Distribution and structural characteristics of the Xi'an Section of the Weihe Fault

The results of seismic deep reflection,high resolution refraction and shallow artificial seismic exploration indicate that the fault on the northern bank of the Weihe river is composed of two faults,one is the Yaodian-Zhangjiawan fault and the other is the Chuanzhang-Zuitou fault.The 22 km long Yaodian-Zhangjiawan fault of EW-striking starts from Chenjiagou via Yaodian town,Qianpai village,Bili village,Wujia town and Zhangjiawan to Jiajiatan.The 15 km long Chuanzhang-Zuitou fault striking near EW starts from Chuanzhang via Mabei to Zuitou.The Weihe fault offset the basement and upper crust,the reflecting layers of TQ,TN,TE and Tg are ruptured at depth of about 15 km.In the deep part,the Weihe fault and the secondary fault form a Y-shaped structure or a synthetic low angle intersection.The Weihe fault is a listric normal fault.The fault has obvious structural characteristics of a reversed-drag normal fault and a normal drag normal fault with the depth of 1 000 m,and also has the characteris-tics of syngenetic sediment.The Weihe fault is one of the faults which control the basin sediment,and it is the boundary fault of Xi’an depression and Xianyang salient.The depth of the fault decreases from the west to east gradually,the deep part intersects with the Lintong-Chang’an fault at the intersection part of Weihe River,Jinghe River and Bahe River and the shallow part connects with the Weinan-Jingyang fault.The seismic exploration re-sults indicate that no fault exists on southern bank of the Weihe River.     

Recognition on the forming-vanishing process and underlying mechanisms of the hypoxia off the Yangtze River estuary

On the basis of compiled multidisciplinary historical data in 2006–2007 and incorporation of relevant simulation results and remote sensing data, we performed an in-depth study of the generation and dissipation process of the hypoxic zone and its distribution morphology and structure off the Yangtze River estuary. Based on the hydrological circulation dynamics, reproduction of phytoplankton(leading to the decomposition of organic matter), and other factors, we comprehensively and systematically investigated the generation and dissipation of the hypoxic zone and underlying mechanisms for the seasonal variation in its position, explored the multi-factorial synergistic reactions during the generation and dissipation process of the hypoxic zone, and revealed the controlled mechanism for the morphology and structure of the hypoxic zone's distribution. Our studies indicate that in the winter and spring seasons, the hydrological environment off the Yangtze River estuary provides a water body with relatively low contents of dissolved oxygen(DO), which is the background for the formation of a hypoxic zone. After entering into the summer season, the hypoxic zone gradually develops towards the north and becomes mature. Because of the impact of the terrain, local decomposition of organic matter, and upwelling of the Kuroshio subsurface water in July–August, the hypoxic zone off the Yangtze River estuary exhibits the characteristics of discontinuous distribution in space and has a south and north "dual-core" structure in the inner continental shelf. In addition, there is a hypoxic core in the eastern outer continental shelf. The degrees of hypoxia vary for different areas; they are strongest overall in the north, next strongest in the south; they are weakest on the outer continental shelf. In summer, the hypoxic zone in the north is related to the northward differentiation of the southern hypoxic zone and results from local development and intensification. In August, the hypoxic zone in the north reaches its peak, and after September, it rapidly retreats southward and disappears because of weakening stratification. In the fall, there is hypoxic zone along the coast of Zhejiang in the south, and there is also a low-DO area to the southwest of Jeju Island, with both zones disappearing rapidly. In addition, the change of dynamic environment also causes the low-DO area of the outer continental shelf to move outward in the fall. The variation in the intensity of the stratification and its cumulative effects as a barrier of vertical DO transportation over long periods of time have a significant impact on the degree of hypoxia in the hypoxic zone. In addition, the seasonal variations in the size of the stratified region, intensity of each current system/water mass, upwelling, front, and high-value area of phytoplankton biomass jointly restrict the extension of the hypoxic zone in the inner continental shelf and latitudinal(south-north direction) movement of its location off the Yangtze River estuary. The combined effect of dynamic factors, such as that of the Kuroshio subsurface water, causes a low-DO core in the outer continental shelf. The bottom cold water to the north of the East China Sea is the dynamic basis for the formation of the low-DO area to the southwest of Jeju Island during the fall season. The special seabed topography and mud area distribution off the Yangtze River estuary have a certain degree of influence on the development of the hypoxic zone. The generation and dissipation of the hypoxic zone and its distribution morphology off the Yangtze River estuary, and seasonal variation of its structure and position are a result of the synergistic effects of various factors.     

