Variations of gravity before and after the Haicheng earthquake, 1975, and the Tangshan earthquake, 1976

Before and after the Haicheng earthquake of magnitude 7.3 which occurred on February 4, 1975, five repeated gravimeter surveys were carried out, three before and two after the earthquake, along a northwest-southeast profile of about 250 km in length not far on the west of the epicenter. The mean-square error of the measurements of the gravity differences between two consecutive points on the profile is less than 40 μGal. From June, 1972 to May, 1973, within a period of about one year, the results of three surveys indicated a clear decrease of the gravity values at points on the southeastern portion of the profile, amounting to about 352 μGal. After the earthquake, the fourth survey, which was carried out in March, 1975, revealed that the gravity values had recovered to the levels of the first survey and continued to increase as was shown by a fifth survey carried out in July of the same year.Variations of gravity were also observed before and after the Tangshan earthquake of magnitude 7.8 which occurred on July 28, 1976, but in this case, gravity was increasing instead of decreasing before the earthquake. Along an east-west profile of about 270 km in length and not far on the north of the epicenter, two gravity surveys were made before and two after the earthquake. The results showed that after the main shock, the gravity values of the whole profile, especially at those points closer to Tangshan, tended to return gradually to their values of the first survey before the earthquake.From these results, there seems to be a close relationship between these gravity variations and the occurrences of earthquakes. Based on results of repeated levelling work done in these regions, the estimated amount of gravity change caused by the change of elevation of the ground surface is far too small to account for the observed value. Therefore we speculate that some large earthquakes might be associated with some sort of mass transfer under ground, within the crust or in the upper mantle. This transfer would cause a large part of the gravity variation observed. We have made a theoretical analysis of this effect and attempted to obtain some estimate of the magnitude of this mass transfer, even though we are not yet clear about the physics of it.     

Results of airborne magnetometer profile from offshore Mangalore to offshore Madras (India) along the 13th degree parallel

Summary A study is made of an airborne magnetometer profile from 130 km offshore Mangalore to 60 km offshore Madras approximately along the 13th degree parallel obtained in May 1967 by the National Geophysical Research Institute. The total length of the profile is about 770 km. A Bouguer gravity anomaly profile along same traverse over the land section has been also studied along with the magnetic profile.Eight major anomalies on the magnetic profile were chosen for depth calculations. Depths of the magnetic bodies were determined by elementary approximation method ofSmellie. It is found that in most of the cases the sources of the magnetic anomalies lie between 5 to 20 km (approximately). Only one anomaly yielded a depth of 2.7 km for its source. These anomalies are accounted for in terms of possible basic intrusions, faults, zones of weakness and ridge-like structures in deeper Crustal levels.N.G.R.I. Contribution Number 131.     

Magnetotelluric survey across the active spreading zone in Southwest Iceland

The results of 10 magnetotelluric soundings, performed along a 110-km-long profile crossing the constructive plate boundary in southwest Iceland, are presented. Apparent resistivities are interpreted by a horizontally stratified earth model to yield a pseudo cross-section along the profile. The crust-mantle interface contains a well conductive layer. The depth to the good conductor increases with age of the crust and the distance from the axial zone. This layer is interpreted as partially molten basalt, at a temperature about 1100°C and a volume fraction of the melt phase in the range 10–20%.The high-conductivity layer probably disappears west of the Borgarnes anticlinal axis, which separates the older (to the west) and younger (to the east) flood basalts in western Iceland, indicating that the temperature below the oldest part of the profile lies below the solidus curve of basalt. Recent seismic crustal investigations in the same area indicate a state of partial melting or a magma chamber, which agrees with the results of the magnetotelluric soundings.     

