Vertical crustal motions from differential tide gauge observations and satellite altimetry in southern Italy

Our goal is to determine vertical crustal movement rates from tide gauge and satellite altimetry measurements. Tide gauges measure sea level, but as they are fixed to the crust, they sense both sea surface height variations and vertical crustal movements. The differential sea level rates of sufficiently nearby stations are a good means to determine differential crustal movement rates, when sea level height variations can be assumed to be homogeneous. Satellite altimetric measurements determine sea surface height variations directly and can be used to separate the crustal signal from the sea surface height variations in tide gauge measurements. The correction of the tide gauge sea level rates for the sea surface height contribution requires collocation of the satellite pass and the tide gauge station. We show that even if this is not the case, the satellite altimetric observations enable correction of differential tide gauge rates for the effects of sea surface rate inhomogeneities.     

Geodetic observation of sea-level change and crustal deformation in the Baltic Sea region

Based on tide gauge observations spanning almost 200 years, homogeneous time series of the mean relative sea level were derived for nine sites at the southern coast of the Baltic Sea. Our regionally concentrated data were complemented by long-term relative sea-level records retrieved from the data base of the Permanent Service for Mean Sea Level (PSMSL). From these records relative sea-level change rates were derived at 51 tide gauge stations for the period between 1908 and 2007. A minimum observation time of 60 years is required for the determination of reliable sea-level rates. At present, no anthropogenic acceleration in sea-level rise is detected in the tide gauge observations in the southern Baltic. The spatial variation of the relative sea-level rates reflects the fingerprint of GIA-induced crustal uplift. Time series of extreme sea levels were also inferred from the tide gauge records. They were complemented by water level information from historic storm surge marks preserved along the German Baltic coast. Based on this combined dataset the incidence and spatial variation of extreme sea levels induced by storm surges were analysed yielding important information for hazard assessments. Permanent GPS observations were used to determine recent crustal deformation rates for 44 stations in the Baltic Sea region. The GPS derived height change rates were applied to reduce the relative sea-level changes observed by tide gauges yielding an estimate for the eustatic sea-level change. For 13 tide gauge-GPS colocation sites a mean eustatic sea-level trend of 1.3 mm/a was derived for the last 100 years.     

相关平差在应变分析中的应用

研究了直接用多期重复大地测量观测资料求剪切应变速率的相关平差方法,简单地讨论了判断计算结果中反映形变信息的可靠性的标准,并对相关平差法与一般方法计算结果的差别进行了比较。通过几个地区的实际计算,认为在地震工作中,为探测大范围的地壳水平运动,迫切需要大幅度提高目前的大地测量精度。      

Wavefield features of teleseismic receiver function in laterally inhomogeneous media

IntroductionSince1980(s,theteleseismicreceiverfunctionhasbeenbroadlyappliedtoexploringthecrustalanduppermantlestructure.Thatthismethoddrawssomanyattentionsisnotonlybecauseitscheapercostandhighverticalresolution,butalsobecauseitcanbeusedforexploringthelateralvariationofthecrustalstructure.IntheUSnationalresearchprogramofthecontinentaldynamicsmuchemphasiswasgiventothereceiverfunctionmethod(Phinney,1989).Uptonow,the1-Dreceiverfunctioninversiontechniquehasbecomesophisticatedandmanypracticalresul…     

Elimination of effects of earthquake in monitoring crustal movement

Monitoring crustal movements is important in understanding the earth surface and in developing theories about plate tectonics. Plate tectonics describes earth crust which consists of a number of plates moving relative to one another. Global plate models suggest that plate movements are constant due to the fact that relative movements of plates were represented by averaged plate motion. However, if an earthquake occurs, the uniform movement of related plates does not follow the regular trend. In analysis by Kalman Filter, the effects of an earthquake occured within crustal movement monitoring period by geodetic method have been eliminated by a proposed approach, Fading Memory Filter. The proposed method was tested using a real/simulated data. The results of the test showed that the effect of a strong earthquake (M_w = 7.1) which had occurred in the Gulf of Eilat/Aqaba was eliminated. Consequently, the proposed method is capable of removing or eliminating a suddan effect in monitoring crustal movements in the analysis by using the Kalman Filter.     