Geometric structure and formation mechanism of Lintong-Chang’an fault zone

The results from investigation of large quantity of fault outcrops and artificial earthquakes suggest that the Lin-tong-Chang’an fault zone mainly consists of two faults. One is the Majie-Nianwan fault that separates a branch of Wangjiabian-Houjiawan fault on the right bank of the Bahe River; the other is the Hujiagou-Shoupazhang fault that separates a branch of Zhongdicun-Tangjiazhai fault in Tongrenyuan and Shaolingyuan. As tensional dip-slip normal faults, the faults distribute with approximately parallel equal intervals in local regions and the profiles drop in a step-like form to the northwest, presenting a Y-shape combination. The result from deep seismic reflection indicates that the fault is about 5~8 km in depth, which is not only a basement fault, but also a listric normal fault in the deep stratum. The Lintong-Chang’an fault is a typical outstretching rift system under the NS-trending ten-sion stress field. At the same time, affected by the sinistral strike slip of the Yuxia-Tieluzi fault, the fault extends like a broom from the northeast to the southwest.     

Climate warming and its impact on natural regional boundaries in China in the 1980s

The global climate warming accelerated in the 1980s has become a focus in the world. Based on the month by month and year by year temperature data from 160 representative stations throughout the country during 1951-1999, this paper analyses annual and four seasons' temperature variations of China since the 1980s. It was found out that the non-equalibrium response with relative great regional and seasonal differences is represented in the country's climate warming. In regional changes a trend of "warm in the north and cold in the south" occurs whereas in seasonal changes, the characters of "warm in winter and cool in summer" present. Significant verification of the temperature variations conducted in terms of mathematical statistics reveals that a confidence level of over 95% has been basically reached in areas north of the Yangtze River. Meanwhile, according to data of diurnal mean temperature steadily passing through accumulated temperature ≥10℃ from 335 stations since 1951 or since the founding of the stations in the early 1950s to 1999, comparative analysis of the data of the last 19 years with that of the first 30 years was conducted and the accumulated temperature ≥10℃ and the variation range of the persistent number of days ≥10℃ were obtained. It was concluded that a general northward shift of central subtropics, north subtropics, warm temperate zone, mesothermal zone and frigid temperate zone of eastern China was observed. The northward shift of north subtropics and warm temperate zone was obvious but changes of south subtropics and marginal tropics were insignificant. In western China, in addition to southwestern Yunnan, the Qinghai-Tibet Plateau and western Inner Mongolia where the temperature zones of each either shifted northward or trended to move upward, not much changes were found in other areas or they shifted southward slightly and declined.     

垂直和倾斜断层围陷波的谱元法数值模拟及其波场特征对比

Fault zone trapped waves( F ZTWs) m ainly travel along the fractured fault zone( F Z)which is of low velocity and high attenuation. FZTWs often carry significant information about a fault  s internal structure,so it is important to understand their wave field characteristics for FZ structure inversion. Most previous simulations are based on vertical faults,while in this paper we implement the FZTW simulations on vertical or inclined faults and compare their wave fields in both time and frequency domains. The results show that the existence of fault zone and inclined angle of fault can significantly influence the features of waves near faults. In amplitude,a fault zone can generate a larger amplitude of waves. The velocity contrast between two walls of fault may lead to amplification of amplitudes in the low velocity fault wall. In frequency,a fault zone tends to influence the waves in the low frequency range. In a pattern of particle polarization of FZTWs,it tends to be single direction for vertical faults but fork to multiple directions for inclined faults,which might provide a new way to study the fault zone with FZTWs. These conclusions may be valuable for FZ structure inversion,and will enhance the knowledge on near-fault strong ground motions.     

