福建若干基础地质问题及研究方向探讨

就当前福建基础地质调查研究中涉及的华夏古陆，岩浆构造活动，地质构造等有关问题研究现状及各自不同认识作初步归纳。并对其进一步研究方向作粗浅探讨。     

利用ERS-2 SAR图像纹理分析方法揭示长白山天池火山近代喷发物空间分布特征

简单介绍了SAR图像的纹理特征以及正交小波变换纹理提取方法。论述了SAR图像的纹理特征参与分类的重要性。以长白山天池火山为例,通过对ERS2SAR图像进行纹理分析,提取了SAR图像两个层次的尺度变化、时频局部化和方向性纹理特征。并将SAR纹理特征与TM图像及DEM进行复合,利用多源信息各自的优势,进行了BP神经元网络分类,从较大范围对长白山天池火山735±15aB.P.大喷发的喷发物空间分布进行评价。获取了长白山天池火山近代喷发物的空间分布及规模。这对长白山天池火山未来喷发危险性初步评价、火山地质制图及火山灾害预测有重要意义。     

沙漠化:从圈层耦合到全球变化

从沙漠化概念提出伊始 ,沙漠化成因机理的研究就一直成为学术界关注的焦点。沙漠化过程究竟有多少是由自然因素所致 ,多少是由人类活动所为 ,诸因素与沙漠化之间有何内在关系 ,沙尘天气发生发展过程及其诱因等 ,当属地球系统科学应解决的难题。文章从全球变化的角度 ,将沙漠系统视作一种类生命的非线性动力系统 ,指出沙漠化源于地球表层系统各圈层之间复杂的相关与耦合作用 :气候变化是控制沙漠进退的首要因素 ;陆表水持续亏损则是导致干旱、半干旱地区沙漠化发展的直接原因 ;岩石圈构造运动塑造了不同的地形地貌单元 ,奠定了沙漠分布的地理格局 ;人类的不当活动和气候变化引起的生物生产力的衰退及土地覆盖的破坏 ,造成了现代沙漠化的快速扩张。     

张北地震区三维构造特征

地震层析成像结果揭示了张北地震区的构造特征。震源区周围分布有相交的北北西和北东向低速异常带。极震区正下方壳内存在高速异常的坚固体 ,其周围被多条低速异常带围限。区域构造具备了大量积累应变能和向高应力区传递应变能的孕震构造特征 ,北北西和北东向的隐伏断裂共同构成了区域的发震构造。该震区在中新生代以来有过多期岩浆活动 ,震中区周围分布有几个新第三纪的火山口。极震区米家沟附近火山的三维剖分图给出了该火山从地表到壳内 10km深度的速度图像 ,中新生代的火山岩浆活动对该区域孕震构造的形成起了重要作用。     

贵州喀斯特地区生态环境恶化的人为因素分析

贵州是喀斯特强烈发育的省区。由于人类活动的影响，近年来贵州喀斯特地区的生态环境出现了日益恶化的趋势，严重影响可持续发展甚至西部大开发战略的顺利实施。本阐述了喀斯地区生态环境恶化的现状，深入探讨了人为因素对喀斯特地区生态环境恶化的影响，为防止喀斯特地区生态环境进一步恶化提供科学依据。     

Slump-initiated and controlled syndepositional sandstone remobilization: an example from the Namurian of County Clare, Ireland

Remobilization of sandstones can dramatically reconfigure original depositional geometries and results in very unusually shaped sandstones, which resemble little, if any, of the original geometry. A number of deformational sandstone bodies, dykes and volcanoes from the upper part of the Carboniferous Ross Formation are described, which offer the opportunity to examine a suite of field-scale reconfigured sandstones. These structures are located in close proximity to the Ross Slide, which outcrops along a 2-km section on the northern coast of the Loop Head Peninsula, County Clare, Ireland. Dome- and ridge-shaped deformational sandstone bodies, dykes and volcanoes are interpreted to be the product of remobilization of a turbiditic sandstone. Liquification and remobilization were triggered by translation, cessation and loading of the underlying turbiditic sandstone by the Ross Slide. Deformational sand body, dyke and volcano development occurred in an asynchronous fashion with deformational sand bodies formed during slump translation. Sand dykes and volcanoes developed after the cessation of slump movement. During slump translation, the minimum principal stress (σ₃) was orientated vertically and the slump behaved in a `ductile' manner. After slump arrest, the minimum principal stress was oriented horizontally, and the unit regained shear strength to behave in a `brittle' manner. The relative change in rheological states with changing applied shear stress is indicative of thixotropic-like behaviour within the slump mass. Ridge-shaped deformational sand bodies are aligned parallel to slump folds, and their morphology is inferred to be controlled by compressional slump deformation associated with heterogeneous cessation of slump movement that was initiated by frontal arrest of the translating mass.     