地幔岩石圈热结构差异与中国大陆岩石圈均衡分析

观测表明,大陆地区存在很多传统均衡模型无法解释的现象,其根本原因在于传统均衡理论中没有考虑地幔岩石圈部分由于热结构差异导致密度差异的影响.本文基于岩石圈尺度的质量平衡模型研究了中国大陆20个构造单元地壳及地幔岩石圈对地形海拔的贡献,以及各块体的均衡状态.计算结果表明,在一些地区,如塔里木盆地、北山和柴达木盆地,尽管岩石圈均衡模型和Airy模型得到了一致的海拔值,但岩石圈均衡模型更能体现均衡过程的物理本质;除青藏高原造山带外的多数块体,岩石圈均衡模型的计算结果更接近观测海拔和地表垂直运动状态;总体上,考虑地幔岩石圈热结构影响后,中国大陆各地区的均衡结果普遍优于传统的均衡模型.通过对均衡状态分析,我们得到以下主要结论：(1)构造稳定地区均衡程度较高;(2) 青藏高原及周边造山带现今地壳运动主要为区域构造过程及深部动力学过程所控制,均衡调整过程不是主要控制因素;(3) 现今地壳垂直运动比较明显的块体处于均衡调整阶段,地表垂直运动的大小反映了该区所受的均衡力作用的程度;(4) 构造稳定地区基于岩石层均衡的理论计算海拔与观测海拔之差值和现今地壳垂直运动速率有较好的相关性,据此我们可以通过均衡分析研究构造块体的运动趋势和动力学性质;(5) 地幔热结构对现今地形、海拔及地壳垂直运动有显著影响,在处理均衡问题时,地幔岩石圈热结构是我们必须考虑的重要因素.     

Sea-level rise: A review of recent past and near-future trends

Global mean sea level is a potentially sensitive indicator of climate change. Global warming will contribute to worldwide sea-level rise (SLR) from thermal expansion of ocean water, melting of mountain glaciers and polar ice sheets. A number of studies, mostly using tide-gauge data from the Permanent Service for Mean Sea Level, Bidston Observatory, England, have obtained rates of global SLR within the last 100 years that range between 0·3 and 3 mm yr^?1, with most values concentrated between 1 and 2 mm yr^?1. However, the reliability of these results has been questioned because of problems with data quality and physical processes that introduce a high level of spatial and temporal variability. Sources of uncertainty in the sea-level data include variations in winds, ocean currents, river runoff, vertical earth movements, and geographically uneven distribution of long-term records. Crustal motions introduce a major source of error. To a large extent, these can be filtered by employing palaeo-sea-level proxies, and geophysical modelling to remove glacio-isostatic changes. Ultimately, satellite geodesy will help resolve the inherent ambiguity between the land and ocean level changes recorded by tide gauges. Future sea level is expected to rise by ～ 1 m, with a ‘best-guess’ value of 48 cm by the year 2100. Such rates represent an acceleration of four to seven times over present rates. Local land subsidence could substantially increase the apparent SLR. For example, Louisiana is currently experiencing SLR trends nearly 10 times the global mean rate. These recently reduced SLR estimates are based on climate models that predict a zero to negative contribution to SLR from Antarctica. Most global climate models (GCMs) indicate an ice accumulation over Antarctica, because in a warmer world, precipitation will exceed ablation/snow-melt. However, the impacts of attritional processes, such as thinning of the ice shelves, have been downplayed according to some experts. Furthermore, not all climate models are in agreement. Opposite conclusions may be drawn from the results of other GCMs. In addition, the West Antarctic Ice Sheet is potentially subject to dynamic and volcanic instabilities that are difficult to predict. Because of the great uncertainty in SLR projections, careful monitoring of future sea-level trends by upgraded tide-gauge networks and satellite geodesy will become essential. Finally, because of the high spatial variability in crustal subsidence rates, wave climates and tidal regimes, it will be the set of local conditions (especially the relative sea-level rise), rather than a single global mean sea-level trend, that will determine each locality's vulnerability to future SLR.     

