Simple stochastic modelling of the eruption history of a basaltic volcano: Nyamuragira,Zaire     

M. L. Burt  G. Wadge  W. A. Scott 《Bulletin of Volcanology》1994,56(2):87-97

Three simple models of the behaviour of a series of basaltic eruptions have been tested against the eruptive history of Nyamuragira. The data set contains the repose periods and the volumes of lava emitted in 22 eruptions since 1901. Model 1 is fully stochastic and eruptions of any volume with random repose intervals are possible. Models 2 and 3 are constrained by deterministic limits on the maximum capacity of the magma reservoir and on the lowest drainage level of the reservoir respectively. The method of testing these models involves (1) seeking change points in the time series to determine regimes of uniform magma supply rate, and (2) applying linear regression to these regimes, which for models 2 and 3 are the determinsstic limits to those models. Two change points in the time series for Nyamuragira, in 1958 and 1980, were determined using a Kolmogorov-Smirnov technique. The latter change involved an increase in the magma supply rate by a factor of 2.5, from 0.55 to 1.37 m³s^-1. Model 2 provides the best fit to the behavior of Nyamuragira with the ratio of variation explained by the model to total variation. R², being greater than 0.9 for all three regimes. This fit can be interpreted to mean that there is a determinstic limit to the elastic strength of the magma reservoir 4–8 km below the summit of the volcano.  相似文献   

Application of numerical wave-propagation techniques to study local soil effects: The case of Benevento (Italy)     

D. Fäh  P. Suhadolc 《Pure and Applied Geophysics》1994,143(4):513-536

Many of the numerical techniques used for seismic zonation studies treat one-dimensional structural models and/or the incidence of plane polarized body waves. These techniques are often not adequate for laterally heterogeneous structures and for sources that are not located beneath the site of interest. In such cases a more rigorous treatment of the combined effects of the source, the path and the site response is needed. This can be accomplished with a hybrid approach combining modal summation and the finite-difference technique. To demonstrate the differences between these techniques, the ground motion in the city of Benevento (Italy) is modelled. We first compare the results obtained with one-and two-dimensional structural models for vertical incidence of plane polarized body waves. These results are then compared with those obtained with the hybrid approach for two-dimensional structural models.The comparisons have allowed us to find important differences in the response obtained with the different modelling techniques. For the same site, these differences consist of strong variations in amplitude and in the shape of the spectral amplifications. For a seismic source which is not located beneath the site, vertical incidence of waves significantly overestimates the local hazard in a laterally homogeneous structure. For a laterally heterogeneous area, we can conclude that one-dimensional modelling fails to estimate the seismic hazard, whereas for a seismic source which is not located beneath the site of interest, two-dimensional modelling with vertical incidence of plane polarized body waves may not allow reliable estimates to be made of the frequency bands at which amplifications occur. The results obtained for two-dimensional structural models are used for a zonation of the city of Benevento.  相似文献   

Geomagnetic depth sounding over the Singhbhum and the surrounding regions of eastern India     

A. N. Hanchinal  P. B. V. Subba Rao  Nandini Nagarajan  D. R. K. Rao  B. P. Singh 《Journal of Earth System Science》1994,103(3):383-400

Magnetovariational studies have been carried out in Singhbhum and surrounding regions during 1987 and 1989. Three deep-seated linear conductors have been identified. One of them is located to the north of Ranchi, Bokaro and Purulia extending in E-W direction coinciding with high heat flow region and Gondwana sediments. The trend of anomaly at Ranchi and Purulia at longer periods suggests a conductivity anomaly due to the mafic and ultramafic intrusions, considered to be responsible for the uplift of Chhotanagpur plateau. The second conductor is associated with the basin margin fault that separates the Singhbhum craton and Chhotanagpur plateau from the West Bengal basin. This conductive zone appears to extend further south and join the high heat flow region of Attri-Tarabalo. This conductor could be isolated only after eliminating the coast effect from the observed induction vectors. The third conductive zone follows the trend of Mahanadi valley located south of the Sukinda thrust. Conductive anomaly associated with the Sukinda and Singhbhum thrust zones could not be resolved due to the interference from neighbouring conductive structures. These two thrusts may not be very deep-seated structures. The Singhbhum granite batholith is found to be highly resistive and seems to extend to greater depths.  相似文献   

Regularized spherical harmonic analysis and the 3-D electromagnetic response of the Earth     

Adam Schultz  Tian Shan Zhang 《Geophysical Journal International》1994,116(1):141-156

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Semi-empirical method for calculation of maximum subsidence in coal mines     

P. P. Bahuguna  Bhawani Singh  A. M. C. Srivastava  N. C. Saxena 《Geotechnical and Geological Engineering》1993,11(4):249-261

Summary Most empirical methods of subsidence prediction are based on the prior knowledge of the maximum possible subsidence, S_max, for a particular coalfield. The subsidence profile and other associated parameters along a desired line may be derived, in most cases, as a function of S _max. These methods are site specific and are not based upon the rational concepts of mechanics. Purely mechanistic methods, on the other hand, could not find wide application because of their limitations in representing the complex behaviour of a rockmass.Therefore a semi-empirical method of calculation of S _max or S, which is an improvement over the empirical method reported elsewhere (Bahuguna et al., 1991a), has been derived from combining the mechanistic and empirical approach. The effects of various parameters are studied by numerical modelling. These qualitative results are then used to influence the development of a more general semiempirical method. The method has been tested for 125 coal mine workings in India and 22 mines of the North Appalachian basin. The method is easily adaptable to other countries.  相似文献   

