Main crustal discontinuities of Morocco derived from gravity data

Sharp linear gradients in maps of potential field data are generally assumed to result from sharp discontinuities or boundaries between rocks having different densities or magnetic susceptibilities and are usually associated with faults or other geological contacts. The computation of the horizontal gradients of the gravity field permits us to localize the limits of such blocks and then the fault locations. The horizontal derivative maxima of the Bouguer anomaly and its upward continuation at several heights show lineaments that could reflect the layout of faults and/or contacts and their dip directions. The application of this method to the Bouguer anomaly map of Morocco (with 19,571 points, using an average crustal density ρ = 2.67 g/cm³) allowed us to perform a multiscale analysis of the gravimetric lineaments of the country. The obtained structural map is consistent with several faults already identified in previous studies, and highlights five new major subsurface faults systems with location and dip: the Saghro fault system; Bou-Arfa Midelt fault system; Sidi Slimane Mezquitem fault; Ksar El Kebir–Chefchaouen fault and the Rifan West Mediterranean fault. In addition, this study suggests a new shape and localization for the Agadir-Oujda trans-Moroccan major fault with a NE-SW direction and 900 km length, subdividing Morocco into two main domains. The results of this study contribute to the improvement of the regional structural map of the north western part of Africa, which is situated within the convergence zone between Africa and Eurasia.     

利用重力资料与GPS数据研究伽师及邻区近期地壳运动特征

利用重力资料和GPS数据研究伽师及邻近地区地壳运动特征,首先对2005～2009年该区域流动重力观测资料进行平差计算,绘制出以1年为尺度的重力场变化的等值线图.分析表明,伽师及邻近地区近年来重力场变化相对活跃,且有一定的规律,最大重力异常值达到150×10 -8m·s-2.其次,利用2005 ～2009年该区域GPS数据在统一框架下进行重新解算,并利用理论计算得到该地区每年的面膨胀值.研究表明,该区域地壳面压缩、面膨胀交替出现,正负交界处往往是剪应变值增大的地区,即地震易孕育区域.最后探讨了重力与GPS(面膨胀)二者在地壳形变、地球动力之间的响应关系.研究表明,震前伽师及邻区重力值主要显示正变化,震后则呈现出大面积负值区域,说明压陷(面收缩)和向北扩张(面膨胀)作用交替进行,这与GPS计算出的面膨胀结果是一致的.     

Toward international cooperation in the study of recent crustal movements in Africa

How might an international plan for crustal-movement studies in Africa be implemented? This was the basic question underlying much of the discussion at the First International Symposium on Crustal Movements in Africa, convened in May 1981 in Addis Ababa at the United Nations Economic Commission for Africa. Three years later, in November/December 1984, the Second Symposium was held in Cairo, Egypt; and in April 1986, some outstanding questions were dealt with at the Third Symposium on Geodesy in Africa held in Yamoussoukro, Ivory Coast, and at the International Symposium on Global Changes in Africa during the Quaternary, held in Dakar, Senegal.The present state and possible development of recent crustal movement studies in Africa are discussed. Finally, the cooperation of all scientists interested in such studies is solicited.     

Some structural and geomorphological aspects of recent crustal movements in estonia

Comparison of some structure contour of northeastern and southeastern Estonia with the bedrock relief patterns leads to the conclusion that direct morphostructures are infrequent in those areas. The formation of bedrock and contemporary relief has been little effected by differential postglacial and recent crustal movements. Morphotectonic analysis enables us to determine the character of those movements only in a few cases.     

Monitoring recent crustal movements in the African continent by means of geodetic methods

The need for geodetic measurements is emphasized, and the requirements in accuracy are indicated. It is argued that the present limitations of the space techniques leave no option but to recourse to terrestrial methods.The necessity of repeating the measurements on schedule over several decades calls for an eleborate co-ordination of activities across national boundaries. The active participation of the IUGG and the IAG can be instrumental in initiating such co-ordination and in sustaining it.     

