Structure of the upper mantle in Asia from phase and group velocities of Rayleigh waves

Dispersion curves of phase velocities of Rayleigh waves are determined by the method of frequency-time analysis in a range of periods of 10–200 s from data of 43 interstation traces in Central Asia. Because the joint use of phase and group velocities significantly decreases the uncertainty in the determination of S wave velocity structures, the same traces were used for calculating group velocities from tomographic reconstructions obtained in [Yanovskaya and Kozhevnikov, 2003, 2006] and determining average velocity structures along these traces. The velocity structures were calculated by the Monte Carlo and linear inversion methods, which gave consistent results. Using velocity values obtained at fixed depths by the 2-D tomography method, lateral variations in velocities at these depths were estimated, which allowed us to construct smoothed vertical velocity structures at some points in the region. The resulting structures were used as initial approximations for constructing local velocity structures solely from previously obtained local dispersion curves of group velocities in the area (32°–56°N, 80°–120°E). Based on these structures, we mapped the lateral distribution of velocity variations at upper mantle depths of 75–400 km and along three vertical profiles. The inferred velocity variations are in good agreement with data on the tectonics of the region.     

Deep structure of the bohemian massif from phase velocities of Rayleigh and Love waves

Summary Phase velocities of Rayleigh and Love waves have been measured between the broadband seismic stations KHC (Kaperské Hory, South Bohemia) and KSP (Ksi, Lower Silesia), a profile that nearly coincides with the Interactional DSS Profile VII. The data for both wave types were separately inverted into models of shear-wave velocity versus depth. Novotný's model KHKS 82[1] for the DSS Profile VII was used as a start model. While the crustal section of Novotný's model is compatible with both of our data sets, our Rayleigh-wave data require smaller shear-wave velocities, on the average by 0.24 km/s, in the top 180 km of the mantle. The average difference between Novotný's model and our Love-wave model in that depth range is only 0.06 km/s. If our identification of the observed Love waves as the fundamental mode is correct, this result indicates the presence of polarization anisotropy in the uppermost mantle.

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Regionality of group velocities of Rayleigh waves in the Pacific and thickening of the plate

The relationship between group velocities of Rayleigh waves and the ocean-bottom age in the Pacific is examined. The Pacific basin is divided into four regions by isochrons determined from geomagnetic lineations. A significant change in group velocities of Rayleigh waves is obtained for these four regions by the use of the least-squares method from data for 27 paths in a period range 40–90 s. The present result and other geophysical observations strongly suggest the “thickening of the oceanic plate”, and are well explained by a simple plate-thickness/age relationl(km) = 7.49 t (m.y.)^1/2 inferred from the “mantle gravity anomaly”.     

Low Q zone at the base of the mantle: Evidence for lower mantle convection?

Temperatures in the lower mantle of the Earth are estimated from the observed Q distribution. A thermal boundary-layer where temperatures rise rapidly is found at the base of the mantle, corresponding to the low Q zone described by Anderson and Hart (1978a,b). The existence of this thermal boundary layer indicates that the lower mantle participates in convection, and also that some of the energy driving the convection is coming from the core.     

Analytical partial derivatives of the phase- and group velocities for Rayleigh waves propagating in a layer on a half-space

The dispersion relation for Rayleigh waves in a layer on a half-space is modified by introducing the quadratic wave number instead of the phase velocity. The implicit function theorem is then used to derive analytical formulae for the group velocity and for the phase- and group velocity partial derivatives with respect to the parameters of the medium. As two examples, the method is applied to the interpretation of dispersion curves for short-period Rayleigh waves observed in a sedimentary basin of the West Carpathians, and in the Moravo-Silesian region, Czech Republic.     

Computation of analytical partial derivatives of phase and group velocities for Rayleigh waves with respect to structural parameters

Summary The analytical expressions used to compute the partial derivatives of phase and group velocity of Rayleigh waves with respect to the P- and S-wave velocity and the density are derived and the related computer code is developed. The results of the analytical computations were satisfactorily tested against numerically determined values. Several examples of partial derivatives for a given structural model are presented.     

扬子与华夏块体缝合带区域的Rayleigh及Love波相速度层析成像

扬子与华夏块体碰撞拼合构成华南大陆, 其间存在多条大型断裂带, 两个块体缝合于哪些断裂尚存争议, 这限制了对华南构造演化的认识.本研究使用布设于扬子和华夏块体之间的184个流动密集台站以及华南地区273个固定台站的三分量连续波形数据(2014年10月至2016年12月), 采用背景噪声层析成像方法获得研究区域内Rayleigh和Love波周期为4~25 s的二维相速度.短周期的相速度与地壳中上部的盆地和褶皱等地质构造单元吻合; 长周期的相速度在东部地区整体表现为高速异常, 可能是受较薄的地壳厚度影响, 西南部地区Rayleigh波呈现高速异常, Love波则呈现显著低速异常, 这种差异可能反映该区域地下介质存在径向各向异性.成像结果显示, 扬子与华夏块体的Rayleigh和Love波相速度存在较大的差异, 结合华南地区已有的地质与地球物理学研究成果, 本研究认为扬子与华夏块体可能在地壳浅层缝合于绍兴—江山—萍乡—永州—河池一线, 深部缝合带则可能由石台—九江—长沙延伸至永州—河池线.

