Transmitting boundary for water-saturated transversely isotropic strata based on the u–U formulation

A new transmitting boundary in a cylindrical coordinate system has been developed for modeling the elastic waves radiating out to an infinite boundary in water-saturated transversely isotropic soil strata over a rigid bedrock. The saturated soil strata are assumed to consist of a porous material and modeled as a transversely isotropic two-phase medium, based on the u−U formulation. The newly developed transmitting boundary is combined with the finite-elements model of the near-field region, using the same u−U formulation, and applied to the study of the dynamics of a rigid circular foundation in porous isotropic or transversely isotropic layered strata, either fully or partly saturated with water. The verification and application examples give valuable insights into new and interesting aspects of the dynamic behavior of rigid circular foundations in fully or partly saturated two-phase ground in terms of permeability, transverse anisotropy, and ground-water table level.     

Caldera subsidence in extensional tectonics

A 45-km-long regional dike was emplaced over a period of 2 weeks in August 2014 at the boundary between the East and North Volcanic Zones in Iceland. This is the first regional dike emplacement in Iceland monitored with modern geophysical networks, the importance of which relates to regional dikes feeding most of the large fissure (e.g., Eldgja 934 and Laki 1783) and lava shield (e.g. early Holocene Skjaldbreidur and Trölladyngja) eruptions. During this time, the dike generated some 17,000 earthquakes, more than produced in Iceland as a whole over a normal year. The dike initiated close to the Bardarbunga Volcano but gradually extended to the northeast until it crossed the boundary between the East Volcanic Zone (EVZ) and the North Volcanic Zone (NVZ). We infer that the strike of the dike changes abruptly at a point, from about N45°E (coinciding with the trend of the EVZ) to N15°E (coinciding with the trend of the NVZ). This change in strike occurs at latitude 64.7°, exactly the same latitude at which about 10 Ma dikes in East Iceland change strike in a similar way. This suggests that the change in the regional stress field from the southern to the northern part of Iceland has been maintained at this latitude for 10 million years. Analytical and numerical models indicate that the dike-induced stress field results in stress concentration around faults and particularly shallow magma chambers and calderas in its vicinity, such as Tungnafellsjökull, Kverkfjöll, and Askja. In particular, the dike has induced high compressive, shear, and tensile stresses at the location of the Bardarbunga shallow chamber and (caldera) ring-fault where numerous earthquakes occurred during the dike emplacement, many of which have exceeded M5 (the largest M5.7). The first segment of the dike induced high tensile stresses in the nearby part of the Bardarbunga magma chamber/ring-fault resulting in radially outward injection of a dike from the chamber at a high angle to the strike of the regional dike. The location of maximum stress at Bardarbunga fluctuates along the chamber/ring-fault boundary in harmony with dike size and/or pressure changes and encourages ring-dike formation and associated magma flow within the chamber. Caldera collapse and/or eruption in some of these volcanoes is possible, most likely in Bardarbunga, but depends largely on the future development of the regional dike.     

Stresses produced by a normal load moving over a transversely isotropic layer of ice lying on a rigid foundation

Summary In this paper, the problem of a normal load moving over the surface of a layer of ice, lying on a rigid foundation, has been considered. FollowingHearmon, the ice is taken to be transversely isotropic. Two cases have been considered-in the first case, the rigid foundation is taken to be smooth, and in the second case, it is taken to be in welded contact with the layer of ice. Expressions have been obtained in both cases for the stresses at any point of the layer. From detailed numerical calculations, it has been found that the value of the normal stress at a point directly below the moving load falls of approximately as that of 1/z with increase in the depthz below the surface. On calculating the normal stress at the boundary between the layer of ice and the foundation, it is found that the normal stress falls off rapidly as we move away along the boundary from the point directly below the load.     

