An indirect boundary element method applied to simulate the seismic response of alluvial valleys for incident P,S and Rayleigh waves

A boundary integral formulation is presented and applied to model the ground motion on alluvial valleys under incident P, S and Rayleigh waves. It is based on integral representations for the diffracted and the refracted elastic waves using single-layer boundary sources. This approach is called indirect BEM in the literature as the sources' strengths should be obtained as an intermediate step. Boundary conditions lead to a system of integral equations for boundary sources. A discretization scheme based on the numerical and analytical integration of exact Green's functions for displacements and tractions is used. Various examples are given for two-dimensional problems of diffraction of elastic waves by soft elastic inclusion models of alluvial deposits in an elastic half-space. Results are displayed in both frequency and time domains. These results show the significant influence of locally generated surface waves in seismic response and suggest approximations of practical interest. For shallow alluvial valleys the response and its resonant frequencies are controlled by a coupling mechanism that involves both the simple one-dimensional shear beam model and the propagation of surface waves.     

Monitoring volcanoes using seismic noise correlations

In this article, we summarize some recent results of measurements of temporal changes of active volcanoes using seismic noise cross-correlations. We first present a novel approach to estimate volcano interior temporal seismic velocity changes. The proposed method allows to measure very small velocity changes (≈ 0.1%) with a time resolution as small as one day. The application of that method to Piton de la Fournaise Volcano (La Réunion Island) shows velocity decreases preceding eruptions. Moreover, velocity changes from noise cross-correlations over 10 years allow to detect transient velocity changes that could be due to long-lasting intrusions of magma without eruptive activity or to pressure buildup associated to the replenishing of the magma reservoir. We also present preliminary results of noise cross-correlation waveform perturbation associated with the occurrence of dike injection and volcanic eruption. We show that such an analysis leads us to locate the areas of dike injection and eruptive fissures at Piton de la Fournaise Volcano. These recent results suggest that monitoring volcanoes using seismic noise correlations should improve our ability to forecast eruptions, their intensity and thus potential environmental impact.     

Reconstructing the Green's function through iteration of correlations

Correlations of ambient seismic noise are now widely used to retrieve the Earth response between two points. In this study, we reconstruct the surface-wave Green's function by iterating the correlation process over the tail of the noise-based correlation function. It has been demonstrated that the so-called C³ function shows the surface-wave part of the Green's function. Using data from 150 continuously recording stations in Europe, the C³ results help in the extraction of the travel-times from noise-based measurements, especially through the suppression of effects caused by non-isotropic source distributions. We present the results of the next iterative step (i.e. C⁵), which show that some coherent signal is still present in the coda of the C³ function, and we investigate the evolution of the reconstruction of the Green's function throughout the iteration process. Finally, we discuss the interest of combining information from the different correlation functions to improve noise-based tomography analysis.     

Tectonic framework of the eastern Alboran and western Algerian Basins,western Mediterranean

Recent progress in the knowledge of the Alboran region subaerial geology and the results of the Garcia del Cid seismic cruises stimulate a reexamination of the tectonic framework of the southwestern part of the Mediterranean. The subaerial geology is governed by a north-south stress field, whereas the marine structures indicate an east-west stress regime. This apparent contradiction results from a difference in age of the tectonics. The main marine extensional features are related to the opening of basins toward the southwest during Burdigalian-Langhian times, while the subaerial structures were developed during a compressional regime from the Tortonian period up to the present.     

Surface-wave propagation across the Mexican Volcanic Belt and the origin of the long-period seismic-wave amplification in the Valley of Mexico

A network of nine broad-band seismographs was operated from March to May 1994 to study the propagation of seismic waves across the Mexican Volcanic Belt (MVB) in the region of the Valley of Mexico. Analysis of the data from the network reveals an amplification of seismic waves in a wide period band al the stations situated in the southern part of the MVB.
The group velocities of the fundamental mode of the Rayleigh wave in the period range 2–13 s are found to be lower in the southern part of the MVB than in its northern part and in the region south of the MVB. The inversion of dispersion curves shows that the difference in group velocities is due to the presence of a superficial low-velocity layer (with an average S -wave velocity of 1.7 km s^-1 and an average thickness of 2 km) beneath the southern part of the MVB. This low-velocity zone is associated with the region of active volcanism.
Numerical simulations show that this superficial low-velocity layer causes a regional amplification of 8–10 s period signals, which is of the same order as the amplification measured from the data. This layer also increases the signal duration significantly because of the dispersion of the surface waves. These results confirm the hypothesis of Singh et al. (1995), who suggested that the regional amplification observed in the Valley of Mexico is due to the anomalously low shear-wave velocity of the shallow volcanic rocks in the southern MVB     

Azimuth dependent wave amplification in alluvial valleys

An extension of the indirect boundary element method (IBEM) to three-dimensional scattering by two-dimensional alluvial valleys is presented. While the valley is two-dimensional, the incident plane waves can arrive outside the 2D plane so the scattering is three-dimensional with coupling of P---SV---SH waves. Such a method makes it possible to take earthquake location into account in the estimation of site effects in alluvial valleys. The method is validated by transparency tests, by comparison with 2D simulations, and by comparison with results of other authors. The advantage of the method is that is combines high accuracy with cost-efficiency in terms of computer-time. It is applied to theoretically estimate site effects across a simplified model of an alluvial valley in the French Alps where azimuth dependence of local amplification has been observed. A parametric study with simulations for a range of azimuths and incidence angles shows that (1) the local amplification depends strongly on both azimuth and incidence of the incoming waves, (2) the global pattern of amplification across the valley is very complex for all azimuths, and (3) it is not possible to predict the 3D response of the valley from 2D modeling. Theoretical spectral ratios are in approximate agreement with observed ones for a station in the center of the valley where the local structure justifies use of a simplified model for the comparison.     

汶川地震区地壳速度相对变化的环境噪声自相关研究

2006年中国地震局地质研究所地震动力学国家重点实验室在川西地区(100°E～105°E, 26°N～32°N)布设了由297个宽频带地震台组成的密集流动地震观测台阵.本文利用川西流动地震台阵29°N以北地区的137个台站2007年1月至2008年10月的连续三分量地震环境噪声记录,研究了汶川地震震前震后地壳速度变化特征.借助川西台阵的密度优势,我们针对单台三分量噪声自相关函数和单台不同分量间的互相关函数,利用互谱移动窗技术,在0.33～1 Hz频带范围内测量了经过50天滑动平均的相关函数与长时间平均参考相关函数的走时变化率,进而求得地壳浅部速度随时间的相对变化,并得到其空间分布特征.本文结果表明,利用环境噪声自相关方法可以得到与互相关方法基本一致的同震速度变化分布图像,同震速度变化分布与同震体应变具有明显相关性.具有更高空间分辨率的噪声自相关研究进一步发现,在鲜水河断裂和龙门山断裂交汇区存在同震速度增加区,这个区域与同震库伦应力变化和地表形变观测预测的周边断层库伦应力增加区一致.我们的研究发现,同震库伦应力增加效应的持续时间大约为2个月左右,在此之后,区域应力场逐渐恢复为普遍的应力下降.