The ties between recent crustal movements and fault tectonics in the Caraptho-Balkan region

To identify the faults controlling the intensity of recent tectonic processes in the region and to study their geometry and dynamics, one of the variants of the statistical method was used-trend analysis. The map of recent vertical crustal movements in the Carpatho-Balkan region (1:2.000.000) published in Budapest in 1979 was used as initial data for establishing these faults.     

Maps of recent vertical crustal movements in Romania: Similarities and differences

A new map of recent crustal vertical movements (RCVM) in Romania, on the basis of high-precision geodetical measurements by the repeated levelling method, had become necessary firstly as a consequence of the great seismic event of March 4, 1977 with foci under the Vrancea Region (the most important seismic zone in the country) and secondly because many years had passed since the previous edition.Certainly, there are similitudes and differences between the two editions, and the authors will try to put these into evidence and explain them.The general view of the major geological units of Romania in both maps is the same: the mountain regions being zones of more or less continuous uplift, the plains showing subsidence movements and the plateau regions presenting a relative stability. Some modifications became apparent owing to the improvement of the national geodetical network on which the repeated levelling measurements were performed and following the choice of the Harbour of Constanţa as the fundamental point.     

Field of tectonic stresses from focal mechanisms of earthquakes and recent crustal movements from GPS measurements in China

Orientations of the principal axes of the tectonic stress field reconstructed from seismological data on focal mechanisms of earthquakes and strain fields determined from GPS measurements in China are compared. The data of GPS measurements used in the paper were obtained by the Crustal Movement Observation Network of China (about 1000 stations) in the period of 1998–2004. On the basis of information on the recent horizontal crustal motions, the strain field is calculated for the study territory by the finite element method. Calculations of the strain tensor using GPS data were carried out with a step of 1° in latitude and longitude. A catalog of earthquake focal mechanisms was used for the reconstruction of tectonic stress field components. Focal mechanisms of earthquakes were calculated with the use of seismological data on signs of first arrivals from the bulletin of the International Seismological Center. To estimate characteristics of the regional stress field, an approach based on the kinematic method proposed by O.I. Gushchenko was applied. The tectonic stress field was reconstructed in depth intervals of 0 < H < 35 km and 35 km < H < 70 km from data on focal mechanisms of earthquakes over the periods of 1998–2004 and 1985–2004. Comparison of directions of the principal strain axes at the surface (according to GPS measurements) and directions of the principal stress axes (reconstructed from focal mechanisms of earthquakes) showed their good convergence. Seismotectonic strains and GPS measurements coincide within a larger part of the territory. The coincidence is best in a depth interval of 0 < H < 35 km. Maximum misfit values are confined to areas of high 3-D gradients of strain axis directions and are possibly related to the structural heterogeneity of the region, zones with strains of the same type along both horizontal axes (compression or extension along all directions), or areas of small absolute values of recent horizontal movements. Areas with invariable directions of the stress axes are recognizable regardless of the depth of initial data. Good reproducibility of results obtained by two different methods made it possible to check the method of stress field reconstruction using data on focal mechanisms of earthquakes.     

Local aspects, models and monitoring of recent crustal movements in central Europe

The paper discusses the contribution of the local geodynamical test-fields in which various methods are applied (geodetic, geological, geophysical and geomorphological) to creating both local and regional geodynamical models. A hypothesis concerning dynamics of the area of Central Europe was advanced by the author in 1987 as a background geodynamical model of the present interaction between Precambrian and Palaeozoic platforms. The model has been adopted as a working hypothesis for the project SAGET (SAtellite GEodynamical Traverses) initiated in Poland in 1986. Concluding the paper, the author resumes her own proposal of 1988 and suggests further expanding the project SAGET towards East and West to adjust it with EUROPROBE.     

