云南地区尾波衰减特征研究

利用22个云南区域数字地震台网近震资料,研究了云南地区地震尾波在1~20 Hz内6个频段的衰减特征. 首先使用单次散射模型计算各台站的尾波衰减系数; 然后利用随机各向同性散射介质均匀分布的多次散射模型, 使用多流逝时间窗分析方法,计算各台站的平均自由程Le和地震反照率B0得到总衰减系数,并从中分离散射衰减系数 、吸收衰减系数 . 云南地区基本衰减特征是Le主要集中在10~30 km内, 在2~6 Hz之间有最大值;B0在1~2 Hz大致为0.5左右,在其它频段则小于0.5. 也即在1~2 Hz, 与 数值相当, 其它频段 逐渐大于,成为尾波总衰减的主要原因. 除1~2 Hz外,其它频段 值接近. 云南地区的Q-1尤其是, 在空间上有较大的差异,滇东略微高于滇西北, 滇西北高于滇西南地区. 与全球其它区域的研究结果比较发现,云南地区 略低于平均水平, 略高于平均水平, 而则处于其它区域结果的中间.      

关中、陕南及周边地区尾波衰减特性（Qc）研究

根据单次散射模型的尾波功率谱分析方法,利用陕西数字地震台网宽频带数字地震波形记录资料,计算了关中陕南及周边地区13个频段的Qc值。结果显示:在2ts~35 s的时间窗内,Q0值在37.54~72.59之间,全区平均值为58.44,在上升运动显著的秦岭山脉值较大,关中盆地和陕南值较小;在2ts~70 s的时间窗内,Q0值在194.74~283.50 之间,全区平均值为186.79,莫霍界面高波速区值较大,低速区和低速梯度带值较小。     

山东地区尾波衰减特性（Qc）研究

基于单次散射模型,利用山东数字地震台网数字地震波形资料,计算了山东地区13个频段的Qc值.结果表明,在1.0-20Hz(短周期)或0.05-20Hz(宽频带)频率范围内,Qc值随频率的变化近似服从幂函数关系Qc=Q0fη,拟合得到山东地区尾波Qc值随频率的变化关系为Qc(f)=118.7±26.8f0.94±0.14,并且选取的流逝时间越长,即时间窗越靠后,Qc值一般也越大.     

山东地区尾波衰减特性（Qc）研究

基于单次散射模型,利用山东数字地震台网数字地震波形资料,计算了山东地区13个频段的Qc值。结果表明,在1．0—20Hz（短周期）或0．05—20HIz（宽频带）频率范围内,Qc随频率的变化近似服从幂函数关系Qc^mQ0f^q,拟合得到山东地区尾波Qc值随频率的变化关系为Qc（f）=118．7±26．8f^0.94±0.14,并且选取的流逝时间越长,即时间窗越靠后,Q值一般也越大。     

不同构造区固有吸收衰减和散射衰减相对强弱的对比研究

Q值是表征地球介质物理化学状态、描述地震波衰减特征的重要参数。根据衰减机理的不同,可以把衰减分为固有吸收衰减和散射衰减。论文首先强调分离地球介质的固有吸收衰减和散射衰减,进而研究固有吸收衰减与散射衰减的相对强弱,对于揭示地球介质不同的物理化学状态和控制地震波能量衰减的机制有重要意义。论文选用S尾波记录来研究介质的固有吸收衰减和散射衰减。在对国内外多种尾波衰减模型的优缺点和适用性范围进行对比的基础上,选用了尾波能量通量模型来分离介质的固有吸收衰减和散射衰减。     

地震波散射研究的新进展——第25届IASPEI大会S13专题报告会“地震波散射与衰减”简介

本文以IASPEI的“地震波的散射与衰减”专题报告会为主,评述了近几年在地震波散射方面的研究进展.述评所涉及的方面有:1.关于地震尾波的起源;2.大震前尾波包络的前兆性变化;3.地震尾波包络衰降的机理;4.区分散射和吸收的新方法;5.新领域的开拓和研究.     

粤西地区的地震尾波Q值和尾波衰减特征

根据Aki的尾波理论,利用短周期仪器记录的地震资料,用两种方法估测了粤西地区的尾波Q值,得到了如下结果;Q_0值为260—370,频率相关系数η值为0.0—0.2。阳江地区的Q_0值较低,其它地区大致相同.Q_0值的分布与地质构造和地震活动有较好的相关性. 研究了粤西地区的尾波衰减特征,得到尾波衰减率K为1.88—3.18,K值与Q值的分布彼此有较好的对应性。以上结果可为地震烈度衰减和地震危险性分析等方面的研究提供参考。     

Characteristics of coda wave attenuation in Yunnan area

The characteristic of seismic coda wave attenuation in Yunnan area in 7 frequency-bands range from 1 Hz to 20 Hz was estimated by using the local earthquake's waveform data recorded from 22 Yunnan digital seismic stations.Coda attenuation Q-c1 of each station was firstly calculated by single scattering method. Then, mean free path Le and seismic albedo Bo of each station were calculated, and scattering attenuation Q-1s and intrinsic attenuation Q-1i were separated from total attenuation Q-1t by multiple lapse time window analysis based on the multiple scattering model in uniform random isotropic scattering medium. The attenuating characteristics in Yunnan show that most value of Le are in 10～30 km, with maximal within 2～6 Hz;Bo are about 0.5 at 1～2 Hz, but less than 0.5at other frequency-bands, which means Q-1i is comparable with Q-1s at 1～2 Hz, and after 1～2 Hz, Q-1i is greater than Q-1s and dominates the attenuation process. Q-1c is close to Q-1i at other frequency bands except 1～2 Hz.Results show that Q-1 especially Qs-1 varies spatially, Q-1 in eastern Yunnan zone is a bit higher than in northwestern Yunnan zone;northwestern Yunnan zone higher than southwestern Yunnan zone. Comparing with other results in global, Qs-1 in Yunnan is lower than the global average value among these results, Q-1i is higher than the global average value, and Q-1t lies the middle among these results.     

