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861.
G. Ramstein Y. Serafini-Le Treut H. Le Treut M. Forichon S. Joussaume 《Climate Dynamics》1998,14(4):233-247
To investigate the cloud response during cold and warm periods, we have performed simulations of the Last Glacial Maximum
(LGM-21ky BP) and of double CO2 concentration using the LMD AGCM model. We observe that the thermal characteristics of these two climates are opposite, but
the cloud response is more complex and does not display the same symmetry When doubling the CO2, the warming of the troposphere and the cooling of the stratosphere are clearly linked with a reduction in low-level clouds
and an increase of high-level clouds associated with relative humidity changes. For the LGM, the cloud response is more complex.
In the inter tropical region, we show that the Hadley cell is reinforced during LGM (+20%) whereas it is reduced (−10%) for
the double CO2 experiments. The most important feature is that we observe an enlarged Hadley cell for LGM climate which strongly modifies
the atmospheric dynamics and water transport. For LGM conditions, the cloud response is then mostly driven by these dynamical
changes at low latitudes though at high latitudes the thermal changes explain a large part of the cloud response. Two different
versions of the model, using different parametrizations for the precipitation show that cloud feedbacks may act differently
for cold and warm climates; and that the cloud response may be more complex that previously expected, but also indicate that
the details of these effects are model dependent. 相似文献
862.
K. Yoshimoto H. Sato Y. Iio H. Ito T. Ohminato M Ohtake 《Pure and Applied Geophysics》1998,153(2-4):489-502
—Borehole seismograms from local earthquakes in the aftershock region of the 1984 western Nagano Prefecture, Japan earthquake were analyzed to measure the frequency-dependent characteristics of P- and S-wave attenuation in the upper crust. The records from a three-component velocity seismometer at the depth of 145m exhibit high S/N-ratio in a wide frequency range up to 100 Hz. Extended coda normalization methods were applied to bandpass-filtered seismograms of frequencies from 25 to 102 Hz. For the attenuation of high-frequency P and S waves, our measurements show Q P -1? 0.052?-0.66 and Q S -1? 0.0034?-0.12 respectively. The frequency dependence of the quality factor of S waves is very weak as compared with that of P waves. The ratio of Q P -1/Q S -1 is larger than unity in the entire analyzed frequency range. 相似文献
863.
B. Muço 《Pure and Applied Geophysics》1998,153(1):151-162
—The northern part of Albania has been the focus of an intense effort by the Seismological Institute of Albania, for in this area two of the country’s biggest reservoirs are located. Three years before the impounding of the Fierza reservoir, a four-station network was installed around it. The possibility of induced seismicity continued after the impoundment of the Fierza reservoir in 1978 and the Komani reservoir, in 1985. The seismicity of the zone and some aspects of induced seismicity including temporal correlation of seismicity with water level changes, spatial patterns in seismicity, frequency-mag nitude relations, fault plane solutions etc., are studied during this period. The presence in this zone of a very important transverse fault, the Shkoder-Peja fault, makes the study of induced seismicity from Fierza and Komani reservoirs even more significant. The studies have shown that the impounding of the Fierza and Komani reservoirs has modified the natural course of microearthquake energy release, increasing the number of swarms in this area.¶The fluctuation of the water level in these two reservoirs, in due course, is a potentially important factor in the evaluation of seismicity for Northern Albania and especially in the hazard assessment of this region. 相似文献
864.
