Crustal structure beneath the Songpan—Garze orogenic belt

The Benzilan-Tangke deepseismic sounding profile in the western Sichuan region passes through the Song-pan-Garze orogenic belt with trend of NNE.Based on the travel times and the related amplitudes of phases in the record sections,the 2-D P-wave crustal structure was ascertained in this paper.The velocity structure has quite strong lateral variation along the profile.The crust is divided into 5layers,where the first,second and third layer belong to the upper crust,the forth and fifth layer belong to the lower crust.The low velocity anomaly zone gener-ally exists in the central part of the upper crust on the profile,and it integrates into the overlying low velocity basement in the area to the north of Ma‘erkang.The crustal structure in the section can be divided into 4parts:in the south of Garze-litang fault,between Garze-Litang fault and Xianshuihe fault,between Xianshuihe fault and Longriba fault and in the north of Longriba fault,which are basically coincided with the regional tectonics division.The crustal thickness decreases from southwest to northeast along the profile,that is ,from62km in the region of the Jinshajiang River to 52km in the region of the Yellow River.The Moho discontinuity does not obviously change across the Xianshuihe fault basesd on the PmP phase analysis.The crustal average velocity along the profile is lower,about 6.30 km/s.The Benzilan-Tangke profile reveals that the crust in the study area is orogenic.The Xianshuihe fault belt is located in the central part of the profile,and the velocity is positive anomaly on the upper crust,and negative anomaly on the lower crust and upper mantle.It is considered as a deep tectonhic setting in favor of strong earthquake‘s accumulation and occurrence.     

Practical VTI approximations: a systematic anatomy

Transverse isotropy (TI) with a vertical symmetry axis (VTI) often provides an appropriate earth model for prestack imaging of steep-dip reflection seismic data. Exact P-wave and SV-wave phase velocities in VTI media are described by complicated equations requiring four independent parameters. Estimating appropriate multiparameter earth models can be difficult and time-consuming, so it is often useful to replace the exact VTI equations with simpler approximations requiring fewer parameters. The accuracy limits of different previously published VTI approximations are not always clear, nor is it always obvious how these different approximations relate to each other. Here I present a systematic framework for deriving a variety of useful VTI approximations. I develop first a sequence of well-defined approximations to the exact P-wave and SV-wave phase velocities. In doing so, I show how the useful but physically questionable heuristic of setting shear velocities identically to zero can be replaced with a more precise and quantifiable approximation. The key here to deriving accurate approximations is to replace the stiffness a₁₃ with an appropriate factorization in terms of velocity parameters. Two different specific parameter choices lead to the P-wave approximations of Alkhalifah (Geophysics 63 (1998) 623) and Schoenberg and de Hoop (Geophysics 65 (2000) 919), but there are actually an infinite number of reasonable parametrizations possible. Further approximations then lead to a variety of other useful phase velocity expressions, including those of Thomsen (Geophysics 51 (1986) 1954), Dellinger et al. (Journal of Seismic Exploration 2 (1993) 23), Harlan (Stanford Exploration Project Report 89 (1995) 145), and Stopin (Stopin, A., 2001. Comparison of v(θ) equations in TI medium. 9th International Workshop on Seismic Anisotropy). Each P-wave phase velocity approximation derived this way can be paired naturally with a corresponding SV-wave approximation. Each P-wave or SV-wave phase velocity approximation can then be converted into an equivalent dispersion relation in terms of horizontal and vertical slownesses. A simple heuristic substitution also allows each phase velocity approximation to be converted into an explicit group velocity approximation. From these, in turn, travel time or moveout approximations can also be derived. The group velocity and travel time approximations derived this way include ones previously used by Byun et al. (Geophysics 54 (1989) 1564), Dellinger et al. (Journal of Seismic Exploration 2 (1993) 23), Tsvankin and Thomsen (Geophysics 59 (1994) 1290), Harlan (89 (1995) 145), and Zhang and Uren (Zhang, F. and Uren, N., 2001. Approximate explicit ray velocity functions and travel times for P-waves in TI media. 71st Annual International Meeting, Society of Exploration Geophysicists, Expanded Abstracts, 106–109).     