1975年海城地震与1976年唐山地震前后的重力变化

1975年2月4日海城7.3级地震前后在震中以西不远的一条长约250公里的北西—南东向剖面上进行了五次重力测量,震前三次,震后两次.剖面上相邻两个测点的重力差的测量均方误差小于40微伽.自1972年6月至1973年5月的一年期间的三次观测结果表明,剖面东南段重力值显著下降,最大达352微伽.地震以后,1975年3月的第四次重力测量发现,剖面东南段的重力值回升到第一次测量时的水平.1975年7月的第五次测量则表明剖面东南段的重力值继续上升.1976年7月28日唐山7.8级地震前后也观测到重力的变化,不过地震前重力是增加而不是减少.震前和震后,在震中以北不远的一条长约270公里的东西向剖面上各进行过两次重力测量.结果表明,主震后整个剖面,特别是靠近唐山的那些测点的重力也有逐渐恢复到震前第一次测量时的数值的趋势.由这些结果可以看到,重力的变化与地震的发生似有密切的关系.根据重复大地水准测量资料估计的地面高程变化所能引起的重力变化远比所观测到的变化为小.因此,我们推测某些大地震可能与地壳和上地幔内的质量迁移有关,认为所观测到的重力变化大部份是质量迁移引起的.我们对质量迁移的重力效应作了理论分析,但是,对迁移的物理过程我们还很不清楚.      

The regularity of vertical crustal movements of the western slopes of the Peter the Great Range

The results of relevellings made in 1948 and 1969–1975 along the Obi-Hingou profile, which is a part of the Garm test area, are analysed. Different characteristics of slow movements in the separate parts of the levelling line divided by faults were obtained by using the displacement velocities of the Earth's surface in each separate section of the profile (from the results of repeated levellings). The uplift of the Peter the Great Range is determined against the background of subsidence in the Tadjik Depression. The exact values of maximum displacements ( ≈ 7 mm ) were determined and the radius of the deformation zone (12–13 km) was established for earthquakes with M = 4.5–5.     

A very public fireball

J C Bridges, J P Schwanethal, V K Pearson, M D Paton, R C Greenwood, J S Watson and G H Morgan report on a determination of orbital elements of a fireball seen over Britain and Northern France.     

Issues in radiocarbon and U-series dating of corals from the last glacial period

Radiocarbon calibration beyond the extent of tree-ring records depends on U-series dating of fossil corals or speleothem, both of which can provide independent calendar ages. Less direct methods rely on layer counting and comparison with other well-dated records. In spite of considerable effort to provide a reliable radiocarbon calibration curve beyond 25,000 years, the majority of the data show large atmospheric radiocarbon peaks which are inconsistent both in magnitude and timing between different determinations. The results of the most recent work [Chiu, T.-C., Fairbanks, R.G., Mortlock, R.A., Bloom, A.L., 2005. Extending the radiocarbon calibration beyond 26,000 years before present using fossil corals. Quaternary Science Reviews 24 (16–17), 1797–1808], from Araki Island fossil corals, indicate a monotonic variation from about 33 to 49 ka, with no radiocarbon peaks, but with some gaps in the data. The difference between this and previous results, from fossil corals, has been attributed to selection of better-quality samples and rigorous analytical methods. However, previous results from Huon Peninsula [Yokoyama, Y., Esat, T.M., Lambeck, K., Fifield, L.K., 2000. Last ice age millennial scale climate changes recorded in Huon Peninsula corals. Radiocarbon 42 (3), 383–401; Cutler, K.B., Gray, S.C., Burr, G.S., Edwards, R.L., Taylor, F.W., Cabioch, G., Beck, J.W., Cheng, H., Moore, J., 2004. Radiocarbon calibration and comparison to 50 kyr BP with paired ¹⁴C and ²³⁰Th dating of corals from Vanuatu and Papua New Guinea. Radiocarbon 46 (3), 1127–1160] show radiocarbon peaks exclusively located within the gaps in the Araki data. The timing of the gaps are not random, but appear to be related to severe climate and sea-level variations associated with Heinrich events initiated in the North Atlantic. We propose that the Huon and Araki data sets are complementary rather than exclusive and that the absence of coral growth at Araki Island during Heinrich events presumably reflect local adverse conditions for coral growth.     

