Phase and Correlation in `Random' Seismic Fields and the Reconstruction of the Green Function

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.     

利用Aki模型对宁夏及邻区尾波Q值的研究

利用单次散射的Aki模型,选取2008年1月-2009年12月宁夏地震台网记录的82次ML≥2.0地震的数字地震波资料,计算宁夏及邻区尾波Q值,拟合Q值对频率的依赖关系.全区域数据结果为Q(f)=(212±87.62)·f0.7584±0.19.与国内其它区域相比,本区域Q值较低、对频率依赖性较高.结合区域地震地质构造...     

The physical implication of coda amplitude ratio and its possible application

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.     

Constituents of Vertical-component Coda Waves at Long Periods

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.     

云鹏水电站及其周围地震尾波Q_c值研究

利用云鹏水电站周边5个数字地震台站记录到的2000年1月至2008年12月间的108个地震的数字波形资料,采用Aki单次散射模型,计算了各台站周围423条地震记录的尾波Q0值。结果显示:云鹏水电站地区尾波的Qc值与频率的关系为Qc(f)=90.2f0.94。该地区的Q0值比云南其它构造活动强烈地区的要高,因此属于中等构造活动地区。云鹏水电站蓄水后弥勒台记录到的108个地震的尾波Q0值及分频Qc值有所下降,但下降幅度不大,表明水库蓄水对库区及其附近区域地下介质性质有所影响,但影响的程度有限。     

利用两种模型对宁夏及邻区尾波 Q 值进行对比研究

分别利用Aki模型和Sato模型,选取2008年1月-2009年12月宁夏地震台网记录的ML≥2.0地震的数字地震波资料,计算了宁夏及邻区平均尾波Q值,并拟合了Q值对频率的依赖关系.结果为:Aki模型为Q(f)=(212±87.62)f0.7584±0.19;Sato模型为Q(f)=(44.81±15.87)f0.9491±0.0805.与国内其他区域相比,两种模型计算结果均显示本区域Q值较低,对频率依赖性较高.研究结果认为两种模型均适合对宁夏及邻区尾波Q值的研究.     

层裂对区域震相Lg波的影响

地下核爆炸Lg波的激发机制, 目前尚未十分清楚. 普遍接受的观点是: 伴随层裂源的补偿线性矢量偶极源激发的Rg波的散射形成的S波, 是Lg波的最主要贡献因素. 本文利用频谱比对方法, 结合合成理论地震图方法, 基于东哈萨克斯坦地区速度模型, 分析了东哈萨克斯坦地壳速度模型下的层裂对区域震相Lg波的影响, 并进一步分析了该地区的实际观测资料. 结果表明, 层裂时间函数对Lg波具有扇贝形调制作用; Lg波振幅谱的低谷点源自于补偿线性矢量偶极（CLVD）源激发的Rg波. 这一结果也表明, 东哈萨克斯坦速度模型下的Lg波受层裂的调制作用, 与内华达试验区的Lg波是一样的, 进一步支持了Patton和Taylor的观点, 对于进一步理解及更好地利用Lg波具有重要理论指导意义.      

基于理论地震图方法的波导结构对 地下核爆炸Lg波传播特性研究

基于频率-波数域算法的理论地震波形图方法, 可以数值模拟频率达到10 Hz、 震中距达1000 km的区域理论地震波形图. 该算法适用于计算大量分层地壳结构中激发的导波Lg波. 本文在前苏联东哈萨克斯坦地下核试验场至我国乌鲁木齐台站间的地球介质速度模型中, 引入速度梯度结构、 速度扰动分布的薄叠加层结构、 降低Q值结构以及速度扰动与Q值变化的综合结构来模拟实际地壳波导结构, 较好地模拟出东哈萨克斯坦地下核爆炸地震在乌鲁木齐台站记录的宽频带地震波形图, 模拟出完整的Lg波序列, 该序列符合Lg波能量分布特征, 且能够解释Lg波波尾的特征. 结果表明, Lg波的形状和峰值结构均依赖于地壳的不同波导结构.