—Uniaxial compression, triaxial compression and Brazialian tests were conducted on several kinds of rock, with particular attention directed to the principal tensile strain. In this paper we aim to clarify the effects of the experimental environment—such as confining pressure, loading rate, water content and anisotropy—on the critical tensile strain, i.e., the measured principal tensile strain at peak load.¶It was determined that the chain-type extensometer is a most suitable method for measuring the critical tensile strain in uniaxial compression tests. It is also shown that the paper-based strain gage, whose effective length is less than or equal to a tenth of the specimen’s diameter and glued on with a rubber-type adhesive, can be effectively used in the Brazilian tests.¶The effect of confining pressure P C on the critical tensile strain ? TC in the brittle failure region was between ?0.02 × 10?10 Pa?1 and 0.77 × 10?10 Pa?1. This pressure sensitivity is small compared to the critical tensile strain values of around ?0.5 × 10?2. The strain rate sensitivities ?? TC /?{log(d|?|/dt)} were observed in the same way as the strength constants in other failure criteria. They were found to be from ?0.10 × 10?3 to ?0.52 × 10?3 per order of magnitude in strain rate in the triaxial tests. The average magnitude of the critical tensile strain ? TC increased due to the presence of water by 4% to 20% for some rocks, and decreased by 22% for sandstone. It can at least be said that the critical tensile strain is less sensitive to water content than the uniaxial compressive strength under the experimental conditions reported here. An obvious anisotropy was observed in the P-wave velocity and in the uniaxial compressive strength of Pombetsu sandstone. It was not observed, however, in the critical tensile strain, although the data do show some variation.¶A "tensile strain criterion" was proposed, based on the above experimental results. This criterion signifies that stress begins to drop when the principal tensile strain reaches the critical tensile strain. The criterion is limited to use within the brittle failure region. The critical tensile strain contains an inelastic strain component as well as an elastic one. It is affected by the strain rate, however, it is relatively insensitive to the confining pressure, the presence of water and anisotropy. 相似文献
865.
Bor-Ming Jahn A.Y. Glikson J.J. Peucat A.H. Hickman 《Geochimica et cosmochimica acta》1981,45(9):1633-1652
Eighteen silicic volcanic rocks of the Warrawoona Group and ten associated plutonic rocks from the Pilbara Block, Western Australia, have been chosen for geochemical and isotopic studies. Silicic volcanics of the UNSB (Upper member of North Star Basalt) are dated at 3.56—3.57 , by both the Rb-Sr and the Sm-Nd methods. The respective 1 (initial isotopic composition) values are 0.7005 ± 5 (Sr) and 0.50810 ± 39 (Nd). This age is consistent with the stratigraphic interpretation that the TalgaTalga Subgroup, in which the North Star Basalt occupies the lowermost position, is overlain by the Duffer Formation, whose age was earlier established at 3.45 by the zircon U-Pb method. The new Rb-Sr data on six silicic lava samples from the Duffer Formation yield an isochron of 3.23 ± 0.28 (2v). Though imprecise, this age agrees with the zircon age within error limits. Rb-Sr ages of 2.3–2.4. obtained for the ‘Panorama’ rocks and the Wyman Formation do not correspond to their initial eruption ages. Chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2.3–2.4 ago.Geochemically, most of these analyzed rocks (volcanic and plutonic) are of tonalite-trondhjemitegranodiorite (TTG) composition, a typical feature found in many other Archean terrains. They generally show fractionated REE patterns, except the Panorama Formation rocks. Furthermore, the Wyman Formation rhyolites and the post-tectonic adamellites show significant negative Eu anomalies, suggesting a similar mode of magma generation and a probable genetic link. Theoretical considerations suggest that most of these TTG rocks could have been generated by partial melting of amphibolitic or basaltic sources, followed by fractional crystallization.Although the Archean granitic gneisses often possess mantle-like Isr values, the trace element data indicate that they could not have been derived by direct melting of upper mantle materials. The immediate tectonic implication is that in any Archean terrain, the formation of Na-rich continental crust of TTG suite must be preceded by the presence of basaltic crust. The occurrence of this basaltic crust is a matter of controversy. Such crust might have been totally destroyed by repeated melting processes, or its remnants are now represented by some of the mafic-ultramafic enclaves within the tonalite-trondhjemite batholiths. 相似文献
866.