Deep structure of the North Vent Area, Great Tolbachik Fissure Eruption of 1975–1976, Kamchatka: Evidence from low-frequency microseismic sounding

Studies were conducted to improve our knowledge of the deep structure of the magmatic system and the plumbing system for the North Vent, Great Tolbachik Fissure Eruption of 1975–1976 based on recordings of background microseismic emission by broadband digital instruments along two parallel lines running through eruptive centers of various ages across the main magma-conducting fault. The method of low-frequency microseismic sounding was used for constructing deep sections down to a depth of 20 km, showing the shear-velocity distributions along these lines. Elements of the magmatic system were revealed beneath both vents in the form of low-velocity anomalies. We identified regions of magma chambers at different depths together with the channelways that connect these. It was found that magma might come to shallow chambers from different deep-seated sources along spatially isolated magma conduits, which is one of the possible causes of the variation in the basalt composition during the eruptions. For the zone of areal volcanism we are the first to demonstrate a change in magma-conducting conduits in the transition from the crystalline basement to the volcanogenic sedimentary rock sequence, with subvertical channels being replaced by inclined forms. It was shown that the elements of the magmatic system beneath both eruptive centers studied here are similar. It is hypothesized that there is a regularity in the configuration of plumbing systems in the middle part of the Tolbachik regional zone of areal volcanism.     

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.     

Verification of the Litho-spheric Structure along Profile Uppsala-Prague Using Surface Wave Dispersion

Experimental dispersion curves of Rayleigh and Love waves along the Uppsala-Prague profile have been determined using records of several Italian earthquakes. To interpret the dispersion data, results of previous geophysical investigations in this region were first analyzed. Seven blocks of the crust and upper mantle were distinguished along the profile on the basis of deep seismic sounding and other seismic data. Layered models were proposed for these blocks. Computation of Rayleigh and Love waves shows a large differentiation of theoretical dispersion curves for the northern (Precambrian) and southern (Palaeozoic) part of the profile. A laterally inhomogeneous model for theUppsala - Prague profile, composed of the seven blocks, satisfies the surface wave data for the profile. Moreover, a mean layered model for the whole profile has also been proposed.     

线形区域的主辅线反射地震调查方法

本文提出主辅线反射地震法, 它由一条主测线和1～2条辅测线共同组成一个反射地震观测系统,其中辅线位于主线的一侧或两侧,大致与主线平行,只在辅线上观测而不放炮.这种系统的成本仅比原常规单线增加20%左右,但是,它却可以同时取得三方面的调查成果：(1)主线常规反射地震剖面,(2)主线与辅线之间区域的表层地震波速层析图,(3)主线与辅线之中线上的反射地震剖面.用比地震剖面略高的费用,主辅线反射地震实现了了解线形区域下方侧向可对比的地质构造变化的目标, 对线形区域的调查尤其适用,而且在中国大陆科学深钻孔区完成的主辅线反射地震调查证实了这一论断.这次主辅线反射地震法调查说明科学深钻孔区地下3000～3600m 深度还可能有规模较大的榴辉岩体存在, 现有金红石矿体储量有可能扩大若干倍.同时,主辅线反射地震法调查提供了一个后备孔位,它是将来扩大矿区储量的首选勘探孔位.     

Axially equilibrated displacement‐based beam element for simulating the cyclic inelastic behaviour of RC members

