Geophysical and geodetic observations in the messina straits

The present note is intended as a contribution to the clarification of the collision process, generally assumed to be active between the African and the Eurasian plates in the area of the Messina Straits.For this purpose detailed analyses are made of the surveys carried out in the Straits of Messina in order to investigate the 1908 earthquake. The magnitude and intensity are established; limits are set for the linear dimension of the fault and of the dislocation that gave rise to the earthquake and an estimate is then given of these parameters. The results are then set out of the investigation into the focal mechanism of the earthquake of 1908. The results are also studied of geodetic surveys carried out at the time for the purpose of measuring the variations in height that occurred during the earthquake.A study is also made of the results of geodetic surveys (some of which are still under way) designed to measure the horizontal movements of Sicily with respect to Calabria in order to study locally the continental collision process in the area of the Messina Straits.It is found that the old and recent geodetic measurements as well as the fault-plane solution of the old and recent earthquakes of that area and the slips associated with those earthquakes indicate a motion towards the north of Sicily with respect to Calabria.     

Strain rate of the South American lithospheric plate by SIRGAS-CON geodetic observations

A multiyear solution of the SIRGAS-CON network was used to estimate the strain rates of the earth surface from the changing directions of the velocity vectors of 140 geodetic points located in the South American plate. The strain rate was determined by the finite element method using Delaunay triangulation points that formed sub-networks; each sub-network was considered a solid and homogeneous body. The results showed that strain rates vary along the South American plate and are more significant on the western portion of the plate, as expected, since this region is close to the subduction zone of the Nazca plate beneath the South American plate. After using Euler vectors to infer Nazca plate movement and to orient the velocity vectors of the South American plate, it was possible to estimate the convergence and accommodation rates of the Nazca and South American plates, respectively. Strain rate estimates permitted determination of predominant contraction and/or extension regions and to establish that contraction regions coincide with locations with most of the high magnitude seismic events. Some areas with extension and contraction strains were found to the east within the stable South American plate, which may result from different stresses associated with different geological characteristics. These results suggest that major movements detected on the surface near the Nazca plate occur in regions with more heterogeneous geological structures and multiple rupture events. Most seismic events in the South American plate are concentrated in areas with predominant contraction strain rates oriented northeast-southwest; significant amounts of elastic strain can be accumulated on geological structures away from the plate boundary faults; and, behavior of contractions and extensions is similar to what has been found in seismological studies.     

Three-dimensional strain analysis in physical models of geological structures

A method is outlined for calculating three-dimensional finite strain in physical models of geological structures containing passive strain markers. This method makes it possible to determine the three-dimensional strain pattern in models of structures that lack any of the types of symmetry (such as that imparted by cylindrical folding) that simplified calculations in previous work. The strain markers in the new method are in the shape of stubby rectangular prisms or cubes. These form a three-dimensional grid or array occupying each of the active layers in a model (e.g., for a simple two-layer gravitationally unstable system, one grid for the overburden layer and one for the buoyant layer). Each of the grids can be described by positions of three families of “strain marker surfaces”, which are contacts between layers of strain markers.

After deformation, the model is serial-sectioned horizontally and the traces of the strain marker surfaces on the sections are digitized. The strain state is calculated at each of several hundred points arranged in a three-dimensional “output grid” extending throughout the mechanically active part of the model. An interpolation procedure is used to estimate the spacing and orientation of the strain marker surfaces in the vicinity of each of the output grid points. The following quantities are determined for each of the three families of strain marker surfaces:

1. (1) the local horizontal orientation of the strain marker surfaces;

2. (2) the local spacing of the surfaces; and

3. (3) the local inclination of the surfaces, calculated from their change in position from the serial section above, to the serial section below, the output grid point.

This information is used to generate a parallelepiped representing the strain marker geometry in the neighbourhood of the output grid point. The edges of the parallelepiped are equivalent to the coefficients of the strain matrix, from which the three principal strain magnitudes and orientations are readily derived.     

Some kinematic features of refracted waves

The equation for the arrival time of a refracted wave is fully stated. This approximates as the arrival of the head wave minus the second and third terms of the equation. Time-distance curves are studied, and examples for carbonates, Precambrian, and crystalline specimens are given. Along with the linear velocity law, the parabolic law is also treated. Peculiarities of propagation of refracted waves in the medium are studied and depth of penetration is considered. Relations for refracted, diffracted, and reflected waves are examined and multiple refracted waves are considered as well. This seems to be a practical and general paper illuminating the refraction phenomenon. — Petroleum Abstracts.     

Some aspects of coupled FEBEM analysis of underground openings

The finite element method (FEM) and the boundary element method (BEM) are two well established numerical methods used for the analysis of underground openings. The advantages of both the methods are utilized by adopting FEBEM in which finite elements are coupled with boundary elements. A coupling procedure is presented in this paper. In using FEBEM, the effect of the location of interface boundary between finite element and boundary element regions, effect of Poisson's ratio and effect of stress ratio are discussed. It is shown that Poisson's ratio and stress ratio have significant effect on the accuracy of the results. Different discretization schemes are discussed to study their effect on accuracy and computation time. The use of different material properties in the FE region is presented. A comparative study is made with FEM for all the cases. It is shown that use of FEBEM is more advantageous than FEM.     