Explosion seismic reflections from the Earth’s core

Recent seismological studies have presented evidence for the existence of a layer with ultra-low seismic velocities at the core-mantle boundary at ca. 2900 km depth. We report high-amplitude, high-frequency, and laterally coherent seismic arrivals from three nuclear explosions in Siberia. With recording station intervals of 15 km, the seismic phases are readily correlated and show the presence of a thin, ultra-low velocity zone in a region where it was not previously reported. The duration and complexity of the arrivals are inconsistent with a simple core-mantle boundary and require a hitherto unidentified, kilometre-scale, fine structure in the ultra-low velocity zone. The observations may be explained by a ca. 7 km thick, two-layer, ultra-low velocity zone with exceptional low velocities, which indicate the presence of high percentages of melt (>15%), in particular in the lower part of the zone. Waveform variation implies lateral change in the thickness and physical properties of the ultra-low velocity zone with a wavelength of less than 100 km.     

变参考慢度Born近似傅氏偏移

针对常规Born近似傅氏偏移方法对于剧烈横向变速介质不能精确成像的状况而提出了变参考慢度Born近似傅氏偏移，理论上解决了任意速度变化地质模型的偏移成像问题。此外，为进一步提高复杂地层的成像精度和波场延拓算子的稳定性，对散射波场的计算公式作了改进。将改进的方法运用于盐丘模型的正演和偏移试验，并与常规Born近似偏移方法相比较，可明显看出变参考慢度Born近似傅氏偏移方法在效果上要优于后者，其处理速度横向变化的能力大大增强。     

Velocity structure of uppermost mantle beneath North China from Pn tomography and its implications

20301 Pn arrival time data are collected from the seismological bulletins of both national and regional seismic networks. Pn travel time residuals are tomographically inverted for the Pn velocity structure of uppermost mantle beneath North China. The result indicates that the average Pn velocity in North China is 7.92 km/s, and the velocity varies laterally from ?0.21 to +0.29 km/s around the average. The approximately NNE trending high and low velocity regions arrange alternatively west-eastward. From west to east we can see high velocity in the middle Ordos region, the Shanxi graben low, the Jizhong depression high, the west Shandong uplift and Bohai Sea low, and the high velocity region to the east of the Tanlu fault. In the southern boundary zone of the North China block, except for the high velocity in the Qingling Mountains region, the velocity is generally lower than the average. Obvious velocity anisotropy is seen in the Datong Cenozoic volcanic region, with the fast velocity direction in NNE-SSW. Notable velocity anisotropy is also seen around the Bay of Bohai Sea, and the fast velocity directions seem to show a rotation pattern, possibly indicating a flow-like deformation in the uppermost mantle there. The Pn velocity variations show a reversed correlation with the Earth's heat flow. The low Pn velocity regions generally show high heat flow, e.g., the Shanxi graben and Bohai Sea region. While the high Pn velocity regions usually manifest low heat flow, e.g., the region of Jizhong depression. This indicates that the Pn velocity variation in the study region is mainly aroused by the regional temperature difference in the uppermost mantle. Strong earthquakes in the crust tend to occur in the region with the abnormal low Pn velocity, or in the transition zone between high and low Pn velocity regions. The earthquakes in the low velocity region are shallower, while that in the transition zone are deeper.     

高温高压下含水矿物脱水对斜长岩纵波速度的影响

在1GPa、室温至880℃条件下对斜长岩的纵波速度进行了测量，并对实验产物进行了鉴定分析，讨论了含水矿物脱水对岩石纵波速度的影响。实验结果表明，在680℃左右，由于斜长岩中的含水矿物绢云母和黝帘石开始脱水，矿物脱水产生的流体及其所引起岩石结构的变化可使岩石纵波速度降低10％左右。这意味着通过矿物脱水这一机制可以形成壳内低速层。     