Structural characteristics of the Tan-Lu fault zone in Cenozoic basins offshore the Bohai Sea

The Tan-Lu fault zone across the eastern margin of the Cenozoic basins offshore the Bohai Sea is a NNE-trending right-lateral strike-slip fault system developed in the Cenozoic basin cover. It cuts through NE-to NNE-striking major extensional faults that controlled the formation of Paleogene basins. Recent petroleum exploration indicates that Cenozoic structural activities of the Tan-Lu fault system have directly or indirectly affected oil and gas distribution offshore the Bohai Sea. As part of a deep fault zone the Tan-Lu fault zone has been activated since the Oligocene,and obviously affected the tectonic evolution of offshore Bohai basins since then. The formation of Paleogene rift basins offshore the Bohai Sea has utilized the pre-existing structural elements of the Tan-Lu fault zone that developed in the late Mesozoic.     

Preliminary Study on Cenozoic Group and Fault Activity in the Sea Area Near the Yangtze River Mouth

By shallow seismic prospecting, the Cenozoic Group in the sea area near the Yangtze Rver Mouth can be divided into five seismic sequences. They correspond to the Quaternary,Pliocene, Upper Miocene, Lower Miocene and Eocene respectively. The Quaternary System covers all the detecting area. The Tertiary System overlaps and thins out from NE to SW. The sedimentary basement mainly consists of volcanic rock (J3) and acidic rock (r35). Paleogene or Late Cretaceous basins are not found there. The faults that have been detected are all normal faults. They can be divided into three groups (NE, NW, near EW) by their trend. The NE and NW-trending faults are predominant, and agree with aeromagnetic anomaly. Their length and displacement are larger than that of the EW-trending faults. The activity of the NEtrending faults is different in different segments. The SW segment is a Quaternary fault, the middle segment is a Neogene fault, The NE is Paleogene. But the segment of the NW-trending fault is not obvious. The average vertical displacement rate is about 0.015mm/a.     

Protolith ages and deformation mechanism of metamorphic rocks in the Zhangbaling uplift segment of the Tan-Lu Fault Zone

Protolith ages and Indosinian deformation mechanism of metamorphic rocks in the Zhangbaling uplift segment of the Tan-Lu Fault Zone are important, unsolved problems. Our LA-ICP-MS zircon dating work indicates that protolith ages of the greenschist-facies Zhangbaling Group are 754–753 Ma, and those of the amphibolite-facies Feidong Complex are 800–745 Ma. These rocks belong to the earliest cover of the Yangtze Plate. Their ages and metamorphic features suggest that the rocks did not come from the Dabie Orogen. The Indosinian structures in the Zhangbaling Group and lower Sinian strata formed in a flatlying ductile detachment zone with a shear sense of top-to-the-SSW whereas those in the underlying Feidong Complex are characterized by ENE-WSW inclined folds developed under a ductile regime. It is suggested therefore that the sinistral Tan-Lu Fault Zone of the Indosinian period is buried under the Hefei Basin west of the Zhangbaling uplift segment and the uplift segment is a displaced block neighboring the fault zone. Detachment deformation between the upper rigid and lower ductile crust during displacement of the Zhangbaling uplift segment resulted in the formation of the flat-lying ductile detachment zone and its underlying drag fold zone of a ductile regime. The protolith ages and deformation mechanism in the Zhangbaling uplift segment further prove sinistral origination of the Tan-Lu Fault Zone during the continent-continent collision of the North China and Yangtze plates and support the indentation model for the two-plate collision that considers the Tan-Lu Fault Zone as an oblique convergence boundary.     

Study on Holocene Paleoearthquakes in Xizhang Trench on the Jiaocheng Fault Zone,Shanxi Province