Some aspects of numerical analysis of creep in layered granular medium

The analysis of viscoelastic multilayered granular medium by application of two types of numerical methods is presented. Viscoelastic problem is first formulated by means of potentials and then by differential operators. Former approach implied the application of recurrent integral formulation, while the latter - the finite difference method. Both methods are used with respect to time variables. In particular, axisymmetric case of practical meaning is analyzed. With respect to spatial variables - the finite element method is applied. Specific features of both numerical methods including stability criteria of the solution are analyzed. The advantages and drawbacks are shown on the basis of numerical results.     

Some guidelines for the practical application of Fry's method of strain analysis

A systematic analysis of patterns in artificial and natural point distributions is presented in order to demonstrate how the nature and degree of anticlustering of point distributions affect the character of Fry diagrams derived from them. Ideally anticlustered artificial point distributions illustrate the nature of the dependence between the degree of anticlustering and the minimum number of points required in such a distribution if it is to reveal strain accurately by Fry's method. In addition, these distributions provide straightforward results which aid in the interpretation of less ideally anticlustered natural point distributions.The results from the artificial distributions may be used to estimate the minimum number of points required for strain determination in natural distributions of varying degrees of anticlustering, as long as the anticlustering is sufficiently strong. However, application of the results from the artificial distributions to less strongly anticlustered natural point distributions shows not only that the existence of a vacancy field is an insufficient criterion for determining whether or not Fry's method is appropriate for strain determination but also that the nature of the anticlustering in these natural distributions limits the effectiveness of enhancing strain definition by increasing the number of points composing the distribution.     

Analysis of crustal deformation in Luzon, Philippines using geodetic observations and earthquake focal mechanisms

We utilize regional GPS velocities from Luzon, Philippines, with focal mechanism data from the Harvard Centroid Moment Tensor (CMT) Catalog, to constrain tectonic deformation in the complex plate boundary zone between the Philippine Sea Plate and Eurasia (the Sundaland block). Processed satellite imagery and digital elevation models are used with existing gravity anomaly, seismicity, and geologic maps to define a suite of six elastic blocks. Geodetic and focal mechanism data are inverted simultaneously to estimate plate rotations and fault-locking parameters for each of the tectonic blocks and faults comprising Luzon. Major tectonic structures that were found to absorb the plate convergence include the Manila Trench (20–100 mm yr^− 1) and East Luzon Trough ( 9–15 mm yr^− 1)/Philippine Trench ( 29–34 mm yr^− 1), which accommodate eastward and westward subduction beneath Luzon, respectively; the left-lateral strike-slip Philippine Fault ( 20–40 mm yr^− 1), and its northward extensions, the Northern Cordillera Fault ( 17–37 mm yr^− 1 transtension), and the Digdig Fault ( 17–27 mm yr^− 1 transpression). The Macolod Corridor, a zone of active volcanism, crustal thinning, extension, and extensive normal and strike-slip faulting in southwestern Luzon, is associated with left-lateral, transtensional slip of  5–10 mm yr^− 1. The Marikina Fault, which separates the Central Luzon block from the Southwestern Luzon block, reveals  10–12 mm yr^− 1 of left-lateral transpression. Our analysis suggests that much of the Philippine Fault and associated splays are locked to partly coupled, while the Manila and Philippine trenches appear to be poorly coupled. Luzon is best characterized as a tectonically active plate boundary zone, comprising six mobile elastic tectonic blocks between two active subduction zones. The Philippine Fault and associated intra-arc faults accommodate much of the trench-parallel component of relative plate motion.     

Statistical analysis of geodetic measurements for the investigation of crustal movements

Geodetic networks are designed to obtain data that can be used to monitor crustal movements. The relative position on the earth's surface is determined from these networks by means of coordinates. The coordinates of stations and its variance—covariance matrix are based on the computational model. In spatial networks at least three points, the base points, should be chosen to define the coordinate system “fixed” to the earth. In monitoring crustal movements these base points are considered to be stationary over the time span of the motion involved. A procedure for testing the stability of the base points, together with other stable points, is described.The coordinate differences between two time epochs, t₀ and t₁ are considered to investigate crustal movements. A statistical test is introduced to determine whether crustal movements have actually occurred.The reliability, i.e., the influence, of nondetected errors in the observations or computations, should be considered. Two types of decisions can be made which may lead to incorrect conclusions. These conclusions are as follows:

1. (1) That no movement has taken place, although a nondetected error leads to the opposite conclusion.

2. (2) That a movement has occurred, although a nondetected error in the observations leads to the opposite conclusion.

The chance of arriving at these conclusions can be computed. Boundary values for assumed crustal motion in specified latitudinal and longitudinal directions give a better insight into the desired specifications for geodetic networks.The testing procedure and the above-mentioned method of computing boundary values can be used for all types of networks e.g., those obtained by conventional triangulation or by a satellite-borne ranging system.     

