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991.
GuoLiang Zhang ZhiGang Zeng XueBo Yin DaiGeng Chen XiaoYuan Wang XiaoMei Wang 《中国科学D辑(英文版)》2008,51(12):1786-1801
Thirty-six basalt samples from near East Pacific Rise 13°N are analyzed for major and trace elements. Different types of zoned plagioclase phenocrysts in basalts are also backscatter imaged, and major element profiles scanned and analyzed for microprobe. Basalts dredged from a restricted area have evolved to different extents (MgO=9.38wt%—6.76wt%). High MgO basalts are modeled for crystalliza-tion to MgO of about 7wt%, and resulted in the Ni contents (≈28 ppm) that are generally lower than that in observed basalts (>60 ppm). It suggests that low MgO basalts may have experienced more intensive magma mixing. High MgO (9.38wt%) basalt is modeled for self-"mixing-crystallization", and the high Ni contents in low MgO basalts can be generated in small scale and periodical self-mixing of new magma (high MgO). "Mixing-crystallization" processes that low MgO magmas experienced accord with recent 226Ra/230Th disequilibria studies for magma residence time, in which low MgO magmas have experi-enced more circles of "mixing-crystallization" in relatively longer residence time. Magma mixing is not homogeneous in magma chamber, however, low MgO magmas are closer to stable composition pro-duced by periodical "mixing-crystallization", which is also an important reason for magma diversity in East Pacific Rise. Zoned plagioclase phenocrysts can be divided into two types: with and without high An# cores, both of which have multiple reversed An# zones, suggesting periodical mixing of their host magmas. Cores of zoned plagioclase in low MgO (7.45wt%) basalt differ significantly with their mantle in An#, but are similar in An# with microlite cores (products of equilibrium crystallization) in high MgO (9.38wt%) basalt, which further shows that plagioclase phenocryst cores in low MgO basalts may have formed in their parental magmas before entering into the magma chamber. 相似文献
992.
993.
嘉峪关地震台地电阻率观测效能评价 总被引:1,自引:0,他引:1
根据中国地震局电磁学科制定的<地电台站观测效能评估方案>,评估了嘉峪关地震台地电阻率观测效能,得到:嘉峪关地震台台址结构条件、电磁环境、台站观测系统符合地电阻率台站建设规范、观测方法标准等规定的技术要求;数字化地电阻率观测精度不低于3‰测值,保证了准确测量不低于1%地电阻率异常的指标需求;该台地电阻率观测对本区及附近中等以上地震有一定监测能力,并记录了对应远距离大震的1年尺度地电阻率异常,其观测数据在本区及附近地震监测、预报和地震科学研究及相关科学研究中有重要价值. 相似文献
994.
应急流动测震台网和遥测地震台网的性质不同,因此在组建方式上也具有差异性。VPN是一种新的网络技术,它能够提供远程访问,外部网和内部网的连接,价格比专线或者帧中继网络要低得多。目前,VPN技术在专用的传输业务中得到了广泛的应用。本文简要介绍了VPN技术原理,以及通过运用该技术借助GPRS和CDMA传输网络来实现流动测震信号的远程传输。 相似文献
995.
This study focuses on the southeast Capital area of North China (38.5–39.85° N, 115.5–118.5° E). Shear-wave splitting parameters
at 20 seismic stations are obtained by a systematic analysis method applied to data recorded by the Capital Area Seismograph
Network (CASN) between the years 2002 and 2005. Although some differences in the results are observed, the average fast-wave
polarization is N88.2° W ± 40.7° and the average normalized slow wave time delay is 3.55 ± 2.93 ms/km. The average polarization
is consistent with the regional maximum horizontal compressive stress and also with the maximum principal strain derived from
global positioning system measurements in North China. In spite of the uneven distribution of faults around the array stations
that likely introduce some amount of scatter in the shear-wave splitting measurements, site-dependent polarizations of fast
shear wave are clearly observed: in the northern half of the study area, the polarizations at CASN stations show E–W direction,
whereas in the southern half the polarizations exhibit a variety of possible azimuths, thus suggesting dissimilar stress field
and tectonic frame in both areas. Comparing the splitting results with those previously obtained in the northwest part of
the region, we find a difference in polarization of about 20° between the southeast and northwest parts of the Capital area;
also, in the southeast Capital area the average time delay is smaller than in the northwest Capital area, thus making clear
that the magnitude of crustal seismic anisotropy is not the same in the two zones. Being the shear-wave splitting polarizations
in the southeast Capital area, which lies on the basin, clearly different from the observed polarizations in the northwest
Capital area, where uplifts and basin converge, it is quite evident that the shear-wave splitting results are consequence
of the tectonics and stress field affecting the two regions. 相似文献
996.
