Conserved quantities and the inverse problem of reflection seismology

We show that the time-dependent wave equation in both one and two spatial dimensions possesses quantities which are globally conserved. We show how these conserved quantities can be used to determine the characteristic impedance, the rock density and the elastic constant of the rock. We also demonstrate that the conserved quantities possess the capability of determining and/or bracketing the unknown component of the direct pressure response, which is required to begin downward continuation algorithms. Further, we demonstrate that the conserved quantities are always available irrespective of the source structure in time. Numerical instability, arising if the filtering due to the source structure is too harsh, can then be controlled to a degree by demanding that the conserved quantities be indeed conserved.     

Comparing historical and contemporary maps - a methodological framework for a cartographic map comparison applied to Swiss maps

Topographic maps are powerful tools for the purpose of identifying land use and cover change (LUCC) as they are among the most reliable representations of past landscapes for the time prior to the existence of aerial photography. In light of the increased availability of historical maps, we argue that there is a need for a standardized process to assess map comparability in a systematic way in order to avoid, or at least minimize, the detection of spurious landscape changes due to incompatible map series. A full understanding of map quality, background and error distributions is fundamental to attain reliable LUCC results. The conceptual framework presented in this study considers the context, distortion and cartographic generalization of topographic maps. Furthermore, it includes an approach to homogenize the level of generalization of landscape elements (e.g. forests) from maps with different scales. To demonstrate its application, we assessed the comparability of seven topographic maps from Canton Zurich covering a time span of 336 years (1664–2000). Overall, for the maps of Canton Zurich, a wall-to-wall comparison of forest cover based on the topographic maps presented here can be problematic for the oldest map from 1664. Based on the results, a wall-to-wall comparison with the later maps is not recommended, due to its substantial distortion. Yet, after re-generalization of natural landscape elements, such as forests, a comparison based on landscape indices is possible, even for the oldest map. Furthermore, our results demonstrate that maps from the mid-19^th century onward possess an acceptable level of accuracy. This framework can be applied to a wide range of maps at regional, national, or global levels, providing the opportunity to look at land cover history over multiple centuries.     

Historical seismicity on the southern margin of the Siberian craton: new data

We discuss historical evidence for seismicity on the southern margin of the Siberian craton collected from old local newspapers. The reported earthquakes vary in magnitude from M = 2.5 to 4.5, and their macroseismic locations agree well with the regional tectonic framework. The new data prove seismic activity in the area and can be used in seismic risk assessment.     

Tjörnes fracture zone. New and old seismic evidences for the link between the North Iceland rift zone and the Mid-Atlantic ridge

The Tjörnes facture zone (TFZ) connects the EW extension of the Mid-Atlantic ridge north of Iceland to the extension of the North volcanic zone (NVZ) of Iceland. Earthquakes up to magnitude 7 (Ms) can occur in TFZ, volcanic eruptions have been observed and large crustal deformations are expected in similar way as have been observed in the NVZ. Most of the zone is below ocean, which limits the historical information and geological observations. For studying the dynamics of the zone we must rely on interpretation and modelling based on seismic observations, especially on microearthquake observations for the last 10 years. In this paper we demonstrate how microearthquakes can be applied to map the details of the plate boundary, and how this information can be applied to find epicenters and fault planes of large historical earthquakes, also how seismic information can be applied in dynamic modelling and to infer spatial and temporal interplay in activity, and to enhance hazard assessment.     

Constraints on crustal and mantle structure of the oceanic plate south of Iceland from ocean bottom recorded Rayleigh waves

From April to July 2002 we carried out a deployment of 6 ocean bottom seismometers and 4 ocean bottom hydrophones in the North Atlantic south of Iceland. During the deployment period we recorded clear Rayleigh waves from 2 regional and 14 teleseismic earthquakes. This corresponds to a Rayleigh wave detection rate of nearly 92% for events with M_W ≥ 6.06.0 and epicentral distance less than 110°, close to detection rate estimates based on noise level variability. We measured Rayleigh wave event-station group dispersion and inter-station phase dispersion for one Mid-Atlantic Ridge event. The group dispersion curve is sensitive to the structure of the North-East Atlantic with an average age of  39 Myr. The phase dispersion curve is sensitive to the structure just south of Iceland (average plate age 33 Myr). Both dispersion curves indicate faster velocities than previously postulated for oceanic plate generated at the Reykjanes Ridge. A grid search approach was used to constrain the range of models fitting the data. The high velocity seismic lid just south of Iceland in the model for the phase dispersion path is slower or thinner than in the group dispersion model, which averages over a larger area and a somewhat older plate age, but the velocities in the low velocity half space are similar. We further consider the residual bathymetry in the experimental area. The residual anomaly decreases by 300–400 m from the Reykjanes Ridge to the  30 Myr old plate south of Iceland. This decrease can be explained by the disappearance of a mantle thermal anomaly associated with the Iceland plume. Both the residual bathymetry and the surface wave data are thus consistent with the notion that the southward spreading of the Icelandic plume is channelised underneath the Reykjanes Ridge and does not spread far outside this channel.Based on the experience from the pilot experiment, we estimate that a minimum recording time of 13–15 months in favourable weather conditions (April–September) is required to record enough data to map the spreading plume with surface waves, and to produce a tomographic image to a depth of 1000 km using body waves. This can be achieved by a continuous deployment of at least  20 months, or by two or three deployments during the spring and summer of consecutive years.     