Present stage of recent crustal movements and seismicity within Greater Cairo area, Egypt

Greater Cairo and the Nile Delta are considered very important, high-density population areas. The subject of the research work is dealing with recent crustal movements and its relation to seismicity and tectonics setting. A Global Positioning System (GPS) network consisting of 11 benchmarks covering Greater Cairo and the southern part of the Nile Delta was established in 1996. Different campaigns surveyed the network. In this study, we used ten measurements collected during the period from 2004 to 2010. The data were processed using Bernese 5.0 software to derive velocity vectors and principal components of crustal strains. The horizontal velocity varies in average between 3 and 6 mm per year across the network. Rate of the accumulated strains in the southern part of Greater Cairo varies from low to moderate. The low strain rates and low level of earthquakes occurrence in the present interval in the Nile Delta area indicated that the rate of the deformation in this area is small. The result from coupling GPS and seismic data indicates that the southern part of the area is seismo-active area when compared with the other parts in the network areas. The paper gives information about the present state of the recent crustal movements within Greater Cairo area to understand the geodynamics of that area. This study is an attempt to build a basis for further development of seismic catastrophic risk management models to reduce a risk of large catastrophic losses within the important area.     

冲绳海槽地区地壳结构与岩石层性质研究

用冲绳海槽地区地质、地球物理、地球化学数据与资料，对横跨大陆边缘—大洋板块的中国东部—冲绳海槽—琉球岛弧地区进行综合研究. 根据地震体波、面波成像结果，空间与布格重力异常及磁力异常的综合解释等结果，利用调和系数法对该研究区的地壳厚度进行了计算，并对地壳属性进行分析. 同时，选择中国东部东北地区的宽甸、华北地区的汉诺坝、华南的闵溪和台湾海峡的澎湖列岛与冲绳海槽和日本岛弧的新生代玄武岩开展元素—同位素对比研究. 结果认为冲绳海槽岩石层明显减薄，上地幔低速层已达到莫霍面，在地幔中已形成交代软流层. 元素—同位素研究表明：它不同于中国东部，与琉球岛弧地区的特点也不相同. 综合分析认为：冲绳海槽地壳属于过渡型地壳，与大洋地壳的许多特点颇为相似.     

A new version of the map of recent vertical crustal movements in the Carpatho-Balkan region based on the collocation method

The paper is concerned with the problem of adequate mapping of resulting discrete values of the movement velocity as a field. The authors used a statistical method (collocation) to represent measurement results for the Carpatho-Balkan region as a field of recent crustal movement velocity. The collocation method gives a possibility of avoiding a strong influence that the available geomorphological and geological knowledge of the area and individual notions of map-makers have on the drawing of isolines of velocity movements. The new version of the map of recent vertical movements in the Carpatho-Balkan region shows some notable differences from the 1979 version.     

Estimates of vertical crustal movements along the coast of Greece,based on mean sea level data

Mean annual sea level (MASL) data for 25 Greek stations were analyzed for the time period 1969–1982. The data from 4 of these were unacceptably poor, and the record of 3 stations showed unexplained step functions that were interpreted as errors. Relative MASL between stations showed crustal stability at 10 of the 18 useful stations. The standard deviation from the long-term average of these stations was ±1.8 cm. We conclude that if station records are carefully kept in this area crustal movements in excess of 5 cm can be detected by relative MASL. A comparison of MASL data with gravity changes measured in the Peloponnese and Central Greece suggests that vertical movements occurred along a gradient equal to or larger than the free air gradient. We conclude that the gravity network should be reoccupied frequently such that the non-tectonic effects to be determined from the probable observed gravity changes, and the tectonic vertical movements may be better understood. A co-seismic subsidence of about 5 cm is inferred to have taken place near Korinth during the 1981,M _s =6.8, earthquake, which occurred 20 km N of this tide gauge (Posidonia). During 2.5 years before the 1968 Thessaloniki,M _s =6.6, earthquake, sea level was lower than average suggesting possible crustal elevation of 3.6 cm at about 28 km epicentral distance. Because of the small amplitude of this change we are not certain that it represents crustal uplift. At station Myrina (on Limnos) a strong and consistent trend of subsidence accumulated a 15 cm change between 1975 and 1980. Chios showed a trend of emergence (total accumulation about +5 cm), while Volos showed a trend of subsidence (approximately ?5 cm total). Kefalinia appears to have subsided about 10 cm during the data period. The six stations along the Hellenic arc plate boundary showed nearly constant MASL, suggesting that crustal stability existed there during the last 14 years. We conclude that MASL data in Greece can be useful for understanding tectonic processes, especially if these data are gathered carefully and at numerous locations, and are cross-correlated to high precision repeat gravity measurements, and geodetic releveling. Also, MASL data on active volcanic islands have excellent potential for detecting uplift before future eruptions.     