Distributed plasticity beam elements are commonly used to evaluate limit state demands for performance‐based analysis of reinforced concrete (RC) structures. Strain limits are often preferred to drift limits because they directly relate to damage and are therefore less dependent on member geometry and boundary conditions. However, predicting accurately strain demands still represents a major simulation challenge. Tension shift effects, which induce a linear curvature profile in the plastic hinge region of RC columns and walls, are one of the main causes for the mismatch between experimental and numerical estimates of local level quantities obtained through force‐based formulations. Classical displacement‐based approaches are instead suitable to simulate such linear curvature profile. Unfortunately, they verify equilibrium only on an average sense due to the wrong assumption on the axial displacement field, leading to poor deformation and force predictions. This paper presents a displacement‐based element in which axial equilibrium is strictly verified along the element length. The assumed transversal displacement field ensures a linear curvature profile, connecting accurately global displacement and local strain demands. The proposed finite element is validated against two sets of quasi‐static cyclic tests on RC bridge piers and walls. The results show that curvature and strain profiles for increasing ductility demands are significantly improved when axially equilibrated rather than classical displacement‐based or force‐based elements are used to model the structural members. Copyright © 2017 John Wiley & Sons, Ltd.     

Intraplate stresses along a section of the Kavali-Udipi DSS profile in the south Indian shield induced by density heterogeneities and topography

Occurrence of intraplate seismicity has been attributed to several causes. The perturbation in the local stress regime, either due to local strength weakening of rock mass or surface and subsurface loading, is considered as a plausible mechanism of intraplate earthquake occurrences. The present work is aimed at analysing the state of stress in a part of south Indian shield induced by existing topography and undulations in the subsurface interfaces. The stresses are computed along a section of well studied Kavali-Udipi profile. The general nature of the stresses is compressive in the upper lithosphere except for a small region of extensional regime at both the ends of the profile. The magnitude of these stresses in comparison to the plate tectonic stresses shows that it also forms a significant component of the prevailing stresses in the region. The computed results also show a lateral variation in the stresses along the profile section. Thus, the role of stresses due to density heterogeneities and topography should be taken into consideration in explaining the microseismicity of the region.     

MT重复测量在地震中短期预报中的应用──祁连山中段MT剖面监测研究

本文研究了青藏高原北缘第一个长约４００ｋｍ的ＭＴ监测剖面上所获得的５次５－７级地震前后的资料。着重分析了在相当于大陆多震层深度范围内壳内介质的多种ＭＴ参数（视电阻率、相位、主轴方位角、扭曲度等）．这些ＭＴ参数在大范围内的同步－同相变化和局部区域之间的超前、滞后变化、相对变化幅度随震中距增大而衰减的特征，可能分别对应着构造孕育型大震的地区性场兆和构造运动型强震的区域性场兆，形成了地震活动大背景叠加小背景的一种链式关系。运用＂ＭＴ监测优显参数＂进行择优监测，将可能为大陆地震的震源物理发展过程和中短期预报提供丰富可靠的判据。     

Increased levels of petroleum hydrocarbons in the surface sediments of Swedish coastal waters

Petroleum hydrocarbons in the sediments of the Bothnian Bay, Baltic proper, Öresund, Kattegatt and Skagerrak were analysed in a survey of oil pollution along the Swedish coast in 1974 and 1975. Sixty-one of these sediment stations were revisited in 1982 and analysed with the methods used in the first survey. The mean content of petroleum hydrocarbons (paraffin-naphthene fraction) in the sediment surface showed a statistically significant increase from 199 to 252 μg g^?1 dry wt. The main increase had taken place in the coastal areas. Within 0–20 km distance of urbanized areas (>20 000 inhabitants) petroleum hydrocarbons had increased from 371 to 447 μg g^?1. Sediment levels increased from 76 to 117 μg g^?1, 20–50 km from urbanized areas. The increase in these areas was equivalent to 0.119 g m^?2 yr^?1.     