Xizhang trench is located 10 km northwest of Taiyuan city, Shanxi Province, in front of a NNW-trending scarp of 4.6m height on the northern segment of the Jiaocheng fault zone. The dimensions of the trench are 108m in length, 8m in width, and 10m in depth. There are 18 horizons revealed in the trench. The upper strata are sandy loam; the upper strata of the downthrown block of the fault are gravels, the lower ones are an interbed of brown loam and sandy loam. The strata on the upthrown block of the fault are sandy loam containing gravel. The trench shows 3 fault planes, and the upper offset point on the fault plane is 1.5m below the ground surface, the newest dislocated stratum is （3.74±0.06） ka BP. The trench reveals a lot of deformation traces, such as fault planes, dislocated strata, colluvial wedges and formation tilting. The relationship between strata and faults in the trench shows that 3 paleoearthquake events have occurred at the Jiaocheng fault zone since the Early Holocene, they are about （3.74±0.06）- （3.06±0.26）ka BP, （8.35±0.09）ka- （3.74±0.06）ka BP, and（ 10.66±0.85） - （8.35±0.09）ka BP. The average interval among the events is 2.6 - 3.6ka. The minimum coseismic vertical displacements of the 3 events are 3.0m, 2.5m and 3.2m, respectively. The significance of Xizhang trench is that the Jiaocheng fault used to be active thousands of years ago, though there is no M ≥ 7.0 earthquake recorded in historical documents. Evidence of new Jiaocheng fault zone activity during the Holocene is important for the earthquake safety assessment of Taiynan city in the future.     

Shear wave velocity structure of the crust and upper mantle in Southeastern Tibet and its geodynamic implications

Southeastern Tibet, which has complex topography and strong tectonic activity, is an important area for studying the subsurface deformation of the Tibetan Plateau. Through the two-station method on 10-year teleseismic Rayleigh wave data from 132 permanent stations in the southeastern Tibetan Plateau, which incorporates ambient noise data, we obtain the interstation phase velocity dispersion data in the period range of 5–150s. Then, we invert for the shear wave velocity of the crust and upper mantle through the direct 3-D inversion method. We find two low-velocity belts in the mid-lower crust. One belt is mainly in the SongPan-GangZi block and northwestern part of the Chuan-Dian diamond block, whereas the other belt is mainly in the Xiaojiang fault zone and its eastern part, the Yunnan-Guizhou Plateau. The low-velocity belt in the Xiaojiang fault zone is likely caused by plastic deformation or partial melting of felsic rocks due to crustal thickening. Moreover, the significant positive radial anisotropy(VSHVSV) around the Xiaojiang fault zone further enhances the amplitude of low velocity anomaly in our VSVmodel.This crustal low-velocity zone also extends southward across the Red River fault and farther to northern Vietnam, which may be closely related to heat sources in the upper mantle. The two low-velocity belts are separated by a high-velocity zone near the Anninghe-Zemuhe fault system, which is exactly in the inner and intermediate zones of the Emeishan large igneous province(ELIP). We find an obvious high-velocity body situated in the crust of the inner zone of the ELIP, which may represent maficultramafic material that remained in the crust when the ELIP formed. In the upper mantle, there is a large-scale low-velocity anomaly in the Indochina and South China blocks south of the Red River fault. The low-velocity anomaly gradually extends northward along the Xiaojiang fault zone into the Yangtze Craton as depth increases. Through our velocity model, we think that southeastern Tibet is undergoing three different tectonic modes at the same time:(1) the upper crust is rigid, and as a result, the tectonic mode is mainly rigid block extrusion controlled by large strike-slip faults;(2) the viscoplastic materials in the middlelower crust, separated by rigid materials related to the ELIP, migrate plastically southward under the control of the regional stress field and fault systems; and(3) the upper mantle south of the Red River fault is mainly controlled by large-scale asthenospheric upwelling and may be closely related to lithospheric delamination and the eastward subduction and retreat of the Indian plate beneath Burma.     

On the synchronism in the events within the core and on the surface of the earth: the changes in the organic world and in the polarity of the geomagnetic field in the phanerozoic

The data on geomagnetic reversals are compared with the changes in the organic world and with the lower-mantle plumes. The times of the formation of plumes and the times of their appearance on the Earth’s surface relate to the intervals characterized by the different frequencies of geomagnetic reversals, i.e., there is no interrelation between the formation of plumes and the frequency of the changes in the geomagnetic field polarity. At the same time, a certain synchronism is observed between the frequency of the geomagnetic reversals and the boundaries of the biostratigraphic ages, i.e., the changes in the organic world in the long-period range. A hypothesis is proposed, which explains the change in the sign of the geomagnetic field by the combined effect of the irregular rotation of the internal core relative to the mantle and the changes in the slope angle of the axis of the Earth’s rotation, which, in turn, results in synchronous events on the Earth’s surface: the rates of changes in the organic world.     

