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排序方式: 共有196条查询结果,搜索用时 31 毫秒
101.
Manuel Carbajal-Romero Esteban Flores-Mendez Norberto Flores-Guzmán Jaime Núñez-Farfán Enrique Olivera-Villaseñor Francisco José Sánchez-Sesma 《Geofísica Internacional》2013,52(1):21-30
The present work shows the propagation of Scholte interface waves at the boundary of a fluid in contact with an elastic solid, for a broad range of solid materials. It has been demonstrated that by an analysis of diffracted waves in a fluid it is possible to infer the mechanical properties of the elastic solid medium, specifically, its propagation velocities. For this purpose, the diffracted wave field of pressures and displacements, due to an initial wave of pressure in the fluid, are expressed using boundary integral representations, which satisfy the equation of motion. The source in the fluid is represented by a Hankel’s function of second kind and zero order. The solution to this wave propagation problem is obtained by means of the Indirect Boundary Element Method, which is equivalent to the well-known Somigliana representation theorem. The validation of the results is carried out by using the Discrete Wave Number Method and the Spectral Element Method. Firstly, we show spectra of pressures that illustrate the behavior of the fluid for each solid material considered, then, we apply the Fast Fourier Transform to show results in time domain. Snapshots to exemplify the emergence of Scholte’s waves are also included. 相似文献
102.
R. Chan M. F. A. da Silva Jaime F. Villas?da?Rocha 《Astrophysics and Space Science》2012,337(1):185-191
Using the conventional gravastar model, that is, an object constituted by two components where one of them is a massive infinitely
thin shell and the other one is a de Sitter interior spacetime, we physically interpret a solution characterized by a zero
Schwarzschild mass. No stable gravastar is formed and it collapses without forming an event horizon, originating what we call
a massive non-gravitational object. The most surprise here is that the collapse occurs with an exterior de Sitter vacuum spacetime.
This creates an object which does not interact gravitationally with an outside test particle and it may evolve to a point-like
topological defect. 相似文献
103.
Results of a study of turbiditic deposits in a sequence of middle to late Holocene marine sediments from Alfonso Basin in
the southern Gulf of California are presented. Turbidites are identified from analyses of core sections, X-ray images, texture
and granulometry. Characterization of distal turbidites could be difficult, particularly for thin fine-grained deposits with
little contrast in grain size, mineralogy and texture. We have here included analyses of microfossil and magnetic mineral
data. Turbiditic currents transport material from various depths in the continental slope into the basin and it can be expected
that turbiditic deposits are characterized by re-worked material, lack of microfossils or mixed assemblages with no content
of paleoceanographic and paleoclimatic record. This may show in lack of internal coherence as quantified by factor analysis
of microfossil assemblages. Turbidites in some cases may also show distinct magnetic properties as compared with those in
the sediment sequence. In Alfonso Basin, turbidites show increased contents of detrital magnetic minerals volcanic-derived
and transported from the basin slope. We identify twelve turbiditic layers through the sequence that present high magnetic
susceptibility and low communalities from factor model to the microfossils data providing characterization of the turbidites. 相似文献
104.
105.
Forest cover change in highland pine-oak forests of Michoacan, Mexico is due to a process of conversion of natural forests to avocado orchards. Privately-owned avocado orchards are found on land that was common forest before the 1992 Reform of the Mexican Constitution. We ask how forest cover change was facilitated by policy changes that affected land tenure rules and existing community forestry programs. We use a comparative case study of four communities, an analysis of forest cover change, and interviews and household surveys. Results show that 33.1% of forest cover was lost over a 16-year-period across the region. However, two forestry case study communities lost 7.2% and 15.1% of forest cover, while two adjacent non-forestry communities lost 86.5% and 92.4%, respectively. Interview data show that the Reform of Article 27 combined with the 1992 Forestry Law led to collapse of local governance, illegal division of common forests, and illegal logging in the two non-forestry communities. 相似文献
106.
Juan Morales Avto Goguitchaichvili Luis M. Alva-Valdivia Jaime Urrutia-Fucugauchi 《Comptes Rendus Geoscience》2003,335(8):661-670
We carried out a detailed rock-magnetic and paleointensity study of the ~187-Ma volcanic succession from northern Chile. A total of 32 consecutive lava flows (about 280 oriented standard paleomagnetic cores) were collected at the Tocopilla locality. Only 26 samples with apparently preserved primary magnetic mineralogy and without secondary magnetization components were pre-selected for Thellier paleointensity determination. Eleven samples coming from four lava flows yielded reliable paleointensity estimates. The flow-mean virtual dipole moments range from 3.7±0.9 to A m2). This corresponds to a mean value of (5.0±1.8)×1022 A m2, which is in reasonably good agreement with other comparable quality paleointensity determinations from the Middle Jurassic. Given the large dispersion and the very poor distribution of reliable absolute intensity data, it is hard to draw any firm conclusions regarding the time evolution of the geomagnetic field. To cite this article: J. Morales et al., C. R. Geoscience 335 (2003). 相似文献
107.
108.
