Hybrid seismic modeling based on discrete-wave number and finite-difference methods     

Jiří Zahradník  Peter Moczo 《Pure and Applied Geophysics》1996,148(1-2):21-38

Any calculation of seismic wave propagation comprising the seismic source, the travel path, and the receiver site in a single finite-difference (FD) model requires a considerable amount of computer time and memory. Moreover, the methods currently available for including point sources in the 2D FD calculations are far-field approximations only. Therefore we have developed a new hybrid method for treating the seismic wave fields at localized 2D near-surface structures embedded in a 1D background medium, and excited by a point source. The source radiation and propagation in the background model is solved by the discrete-wave number (DW) method, while the propagation in the local 2D structure is calculated by the FD method. The coupling between the two sets of calculations is performed on a rectangular excitation box surrounding the local structure. We show the usefulness of the method in ground-motion studies where both near-field source effects and local site effects are important. Technical problems connected with the inconsistency between the 3D source radiation and the 2D FD calculation are minor for the relatively distant in-plane point explosive sources, but are more serious for the in-plane dislocation sources.  相似文献   

Accelerating ground warming in the Canadian Prairie provinces: Is it a result of global warming?     

Jacek A. Majorowicz 《Pure and Applied Geophysics》1996,147(1):1-24

The results of precision temperature logs made to depths of several hundred meters in some 80 wells in Western Canada, most of which are located in the Prairie Provinces, show evidence of warming at the ground surface in the 0.5 K to 3.5 K range (average=2.2±0.7 K, for 80 unevenly distributed sites). Modeling shows that this warming mostly pertains to this century and it has been most substantal in the last four decades if the ramp function of the linear increase of surface temperature is assumed. Using the step function model's increase of surface temperature (land clearing, forest fires, etc.) the calculated onset of warming would pertain mostly to the last two decades. Contour maps of ground temperatures currently and previously and a contour map of the ground warming magnitude dilineate a large regional character of the ground temperature change at the southern marigin of permafrost for the large area of the Prairie Provinces. In many cases however, the magnitude of ground warming is much larger than the magnitude of air warming. This is especially evident for the northern areas of Alberta in the boreal forest ecoprovince. The magnitude of ground warming is equal to the magnitude of surface air warming in southern Alberta in the grassland and aspen parkland ecoprovinces. The analysis of the temperature depth response to the surface warming from well data shows the integrated effect of surface air warming together with the increases in ground temperature due to natural terrain effects and other anthropogenical changes to the surface of the earth.  相似文献   

Blind prediction of the site effects at Ashigara Valley,Japan, and its comparison with reality     

J. Zahradník  P. Moczo  F. Hron 《Natural Hazards》1994,10(1-2):149-170

Weak and strong ground motions were numerically predicted for three stations of the Ashigara Valley test site. The prediction was based on the records from a rock-outcrop station, one weak-motion record from a surface-sediments station, and the standard geotechnical model. The data were provided by the Japanese Working Group on the Effects of Surface Geology as a part of an international experiment. The finite-difference method for SH waves in a 2-D linear viscoelastic medium (a causalQ model) was employed.Comparison with the real records shows that at two stations the predictions fit better than at the third one. Strangely, the two better predictions were for stations situated at larger distances from the reference rock station (one station was on the surface, the other in a borehole). The strong ground motion (the peak acceleration of about 200 cm s^–2) was not predicted qualitatively worse than the weak motion (8 cm s^–2). A less sophisticated second prediction (not submitted during the experiment), in which we did not attempt to fit the available weak-motion record at the sedimentary station, agrees with the reality significantly better.  相似文献   

The project “spice”     

D. Rouan  N. Epchtein  M. De Muizon  F. Lacombe  P. Puget  D. Tiphene  F. Boulanger  X. Desert  G. Guyot  L. D'Hendecourt  J -M. Lamarre  A. Leger  F. Pajot  J -L. Puget  E. Caux  M. Giard  G. Serra  C. Cesarsky  L. Vigroux  A. Omont 《Astrophysics and Space Science》1994,217(1-2):41-44

Proposed to both the French and the European Space Agency as one possible small mission, SPICE is a project for a dedicated small satellite for a near-IR spectroscopic all-sky survey. The instrument would cover the spectral range 1.8 - 3.6µm, possibly extended to 1.8 - 7µm, at a resolution of 100, with pixels of 1 arc-min. The excellent sensitivity (0.02 MJy sr-1) results from: i) the quasi-zero level of background due to the efficient passive cooling of the whole experiment; ii) the use of large format arrays; iii) the non-stop observing mode (drift-scanning). The spectral domain, complementing the one of ISO, partially opaque from the ground, is specially rich in spectral features tracing stars and all components of the Interstellar Medium (molecular, atomic and ionized gas, dust). With a cooling below 80 K of the focal instrument, then it becomes possible to consider doubling the spectral domain and to cover the whole 1.8 - 7µm range.  相似文献   

The emplacement of intermediate volume ignimbrites: A case study from Roccamonfina Volcano,Southern Italy     

