Applied Geophysics》1988,126(2-4):465-497
The study of coda waves has recently attracted increasing attention from seismologists. This is due to the fact that it is viewed as a new means by which the stress accumulation stage preceding a large earthquake can be measured, since the scattering paths nearly uniformly cover a fairly large region around the focus and observation stations, compared with the direct ray paths. To date, we have had many reports on the temporal variation of the relation between coda duration and amplitude magnitude, and that of the coda attenuationQ
c
–1
which is estimated from coda amplitude decay. Some of these have shown a precursor-like behavior; however, others seem to have shown a coseismic change. We have critically reviewed these reports, and discussed what these observational facts tell us about the change in the heterogeneous crust. We found significant temporal variations, not only in the mean but also in the scatter ofQ
c
–1
, associated with the mainshock occurrence. The formation of new cracks, the reopening and growing of existing cracks, the interaction of these cracks, and the pore water movement through these cracks might correspond to such variations. In addition, we may expect an inhomogeneous distribution of crack clusters in a fairly large region, compared with the aftershock region. The gradual appearance of such crack clusters seems to be the most plausible mechanism by which coda decay gradients are caused to largely scatter in the stress accumulation stage. 相似文献
20.
利用二维非均匀介质地震波传播的伪谱和有限差分混合方法,通过数值计算,讨论了松软覆盖层对隐伏断层带围陷波特征的影响.在没有覆盖层的情况下,围陷波振幅和围岩上相比明显增加,持续时间变长.覆盖层造成围岩上地面运动振幅增大,围陷波的部分能量传播到覆盖层中,使得围陷波的能量变小.随着覆盖层厚度增加,围陷波的振幅越来越小,和围岩上... 相似文献