汶川MS8.0地震：地壳上地幔S波速度结构的初步研究

2008年5月12日我国四川省汶川地区发生了震惊世界的M_S8.0地震.历史上,同类地震在大陆内部极为罕见.该地震深部构造背景的研究对理解其成因极为重要.本文利用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布设的大规模密集流动宽频带地震台阵记录的远震P波波形数据和接收函数非线性反演方法,得到了沿北纬31°线的19个台站下方120 km深度范围内的S波速度结构及台站下方地壳的平均泊松比.该观测剖面穿越了主震区,总长度约为420 km. 我们的结果揭示了川滇地块、松潘-甘孜地块和四川盆地三个不同地块构造差异.上述三个地块的地壳结构特征可以概括为：（1）四川盆地前陆壳幔界面向西侧倾斜并有较为明显的横向变形,地壳厚度存在46～52 km的横向变化,中下地壳S波速度存在横向变化,地壳平均泊松比值较高（0.28～0.31）,但在龙门山断裂带附近,显示了坚硬地壳的特征,地壳平均泊松比仅为0.2;（2）松潘-甘孜地块地壳厚度由西侧靠近鲜水河断裂的60 km,向东减薄为52 km,在14～50 km深度范围内存在S波速度2.75～3.15 km/s的楔状低速区,其厚度由西侧的～30 km向东逐渐减薄为～15 km,相应区域的地壳平均泊松比高达0.29～0.31; （3）鲜水河断裂西侧,川滇地块地壳结构相对简单,地壳厚度为58 km,并在26 km深度存在约10 km厚度的高速层,地壳内平均泊松比约为0.25;（4）汶川大震区在12～23 km深度上具有近乎4.0 km/s的S波高速结构,而其下方的地壳为低速结构,地壳平均泊松比0.31～0.32,汶川大震的余震序列主要分布在高速介质区域内. 本文的结果表明松潘-甘孜地块的地壳相对软弱;而且并不存在四川盆地向西侧的俯冲.我们认为在青藏高原东向挤压的长期作用下,四川盆地强硬地壳的阻挡作用可导致松潘-甘孜地块内部蓄积很大的应变能量以及上、下地壳在壳内低速层顶部边界的解耦,在龙门山断裂带附近形成上地壳的铲形逆冲推覆.汶川大地震及其邻近区域所具有的坚硬上地壳和四川盆地的阻挡作用为低应变率下的高强度应力积累创造了必要条件,而松潘-甘孜地块长期变形积累的高应变能构成了孕育汶川大地震的动力来源.     

新疆和田小孔径地震台阵的建设

从地震台阵的布局设计、技术思路、工程建设、数据传输和台阵监测能力等方面介绍了新疆和田小孔径地震台阵的建设情况.建成后的小孔径台阵填补了新疆地震监测台网无地震台阵的空白,提高了新疆台网对南部塔里木盆地、昆仑山地区、阿尔金山及藏北等台网布局"盲区"的地震监测能力.     

Comparison of three different methods in investigating shallow shear-wave velocity structures in Ilan,Taiwan

The Central Weather Bureau (CWB) and National Center for Research on Earthquake Engineering (NCREE) launched a project to build an engineering geological database for strong-motion stations in Taiwan in 2000. The project measures shear-wave velocity using the suspension PS-logging method. In this study, we conduct array measurements of microtremors and apply the stress wave propagation method (SWPM) at seven free-field strong-motion stations in Ilan County to estimate shallow shear-wave velocity structures. We focus on the sediment layers of the top 100 m to compare the shear-wave velocity structures of the three different methods. There are some misfits among the results of the three different methods; so we calculate the values of Vs₃₀, Vs₁₀₀ and plot S-wave travel-time curves of these methods for each site to analyze the misfits effectively. This analysis helped us to prove the efficiency of the microtremor array method in investigating shear-wave velocity structures in the shallow subsurface. Moreover, the horizontal-to-vertical ratios of microtremors for each survey point show the existence of divergence at the same site. We considered this as evidence that misfits are caused by the heterogeneous nature of sediments and also due to the nature of the methods as being one-, two- and three-dimensional. Furthermore, the average shear-wave velocity structure of microtremor arrays may be more representative of the whole site.     

3D soil structure of the Mygdonian basin for site response analysis

The 3D structure of the Mygdonian sedimentary basin (N. Greece) is investigated. The aim of this study is to propose a 3D model of this sedimentary structure that can later be used to model the seismic records currently being obtained by the permanent accelerograph network operating in the area. This model builds on previous efforts and incorporates new data. The geometry and dynamic properties of the soil layers were inverted using data from microtremor array measurements, seismic refraction profiles, boreholes, and geotechnical investigations. Phase-velocity dispersion curves of Rayleigh waves were determined at 27 sites in the basin using the spatial autocorrelation method (SPAC) introduced by Aki [1]. S-wave velocity profiles were inverted from these dispersion curves and the whole valley structure was interpolated using our new results and all previously available data. The proposed 3D model describes the geometry and shear-wave velocities of the Mygdonian and pre-Mygdonian sedimentary systems, and the top bedrock surface. Our results indicate that this 3D model correctly reflects the geometry and dynamic properties of the sedimentary layers. The case of Euroseistest, where the subsoil structure is the result of bringing together many disparate data, could be used as an example for similar alluvial basins throughout the world, where usually only scarce data is available.     