P. D. Cole  J. E. Guest  A. M. Duncan 《Bulletin of Volcanology》1993,55(7):467-480

A model is presented for the emplacement of intermediate volume ignimbrites based on a study of two 6 km³ volume ignimbrites on Roccamonfina Volcano, Italy. The model considers that the flows were slow moving, and quickly deflated from turbulent to non-turbulent conditions. Yield strength and density increased whereas fluidisation decreased with time and runout of the pyroclastic flows. In proximal locations, on the caldera rim, heterogeneous exposures including discontinuous lithic breccias, stratified and cross-stratified units interbedded with massive ignimbrite suggest deposition from turbulent flows. In medial locations thick, massive ignimbrite occurs associated with three types of co-ignimbrite lithic breccia which we interpret as being emplaced by non-turbulent flows. Multiple grading of different breccia/lithic concentration types within single flow units indicates that internal shear occurred producing overriding or overlapping of the rear of the flow onto the slower-moving front part. This overriding of different parts of non-turbulent pyroclastic flows could be caused by at least two different mechanisms: (1) changes in flow regime, such as hydraulic jumps that may occur at breaks in slope; and (2) periods of increased discharge rate, possibly associated with caldera collapse, producing fresh pulses of lithic-rich material that sheared onto the slower-moving part of the flow in front.We propose that ground surge deposits enriched in pumice compared with their associated ignimbrite probably formed by a flow separation mechanism from the top and front of the pyroclastic flow. These turbulent clouds moved ahead of the non-turbulent lower part of the flow to form stratified pumice-rich deposits. In distal regions well-developed coarse, often clast-supported, pumice concentrations zones and coarse intra-flow-unit lithic concentrations occur within the massive ignimbrite. We suggest that the flows were non-turbulent, possessed a relatively high yield strength and may have moved by plug flow prior to emplacement.  相似文献   

近40年黄河流域资源环境格局变化分析与地质工作建议          下载免费PDF全文

王尧  陈睿山  郭迟辉  夏子龙 《中国地质》2021,48(1):1-20

20世纪80年代以来,自然演变与强烈的人类活动叠加,致使黄河流域资源环境格局发生了显著变化,进而影响其开发利用布局,面临诸多资源短缺、环境退化和生态破坏问题,急需从地球系统科学的视角深入研究流域资源环境的格局变化特点,识别重大资源环境问题,提出解决方案,为黄河流域可持续发展提供决策依据。本研究以"山水林田湖草"生命共同体理念和地球系统科学理论为指导,利用多源多时相调查研究数据,分析黄河流域1970—2015年近40年气候、山区、水文等十一大类资源环境格局变化及区域分异规律,识别重大地质资源环境问题,提出地质工作建议。研究结果表明:黄河流域气候整体暖干化趋势明显,局部出现暖湿现象;山区生态系统脆弱、逐步恢复,局部人类活动影响剧烈;水体与湿地面积总体减少,近年呈增加趋势,径流量呈减少趋势,部分干支流污染严重,水生生物多样性减少;农田面积呈下降趋势,下游地区农田生产潜力增加明显;植被覆盖呈现整体缓慢升高、局部退化趋势;草地面积持续减少,草地生态系统退化明显;荒漠化扩展态势得到遏制,总体形势依然严峻;自然灾害频发,水害严重,地质灾害聚集分布,形成陇中黄土高原和陇南山地两个高发区;城镇建设用地面积持续上升,由下游地区向中上游扩散,中上游呈现出由轴线连接的多个核心的组团分布特征;人口分布重心进一步向东偏离,沿中心城市、干流、交通干线增加;GDP整体呈现上涨趋势,呈现由东向西递减、由干流或主支流沿岸向两边递减、由中心城市向周围递减。为保护黄河流域生态环境、促进高质量发展,解决黄河流域面临的复杂多样的地质资源环境问题,建议地质工作以地球系统科学理论为指导对黄河流域重大地质资源环境问题进行系统调查研究;以地球系统科学研究方法为框架,进行跨学科协作,整合区域性地质调查工作。  相似文献   

河北省唐山地区丰台—野鸡坨断裂第四纪活动性——来自¹⁴C和磁性地层年代学的证据          下载免费PDF全文

陈宏强  专少鹏  陈超  赵华平  杨瑞  何娇月  胥勤勉 《中国地质》2021,48(2):605-617

丰台—野鸡坨断裂为唐山地区主要断裂之一，西侧为鸦鸿桥凹陷，东侧为唐山凸起，断裂两侧第四系厚度之差巨大。本文依据该断层两侧钻孔对其第四纪以来活动性进行初步的探讨。通过对丰台—野鸡坨断裂上下两盘PZK14和PZK20孔磁性地层学研究，并结合钻孔岩石地层，及浅部光释光和¹⁴C测年结果，建立第四纪地层格架。结果表明：两孔底部“泥包砾”为新近纪沉积；PZK14孔下更新统底界埋深为387 m，中更新统底界埋深为114 m，上更新统底界埋深为71 m，全新统底界埋深为6 m；PZK20孔下更新统底界埋深为155 m，中更新统底界埋深为73 m，上更新统底界为36 m，无全新世地层。丰台—野鸡坨断裂活动在早更新世时表现为逐渐增强的特点，活动速率由早期的5.4 cm/ka增加到13.9 cm/ka。中更新世断裂活动基本处于停滞状态，活动速率为1.0 cm/ka。晚更新世以后，断裂重新活动，且更加剧烈，活动速率达到了54.5 cm/ka。  相似文献   