Assessing landslide movements in volcanic islands using near-shore marine geophysical data: south Pico island,Azores

Marine geophysical data from around the submarine flanks of volcanic islands can potentially help to resolve whether large-scale instability of an edifice has been geologically recently active. We use geophysical data to investigate part of the coast of Pico Island of the Azores where, above sea-level, a major slump of Topo volcano has been interpreted previously from arcuate escarpments and a rugged irregular topography seaward of them. Multibeam echo-sounder data collected offshore of this feature show remarkably little evidence for slump fault movements in the island's submarine slope. Mid-slope benches, like those associated with the Hilina Slump of Kilauea, are absent. The high-resolution data extends onto the island's shelf, allowing us to evaluate evidence for continuing activity there. In particular, as the shelf's rock platform will have been last modified by surf erosion during the postglacial period of sea-level transgression, it provides a reference surface of intermediate age (7–19 ka) that can potentially reveal whether any movements occurred in pre-historic times. Where the arcuate escarpments are continued offshore, the modern seabed shows no bathymetric evidence for active faults where the shelf rock platform crops out in the multibeam data. Elsewhere, mobile shelf sediments could be disguising evidence for active faulting so we examined boomer profiles able to image the rock platform beneath them. The data reveal a platform that is steep (6.6°) compared with the dips of platforms that we have studied previously around the coast of adjacent Faial Island and steeper than the platform outside the proposed slump. This suggests that it was created by coastal erosion over a shorter period and hence is consistent with a younger age of the coastline. As with the multibeam data, where escarpments are continued offshore onto the shelf, the rock surface imaged with these boomer data also shows no clear evidence of major slump-related fault displacements. This study therefore illustrates how high-resolution boomer seismic and multibeam data could usefully contribute to hazard assessment of volcanic islands, by helping to evaluate areas with no historical movements. Explanations to reconcile the onshore and offshore data here are also put forward.     

Studies on the application of combining models for vertical crustal movements

There are several kinds of analyzing models for vertical crustal movements, every one of them has different advantages and disadvantages, and adapts to different situations. This paper deals with the theory of the optimizing combining models for vertical crustal movements. The purpose for this optimization is to improve the fitting ability of models. This research was supported in part by the National Science Foundation of China.     

汶川8.0级地震前区域地壳运动与应变场动态特征

利用中国地壳运动观测网络GPS资料, 通过获取水平相对运动、 水平应变场分布变化等, 研究了汶川8.0级地震前的区域水平运动与应变率场变化, 以及大尺度地壳运动动态特征等。 研究结果表明, 发生汶川8.0级地震的龙门山断裂带, 由于受到其西侧巴颜喀拉地块向东运动的构造动力作用, 处于缓慢的应变积累状态。 在汶川地震前龙门山断裂带相对华南地块的差异运动小于GPS观测误差。 川滇地区应变场图像显示2004-2007年面应变率负值最高区出现在汶川8.0级地震震中区及其附近, 可能反应了局部挤压增强。 GPS基准站资料反映的大尺度北东向地壳缩短的相对运动增强, 也形成了促进龙门山断裂带发生大破裂的区域构造动力增强的背景条件。     

Recent and Late Holocene vertical crustal movements in the Southeast Baltic—The transition zone between the Fennoscandian shield and the Russian plate

The paper analyzed recent vertical crustal movements and compared them with Late Holocene ones (over several thousands of years) with the case study of the transition zone between the Fennoscandia shield and the Russian plate. The study of recent and young movements is based on long-term sea level and repeated geodetic measurements and on geologic geomorphic data, respectively. The data set makes it possible to establish that a weak uplift of the territory gives way with regularity to a weak subsidence after going across the zone of zero movements that agrees with the modern geodynamic constructions and can serve as the basis of long-term prognostic estimations. The results are compared with the newest GPS measurements and with the theoretical model of viscoelastic deformations in the periphery of the past glaciation area during its postglacial isostatic adjustment     