长白山火山公元1199～1201年喷发的历史记录

长白山天池火山是世界著名的火山之一,是我国规模最大、最具有潜在喷发危险的一座近代活动火山。目前,国内外许多火山学者为了研究天池火山最近一次大喷发年代问题,做了大量工作,并取得了一些年代资料,但一直没取得有关这次大喷发历史记载的证实。笔者等再次收集和查阅了一亿七千万字的文史古籍资料,通过整理、筛选、考证、对比和分析处理, 发现了长白山天池火山,于公元1199～1201年一次大喷发的历史记录。这对于长白山天池火山未来喷发危险性的研究具有重要的意义。     

Natural hazards in the Bavarian Alps: a historical approach to risk assessment

The Bavarian Alps region is strongly affected by various natural hazards, mainly hydrological events (floods, debris flows), geomorphic/geological events (landslides, rock falls), and avalanches. Extraordinary floods, like in 2002 or in the summer of 2005 in south Bavaria, have again posed the question of the possible extent and frequency of recurrence of catastrophic events. To put risk assessment on a broader basis historical data about all kinds of past natural hazards were detected in the archives of local authorities and administrative offices for water management. More than 10,000 sources (written accounts, maps, and photographs) were collated in a database. The majority of this information reaches back to the middle of the 19th century. In addition, many documents referring to events dating back even as far as the middle age were found. The Historische Analyse von NaturGefahren (HANG, historical analysis of natural hazards) project at the University of Eichstaett mainly focuses on a small-scale examination of the data. Initial results of the data analysis show that most catastrophic events in the Bavarian Alps only affect parts of the area, but not the whole region. Therefore it is necessary to assess the risk potential on a local scale like valleys, the catchment areas of mountain streams, or even single streams. Firstly the presented data is aimed to help engineers in future planning of hazard-protection measures. Secondly the information can form a vital component to enhance our knowledge of hydrological and geomorphic/geological dynamics in the Alps.     

人类活动对公元1194年以来黄河河口延伸速率的影响

以近 50年来的观测资料和公元 1 1 94年以来苏北废黄河和现代黄河的历史文献资料为基础,在宏观时间尺度上 (1 0 2 ～ 1 0 3年 )研究了人类活动对黄河口延伸速率的影响。结果表明,废黄河口的延伸可以分为两个阶段,1 1 94～ 1 578年为第一阶段,河口延伸速率较慢 ;1 579～ 1 855年为第二阶段,河口延伸速率大大加快。 1 855年以来现代黄河河口延伸速率的变化曲线可以分为 3个阶段,分别用 3条斜率不同的直线来拟合。 3条直线之间的两个转折点可以代表人类活动的方式和强度发生改变的临界点。由此建立了人类活动影响下现代黄河三角洲河口延伸过程的三阶段模式。     

历史地理学信息化若干问题的探讨

信息技术应用的日益普及为中国历史地理学发展提供了可选择的途径。在社会需要、信息源、技术方法等因素的支持下 ,开展历史地理学信息化已具备了可行性 ,应结合具体实例在历史政区、气候、人口、土地等专题开展 GIS技术应用 ,以及推广数字地图、图像处理技术应用。在实践中不断解决历史文献的信息精度等问题 ,推动学科方法论的进步。     

Chaotic ray behaviour in regional seismology

Rays propagating through strongly laterally varying media exhibit chaotic behaviour. This means that initially close rays diverge exponentially, rather than according to a power law. This chaotic behaviour is especially pronounced if the medium contains laterally varying interfaces. By studying simple 2-D and 3-D versions of models with laterally varying interfaces, the importance of chaotic ray behaviour is determined. A model of the Moho below Germany produces sharp variations with epicentral distance of the number of arrivals. In addition, the number of caustics grows dramatically: up to 1200 caustics are present between a distance of 0 and 800 km. Using the theory of Hamiltonian systems, a more in-depth study of the chaotic character of the ray equations is obtained. It is found that for realistic heterogeneous models most of the relevant rays will exhibit chaotic behaviour. The degree of chaos is quantified in terms of predictability horizons. Beyond the predictability horizons ray tracing cannot be carried out accurately. For the models under consideration, the length from the source to the predictability horizon has an order of magnitude of 1000 km. The chaotic behaviour of the rays makes it necessary to use extensions of asymptotic ray theory, such as Maslov theory, to compute seismic waveforms. It is shown that pseudo-caustics, an important obstacle in computing Maslov synthetics, are a generic feature of the 2-D laterally varying models that are studied. Eventually, the use of asymptotic methods is restricted because of the inaccuracy in the computation of the ray paths.