The role of high-precision gravimetry in detecting and monitoring recent crustal movements in view of the African situation

High precision relative gravimetry can play a dual role in the study of vertical recent crustal movemen: exploring and monitoring. A number of G-meters of LCR type have been calibrated over the full measurement range so that they can be used in almost any part of African. Recent high precision measurements in desert, tropical and subtropical zones reveal clearly that even under unfavorable climatical and transportation conditions precise gravity is available within ± 10 to 20 microgal accuracy.     

Velocity properties of the lithosphere in the ocean-continent transition zone in the Kamchatka region from seismic tomography data

Arrival times of P and S waves from local earthquakes in the Kamchatka area of the Kurile-Kamchatka Island Arc are used for calculating a spatial model of the elastic wave velocity distribution to a depth of 200 km. The lithosphere is shown to be strongly stratified in its velocity properties and laterally heterogeneous within the mantle wedge and seismic focal zone. A lower velocity layer (an asthenospheric wedge) is identified at depths of 70–130 km beneath the Eastern Kamchatka volcanic belt. The morphology of the Moho interface and the velocity properties of the crust are studied. The main tectonic structures of the region are shown to be closely interrelated with deep velocity heterogeneities. Regular patterns in the statistics of the earthquakes are analyzed in relation to variations in the elastic wave velocities in the focal layer. A mechanism of lithospheric block displacements along weakened zones of the lower crust and upper mantle is proposed.     

云南腾冲火山区地壳及岩石圈厚度研究

使用云南腾冲火山监测台网9个宽频带地震台站的远震数据,采用P波和S波接收函数的方法研究了腾冲火山区的地壳厚度、泊松比值以及岩石圈和软流圈分界面(LAB)深度.研究结果表明: 1) 云南腾冲火山区的地壳厚度约在33.5~38.0 km之间; 2) 火山区的泊松比主要集中范围为0.26~0.32,其中6个台站均大于0.29,推测与地壳镁铁质成分的增加有关并且可能存有2个岩浆囊; 3) 火山区的岩石圈厚度在78.2~88.0 km,较周边地区明显隆起且横向差异较大.腾冲火山区岩石圈的明显穹隆,由软流圈上涌(地幔热物质上升)引起岩石圈的拉张与减薄所致.     

Inhomogeneous structure of the Voronezh shield lithosphere from explosion seismology data

This paper presents the results of a seismological investigation of the Voronezh shield. The use of industrial quarry blasts, as the most economic seismic energy source, and telemetric arrays of the Taiga type for deep seismic sounding observations has enabled the Geophysical Research Group of the Voronezh State University to obtain complete and valid information concerning the deep structure of the shield.Seismic records of refracted and reflected P and S phases from the intracrustal and subcrustal discontinuities allow the determination of independent and compatible velocity models V_P(Z) and V_S(Z).Analysis of the travel-time and amplitude curves derived from the deep seismic sounding data reveals a complex and inhomogeneous structure of the Voronezh shield lithosphere. The major discontinuities are found in the middle part of the consolidated crust. Several rather thick layers can be resolved, which are separated by first-order seismic boundaries.In the consolidated crust beneath the southeastern part of the shield at depths from 6–7 to 11–13 km, a low-velocity layer is clearly established from the compressional wave data. Within the low-velocity channel, V_P appears to decrease by 5–6%. The low-velocity channel in the uppermost consolidated crust is assumed to be due to metamorphogeneous granitization. This is revealed by the Precambrian basement structure.Poisson's ratio calculated from the compatible velocity models V_P(Z) and V_S(Z) shows a systematic increase down to the crustal basement and becomes discontinuous only in the low-velocity channel. At the top of the mantle, Poisson's ratio decreases on all the deep seismic sounding profiles. The systematic variation of Poisson's ratio is related to changes in the composition and mineralogy of the Voronezh shield lithosphere.