Two and Half Dimensional Simulation of Ridge Effects on the Ground Motion Characteristics

— Seismic responses of weathered and non-weathered ridge models were simulated to study the ridge effects on the ground motion characteristics. The range of ridge slope from 19.98° to 45° was considered to produce a possible set of generalized results. 2.5-D modeling based on parsimonious staggered grid approximation of elastodynamic wave equations was adopted in simulations. Computed results reveal an increase of amplitude of incoming waves with both elevation and the slope of the ridge. Further, the characteristics of surface waves are highly ridge slope dependent. The analysis of responses of weathered and non-weathered ridge models reveals that ridge has caused a strong generation of surface waves near its top. The surface waves are not dominating on the top of the ridge but at some lower elevation. The increase of weathering of ridge further intensified the ridge effect. Analysis in frequency domain, based on spectral ratio method, does not indicate any pattern in the spectral amplification factor and is very much sensitive to slope, source focal mechanism and location. However, on an average there is a continuous decrease of amplification with slope in the vertical component and increase in the transverse component, and it is increasing in the radial component up to slope =38.0° and thereafter decreasing.     

Lateral tidal mixing in the Antarctic marginal seas

Tidal mixing plays an important role in the modification of dense water masses around the Antarctic continent. In addition to the vertical (diapycnal) mixing in the near-bottom layers, lateral mixing can also be of relevance in some areas. A numerical tide simulation shows that lateral tidal mixing is not uniformly distributed along the shelf break. In particular, strong mixing occurs all along the Ross Sea and Southern Weddell Sea shelf breaks, while other regions (e.g., the western Weddell Sea) are relatively quiet. The latter regions correspond surprisingly well to areas where indications for cross-shelf exchange of dense water masses have been found. The results suggest that lateral tidal mixing may account for the relatively small contribution of Ross Sea dense water masses to Antarctic Bottom Water.     

SHRIMP U-Pb zircon age,geochemistry and Nd isotope of the Guandaoshan pluton in SW Sichuan: Petrogenesis and tectonic significance

SHRIMP U-Pb zircon age, geochemical and Nd isotopic data are reported for the Neo-proterozoic Guandaoshan pluton in the Yanbian region, SW Sichuan. This pluton is of typical I-type granite and emplaced at (857 ± 13) Ma. Geochemical and Nd isotopic characters suggest that the pluton was generated by partial melting of pre-existing, young (late Mesoproterozoic to early Neo-proterozoic) low-K tholeiitic protolith within an intraplate anorogenic setting. The Guandaoshan pluton probably records the earliest magmatism induced by the proposed ca. 860–750 Ma mantle superplume beneath the supercontinent Rodinia.     

Integration and Sharing of Geo-spatial Data Based on Data Engine

Through analyzing the principle of data sharing in the database system, this paper discusses the principle and method for integrating and sharing GIS data by data engine, introduces a way to achieve the high integration and sharing of GIS data on the basis of VCT in VC , and provides the method for uniting VCT into RDBMS in order to implement a spatial database with object-oriented data model.     

高原交通干线对土地利用和景观格局的影响——以兰州至格尔木段为例

运用景观生态学方法和Arc/Info软件，研究1995年至2000年兰州至格尔木铁路沿线的区域土地利用和景观格局变化，分析交通干线（公路和铁路）的影响程度与范围。主要结果如下：（1）东线土地利用和景观格局没有发生显著变化，西线土地利用和景观格局变化较大；（2）东线缓冲带土地利用程度综合指数没有显著变化，西线缓冲带土地利用程度综合指数明显升高。缓冲带综合土地利用动态度的变化反映了交通干线的轴向影响规律；兰州至西宁铁路对土地利用变化的显著影响范围是5km，青藏铁路对土地利用变化的显著影响范围是7km；（3）9个城市缓冲带的土地利用变化分析表明：城市扩展主要发生在1km缓冲带内。     

