Seismic rays in an ellipsoidal earth model: theoretical analysis and estimation of deviations due to the ellipticity     

Jean-Francois Guéguen  Pho Hoang-Trong  Hilaire Legros 《Geophysical Journal International》1987,89(3):839-855

Summary. A first-order form of the Euler's equations for rays in an ellipsoidal model of the Earth is obtained. The conditions affecting the velocity law for a monotonic increase, with respect to the arc length, in the angular distance to the epicentre, and in the angle of incidence, are the same in the ellipsoidal and spherical models. It is therefore possible to trace rays and to compute travel times directly in an ellipsoidal earth as in the spherical model. Thus comparison with the rays of the same coordinates in a spherical earth provides an estimate of the various deviations of these rays due to the Earth's flattening, and the corresponding travel-time differences, for mantle P -waves and for shallow earthquakes. All these deviations are functions both of the latitude and of the epicentral distance. The difference in the distance to the Earth's centre at points with the same geocentric latitude on rays in the ellipsoidal and in the spherical model may reach several kilometres. Directly related to the deformation of the isovelocity surfaces, this difference is the only cause of significant perturbation in travel times. Other differences, such as that corresponding to the ray torsion, are of the first order in ellipticity, and may exceed 1 km. They induce only small differences in travel time, less than 0.01s. Thus, we show that the ellipticity correction obtained by Jeffreys (1935) and Bullen (1937) by a perturbational method can be recovered by a direct evaluation of the travel times in an ellipsoidal model of the Earth. Moreover, as stated by Dziewonski & Gilbert (1976), we verify the non-dependence of this correction on the choice of the velocity law.  相似文献   

Modele circulaire de source sismique     

Hoang Trong Pho  Hilaire Legros 《Pure and Applied Geophysics》1974,112(2):380-394

Résumé On donne l'expression du déplacement dû à la propagation d'une dislocation uniforme partant du centre d'une faille circulaire, avec une vitesse de propagation finie ou infinie. Dans cedernier cas on se trouve dans une situation analogue à celle de la diffraction de Fraunhofer en Optique. Le spectre d'amplitude de l'ondeP est modulé dans son détail par la fonction de Bessel d'ordre1. Quand le rayon du cercle est relativement faible, on retrouve les expressions d'une source ponctuelle, dans lesquelles la densité surfacique de moment intégré est remplacé par le moment sismique total. On met en évidence une fréquence de coupure qui augmente avec la profondeur du foyer et diminue quand le rayon de la faille augmente. On donne enfin une expression du moment sismique total en fonction de l'amplitude spectrale aux basses fréquences.

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Summary We have examined the case of the radial propagation of a uniform dislocation and stated in detail the expression of the displacement. The rupture velocity is assumed finite or infinite. The last case shows some analogies with the Fraunhofer diffraction. TheP wave amplitude spectrum is modulated in its detail by theJ ₁ Bessel function. With small values of the radius, one can obtain the displacement by replacing in the expression corresponding to point sources the integral of the moment density by the total seismic moment. The corner frequency increases with the focal depth, and decreased when the fault radius increases. Finally, we give an expression of the total seismic moment deduced from the low frequency spectral amplitude components.

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Geochemistry of surface waters in the vicinity of open pit mines at the Cay Cham deposit, Thai Nguyen province, northern Vietnam     

Pham Tich Xuan  Nguyen Van Pho  O. L. Gas’kova  S. B. Bortnikova 《Geochemistry International》2013,51(11):931-938

This study provides an assessment of the environmental impact of open pit mining operations at the Cay Cham titanomagnetite-ilmenite deposit (northern Vietnam). The results of surface water sampling indicate the formation of acid mine drainage and contamination of adjacent areas by heavy metals (Cu, Zn, Ni, and Mn). The acid mine drainage is produced by oxidation leaching of sulfide minerals associated with primary mineralization owing to the low neutralization potential of the natural waters in the humid environment of tropical rainforest. The study showed that alternating dry and wet seasons typical of this climatic region promote the generation of stored acidity leading to a sharp decrease in pH of drainage water during the wet season and result in the negative impacts of this mine on both flowing and stagnant surface waters.  相似文献   

Some medium properties of the Hohenzollerngraben (Swabian Jura,W. Germany) inferred from Q_P/Q_S analysis     

P. Hoang-Trong 《Physics of the Earth and Planetary Interiors》1983,31(2):119-131

High-quality three-component records of some very low-magnitude aftershocks (M_L < 2) of the September 3, 1978 Swabian Jura earthquake have been used to estimate the crustal ratio Q_P/Q_S in this seismically active area. The slopes of the P and S spectra at high frequency (20–40 Hz) are the basic data used in this investigation. Q_P/Q_S ratios depend upon the area crossed by the corresponding ray path. In particular, small ratios are observed for rays crossing the Hohenzollerngraben. This result suggests that the crystalline basement has been tectonized under the graben structure and has experienced microcracking and/or grain size reduction.  相似文献   

Identification of anthropogenic radionuclides in the Song Hong (Red) and Song Da (Black) river basins of Vietnam     

Nguyen Trung Minh  L. L. Kashkarov  Cu Sy Thang  Doan Dinh Hung  Nguyen Duc Chuy  Nguyen Trung Kien  Nguyen Thi Kim Thuong  Nguyen Thi Thu  Nguyen Van Pho 《Geochemistry International》2014,52(11):925-938

The concentration and distribution of U were measured in zircon samples separated from the streambed sediments collected at different sites along the rivers of northern Vietnam using semiconductor alpha spectrometry and alpha- and fission-track autoradiography. Based on the absolute U content and distribution, we distinguished two groups of zircon grains of different origin. Group 1 zircons have relatively low and uniform U contents ranging from 280 to 440 μg/g. Group 2 zircons are characterized by heterogeneous U distribution and U contents ranging from 490 to 2040μg/g. The determination of absolute alpha activity values and alpha spectrometric characteristics for a number of group 2 zircon grains indicated the presence of relatively short-lived alpha-emitters. These radionuclides are probably ²¹⁰Po, ²³²U, ²³⁶Pu, ²⁴¹Am, and ²⁴³Am, and all of them can originate exclusively from anthropogenic sources. A comparison of the observed alpha activity values for group 2 zircons with those of group 1 zircons and zircons separated from the suspended colloids in river water samples provided conclusive evidence for traces of anthropogenic radioactive contamination in the Lao Cai, Yen Bai, Tuyen Quang, Thac Ba, Viet Tri, and Hanoi streams flowing in the Song Hong (Red) and Song Da (Black) river basins of northern Vietnam and sourced in the southern regions of the neighboring state.  相似文献