ADV measurements of velocity distributions in a gravel‐bed flume

Velocity measurements carried out by an acoustic doppler velocimeter (ADV) in a rectangular laboratory ?ume having a gravel bed are presented. The velocity pro?les are measured in six verticals of the channel cross‐section having an increasing distance (from 4 to 38·5 cm) from the ?ume wall. The experimental runs are carried out for ?ve different bed arrangements, characterized by different concentrations of coarser elements, and for the two conditions of small‐ and large‐scale roughness. For both hydraulic conditions, the velocity measurements are ?rst used to test the applicability of the Dean pro?le and of the logarithmic pro?le corrected by a divergence function proposed in this paper. Then, for each value of the depth sediment ratio h/d₈₄, the non‐dimensional friction factor parameter is calculated by integration of the measured velocity distributions in the different verticals of the cross‐section. Finally a semi‐logarithmic ?ow resistance equation is empirically deduced. Copyright © 2003 John Wiley & Sons, Ltd.     

Geomechanical Properties of Shear Zones in the Eastern Aar Massif,Switzerland and their Implication on Tunnelling

Summary ¶Rock zones containing a high fracture density and/or soft, low cohesion materials can be highly problematic when encountered during tunnel excavation. For example in the eastern Aar massif of central Switzerland, experiences during the construction of the Gotthard highway tunnel showed that heavily fractured areas within shear zones were responsible for overbreaks in the form of chimneys several metres in height. To understand and estimate the impact of the shear zones on rock mass behaviour, knowledge concerning the rock mass strength and deformation characteristics is fundamental. A series of laboratory triaxial tests, performed on samples from granite- and gneiss-hosted shear zones revealed that with increasing degree of tectonic overprint, sample strength decreases and rock behaviour shows a transition from brittle to ductile deformation. These trends may be explained by increasing fracture densities, increasing foliation intensity, increasing thickness of fine-grained, low cohesion fracture infill, and increasing mica content associated with the increasing degree of tectonic overprint. As fracture density increases and the influence of discrete, persistent discontinuities on rock mass strength decreases, behaviour of the test samples becomes more and more representative of rock mass behaviour, i.e. that of a densely fractured continuum. For the purpose of numerical modeling calculations, the shear zones may be subdivided with respect to an increasing fracture density, foliation intensity and mica content into a strongly foliated zone, a fractured zone and a cohesionless zone, which in turn exhibit brittle, brittle-ductile and ductile rock mass constitutive behaviour, respectively.Received December 17, 2001; accepted January 9, 2003 Published online April 29, 2003     

Empirical correlations of soil parameters based on Cone Penetration Tests (CPT) for Greek soils

This study presents the results of Cone Penetration Tests (CPT) and estimated correlations with the results of other in-situ tests (standard penetration test, cross-hole, seismocone) and laboratory tests (classification and undrained shear strength tests). These tests cover nearly all types of penetrable soils in Greece and, together with the formulation of empirical correlations, complement and improve on previous studies due to the significant number of CPT measurements.     

农田生态系统大气硫沉降通量的观测研究

1998年 11月～ 1999年 10月间在中国科学院红壤站 (江西鹰潭 )农田小气候站进行了大气二氧化硫 (SO2 )、硫酸盐粒子 (SO2 -4)浓度采样和雨水样本收集 ,利用阻力模式和全年逐时气象资料计算 SO2 和 SO2 -4的干沉降速度 ,估算干沉降通量 ,利用降水资料和雨水中硫酸根离子浓度估计大气硫的湿沉降 ,从而定量研究大气硫沉降输入农田生态系统的通量 ,结果表明 :农田下垫面上 SO2 和 SO2 -4干沉降速度的年平均值分别是 0 .373± 0 .170 cm· s-1(月均值 0 .16 1～ 0 .5 45 cm·s-1)、0 .198± 0 .12 3cm· s-1(月均值 0 .15 2～ 0 .2 6 9cm· s-1)。农田下垫面硫年总沉降量为 10 .3g· m-2 ,其中干沉降占总沉降的 83.3%。硫的干沉降又以 SO2 的干沉降为主 ,占年干沉降总量的 92 .2 %。大气硫沉降输入占农田生态系统输入总量的 90 %以上 ,是农田生态系统获取硫素的一个重要途径