Comparison Between the Nutations of the Planet Mars and the Nutations of the Earth

The nutations of the planets Mars andEarth are investigated and compared. Alarge number of interior structureparameters are involved in the nutationcomputations. The comparison between the observations and the computationsprovides several constraints on these parmeters andtherefore allows a better understanding of the physics of the interior of theplanet. For the Earth, the high precision of the observations of the nutationshas led to a very good determination of interior properties of the planet. ForMars, observations of nutations are not yet available, and we review how theamplitude of the Martian nutations depends on the hypotheses consideredfor its interior. Although Mars is very similar to the Earth, its interior is not well known;for example, we don't knowif its core is liquid or solid. Only if the core is liquid,the Free Core Nutation (FCN) normal mode exists and can alter the nutationswhich are close to the resonance. From the observed geoids, it is known thatboth planets are not in hydrostatic equilibrium. The departure is larger forMars than for the Earth, and consequently, the implication of considering a convective mantle instead of a mantle in hydrostatic equilibrium described byClairaut's equation for the initial equilibrium state of the planet is largeron the Martian nutations than on the Earth nutations. The consequences of theuncertainty in the core dimensions are also examined and shown to be of a veryhigh influence for Mars if the core is liquid, due to the potential changes inthe FCN resonance. The influence of the presence of an inner core, which isknown to exist for the Earth, could be more important for Mars than for theEarth if the inner core is large. Due to the presence of Tharsis on Mars, thetriaxiality of this planet has, additionally, larger effects than on Earth.     

太平洋中部地磁台11年周期变化的研究

综合APIA、HONOLULU、PAMATAI 3个地磁台三分量的11年周期变化的形态后,可定性地提出该变化源于内场。如内源为核幔边界的电涡流,则其中心点应位于3台之间。     

On the use of the autocorrelation method for estimating the depth to the magnetic basement

The depth to the top of magnetic dykes can be estimated from total field aeromagnetic data using the relation between the depth to magnetic sources and the autocorrelation function of magnetic data. By using synthetic anomalies we show that in the ideal case, depth can be determined to an accuracy of 10% or better, when the anomaly sources are two-dimensional dykes. However, the estimated depths depend on the width of the dykes. The estimated depth is about 0.6 times the actual depth to the top of thin dykes, and around the true depth for thick dykes having width-to-depth ratio around 3. The depth is considerably overestimated for very thick dykes (e.g., contacts, which is a special case of the thick dyke). Thus, the autocorrelation method requires that the width-to-depth ratio of the dyke is estimated independently to correctly estimate the depths. Alternatively, it must be assumed that the width-to-depth ratio for the two-dimensional source body is between 1.5 and 4.     

固体潮的地震预测研究与地球动力学研究之分析比较

本文分析了现行固体潮地震研究中的历史影响,对比研究了固体潮地震预测研究和固体潮地球动力学研究二者在振动频率、振动源体积、传输距离与介质等方面的地球物理特点与本质区别,强调了固体潮地震预测研究中最根本的工作是建立地震预测的目标和基本概念.     

On the Mechanism of the Secular Tidal Acceleration of the Solid Inner Core and the Viscosity of the Liquid Core

As is known, the secular deceleration of the Earth's diurnal rotation is explained mainly by the tidal friction in the ocean. Below we consider this mechanism in some detail, taking into account also elastic deformations of the mantle under the action of ocean loading and the interaction between the tide-generating body, ocean tidal wave, liquid outer core, and solid inner core. It is shown that elastic displacements of the core-mantle boundary under the action of ocean loading are of about the same amplitude and phase as the elastic loading displacements of the Earth's outer surface. As a result, side by side with the mechanism of secular deceleration of diurnal rotation of the mantle, there are also (1) the opposite mechanism of secular acceleration of diurnal rotation of the outer liquid core and of the solid inner core and (2) the mechanism of excitation of differential rotation in the liquid core. Taking these effects into account, we compare theoretical and modern observed data on the eastward drift of the solid inner core. It is shown that the best agreement may be obtained if the turbulent viscosity of the liquid core is about 2 × 10 ³ Poise     