Magnetic activity inside the auroral zones and field-aligned currents

Big perturbations of the magnetic field (amplitudes larger than 250 nT) are simply detected by subtracting the values of a model from the measurements of CHAMP satellite. Taking a full year of CHAMP data and organizing them in four subsets of three months length (spring, summer, autumn, winter), it is found that: (a) the two domains where such big perturbations mainly exist are limited, in both hemispheres, by a parallel of high latitude of the corrected geomagnetic coordinates system; (b) a conspicuous seasonal (annual) variation affects the density of the perturbations and is opposite in the two hemispheres. We hold that these perturbations are linked to the midday magnetic activity within the auroral zone, long ago described by one of us (Mayaud, 1956). The source of the perturbations observed at the satellite altitude would be field-aligned currents resulting from the penetration of the solar wind into the magnetospheric cusps. To cite this article: J.-L. Le Mouël et al., C. R. Geoscience 335 (2003).     

Groundwater residence times and flow paths in fractured rock determined using environmental tracers in the Mission Tunnel; Santa Barbara County,California, USA

Multiple geochemical tracers [ion chemistry, stable isotopes of water, chlorofluorocarbons (CFC), tritium] and a 25-year-long record of discharge were used to understand residence times and flow paths of groundwater seeps in the fractured rock aquifer surrounding the Mission Tunnel, Santa Barbara, California. Tritium data from individual seeps indicate that seep waters are a mixture of >45-year-old (recharged prior to the nuclear bomb tests) and young groundwater. CFC data support this interpretation, however, a two-end member mixing model cannot completely explain the age tracer data. Microbial degradation and partial re-equilibration complicate the CFC signal. Spectral analysis of precipitation and groundwater seepage records shows that seepage lags precipitation by 3 months. This delay is related to the advancement of the wetting front and increasing the number of active flow paths. Additionally, the amount of seepage produced by precipitation is less during extended periods of drought than during normal or wet periods, suggesting antecedent conditions strongly affect flow through this fractured rock aquifer.     

Lower crustal granulite xenoliths from the Pannonian Basin,Hungary, Part 2: Sr–Nd–Pb–Hf and O isotope evidence for formation of continental lower crust by tectonic emplacement of oceanic crust

Mafic granulite xenoliths from the lower crust of the Pannonian Basin are dominated by LREE-depleted bulk-rock compositions. Many of these have MORB-like ¹⁴³Nd/¹⁴⁴Nd but ⁸⁷Sr/⁸⁶Sr is elevated relative to most MORBs. Their '¹⁸O values cover a wide range from +3.8 to +9.5‰. A group of LREE-enriched mafic granulites have higher ⁸⁷Sr/⁸⁶Sr (0.704-0.708) and lower ¹⁴³Nd/¹⁴⁴Nd (0.5128-0.5124), with higher '¹⁸O values on average (+7.8 to +10.6‰) than the LREE-depleted granulites. The LREE-enriched granulites are, however, isotopically similar to newly discovered metasedimentary granulite xenoliths. A sublinear correlation in )Hf-)Nd isotope space has a shallower slope than the crust-mantle array, with the metasedimentary rocks forming the low )Hf end member; the radiogenic end is restricted to the LREE-depleted granulites and these overlap the field of MORB. Pb isotopes for the LREE-depleted samples are less radiogenic on average than those of the LREE-enriched and metasedimentary xenoliths, and metasedimentary granulites have consistently higher ²⁰⁸Pb/²⁰⁴Pb. The wide range in '¹⁸O over a restricted range in Nd and Sr isotope values, in combination with the predominance of LREE-depleted trace-element compositions, is consistent with an origin as a package of hydrothermally altered oceanic crust. The existence of '¹⁸O values lower than average MORB and/or mantle peridotite requires that at least some of these rocks were hydrothermally altered at high temperature, presumably in the oceanic lower crust. The low ¹⁴³Nd/¹⁴⁴Nd of the LREE-enriched mafic granulites cannot be explained by simple mixing between a LREE-depleted melt and an enriched component, represented by the recovered metasediments. Instead, we interpret these rocks as the metamorphic equivalent of the shallowest levels of the ocean crust where pillow basalts are intimately intercalated with oceanic sediments. A possible model is accretion of oceanic crustal slices during subduction and convergence, followed by high-grade metamorphism during the Alpine orogeny.