High-resolution Induan-Olenekian boundary sequence in Chaohu,Anhui Province

The Lower Triassic of Chaohu has been exten-sively studied owing to its classic stratigraphic se-quence formed in a unique paleogeography, very easyaccessibility and comfortable working conditions inthe area, and the Induan-Olenekian boundary stratareceived special attention in recent years to meet therequirements of defining the GSSP of the boundary.The best-studied boundary sequence in Chaohu is atthe West Pingdingshan Section, which was recom-mended as a GSSP candidate for the Induan-…     

On stresses and displacements due to a moving line load over the plane boundary of a heterogeneous elastic half-space

Summary The problem of concentrated line load moving with supersonic speed along the boundary of an isotropic heterogeneous medium has been solved as a plane strain problem. The stresses and displacements in the heterogeneous case considered are found to decay exponentially with distance.     

中国大陆微破裂过程和应力图案的变化

本文通过对我国大陆东经97°以东地区,1972—1978年发生的17,000多次M≥2.0地震的活动率(d值)和震级-频度关系(b值)的时空扫描,研究了我国大陆的微破裂过程及其与大地震的关系,并认为区域性的b值扫描图在很大程度上反映了区域应力图案的变化     

Note on stresses produced by a shearing force moving over the boundary of a semi-infinite transversely isotropic solid

Summary Two dimensional problem of distribution of stresses produced by a pulse of shearing force moving uniformly on the boundary of a semi-infinite transversely isotropic solid has been studied here. It is observed that the shearing stress in the direction of propagation of pulse attains maximum value at a point directly below the point of application of the shearing force on the boundary.     

Dike propagation within active central volcanic edifices: constraints from Somma-Vesuvius,Etna and analogue models

Dikes within stratovolcanoes are commonly expected to have radial patterns. However, other patterns may also be found, due to regional stresses, magmatic reservoirs and topographic variations. Here, we investigate dike patterns within volcanic edifices by studying dike and fissure complexes at Somma-Vesuvius and Etna (Italy) using analogue models. At the surface, the dikes and fissures show a radial configuration. At depths of tens to several hundreds of metres, in areas exposed by erosion, tangential and oblique dikes are also present. Analogue models indicate that dikes approaching the flanks of cones, regardless of their initial orientation, reorient to become radial (parallel to the maximum gravitational stress). This re-orientation is a significant process in shallow magma migration and may also control the emplacement of dike-fed fissures reaching the lower slopes of the volcano.     

IMPACTS OF DIMENSION AND SLOPE OF SUBMERGED SPUR DIKES ON LOCAL SCOUR PROCESSES - AN EXPERIMENTAL STUDY

1 INTRODUCTION In alluvial streams bed scour often occurs if the sediment load is less than the transport capacity of the flow. Two types of scour are identified, namely local scour and channel bed scour. Channel bed scour can be further classified accord…     

The pattern of circumferential and radial eruptive fissures on the volcanoes of Fernandina and Isabela islands,Galapagos

Maps of the eruptive vents on the active shield volcanoes of Fernandina and Isabela islands, Galapagos, made from aerial photographs, display a distinctive pattern that consists of circumferential eruptive fissures around the summit calderas and radial fissures lower on the flanks. On some volcano flanks either circumferential or radial eruptions have been dominant in recent time. The location of circumferential vents outside the calderas is independent of caldera-related normal faults. The eruptive fissures are the surface expression of dike emplacement, and the dike orientations are interpreted to be controlled by the state of stress in the volcano. Very few subaerial volcanoes display a pattern of fissures similar to that of the Galapagos volcanoes. Some seamounts and shield volcanoes on Mars morphologically resemble the Galapagos volcanoes, but more specific evidence is needed to determine if they also share common structure and eruptive style.     