内蒙古色尔腾山山前断裂(乌句蒙口-东风村段)的断层活动与古地震事件

通过对色尔腾山山前断裂乌句蒙口 -东风村段的遥感资料解释、野外地质地貌考察 ,并通过对重点地段的古地震探槽开挖 ,获得了该断裂段晚更新世晚期以来的垂直位移速率是 0 88～ 1 83mm a ,全新世中期以来的垂直位移速率是 0 89mm a。通过 2个大型探槽的开挖、古地震事件分析和相关堆积物的断代研究 ,以及用逐次限定方法分析整个断层段上的古地震事件 ,认定该断裂段上全新世以来发生了 5次古地震事件 :事件 1发生在距今 90 0 0± 130 0年 ,事件2发生在距今 6 5 0 0± 5 0 0年 ,事件 3发生在距今 5 5 70年左右 ,事件 4发生在距今 4 2 0 0± 30 0年 ,事件 5发生在距今 32 5 0± 2 5 0年。晚更新世晚期到距今 1万年之间 ,古地震事件很不完整。全新世以来的 5次古地震事件表现出一定的丛集特征。最早的一丛事件发生在距今 890 0年左右 ,第2丛发生在距今 6 5 0 0～ 5 70 0年之间 ,第 3丛事件发生在距今 32 5 0～ 4 2 0 0年之间。第 1丛与第 2丛古地震事件之间间隔为 2 4 0 0年左右 ,而第 2丛与第 3丛古地震事件之间仅间隔 15 70年左右。距今 32 5 0年以来 ,该断裂段上还没有发生过错断地表的地震事件 ,已经超出了古地震丛之间的重复间隔。因此 ,它是色尔腾山前活动断裂带上具备潜在危险的一个活动断裂段。     

The use of modern statistical theories in the assessment of earthquake hazard, with application to quiet regions of eastern North America

It is generally accepted that both deterministic and statistical approaches are useful for the characterization of earthquake hazard. Although the most reliable estimates of seismic hazard can only be based on an improved understanding of the earthquake mechanism, efficient utilization of the appropriate methods provided by recent statistical theories is also important in seismic risk analysis. This is especially true in regions where the connection between seismicity and geologic structure is tenuous at best. We are particularly interested in developing better statistical treatments of data for regions with little known seismic activity. To this end, we have applied three statistical methods to the historical record of seismicity in relatively quiet regions of eastern North America. These are: (1) the threshold method for tail inference, a new theory for modeling earthquakes with sizes above a given threshold, (2) the ‘bootstrap’ technique in which the characteristics of an unknown population are simulated by replacing the true population by an estimated one, and (3) a technique to estimate the number of earthquakes below a given size, in order to compensate for the under-reporting of small earthquakes in most catalogs. A combination of these techniques has been used to estimate the probabilities of future large earthquakes for the regions studied. Because of limitations imposed by existing catalogs, the size estimate used has been maximum intensity.     

ELF propagation parameters for uniform models of the Earth–ionosphere waveguide

Uniform models for the Earth–ionosphere cavity are considered with particular attention to the physical properties of the ionosphere for the extremely low frequency (ELF) range. Two consistent features have long been recognized for the range: the presence of two distinct altitude layers of maximum energy dissipation within the lower ionosphere, and a “knee”-like change in the vertical conductivity profile representing a transition in dominance from ion-dominated to electron-dominated conductivity. A simplified two-exponential version of the Greifinger and Greifinger (1978) technique widely used in ELF work identifies two slopes in the conductivity profile and, providing accurate results in the ELF communication band (45–75 Hz), simulates too flat a frequency dependence of the quality factor within the Schumann resonance frequency range (5–40 Hz). The problem is traced to the upward migration, with frequency increasing, of the lower dissipation layer through the “knee” region resulting in a pronounced decrease of the effective scale height for conductivity. To overcome this shortcoming of the two-exponential approximation and still retain valuable model analyticity, a more general approach (but still based on the Greifinger and Greifinger formalism) is presented in the form of a “knee” model whose predictions for the modal frequencies, the wave phase velocities and the quality factors reasonably represent observations in the Schumann resonance frequency range.     

Moment-tensor statistics of the 1997 Long Valley microearthquake swarm

Abstract. A simple, fast, moment-tensor inversion method using bandpass-filtered P-amplitudes was used to study the moment-tensor statistics of Long Valley caldera microearthquakes. The events were recorded in the summer of 1997, during a swarm in the caldera. The swarm was associated with geodetic extension, uplift, and subsequent moderate earthquake activity. The moment tensor solutions for 1,993 events were calculated using the new method. The majority of the resulting focal mechanisms appear to be explained in terms of double couple mechanisms. Since some events did exhibit considerable deviation from double-couples, the moment data were studied for their statistical significance. The moments of the actual data were compared to the moments of synthetic data with varying degrees of random noise in their spectra. The results of this study suggested that unless data from more than 20 stations are used and the earthquake epicenter is located inside or very close to the network area, moment-tensor inversion does not correctly resolve the non-double-couple components of microearthquakes. Analysis of the inversion residuals shows that the average noise in the P-wave spectra was close to 20%. The fluctuations of the volumetric components of the moment-tensor are in good agreement with those of the synthetic pure double-couples with 20% of added noise. Thus the moment-tensor statistics suggests that little if any volume change is required to explain the observed seismic energy release in the swarm. However, the statistics do show that a significant compensated-linear-vector-dipole component maybe present in the bulk of the seismicity. Given the network used in this study, such a component could not be precisely resolved for individual earthquakes. This possibility deserves further investigation because of its bearing on the nature of fluid-fault-earthquake processes in swarms.