Y. Chung 《Earth and Planetary Science Letters》1981,55(2):205-216
210Pb and226Ra profiles have been measured at five GEOSECS stations in the Circumpolar region. These profiles show that226Ra is quite uniformly distributed throughout the Circumpolar region, with slightly lower activities in surface waters, while210Pb varies with depth as well as location or area. There is a subsurface210Pb maximum which matches the oxygen minimum in depth and roughly correlates with the temperature and salinity maxima. This210Pb maximum has its highest concentrations in the Atlantic sector and appears to originate near the South Sandwich Islands northeast of the Weddell Sea. Concentrations in this maximum decrease toward the Indian Ocean sector and then become fairly constant along the easterly Circumpolar Current.Relative to226Ra, the activity of210Pb is deficient in the entire water column of the Circumpolar waters. The deficiency increases from the depth of the210Pb maximum toward the bottom, and the210Pb/226Ra activity ratio is lowest in the Antarctic Bottom Water, indicating a rapid removal of Pb by particulate scavenging in the bottom layer and/or a short mean residence time of the Antarctic Bottom Water in the Circumpolar region.226Ra is essentially linearly correlated with silica and barium in the Circumpolar waters. However, close examination of the vertical profiles reveals that Ba and Si are more variable than226Ra in this region. 相似文献
867.
868.
Q.Y. Wei T.C. Li G.Y. Chao W.S. Chang S.P. Wang 《Physics of the Earth and Planetary Interiors》1981,25(1):107-112
Curves of the variation of the inclination and declination of the geomagnetic field for the Loyang region during the last 2400 years are presented. An I-D diagram of secular variation, like that for London, has been drawn for Loyang. 相似文献
869.
Variations in metamorphic grade, structural style, isotopic ages and granite geochemistry observed within the Yilgarn craton, and between the Yilgarn and Pilbara cratons, Western Australia, are interpreted in terms of vertical zonation of the Archaean crust. We correlate the gneiss-granulite suite of the Wheat Belt (southwestern Yilgarn) with concealed coeval infracrustal roots of the low-grade granite—greenstone Kalgoorlie terrain (eastern Yilgarn). Differences between the Pilbara, Southern Cross and Laverton granite—greenstone blocks and the downfaulted linear greenstone belts of the Kalgoorlie block are interpreted in terms of deeper-level exposure in the first three blocks.Ultramafic—mafic volcanic sequences in the Yilgarn craton can be divided into at least two major groups — the lower greenstones, regarded as relicts of a once extensive simatic crust, and the significantly younger upper greenstones, believed to have formed within linear troughs following the intrusion of Na-rich granites.At least three major Archaean granite phases occur in Western Australia: (1) 3.1-2.9 b.y. old (recognized to date only in the western Yilgarn and in the Pilbara craton); (2) 2.8-2.7 b.y. old, and (3) 2.6 b.y. old (the two latter phases can only be separated from each other in the eastern Yilgarn, and phase (3) is also identified in the Pilbara). In the main, granites of phases (1) and (2) are Na-rich and those of phase (3) are K-rich. There is evidence for a secular increase in Rb levels and initial 87Sr/86Sr ratios. It is suggested that the K-rich granites grade down into Na-rich granites, and the former were generated by ensialic anatexis resulting in upward migration of K, Rb, U, and Th-enriched magmas.A review of data from several Archaean cratons in other continents suggests that evidence from these regions can be interpreted in terms of the general model of crustal evolution proposed for Western Australia. Implications of this model concerning petrogenesis of Archaean plutonic and volcanic suites, geothermal gradients and tectonic evolution of greenstone belts are discussed. Partial melting associated with mantle diapirism is thought to have given rise to the ultramafic—mafic volcanic cycles. Widespread subsidence and partial melting of this crust yielded Na-rich acid magmas. The development of the upper greenstones was confined to linear belts in a partly cratonized crustal environment. About 2.6 b.y. ago a rise in the geothermal gradient resulted in regional metamorphism and crusctal anatexis which gave rise to the K-rich granites. 相似文献
870.
A simple and highly accurate finite-strain measurement technique is presented. The principle behind the technique is that the longer the dimension of a strain indicator in a certain orientation, the higher the probability of its being intersected by a plane perpendicular to this orientation. Strain indicators have to be counted in any three mutually perpendicular orientations and their populations are analytically transferred into the parameters describing the strain ellipses in the three orientations. Strain indicators might be of any shape, but they must have had predeformational random distribution and random orientation.The geological implications of the technique are discussed. The technique is applied to deformed Upper Cretaceous chalk from Dorset (southern England) using microfossils as strain indicators. 相似文献