雄安新区地热地质模型探究：来自地球物理的证据          下载免费PDF全文

王凯  张杰  白大为  吴新刚  岳航羽  张保卫  王小江  张凯 《中国地质》2021,48(5):1453-1468

雄安新区内地热资源丰富，区内有牛驼镇地热田、容城地热田和高阳地热田，地热资源开发利用较早，但是对其深部热源机制仍未形成统一观点。为了研究雄安新区内地热田深部热源机制，在新区及外围进行了深反射地震和长周期大地电磁探测，对取得的同剖面的深反射地震和大地电磁数据进行处理和综合解释，探明了研究区从地表至莫霍面范围内地质构造和电性结构。下地壳结构在深反射地震剖面与大地电磁剖面上有很好的对应关系。电阻率低值区对应着在深反射地震剖面上存在一系列反射同相轴，且同相轴可以延续到莫霍面，电阻率高值区对应着在深反射地震剖面上无明显连续反射同相轴，尤其是在莫霍面之上呈现地震反射近似"空白区"。结合区域地热资料构建了研究区深部地热地质模型，对新区内深部地热机制进行了解释。该模型为"二元"生热模型，其热源包含两个部分，深部地幔热源和地壳放射性元素衰变生热。放射性元素衰变生热占地表热流的接近30%，而幔源热流在地表热流中的占比可达约70%。在牛驼镇下方，莫霍面以上，由于地幔热物质上涌造成下地壳上隆，幔源岩浆底侵作用于下地壳形成了局部热异常，该热异常具有低速高导的地球物理特征，认为是牛驼镇地热田和容城地热田的深部热源；以区域断裂为热通道，大地热流由深部向上传导、扩散到牛驼镇凸起和容城凸起顶部，对碳酸盐岩储水层进行加热，形成地热储层；上覆新近系沉积地层是良好的热盖层。  相似文献   

快速城镇化进程中珠江三角洲高铵地下水赋存环境及驱动因素          下载免费PDF全文

吕晓立  刘景涛  韩占涛  朱亮  杨明楠  李海军 《中国地质》2021,48(6):1770-1780

地下水中高浓度的铵态氮对生活饮用水安全及生态环境存在潜在威胁。相比较硝态氮，高浓度的铵态氮不仅有各种人为来源，天然沉积环境更是造成高铵地下水的主要成因。本文以城镇化快速发展的珠江三角洲为研究区，运用数理统计、主成分分析等方法深入探讨了研究区高铵地下水的赋存环境特征及驱动因素。结果表明，研究区地下水中NH₄⁺质量浓度介于未检出~180 mg/L。研究区1539组地下水样品中，NH₄⁺质量浓度大于10 mg/L的高铵地下水69组，其中含NH₄⁺质量浓度大于30 mg/L的高铵"肥水"23组。对比2005-2008年历史水化学数据，2009-2018年新增建设用地孔隙含水层高铵地下水样品比例增加25%。高铵地下水呈斑块状分布于三角洲平原区第四系底部低洼的基底、洼地等退积层序发育的淤泥质含水层中。淤泥层等富含有机质和总有机碳的沉积层是珠江三角洲地区的"生铵层"，有机氮的矿化是三角洲平原区城市化孔隙含水层中高铵地下水的主要驱动力。城镇化扩张引起生活污水及富铵工业废水的泄漏入渗是城乡结合部高铵地下水铵氮的重要来源。三角洲平原区中性至弱碱性富含有机质的还原环境是高铵地下水的主要成因。风化溶滤、阳离子交换吸附、海陆交互作用是珠江三角洲高铵地下水质演变的主要水文地球化学过程。  相似文献   

施工顺序对软土地坪复合地基的影响探讨     

魏海涛  蔡智 《岩土工程技术》2021,(1)

场地回填土与搅拌桩施工顺序将直接影响软土地坪复合地基的处理效果,其中沉降控制是影响地坪安全运行的关键。以近海软土地区工业厂房水泥土搅拌桩复合地基工程为例,探讨了复合地基水泥土搅拌桩、回填土两种不同施工顺序的影响因素及相应处理效果;通过数值模拟试验,计算分析了不同填土厚度下两种工况的地基固结总沉降、施工沉降、工后沉降。结果表明,采用“先土后桩”的施工顺序的施工期沉降大于“先桩后土”,但工后沉降大大减小,提高了软土地坪地基处理效果,有利于处理后地坪的安全运行,验证了前述分析结论,为合理设计方案的确定提供了重要的参考依据。  相似文献