Sensitivity test of the geodetic network in Las Cañadas Caldera, Tenerife, for volcano monitoring

A 17-benchmark geodetic network in the volcanic area of the Teide Caldera, Canary Islands has been utilised several times since 1982 to detect crustal movements associated with volcanic activity within the network, as well as a procedure for solving configuration problems. The network is located on the mid-western side of the Caldera, where there are two different morphological zones that both have benchmarks. The authors performed a sensitivity test of this geodetic network for volcano monitoring purposes. To do so, we use a deformation model to calculate surface displacement caused by a dike intrusion in a homogenous half space. The depth and location of dike are changed to study the variation of the effects produced (displacements). The size and location of the intruded dike are found to play a major role in determining both the displacement pattern and magnitude. When the dike is close to the surface, there is an inversion of the surface displacement pattern and very large surface displacement at certain benchmarks. Such phenomena can serve as precursors of such dike eruptions. Our study show a clear need to extend the existing geodetic network to cover the full island for volcano monitoring purposes.     

APPLICATION OF TOPOGRAPHIC SLOPE AND ELEVATION VARIATION COEFFICIENT IN IDENTIFYING THE MOTUO ACTIVE FAULT ZONE

The eastern Himalaya syntaxis is located at the southeastern end of the Qinghai-Tibet Plateau and is the area where the Eurasian plate collides and converges with the Indian plate. The Namjabawa is the highest peak in the eastern section of the Himalayas, and the Yarlung Zangbo River gorge is around the Namjabawa Peak. The NE-striking Aniqiao Fault with right-lateral strike-slip is the eastern boundary fault of the Namjabawa syntaxis. Motuo Fault is in the east of and parallel to the Aniqiao Fault, distributing along the valley of the Yarlung Zangbo River. The section of Yarlung Zangbo River valley at the eastern side of the Namjabawa area is located in the southern foothills of the Himalayas and belongs to the subtropical humid climate zone with dense tropical rainforest vegetation. Dense vegetation, large terrain elevation difference, strong endogenetic and exogenic forces, and abundant valley deposition bring enormous difficulty to the research on active faults in this area. Since 1990s, surface morphology can be quantitatively expressed by digital elevation models as the rapid development of remote sensing technology. Geomorphic types and their characteristics can be quantified by geomorphological parameters which are extracted from DEM data, describing geomorphologic evolution and tectonic activity. But to date, researches based on quantitative geomorphic parameters are mainly focus on the differential uplift of regional blocks. In the study and mapping of active faults, surface traces of active faults are acquired by visual interpretation of remote sensing images. It has not been reported to identify the location of active faults via the change of quantitative geomorphic parameters. The distribution map of topographic elevation variation coefficient is suitable to reflect the regional erosion cutting and topographic relief, and the places with higher topographic elevation variation coefficient are more strongly eroded. In this paper, we attempt to identify the active faults and explore their distribution in the Yarlung Zangbo Gorge in the east of the Namjabawa Peak based on the application of two quantitative geomorphic parameters, namely, the topographic slope and the elevation variation coefficient. Using the DEM data of 30m resolution, two quantitative geomorphic parameters of topographic slope and elevation variation coefficient in Namjabawa and its surrounding areas were obtained on the ArcGIS software platform. On the topographic slope distribution map, the slope of the eastern and western banks of the Yarlung Zangbo River near Motuo is steep with a slope angle of more than 30°. Under the background of steep terrain, there are gentle slope belts of 5°~25° distributing intermittently and NE-striking. On the distribution map of topographic elevation variation coefficient, the elevation variation coefficient of the Yarlung Zangbo River near Motuo is greater than 0.9. On the background of the high topographic fluctuation area, it develops gently topographic undulating belts with elevation variation coefficient of 0.2~0.9. The belts are intermittently distributed and northeastern trending. Through the field geological and geomorphological investigation and trench excavation, it is found that the abnormal strips of the above-mentioned geomorphological parameters are the locations where the active faults pass. The above results show that the quantitative analysis of the topographic slope and the coefficient of variation of elevation can help us find active faults in areas with large terrain slope, serious vegetation coverage and high denudation intensity.     