MT重复测量在地震中短期预报中的应用──祁连山中段MT剖面监测研究

本文研究了青藏高原北缘第一个长约４００ｋｍ的ＭＴ监测剖面上所获得的５次５－７级地震前后的资料。着重分析了在相当于大陆多震层深度范围内壳内介质的多种ＭＴ参数（视电阻率、相位、主轴方位角、扭曲度等）．这些ＭＴ参数在大范围内的同步－同相变化和局部区域之间的超前、滞后变化、相对变化幅度随震中距增大而衰减的特征，可能分别对应着构造孕育型大震的地区性场兆和构造运动型强震的区域性场兆，形成了地震活动大背景叠加小背景的一种链式关系。运用＂ＭＴ监测优显参数＂进行择优监测，将可能为大陆地震的震源物理发展过程和中短期预报提供丰富可靠的判据。     

位于构造活跃区的小湾水库地震活动特征——基于地震精定位的分析

针对我国在西南构造活跃区修建的水库蓄水与地震活动的关系,本文对蓄水已长达7年并在高水位运维多个周期的云南小湾水库,采用结合波形互相关技术的双差地震定位法对水库库区及周边地区2005年7月至2014年12月发生的M≥1.0级地震进行了精定位处理,结果显示出明显的地震成丛活动特征,库区内外的地震震源深度差别较大.对地震震源深度、地震活动与水库蓄水水位及b值分析结果表明:小湾水库蓄水后地震活动明显增多,有水库触发地震发生,触发地震主要分布在沿黑惠江(A)和澜沧江流域(B、C)的3组地震丛中,且3个区的触发地震类型均为快速响应型;在水库蓄水响应活动最明显的地震丛集区A,展现出明显的随水库蓄水水体渗透发生地震"迁移"活动的现象;但库区内也存在着与蓄水关系不大的可能属于正常构造地震的活动,而库区外的地震活动与水库蓄水没有什么相关性,很可能是属于正常的构造地震.综合断层展布、岩性分布及震源深度分析,认为水库蓄水引起的溶岩作用和渗透作用及断层活动可能是小湾水库触发地震的主控因素.     

Longitudinal distribution of periphyton algae in the Moskva river under eutrophication

Variations of the species structure of periphyton algae community on submerged plants have been studied along the longitudinal profile of the Moskva R. with the use of multivariate statistical methods. The results of principal-coordinates analysis show the flow velocity and biogenic load to be the major factors governing the spatial variations of periphyton structure. Mantel test shows the difference in the species structure to be correlated more closely with local variations in the environmental factors than with the distance along river channel. Therefore, the spatial distribution of periphyton in the Moskva R. agrees better with the concept of patch dynamics than the concept of river continuum. Estimation of the indicator potential of three ecological guilds of diatoms (low profile, high profile, and motile) has shown a positive response of motile diatoms to an increase in the concentration of biogenic compounds. The effect of other factors, such as light intensity in macrophyte stands and the microdistribution of diatoms in periphyton layer, can disturb the relationship between biogenic substance concentrations and the abundance of low profile and high profile diatoms.     

准噶尔盆地北部基底结构与属性问题探讨

准噶尔盆地的基底结构与属性一直是地学界关注的焦点问题之一.横跨准噶尔盆地北部,走向近东西的克拉玛依—喀姆斯特地震剖面提供了该盆地北部详细的地壳与上地幔顶部的速度结构与构造,特别是基底顶界面的速度.沿剖面发现了数条走向近南北的“H”型超壳断裂,它们没有明显的断差,断裂处反射系数明显降低,介质的Q值减小,推测具“开裂”性质;利用盆地内1:20万重磁数据完成了重磁联合反演,获得了沿剖面的地壳与上地幔顶部的二维密度结构与二维磁性结构.根据在一定深度范围内介质的速度-密度-岩性之间的关系,确定了盆地北部基底岩性分布.结果表明,准噶尔盆地北部的基底多处为基性和超基性物质,推测为深部（上地幔）物质沿超壳断裂进入地壳内部并对地壳物质进行改造的结果.这一推断得到盆地内部高磁性、高重力异常的支持,也与盆地具有较高的地壳平均速度相一致.综合其他地球物理与地质资料综合分析,给出了综合地质解释剖面,建立了准噶尔盆地北部基底结构与属性的动力学模型.     

The distribution characteristics of short period geomagnetic variations in Shaanxi region

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