     

Joint inversion of Rayleigh group and phase velocities for S-wave velocity structure of the 2021 MS6.0 Luxian earthquake source area,China

On September 16, 2021, a M_S6.0 earthquake struck Luxian County, one of the shale gas blocks in the Southeastern Sichuan Basin, China. To understand the seismogenic environment and its mechanism, we inverted a fine three-dimensional S-wave velocity model from ambient noise tomography using data from a newly deployed dense seismic array around the epicenter, by extracting and jointly inverting the Rayleigh phase and group velocities in the period of 1.6–7.2 s. The results showed that th...     

基于勒夫波的鲁甸MS6.5地震震源复杂性分析

2014年8月3日,云南省鲁甸县发生了M_S6.5地震,造成600余人死亡,震害之大出人预料. 我们利用经验格林函数技术从国家地震台网记录的宽频带勒夫波提取了视震源时间函数,通过分析其方位依赖性获得了关于震源结构与能量辐射复杂性的认识,进而讨论了它们与震害之间的关系. 对视震源时间函数的分析表明,能量释放的时间过程比较简短,83%的能量集中释放于前10 s,但这次地震发生于两个走向分别为60°和150°左右的相互交叉的断层,破裂开始于60°取向的断层,起初以双侧破裂为主,后来向60°方位的传播占优势,但很快触动了150°取向的断层,同样,起初以双侧破裂为主,后来向150°方位的传播占优势. 由于两条断层同时参与了这次地震,近场能量辐射具有较强的方位复杂性,其中60°~210°方位能量辐射较强. 所以,能量释放时间过程之简短但交叉的震源结构和依赖于方位的能量辐射应该是造成如此震害的主要原因.     

Seismic Q for the lithosphere of the Montenegro region (Yugoslavia): frequency,depth and time windowing effects

The availability of accelerometric data for the Montenegro earthquake of 15th April 1979 makes it possible to investigate seismic Q of the lithosphere in that region, in particular, its dependence on frequency, on the depth reached by seismic waves, and on the length of time windows in which signals are processed. Two different spectral methods, S phase energy ratio and coda envelope decay, are applied, respectively, to direct and scattered shear waves. Similar results are obtained using different portions of the recordings, i.e., coda waves for the envelope decay fit and the S wave train, with a significant duration of ～ 10 s, for the energy ratios. The same apparent Q (Q ～ 40 f, where f is the frequency expressed in Hz) that is found for other neighbouring central Mediterranean regions (e.g., Ancona, on the central Italian Adriatic coast; Valnerina, in the central Apennines; Irpinia, in the southern Apennines) is also found for the southern Yugoslavian coast, in the band 1–25 Hz up to a maximum range of ～ 120 km from the focus. This strong frequency dependence is probably connected with the type of small-scale heterogeneity and the same geological age and level of tectonic activity peculiar to all these seismotectonic areas.In order to compare the apparent Q of the whole S wave train, ～ 10 s long, with the (intrinsic) apparent Q of the single direct S wave (usually 1 s or less), the maximum entropy method is applied in the energy spectrum computation for shorter wave trains. The use of shorter time windows does not reveal any significant variation in the tendency of Q to increase linearly with frequency as the length of the time window containing the sample of the S waves decreases. This seems to indicate that scattering-dependent Q is generally inseparable from intrinsic Q in the lithosphere when estimates based on variations with distance of the seismic signal spectrum are used. While the type of linear growth with frequency does not seem to undergo any variations (it remains of the Q = qf type), the data show there are a considerable decrease in the coefficient of proportionality Q with decreasing duration of the window of S waves analysed, probably as a result of variations in seismic attenuation with depth.     

Subionospheric VLF/LF monitoring of ionospheric perturbations for the 2004 Mid-Niigata earthquake and their structure and dynamics

This paper reports on the detailed spatial structure and temporal dynamics of the ionospheric perturbations associated with the huge 2004 Mid-Niigata earthquake in Japan. The terminator time method in VLF/LF subionospheric propagation has been used to deduce the presence and dynamics of the seismo-ionospheric perturbations by making full use of our Japanese VLF/LF network observations. It is found that significant shifts were observed in terminator time for some selected paths, a few to several days before the earthquake, and we obtained the anisotropic shape of the inferred ionospheric perturbation likely elongated along the fault lines there. The temporal dynamics of inhomogeneity of the perturbations is suggested on the basis of a comparison of the observed terminator time shift and theoretical full-wave computations (2D FDTD and 3D full-wave scattering).     

两种短期概率预测模型在2017年九寨沟7.0级地震中的应用和比较研究

本文选用"传染型余震序列"（ETAS）模型和Reasenberg-Jones（R-J）模型,分别对九寨沟MS7.0地震序列的模型参数稳定性、余震发生率预测和余震概率预测进行了比较研究,并利用"地震信息增益"（IGPE）、N-test和T-test检验方法对预测效果进行了评价.研究结果表明,ETAS模型和R-J模型的序列参数分别在震后t2=2.0天和t2=1.50天后趋于稳定,此次九寨沟MS7.0地震序列的衰减较为正常;对未来1天的余震发生率预测和余震概率连续滑动预测表明,ETAS模型给出的余震发生率和余震概率数值均低于R-J模型预测结果;IGPE结果显示,ETAS模型在95%的置信区间上预测效果明显优于R-J模型;统计检验结果表明,在序列参数较不稳定的震后早期阶段,ETAS模型预测失效而R-J模型预测效果较好,在序列参数稳定阶段,ETAS模型预测效果较好而R-J模型预测失效.根据上述分析,在与此次九寨沟MS7.0地震类型相同的地震的余震预测策略上,如可在序列参数不稳定的震后早期阶段使用R-J模型、在此后使用ETAS模型,或可取得较好的预测效果.