长江三峡工程构造应力场数值模拟     

谭成轩 《水文地质工程地质》1993,20(4):7-9,56

本文分别进行了长江三峡工程区域1/100万现今构造应力场有限元数值模拟和长江三峡工程库首区1/50万燕山期、新构造期和现今构造应力场演化规律模拟和分析。区域模拟结果表明:坝区不存在发生大于5级地震的构造应力场背景,库首区模拟结果表明:坝区不存在发生大于3级地震的构造应力场背景。  相似文献   

Investigation of the earth's crustal structure using earthquake and deep seismic sounding data obtained for the Carpathians     

N. V. Kondorskaya  L. B. Slavina  N. B. Pivovarova  V. B. Sollogub  A. V. Chekunov  E. A. Sagalova  Y. K. Shchukin 《Pure and Applied Geophysics》1981,119(6):1157-1166

The joint analysis performed for the Carpathian region from seismological data (three-dimensional fields of velocities, areas of concentration of earthquake foci in the Vrancea focal zone) and interpretation of DSS data and those of other geophysical fields have permitted to obtain new evidence for the earth's crust and mantle structure in this region.  相似文献   

On the resolving power of the VLF method     

Václav Bezvoda  Karel Segeth 《Pure and Applied Geophysics》1982,120(2):348-364

In the paper, the mathematical tools, used for the modelling of the electromagnetic field of the harmonic plane wave in the two-dimensional inhomogeneous medium (the case ofE-polarization), are presented. Further, the resolving power of some parameters, that are measured in the VLF and VLF-R methods, is compared in the case of two vertical conductive dykes. To this aim, all the usual parameters of the VLF method and the most important parameter of the VLF-R method () have been chosen. Two groups of models that differ in resistivities have been considered. In each of the groups the distance of the dykes is variable. The parameters |H _y|, which has shown most promising in this respect, is examined also from the point of view of the sensibility to geological noise. The results of the modelling are illustrated by two examples of the results of field measurements.  相似文献   

Laboratory modelling of gravel braided stream morphology     

Peter E. Ashmore 《地球表面变化过程与地形》1982,7(3):201-225

Hydraulic modelling principles, together with a knowledge of channel pattern thresholds, allow the development of a small scale model of a gravel braided stream with flow characteristics and equivalent dimensions of a natural river. The forms and processes of natural gravel braided rivers are reproduced by imposing a constant flume discharge and slope, and maintaining approximate equilibrium with an adjustable sediment feed. Beginning from a straight trough, braiding is initiated by development of a series of alternating bars and scour pools which produce bends of increasing amplitude, leading finally to channel division. These lobate bars accrete downstream by deposition of bed material at their margins, often in the form of avalanche faces. Together with the scour pools with which they are necessarily closely associated, these bars are the fundamental elements of the channel pattern. Channel migration and division is a response to the development of bars, and these adjustments leave portions of the originally active bars in the form of exposed and eroded remnants. Complex flats built from these lobate forms show varying degrees of preservation of the original depositional units, but the model allows observation of the systematic construction of some flats. Sorting of sediment on active bars with avalanche faces shows a distinct fining downstream. This may be the result of the accretion of fining upwards avalanche faces along the bar margins rather than a ‘winnowing out’ of fine material. The processes and forms observed in the model appear to be very similar to those occurring in natural gravel braided streams during peak flows.  相似文献   

Insights about the structure and development of Zhongsha Bank in the South China Sea from integrated geophysical modelling     

Haibo Huang  Enyuan He  Xuelin Qiu  Jianke Fan  Xunhua Zhang 《International Geology Review》2020,62(7-8):1070-1080

ABSTRACT

We construct a complete density transection based on the velocity structures across the Zhongsha Bank in the South China Sea. Gravity modelling of the lateral density contrasts between tectonic units helps us to determine the structural attributes and boundaries between continental blocks and deep basins. The configuration of the continent–ocean boundary (COB) around the Zhongsha Bank is mapped based on the gravity/magnetic anomaly and crustal structures. A low-density mantle is found beneath the Zhongsha Bank and the oceanic basins, and this mantle is associated with the high heat-flow background. The COB orientation is northeast-east in the north of the bank, with faulted linear structures. In further southeast, where there is a more intact crust, the COB orientation changed to north-northeast. The reconstructed density model and gravity/magnetic map indicate that the Zhongsha Bank is conjugated with the Liyue Bank by a rifted basin, where the crust had experienced localized deformation before the seafloor spreading. Because of the insufficient magmatism in the oceanic basin, the spreading ridge propagates into the weakened continental lithosphere between the two continental blocks, thus completely separating the Zhongsha Bank from the Liyue Bank. Seafloor spreading ridge jumps within the South China Sea may also be affected by the heterogeneous lithosphere beneath the continental blocks and oceanic basins.  相似文献