Geochemical changes across the Ordovician-Silurian transition on the Yangtze Platform,South China 总被引:1,自引:0,他引:1
The trace element and rare earth element (REE) variations across the Ordovician-Silurian succession are presented from two
outcrop sections on the Yangtze Platform: the Nanbazi section, Guizhou Province, deposited in a shallow platform interior
setting, and the Wangjiawan section, Hubei Province, deposited in a deeper basinal environment. Geochemical analysis of closely
spaced samples through three intervals, the Wufeng, Guanyinqiao and Longmaxi, revealed vast palaeoceanographic changes. Some
geochemical proxies, including Th/U, Ni/Co, V/Cr, and V/(V+Ni) ratios, together with sedimentary facies and biotic data, indicate
that an anoxic condition on the most of the Yangtze Platform during the Wufeng and Longmaxi intervals, but an oxic condition
during the Guanyinqiao time. The shift of the anoxic to the oxic environment during the Guanyinqiao time coincided with a
global sea-level lowstand, in parallel with the global glaciation. The Longmaxian anoxic environment was a result of a global
sea-level rise, which may be synchronized with a mainly catastrophic event in the latest Ordovician. Although the two sections
generally show similar variation patterns of trace and REE concentrations and some element ratios, a minor difference occurs
between the Wangjiawan and Nanbazi sections, likely reflecting a difference in depositional setting during the accumulation.
Such an oceanic oxygen-level variation may add a useful constraint to the current arguments on the cause and consequence of
the latest Ordovician mass extinction.
Supported by National Basic Research Program of China (Grant No. 2005CB422101) 相似文献
997.
Cropland cover change in Northeast China during the past 300 years 总被引:15,自引:0,他引:15
Land use/cover change induced by human activities has emerged as a “global” phenomenon with Earth system consequences. Northeast
China is an area where the largest land cultivation activities by migrants have happened in China during the past 300 years.
In this paper, methods including documentary data calibration and multi-sourced data conversion model are used to reconstruct
historical cropland cover change in Northeast China during the past 300 years. It is concluded that human beings have remarkably
changed the natural landscape of the region by land cultivation in the past 300 years. Cropland area has increased almost
exponentially during the past 300 years, especially during the past 100 years when the ratio of cropland cover changed from
10% to 20%. Until the middle of the 19th century, the agricultural area was still mainly restricted in Liaoning Province.
From the late 19th century to the early 20th century, dramatic changes took place when the northern boundary of cultivation
had extended to the middle of Heilongjiang Province. During the 20th century, three agricultural regions with high ratio of
cropland cover were formed after the two phases of spatial expansion of cropland area in 1900s–1930s and 1950s–1980s. Since
1930s–1940s, the expansion of new cultivated area have invaded the forest lands especially in Jilin and Heilongjiang Provinces.
Supported by National Natural Science Foundation of China (Grant No. 40571165) and Innovation Foundation of Chinese Academy
of Sciences (Grant No. KZCX2-YW-315) 相似文献
998.
999.
在较大地震的孕育过程中,随着弹性应变能的不断积累,孕震区有可能从原来的无序状态转变为一种在时间、空间上的有序状态,与之相联系的各种信息量的有序程度都有可能增加。本文引入信息熵来描述信息量的有序程度,结果表明,在研究范围内的多数地震前出现了熵减的过程,说明地震前确实存在一个从无序状态向有序状态转化的现象。文中利用信息熵方法,对福建省4个水氡观测资料进行分析,认为该方法,对各台的映震效能不同,虽然都通过了R值检验,但由于漏报率、虚报率的差异,从而影响了各台的R值评分;另外,用该方法提取的异常所反映出来的特征也不相同,如各台的异常持续时间和异常幅度同闽台地震的关系存在差异。 相似文献
1000.