Macrobenthic communities of the Pechora Sea: the past and the present on the threshhold of the prirazlomnoye oil-field exploitation

On data of bottom sampling, carried out by means of grab, trawl and underwater photography in August–September of 1993 in the area of the Pechora Sea, quantitative regularities of macrobenthos distribution are described for the ecological monitoring purposes. Maps of -biodiversity and biomass indices, bottom communities and trophic zones, singled out by dominant method, are presented. Assessment of structure changes of the investigated area bottom communities during the last 60–70 years is fulfilled. It is shown that the described communities on the whole are of natural undisturbed character and that the revealed changes are within the natural fluctuations in the abundance of benthic populations or may be interpreted in terms of methodical differences in the data analysis and generalization by different authors.     

动水压力对深水桥墩地震响应影响的研究综述

地震激励下处于深水中的桥墩和周围水体的相互作用将对桥梁结构的动力响应产生较大影响。首先对地震作用下水-桥墩的相互作用理论做了概括,给出了动水压力对桥墩的作用效应及各自的适用范围;对主要的三种考虑流固耦合效应的分析方法做了对比,探讨了地震作用下影响水-结构相互作用的主要因素,并对今后的研究提出了建议。     

On the maintenance of the axisymmetric part of the flow in the atmosphere

Summary The maintenance of the axisymmetric component of the flow in the atmosphere is investigated by means of a steady-state, quasi-geostrophic formulation of the meteorological equations. It is shown that the meridional variations in the time-averaged axisymmetric variables can be expressed as the sum of three contributions, one being due to the eddy heat transport, another to the eddy momentum transport, and a third to the convective-radiative equilibrium temperature which enters the problem through the specification of a Newtonian form of diabatic heating. The contributions by the large scale eddies are evaluated through the use of observed values for the eddy heat and momentum transports.The contributions from each of the three forcing mechanisms to the temperature and zonal wind fields are invstigated individually and found to be of about equal importance. The sum of the three contributions are also presented for the temperature, the zonal wind, the stream function associated with the mean meridional circulation and the corresponding vertical motion. Although the results fail to reproduce the main observed features of the lower stratosphere, they are found to be in good agreement with observations in the middle latitude troposphere. At any pressure level, for example, the computed mean zonal wind has a jet-like profile and the axis of the jet is found to slope to the south with height, as observed in the atmosphere.Based in part on a thesis submitted by the first author as partial fulfillment of the requirements for the Ph.D. degree at the University of Michigan. — Publication No. 194 from the Department of Meteorology and Oceanography, The University of Michigan.     

环太平洋俯冲带内双地震带及其成因机制研究进展

俯冲带作为地球最为庞大的循环系统的重要组成部分,已成为地球科学的研究热点之一.很多俯冲带,特别是环太平洋俯冲带内的中深源地震,在空间上呈明显的分层分布,并且各层地震具有不同的震源机制,即所谓的双地震带现象.本文简要介绍了环太平洋双地震带形态特征与震源机制的空间分布,并回顾了双地震带的几种成因模型.根据形态特征和震源机制的差异,中源深度的双地震带可以分为两类,其中,一类双地震带对应上、下二层分别为压缩和张性的地震分布;另一类双地震带的震源分布较浅,且其浅部地震以横向压缩为主.此外,日本本州东北俯冲带的地震分布可能是由三层地震带组成的,而且汤加、伊豆-小笠原地区还发现深源深度的双地震带.通过对双地震带的形态特征以及其热力学条件的研究,人们从抗弯作用、脱水脆化、相变断层等多方面,尝试建立解释双地震带成因的模型.目前,大多数研究利用数值计算结果,结合蛇纹石脱水脆化、相变断层模型,能够不同程度地分别解释中源和深源双地震带成因.不过,这些模型几乎相互独立,并不能同时解释中源和深源双地震带.有人试图尝试用统一模型解释中深源地震成因,例如,先前存在的断层模型,不过该模型还不很具有说服力.也可能是多种因素的联合作用,共同影响着俯冲板内的温度场、应力场分布.