Study of the Earth’s free core nutation by tidal gravity data recorded with international superconducting gravimeters

By stacking high-precision tidal gravity observations obtained with superconducting gravimeters at six stations in China, Japan, Belgium, France, Germany and Finland, the local systematical discrepancies in the parameter fitting, caused by atmospheric, oceanic tidal loading and the other local environmental perturbations, are eliminated effectively. As a result, the resonance parameters of the Earth’s free core nutation are accurately determined. In this study, the eigenperiod of free core nutation is given as 429.0 sidereal days, which is in agreement with those published in the previous studies. It is about 30 sidereal days less than those calculated in theoretical models (about 460 sidereal days), which confirms the real ellipticity of the fluid core of the Earth to be about 5% larger than the one expected in assumption of hydrostatic equilibrium. The quality factor (Q value) of free core nutation is given as about 9543, which, compared with those determined before based on the body tide observations, is much larger, but more close to those obtained using the VLBI observations. The complex resonance strength is also determined as (−6.10×10⁻⁴, −0.01 ×10⁻⁴)°/h, which can principally describe the deformation characteristics of an anelastic mantle.     

Natural hydraulic cracking: numerical model and sensitivity study

Natural hydrofracturing caused by overpressure plays an important role in geopressure evolution and hydrocarbon migration in petroliferous basins. Its mechanism is quite well understood in the case of artificial hydraulic fracturing triggered by high-pressure fluid injection in a well. This is not so for natural hydraulic fracturing which is assumed to initiate as micro-cracks with large influence on the permeability of the medium. The mechanism of natural hydraulic cracking, triggered by increasing pore pressure during geological periods, is studied using a fracturing model coupled to the physical processes occurring during basin evolution. In this model, the hydraulic cracking threshold is assumed to lie between the classical failure limit and the beginning of dilatancy. Fluid pressure evolution is calculated iteratively in order to allow dynamic adjustment of permeability so that the fracturing limit is always preserved. The increase of permeability is interpreted on the basis of equivalent fractures. It is found that fracturing is very efficient to keep a stress level at the rock’s hydraulic cracking limit: a fracture permeability one order of magnitude larger than the intrinsic permeability of the rock would be enough. Observations reported from actual basins and model results strongly suggest that natural hydraulic cracking occurs continuously to keep the pressure at the fracturing limit under relaxed stress conditions.     

Sedimentary cycles of trace elements in Salawusu River Valley since 150 ka BP

The paper makes some analyses on 11 trace elements in the Milanggouwan stratigraphical section in the Salawusu River valley, which is regarded as a prototype geology-palaeoclimate record since 150 ka BP. The results show that the content and variation of trace elements has experienced remarkably regular changes in the pace with coarse and fine sedimentary cycles of palaeo-aeolian sands to its overlying fluvio-lacustrine facies or/and palaeosols. The trace elements with chemical properties of relatively active (V, Sr, Cu, Ni, As) and relatively stable (P, Pb, Rb, Mn, Nb, Zr) are a manifestation of the corresponding 27 changeable cycles between peak and valley values, appearing a multi-fiuctuational process line of relative gathering and migration since then. The low numerical value distribution of these two types of trace elements in the aeolian sand facies represents erosion and accumulation under wind force during the cold-dry climate. Whereas their enrichments in both fluvio-lacustrine facies and palaeosols are related to the valley’s special low-lying physiognomic position between the Ordos Plateau and the Loess Plateau under the warm and humid climate conditions. The above relatively migrated and gathered change of the trace elements is the result of 27 climatic cycles of cold-dry and warm-humid, which is probably caused by repeated alternations of winter monsoon and summer monsoon in the Mu Us Sandy Land influenced by the climate vicissitudes in northern hemisphere during glacial and interglacial periods since 150 ka BP.     

Mixing, sediment accumulation and focusing using 210Pb and 137Cs

The Portil lagoon is a natural freshwater reservoir located at the southwest of Spain, near the coast. In its surroundings an important tourist complex has been developed since the earlier 1970s. This has resulted in increased loads of nutrients, specially during summer months. In order to evaluate the impact from humans on the lagoon caused by anthropogenic activities, we have determined vertical profiles of excess ²¹⁰Pb and ¹³⁷Cs in a sediment core taken from its bottom. Vertical profile of excess ²¹⁰Pb seems to indicate that the sediment core was affected by large-scale mixing processes. Nevertheless this possibility was ruled out based on the shape of the ¹³⁷Cs vertical distribution after modelling the ¹³⁷Cs profile considering large-scale mixing. The chronology developed from ²¹⁰Pb and ¹³⁷Cs has allowed us to calculate the temporal evolution of the mass sedimentation rate during the last century. This mass sedimentation rate increased in 1973: from 0.08(2) g cm^-2 a^-1 in the period 1900-1973 to 0.17(5) g cm^-2 a^-1 in the period 1973-1995. This increase could be related to land-movements in the drainage area and to permitted sewage inputs into the lagoon, both with its origin in the growing/running of the surrounding tourist complex.     

TDP-0844地震前兆数据综合采集器的研制

介绍了TDP—0844型地震前兆数据综合采集器采用的设计方案及工作原理，及各个功能模块的设计准则，并列出了采集器实现的功能。文章还给出了采集器的部分应用图例，证明该采集器能够满足地震前兆观测需要。     

Introduction of structural aseismic research in China in recent four years

Introduction Greeting the coming of the 21st century, Professor HU Yu-xian and other Chinese scholars briefed the trend of earthquake engineering in China and aboard (HU, 1999; HU, ZHOU, 1999). The experiences and lessons learning from the destructive earthquakes in China and abroad in re-cent years, the damage action of the large velocity impulse in ground motion in near field in seis-mic design, numerous earthquake examples show that there are many weaknesses in aspects of earthquake p…     

地震数据IP传输及存储技术应用

介绍一种IP传输方案。该方案通过地震数据IP传输及存储单元(ITU)、IP流多串口服务器完成，能实现单个台站与多个台网中心的Internet传输、单个台站与多个台网中心的专线传输、地震台网中心之间的Internet连接、远程数据下载及参数设置等多种传输方式，该传输方案具备实时性、可压缩性、多路服务、安全性、数据在线保存、传输费用低、操作简单等特点。     

ADP数字地震前兆综合观测台网系统的设计

介绍了ADP数字地震前兆综合观测台网系统的功能与结构。ADP系统已在广东省地震局连续运行，满足前兆台网综合化、数字化、自动网络化的要求。     

ADAPT数字遥测地震台网中传递函数的辨识

Adapt系统可以利用脉冲标定波形得到系统的传递函数，它将复杂的传递函数计算过程简化了，用户可以将传递函数的测定作为一种日常工作来做。     

武汉基准台超导重力仪重力潮汐观测结果

本文采用重力残差信号对武汉基准台超导重力仪的原始观测数据（２０秒采样）作预处理，将原始观测信号分解成重力潮汐和残差两部分，在残差信号中消除一些已知的干扰因素的影响。利用ＥＴＥＲＮＡ软件作调和分析，两主波（Ｏ１和Ｍ２）的重力潮汐参数（振幅因子和相位滞后）为：Ｏ１：１．１７８８±０．０００６，－０．５９７５°±０．０２８°；Ｍ２：１．１７１３±０．０００２，－０．５８７３°±０．００８°经海潮改正以后，Ｏ１和Ｍ２波的潮汐参数与Ｗａｈｒ－Ｄｅｈａｎｔ潮汐模型值相比，振幅因子的偏差优于０．１％。