Some aspects of the geology of Svalbard

Svalbard has exceptional geological diversity within a relatively small area. The islands are situated between 76°N and 81°N, longitude 10°E and 35°E, with a total land area of some 63 000 km², about twice the size of Belgium. Around 60 per cent is covered by glaciers. It is one of the few places in the world where stratigraphical sections representing much of the Earth's history are relatively easily accessible for study.     

Some aspects of the seismic hazard in Panama City

The definition of earthquake sources in the Panama region on the basis of both tectonics and average seismicity rates, have recently led to the concept of a microplate surrounded by seismically active areas. The effects of these earthquakes on the place where the most important concentration of investments and population is located, the capital city of Panama, are analyzed in this paper using statistical approaches.The parameters of Gumbel's Type-I distribution of extreme values for a continuous interval of 60 yr annual maximum magnitudes were used to make probabilistic estimations of the seismic hazard in Panama City. An earthquake with magnitude 7.5 is capable of producing a modified Mercalli intensity VII in Panama City, provided the source distance is of the order of 100 km. This earthquake has a probability of occurrence of 69% in 50 yr.     

太古宙地层研究中应注意的几个问题

太古宙地层与显生宙地层相比,具有很大的复杂性,研究难度大。研究太古宙地层,在学术思路上应以岩石、地层、构造和地质事件综合研究为基础,配合多种新技术新方法特别是同位素年代学定年为依托。另外,要注意解决好如下6个方面的问题：（1）在“单斜”地层中注意研究有无复杂同斜褶皱;（2）正确判定岩石类型及其属性;（3）多次变形叠加如何与区域变质作用相联系的问题;（4）地层定年中应该注意的问题;（5）特殊岩石单位的定名问题;（6）正确区分变质火山－沉积地层层序和构造接触问题。     

Some observations on the influence of faults in mininginduced seismicity

The effect of faults on seismicity has been investigated for mining-induced seismicity associated with coal extraction in the North Staffordshire Coalfield, U.K. The studies show that seismic events occur if the mine workings cause the redistribution of strata pressure in such a way that sliding movements take place along major faults. In particular, movements occur when the active longwall face is driven in the footwall of the faults and parallel to the fault plane.     

The Kyrgyz-Ata Synform in the South Tien Shan: Structural and kinematic aspects of its evolution

The complexes of metamorphic rocks that take part in the tectonic structure of the South Tien Shan are mostly confined to the cores of large synforms and occupy high structural position therein. Problems of their age, origin, metamorphic conditions, and relationships with the adjacent unmetamorphosed Paleozoic sequences have remained debatable throughout the history of geological investigations. The results of detailed studies of the Kyrgyz-Ata Synform, a typical structure of the Turkestan-Alay Zone in the South Tien Shan, are reported in this paper. The metamorphic rocks of the Kan Group participate in the structure of this synform. On the basis of the study of structural assemblages and the kinematic analysis, it is concluded that the multistage evolution of this tectonic unit proceeded under transpressive conditions of volcanic trough “crushing.” The detailed study of rock alteration in the contact zone between the Kan Group and the Middle Devonian basic volcanic rocks allowed us to suggest that the metamorphic Kan Group is not a self-dependent stratigraphic unit but represents an infrastructure (a metamorphosed analogue of the volcanic sequence and underlying rocks) displaced tectonically over its own suprastructure.     

Seismotectonic deformations related to the 2010 Maule earthquake at different stages of the seismic cycle from satellite geodetic observations

Doklady Earth Sciences - The interpretation of multiannual satellite geodetic observations after the 2010 Maule earthquake is given on the basis of the keyboard concept of the subduction region...     

Some aspects of coaxial refolding

The author has observed many examples of coaxial refolding in the Moinian rocks of the North-West Highlands of Scotland. This coaxial nature of refolding is not the result of the continuation of a single movement phase, but developed where a later fold refolded both limbs of an earlier fold. It is concluded that the coaxiality results from a strong flexural component in the later folding.

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Zusammenfassung Der Autor untersuchte zahlreiche Beispiele gleichachsiger Wiederfaltungen in dem Moinian der Schottischen Nordwesthochlande. Diese Art Parallelismus der Faltenachsen ist nicht eine Folge der Fortsetzung einer einzigen Bewegungsphase, sondern folgte aus der Überprägung der beiden Flügel der vorhandenen Falte einer späteren Faltung. Man kommt zu dem Schluß, daß die Gleichartigkeit auf eine starke flexurelle Komponente der überprägenden Spätfaltung zurückzuführen ist.

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Résumé Nous avons observé plusieurs examples de replissement coaxial dans les roches Moinianes des hautes montagnes Nord-Ouest Ecosse. Les replissements co-axials ne se forment pas par la suite d'un seul mouvement, mais se développent où un pli postérieur replie les deux flancs de raccordement d'un pli antérieur. Nous constatons que la co-axialité résulte d'un composant de flexion très forte dans les plissements postérieurs.

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河相地层研究的几个问题

河相地层是寻找非构造油气藏的重要目标。笔者对陆区基准面、陆上可容空间、河流随基准面变化的沉积响应以及河流相地层模式进行了详细的研究。认为采用高分辨率层序地层学得到基准面原理和方法能提高河流相储层预测的精度