横向非均匀介质远震体波接收函数的波场特征

系统研究了台站下方存在倾斜界面时,远震体波接收函数的波场特征及多方位接收函数对台站下方介质横向非均匀特征的反映能力．我们利用合成三维横向非均匀介质接收函数的Maslov方法,具体模拟了台站下方存在倾斜界面时的接收函数响应．结果表明,当台站下方存在倾向一致的倾斜界面时,远震体波接收函数的径向分量和切向分量分别随震源方位角的变化呈现对称和反对称的规律性变化．利用不同震源方位角接收函数径向分量和切向分量的变化规律,可以估计台站下方界面的倾向和倾角．当台站下方各界面的倾向不同时,随方位角的变化,接收函数只能直观给出界面的整体倾向．实际观测数据的分析结果表明,对于利用单个台站接收函数研究台站下方介质的横向非均匀特征来说,简单倾斜界面可以看作较好的一级近似．      

Dynamics of interaction between fields of seismicity and surface deformations (Bishkek geodynamic test area)

The spatial-temporal dynamics of surface crustal movements revealed from GPS data is compared with seismicity in the Bishkek geodynamic test area documented in the regional KNET catalog. The geological information system (GIS) GeoTaim 2.0 is substantially improved, which allowed variations in seismicity and deformation fields to be analyzed in the 3D raster. It is shown that seismicity and surface deformations are correlative in the test area. The periods with extreme values of contraction and the extension rates of the Earth’s surface areas are accompanied by enhanced seismicity and strong earthquakes. The increase in the spatial gradient of surface crustal movements coincides with changes in the azimuths of compression axes indicated by mechanisms of earthquakes that occurred at depths of up to 25 km. For a better geological—geophysical interpretation of interactions between deformation and seismicity fields in the Bishkek geodynamic test area, the spatial system GPS stations and measurement frequency need substantial improvement.     

Long-term sea-level variations in the central Great Barrier Reef

Low frequency sea-level variations and associated geostrophic currents in the central Great Barrier Reef (GBR) region near Townsville are studied using optimally-lagged multivariate regression. The analyses show that pressure-adjusted coastal sea levels and mid-shelf geostrophic currents are influenced predominantly by local along-shelf wind stress at the weather time-scale, and by climatic variables, such as atmospheric pressure and temperature, at seasonal and inter-annual time-scales. These forcing variables can specify sea levels over annual and inter-annual time-scales with a forecasting skill of 0.53 and 0.22, respectively (where 1.0 is perfect skill). Associated along-shelf geostrophic currents can be forecast with a skill of 0.57 over an annual time scale. If, instead, absolute coastal sea levels or offshore sea-level differences are used to specify the along-shelf geostrophic current, the forecasting skill is 0.75. A characteristic El Niño/Southern Oscillation (ENSO) response is detected for time periods up to 25 years in monthly sea-level both at Townsville and at western Pacific island sea-level stations. This spatially coherent response varies in intensity and phase within the Coral Sea. Sea-level differences show a pattern which characterizes known features of the large-scale circulation of the Coral Sea. These very low frequency sea-level variations in the Coral Sea must be taken into account to obtain accurate predictions of along-shelf geostrophic current variations on seasonal and inter-annual time scales. Regression analysis and a diagnostic river plume model show that the influence of the major rivers can produce sea-level changes due to buoyancy of order 5 cm. The corresponding errors in geostrophic velocities estimated using pressure-adjusted Townsville sea-level data alone are of order 5 cm s⁻¹ rms.     