Rock-mass characterization using intrinsic and scattering attenuation estimates at frequencies from 400 to 1600 Hz

Intrinsic and scatteringS-wave quality factors (Q ) were estimated using the Multiple Lapse Time Window Analysis (MLTWA) for microseismic events (M<–1) with source-sensor distances of 45 to 120 m, associated with an excavation at 630 m depth in Strathcona Mine, Sudbury, Canada. Additional information on the rock mass was provided by underground structural mapping data. IntrinsicQ values, at 800 Hz, were on the order of 140, similar to quality factor values obtained in previous studies using Spectral Decay and Coda-Q methods (120 to 170). The scattering quality factor at this frequency was about 520. An observed frequency dependence of the scattering attenuation suggested that a decrease in the density of scatterers, with scale lengths on the order of 2 m, exists at the site. Characteristic fracture scale lengths were considered to range from 4 to 6 m as identified in the mapping data. These observations were supported by the increase in scattering found for seismic waves with frequencies less than 1000 Hz. By assuming that the identified scatters are characteristic faults, these scatterers can then be considered to increase nonsimilar behavior in source scaling. Overall, our results suggest that MLTWA provides a practical method for remotely characterizing the quality of a rock mass when visual observations are not attainable.     

CodaQ as a combination of scattering and intrinsic attenuation: Numerical simulations with the boundary integral method

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.     

Intrinsic absorption and scattering attenuation in the southern part of the Netherlands

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.     

Lapse time and frequency-dependent attenuation characteristics of coda waves in the Northwestern Himalayas

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.     

The coda attenuation of the Yao''''an area in Yunnan Province

IntroductionThetravellingqualityfactorQofseismicwaveisoneofbasicphysicalparametersformeasuringthemediaattenuation(1/Q)intheearthandreflectionofinhomogeneityandinelasticityofthemedia,whichisusedinstudiesoffocalphysicsandengineeringearthquake.Qiscloselyrelatedtotheregionaltectonicactivityandseismicity.Thisfeaturehasbeenstudyingasoneoffactorsofearthquakeprediction(Chouet,1979;JinandAki,1986,1989).InYunnanregion,QIN(1992)andQIN,etal(1995)hadstudiedthefeaturesofregionalvariationofcodaQcandflu…     

邯郸-邢台地区尾波衰减特征研究

利用红山数字地震台和邯郸数字台网记录到的2001年3月~2006年7月近震波形资料,采用Aki的尾波单次散射模型计算了邯郸—邢台地区不同尾波窗长的Qc值,窗长为20s时该区域的平均值为Qc=78f0.92,40s时Qc=144f0.88。在研究范围内呈现出北高南低的态势。     

随机弹性介质中地震波散射衰减分析(英文)

地震波衰减一直是许多学科研究的热点,因为可以反映介质的特性。导致地震波衰减的因素很多,如:传播过程中由于能量扩散导致的几何衰减,固体岩石内部晶粒间相对滑移导致的摩擦衰减,岩石结构不均匀引起的地震波散射衰减。本文主要从统计的观点出发,通过多次数值模拟的方法研究纵波散射在随机弹性介质中所引发的衰减。首先用随机理论建立了二维空间随机弹性介质模型,然后用错格伪谱法的数值方法模拟了波在随机介质中的传播,再通过波场中虚拟检波器的记录,用谱比法估计了弹性波在随机介质中的散射衰减。不同非均匀程度随机弹性介质中的数值结果表明:介质不均匀程度越高,散射衰减越大;在散射体尺寸小于波长的前提下,不同散射体尺寸的计算结果说明:散射体尺寸越大,弹性波衰减越明显。最后提出了一种不均匀孔隙介质中流体流动衰减的方法。通过对随机孔隙介质中地震波的总衰减和散射衰减分别进行了计算,并定量得出了随机孔隙介质中流体流动衰减,结果表明:在实际地震频段下,当介质不均匀尺度101米量级时,散射衰减比流体流动衰减要大,散射衰减是地震波在实际不均匀岩石孔隙介质中衰减的主要原因。     

在成像空间中衰减多次波方法研究

在偏移后成像空间中的共成像点道集中可以对多次波进行衰减,对于给定的偏移速度模型,一次波与多次波在叠前偏移后的共成像点道集中具有不同的动校时差,这样我们就可以使用类似于偏移前衰减多次波的方法将一次波和多次波进行分离.本文在成像空间中应用抛物Radon变换分离多次波和有效波,由于每个共成像点道集都包含了复杂三维波场传播效应,所以本文方法具有处理三维数据和复杂地下构造的能力.相比于SRME以及传统Radon变换衰减多次波方法,本文方法能够在保持较小的计算量的同时,保证了衰减多次波的准确性.通过对模型数据试算和对实际数据的处理验证了本文方法在叠前时间偏移后衰减多次波的能力,并取得了很好的成像效果.     

震后趋势判断中尾波振幅衰减特征k值的研究

利用唐山、浑源、岫岩和浙江省的鄞县、珊溪水库地震等不同地震类型的数字地震波资料，经过筛选和整理，对每一个地震序列求出几个台站的尾波振幅衰减特征，k值随时间序列的变化特征。结果表明，虽然k值的变化趋势比较复杂，但还是发现不同震型的k值的变化特征有所不同。从趋势上来说，在主震前后，主震一余震型地震的k值显示先低后高的趋势，震群型地震显示先高后低的趋势。我们可以根据这个特征来判断所发生地震的震型，用于震后地震趋势快速判断。