Continuous pollen and sediment records from two ∼8.5-m-long cores document late Pleistocene and Holocene sedimentation and vegetation change in the Ballston Lake basin, eastern New York State. Pebbles at the base of both cores and the geomorphology of the watershed reflect the presence of the Mohawk River in the basin prior to ∼12,900 ± 70 cal yr B.P. Ballston Lake formed at the onset of the Younger Dryas (YD) by an avulsion of the Mohawk River. The transition from clay to gyttja with low magnetic susceptibility (MS), low bulk density, and high organic carbon indicates rapid warming and increased lake productivity beginning 11,020 cal yr B.P. MS measurements reveal that the influx of magnetic particles, associated with pre-Holocene clastic sedimentation, ceased after ∼10,780 cal yr B.P. The pollen record is subdivided into six zones: BL1 (12,920 to 11,020 cal yr B.P.) is dominated by boreal forest pollen; BL2 (11,020 to 10,780 cal yr B.P.) by pine (Pinus) forest pollen; BL3 (10,780 to 5290 cal yr B.P.) by hemlock (Tsuga) and mixed hardwood pollen; BL4 (5290 to 2680 cal yr B.P.) by mixed hardwood pollen; BL5a (2680 cal yr B.P. to 1030 cal yr B.P.) by conifer and mixed hardwood pollen; and BL5b (1030 cal B.P. to present) by increasing ragweed (Ambrosia) pollen. A 62% decrease in spruce (Picea) pollen in <320 cal years during BL1 reflects rapid warming at the end of the YD. Holocene pollen zones record more subtle climatic shifts than occurred at the end of the YD. One of the largest changes in the Holocene pollen spectra began ∼5300 cal yr B.P., and is characterized by a marked decline in hemlock pollen. This has been noted in other pollen records from the region and may record preferential selection of hemlock by a pathogen or parasites. 相似文献
109.
An integrated paleomagnetic study of Rio Grande de Santiago volcanic succession (trans-Mexican volcanic belt): revisited 总被引:1,自引:0,他引:1
Avto Goguitchaichvili Luis Alva-Valdivia Jose Rosas-Elguera Jaime Urrutia-Fucugauchi Jose Antonio Gonzalez Juan Morales Jesus Sol 《Physics of the Earth and Planetary Interiors》2002,130(3-4):175-194
We carried out an integrated paleomagnetic, rock-magnetic and paleointensity study of Miocene volcanic succession from the trans-Mexican volcanic belt (TMVB) north of Guadalajara. A total of 37 consecutive basaltic lava flows (326 oriented standard paleomagnetic cores) were collected at Lazo locality. Continuous susceptibility measurements with temperature and hysteresis experiments yield in most cases reasonably reversible curves with Curie points close to that of pseudo-single-domain magnetite. Two geomagnetic reversals were observed in the 300 m thick composite section. Paleosecular variation was lower than the one observed in general during Miocene. It appears that the volcanic units have been emplaced during a relatively short time span of about 1 Ma. The mean paleomagnetic directions obtained from this study do not differ significantly from that expected for the middle Miocene. The mean paleomagnetic direction calculated from all data is I=31.1°, D=354.6°, k=124 and 95=2.1°, N=37. Seventy-two samples with apparently preserved primary magnetic mineralogy and without secondary magnetization, mostly belonging to reverse polarity chron were pre-selected for Thellier paleointensity determination. The flow-mean paleointensity values are ranging from 22.4±3.4 to 53.8±6.0 μT and the corresponding virtual dipole moments (VDMs) are ranging from (5.4±0.8) to (12.0±1.4)×1022 A m2. This corresponds to mean value of (7.7±2.2)×1022 A m2, which is close to present day geomagnetic field strength. Altogether, our data suggest the existence of relatively high geomagnetic field strength undergoing low fluctuations. 相似文献
110.
Luis M Alva-Valdivia Avto Goguitchaichvili Manuel Grajales Antonio Flores de Dios Jaime Urrutia-Fucugauchi Carmen Rosales Juan Morales 《Comptes Rendus Geoscience》2002,334(11):811-817
We report rock-magnetic and magnetostratigraphic results from one of the best exposed Permo-Carboniferous outcrops in Mexico, in order to determine some decisive magnetostratigraphy constraints. Some spinels, most probably titanomagnetites, seem to be responsible for aimantation, although greigite may also exist judging from thermal desaimantation of isothermal remanence. Remanence analyses indicate that only one remanent component could be recognized with minor secondary overprint, which were easily removed applying 100–180 °C. Six normal and four reverse magnetozones were recognized from bottom to top in Patlanoya section between 340 and 280 Myr. Both normal and reverse polarity rocks were found in Carboniferous time around 340 Myr, in agreement with previous paleomagnetic works. Our record revealed two normal subchrons within PCRS (Kiaman), the dominantly reverse superchron, at 305 and 280 Myr, respectively. These, which may be speculatively correlated to normal aimantation, occasionally occurred during PCRS interval. More detailed studies are needed in order to establish a more precise magnetostratigraphy for Permo-Carboniferous time. To cite this article: L.M. Alva-Valdivia et al., C. R. Geoscience 334 (2002) 811–817. 相似文献