Spatial vent opening probability map of Etna volcano (Sicily, Italy)

We produce a spatial probability map of vent opening (susceptibility map) at Etna, using a statistical analysis of structural features of flank eruptions of the last 2?ky. We exploit a detailed knowledge of the volcano structures, including the modalities of shallow magma transfer deriving from dike and dike-fed fissure eruptions analysis on historical eruptions. Assuming the location of future vents will have the same causal factors as the past eruptions, we converted the geological and structural data in distinct and weighted probability density functions, which were included in a non-homogeneous Poisson process to obtain the susceptibility map. The highest probability of new eruptive vents opening falls within a N-S aligned area passing through the Summit Craters down to about 2,000?m?a.s.l. on the southern flank. Other zones of high probability follow the North-East, East-North-East, West, and South Rifts, the latter reaching low altitudes (～400?m). Less susceptible areas are found around the faults cutting the upper portions of Etna, including the western portion of the Pernicana fault and the northern extent of the Ragalna fault. This structural-based susceptibility map is a crucial step in forecasting lava flow hazards at Etna, providing a support tool for decision makers.     

Apparent stress and stress drop for intraplate earthquakes and tectonic stress in the plates

The magnitude of shear stress in the lithosphere is bounded from below by the apparent stress and stress drop during intraplate earthquakes. Apparent stresses and stress drops for a number of mid-plate earthquakes are calculated from the earthquake magnitude, SH wave amplitude spectra, and estimates of the length of the fault zone. Apparent stresses vary between 0.1 and 2 bars, ifm _b is used as a measure of seismic energy, and stress drops lie between 2 and 70 bars. There is no systematic difference in either apparent stress or stress drop between these intraplate events and typical plate boundary earthquakes. These bounds on intraplate shear stresses are consistent with the inference from current models of plate tectonic driving forces that regional stress differences in the plates are typically on the order of 100 bars. The highest stress drops measured for midplate earthquakes under this model represent nearly total release of local tectonic stress.     

Dayside isotropic precipitation of energetic protons

Recently it has been shown that isotropic precipitation of energetic protons on the nightside is caused by a non-adiabatic effect, namely pitch-angle scattering of protons in curved magnetic field lines of the tail current sheet. Here we address the origin of isotropic proton precipitation on the dayside. Computations of proton scattering regions in the magnetopheric models T87, T89 and T95 reveal two regions which contribute to the isotropic precipitation. The first is the region of weak magnetic field in the outer cusp which provides the 1–2° wide isotropic precipitation on closed field lines in a 2–3 hour wide MLT sector centered on noon. A second zone is formed by the scattering on the closed field lines which cross the nightside equatorial region near the magnetopause which provides isotropic precipitation starting 1.5–2 h MLT from noon and which joins smoothly the precipitation coming from the tail current sheet. We also analyzed the isotropic proton precipitation using observations of NOAA low altitude polar spacecraft. We find that isotropic precipitation of >30 to > 80 keV protons continues around noon forming the continuous oval-shaped region of isotropic precipitation. Part of this region lies on open field lines in the region of cusp-like or mantle precipitation, its equatorward part is observed on closed field lines. Near noon it extends 1–2° below the sharp boundary of solar electron fluxes (proxy of the open/closed field line boundary) and equatorward of the cusp-like auroral precipitation. The observed energy dispersion of its equatorward boundary (isotropic boundary) agrees with model predictions of expected particle scattering in the regions of weak and highly curved magnetic field. We also found some disagreement with model computations. We did not observe the predicted split of the isotropic precipitation region into separate nightside and dayside isotropic zones. Also, the oval-like shape of the isotropic boundary has a symmetry line in 10–12 MLT sector, which with increasing activity rotates toward dawn while the latitude of isotropic boundary is decreasing. Our conclusion is that for both dayside and nightside the isotropic boundary location is basically controlled by the magnetospheric magnetic field, and therefore the isotropic boundaries can be used as a tool to probe the magnetospheric configuration in different external conditions and at different activity levels.     