A modified normalized input-output minimization （Mod-NIOM） method for seismic wave propagation modeling

A new method for wave propagation modeling is introduced in this paper. By using the constraint optimization (Lagrange multiplier) method, the sum of weighted squared Fourier amplitudes is minimized when subjected to a constraint. The sum of the maximum amplitudes obtained from all output models is normalized to unity and is taken as a constraint. In this method, all the actual time histories are considered as outputs and dealt with equally. Independently of the combinations of time histories (or the first ...     

A new stacking technique in ambient noise tomography

We have developed a new stacking technique in ambient noise tomography to obtain high-quality dispersion curves of Rayleigh waves.This technique is used to stack the vertical components of the Estimated Green Functions(EGFs) obtained respectively from cross correlation of the ambient noise data recorded by a remote seismic station and one of the short distance seismic stations of a seismic array.It is based on a phase-matched filter and is implemented by a four-step iterative process:signal compression,stacking,signal extraction and signal decompression.The iterative process ends and gives the dispersion curve of Rayleigh wave when the predicted one and the processing result converge.We have tested the method using the vertical components of synthetic Rayleigh wave records.Results show that this new stacking method is stable and it can improve the quality of dispersion curves.In addition,we have applied this method to real data.We see that the results given by our new technique are obviously better than the ones employing the traditional method which is a three-step process:signal compression,signal extraction and signal decompression.In conclusion,the new method proposed in this paper can improve the signal to noise ratio of EGFs,and can therefore potentially improve the resolution of ambient noise tomography.     

Wave propagation through dense vertical cylinder arrays: Interference process and specific surface effects on damping

The purpose of this research work is to study the effect of specific surface s, the fluid–solid contact surface per volume unit, on the wave energy dissipation by porous structures consisting in dense arrays of emergent vertical cylinders. Experiments have been carried out in a 10 m long wave flume. Three cylinder diameters D are considered in order to study the effects of the specific surface while keeping the porosity constant. In a first series, the length of the porous zone is kept constant for the three cylinder diameters tested. The measurements, which include various wave steepness conditions, demonstrate the role of specific surface s on both wave attenuation and interference processes. The larger the specific surface is, the stronger the wave damping is. Damping is found to be almost proportional to 1/D when laminar, turbulent and inertial effects are of same order. Results are compared to numerical calculations based on either a constant rate of wave damping within the porous medium per unit wavelength or a quadratic damping developed using a force expression based on the work of [26]. This latter model, calibrated with drag and inertia coefficients, shows a good agreement with measurements. In a second series, both porous length and water depth are kept proportional to the cylinder diameter for the three diameters. Scale effects are then discussed and underline the importance of the flow regime within the porous medium.     

Clear water scour around a finite array of cylinders

This experimental study presents clear-water scour and deposition patterns around hexagonal arrays of circular cylinders in steady flow conditions. Understanding the scour processes around such configurations could facilitate the design of several hydraulic and marine engineering structures, such as bridge piers and piles. The flow alteration caused by the examined porous obstacles depends on the solid volume fraction of the obstacles and on the angle of attack of the incoming flow, due to the limited number of cylinders constituting the array. Flume experiments with erodible bed were carried out for four array densities (solid volume fractions: 0.14, 0.20, 0.32 and 0.56) under three different orientations (regular, angled and staggered configurations). The scour/deposition characteristics were obtained by means of laser scanner and the results were compared to solid cylinders of equal circumambient diameter. Different angles of attack of the incoming flow lead to different blockage ratios, which have direct impact on the scour characteristics and deposition patterns. The arrays with the higher solid volume fraction generated scour/deposition patterns similar to solid cylinder, while in the arrays with the lower solid volume fractions, local scour around the individual small cylinders became evident. Finally, considering that the load bearing capacity of a pier basically depends on the area of its cross-section, a comparison of the maximum induced scour depth and volume by the cylinder arrays and the solid cylinder with equal solid cross-sectional area is presented, in order to introduce an alternative pier configuration that induces less scour. The results showed that the array of cylinders could generate 27% less scour volume and 22% less scour depth compared to its single solid cylinder counterpart.     

Characterization of the contractive and pore pressure behavior of saturated sand deposits under seismic loading

This paper presents an identification technique to characterize the contractive and pore pressure behavior of loose sandy soils under seismic excitation. The technique relies on acceleration and pore pressure records provided during excitation by vertical arrays of accelerometers and pore pressure sensors. The technique employs non-parametric estimates of shear stresses and strains. A multi-surface plasticity approach is used to model the soil response. A reduced scale centrifuge model and a large scale experiment are used to demonstrate the capabilities of the developed technique. The technique allows for a more complete interpretation of the coupled shear–volume behavior of a soil deposit.     

关于前兆台阵的概念及地下流体前兆台阵建设的方案讨论

中国地震局在“十五”期间把建设地震前兆台阵列入计划之中,为了使该前兆台阵建设具有创造性,科学性与实用性,笔者提出了前兆台阵的概念与科学目标,建设台阵的科学思路与地下流体台站建设的建议,以便与同行专家进行讨论。