新疆西天山乌孙山地区石英正长斑岩形成时代及其地质意义

新疆西天山乌孙山地区科库萨依系列石英正长斑岩呈岩瘤状、岩脉状产出,侵入于早石炭世大哈拉军山组地层中。其LA-ICP-MS锆石U-Pb年龄为314.4±3.7 Ma（MSWD=0.51）,属石炭世晚期,为本区岩浆活动及构造环境探讨提供年代学依据。     

中国布格重力异常水平梯度图的判读和构造解释

分析和对比中国大陆布格重力异常水平梯度的彩色图像,得到一些新的构造特征信息. 大致以105°E为界,图像的东部以蓝色低梯度为背景,出现中、高梯度带状构造和大梯度差值边界的线状构造;西部以高、中梯度的带状构造为格架,镶嵌着低梯度区,区内有阴影线状构造. 总体上,重力异常水平梯度构造可概括为三大组. 第一组是NNE向的大兴安岭、太行山、武陵山高梯度带与斜切东部全区的郯庐大断裂带和NNW向的斜切西部全区的三条大断裂带;第二组为西部的NWW向与NEE向高、中梯度带的交叉构造格局,以及东部近于等间距的三条NWW向断续的阴影构造带和一条单一的构造线,而华南则仅有NE向的低梯度构造线;第三组是横贯全区的的几条近EW向和中部SN向宽而复杂的构造带,在后者的东、西两侧散臣着大间距、小长度、断续的SN向阴影构造线. 这三组重力梯度构造不仅反映了地貌起伏的信息,而且显示了大区域构造,特别是深部构造的密度差. 各组线状构造带的内部结构和各组的形成时间及演变历史可能不同,但它们相互复合构成现今总体的构造格局,对大尺度现代构造应力场作用都会有不同的响应,因此导致主应力方向的区域性变化.     

Vertical and horizontal vibrations of a rigid disc on a multilayered transversely isotropic half-space

A half-space containing horizontally multilayered regions of different transversely isotropic elastic materials as well as a homogeneous half-space as the lowest layer is considered such that the axes of material symmetries of different layers and the lowest half-space to be as depth-wise. A rigid circular disc rested on the free surface of the whole half-space is considered to be under a forced either vertical or horizontal vibration of constant amplitudes. Because of the involved integral transforms, the mixed boundary value problems due to mixed condition at the surface of the half-space are changed to some dual integral equations, which are reduced to Fredholm integral equations of second kind. With the help of contour integration, the governing Fredholm integral equations are numerically solved. Some numerical evaluations are given for different combinations of transversely isotropic layers to show the effect of degree of anisotropy of different layers on the response of the inhomogeneous half-space.     

Torsional vibration of a finite cylindrical cavity in a two-layer transversely isotropic half-space

The aim of this paper is to present a rigorous investigation for a two-layered transversely isotropic linear elastic half-space containing a circular cylindrical cavity of length equal to the top layer undergoing mono-harmonic ring shape shear stress applied either on the vertical cylindrical surface or on the base of the cavity. To this end, a combination of Fourier cosine integral transform for depth and Hankel integral transform for radial distance are used, which translate the boundary value problem to a singular integral equation for the shear stress comes out from the continuity of two layers. The integral equation is solved for some collocation points with a smoothed variable of distance, which is adapted with the use of a free parameter. It is shown that, although the shear stress is highly singular, it does not highly depend on this free parameter. Both the analytical and numerical results are verified with both the static isotropic and dynamic transversely isotropic homogeneous cases. In addition, some new graphical results are presented for more understanding in engineering point of view.     

Central Europe: Active or residual tectonic stresses

The regional stress field in the Western Alps and their northern foreland has been investigated byin situ stress determinations. More than 600 strain relief measurements were made with resistance strain gages in boreholes carried out in mines, tunnels and quarries. The stresses calculated and data obtained from other papers were used to get a detailed idea of the stress conditions in Central Europe.The measurements confirm a continuous flux of compressive stress from the Alps to the northern foreland east of the Rhinegraben. The largest stresses are observed in the Central Alps, the lowest in the Rhinegraben rift system. The horizontal stresses exceed at nearly all places the vertical ones. Evidently the excess of horizontal stress is generated by active plate tectonics in the Alps. A tectonic model to explain the observed stress pattern is presented.