A three-dimensional gravity study of the 95.5°W propagating rift in the Galapagos spreading center

Seafloor at the Galapagos 95.5°W propagating rift (PR) has a varied morphological expression that can be spatially correlated with the predicted kinematic history of the PR. A median valley-like depression occurs near the tip of the growing ridge axis. To test if this bathymetry is a dynamic feature supported by mantle or lithosphere strength or if it is due to isostatically compensated crustal thickness variations, we use three-dimensional gravity modelling to constrain the crustal structure in this region, from data collected by Hey in 1979 and 1982. The gravity anomaly at the PR tip depression suggests that the tip depression is not caused by crustal thinning. The data are consistent with a stress-supported PR tip depression caused by asthenospheric along-axis flow into the growing ridge axis (Phipps Morgan and Parmentier [1]). In contrast to the tip depression, seafloor in the sheared zone of material transferred through transform migration from the Cocos to Nazca plate is anomalously shallow and has a pronounced regional 300–400 m tilt towards the growing ridge axis over the 20 km width of the sheared zone. The gravity data also suggest that the sheared zone is not compensated by crustal thickening.     

北天山中段近期重力场变化特征研究

利用北天山中段最新的流动重力观测数据,通过整理计算平差,绘制该地区半年和一年尺度的重力场变化图像;选取横跨北天山中段乌鲁木齐、独山子两条测线,并分别绘制其重力剖面变化图;结合北天山中段地区动力构造环境,剖析该地区重力变化特征及其与地震孕育之间的关系。分析表明:北天山中段地区重力异常变化值不大,重力变化等值线图和剖面变化图均能较好地反映重力场随时空变化特征;北天山中段动态重力变化特征基本反映了该地区动力构造运动的外部环境;研究区域内的重力场空间格局分布特征较清晰,其重力变化具有明显的分区特征,山体和盆地之间的重力变化具有显著差异,山体与盆地边缘正负重力变化交替出现,且变化相对较平稳。     

A new determination of recent vertical movements of the Earth's crust in Poland

A new determination of recent vertical movements of the Earth's crust in Poland is described in this paper. It resulted from the joint adjustment of the network of repeated levelling in 7 countries of Eastern and Central Europe. The author presents the characteristics of the levelling data sets used (measurements made in 1953–1957 and 1974–1979), results of the repeated levelling, the method of determination of the recent crustal movements and of the preparation of the map at a scale of 1:2.500.000 presenting these movements for the area of the whole country. The results of the determination of the Earth's crustal movements are discussed, and causes of the prevailing appearance of sinking movements are suggested. Differences between the values of vertical movements of the Earth's crust obtained during various determinations are presented, as well as the maximum mean error of the difference between new and old determinations. The conformity of these determinations is assessed.     

温度变化引起跨断层位移测量结果的年周期变化

本文首先给出了北京地区若干跨断层位移测量结果的年周期变化.而后,用二维非线性有限元方法,模拟了温度变化引起的跨断层位移测量结果的年周期变化.计算得到的应变量与野外观测大体一致.结果表明,在地壳介质不均一条件下,地表温度变化足以引起已观测到的种种复杂现象,诸如:存在两类基线,一类是当地表温度升高时增长;另一类是当地表温度升高时缩短.同时,存在两类水准变化,一类是断层热胀系数大的一盘,当地表温度上升时抬升;另一类则相反等等.一般说来,地壳介质是不均一的,因而温度对跨断层位移测量的干扰是不容忽视的.      

Estimates of vertical crustal movements along the coast of Greece,based on mean sea level data

Mean annual sea level (MASL) data for 25 Greek stations were analyzed for the time period 1969–1982. The data from 4 of these were unacceptably poor, and the record of 3 stations showed unexplained step functions that were interpreted as errors. Relative MASL between stations showed crustal stability at 10 of the 18 useful stations. The standard deviation from the long-term average of these stations was ±1.8 cm. We conclude that if station records are carefully kept in this area crustal movements in excess of 5 cm can be detected by relative MASL. A comparison of MASL data with gravity changes measured in the Peloponnese and Central Greece suggests that vertical movements occurred along a gradient equal to or larger than the free air gradient. We conclude that the gravity network should be reoccupied frequently such that the non-tectonic effects to be determined from the probable observed gravity changes, and the tectonic vertical movements may be better understood. A co-seismic subsidence of about 5 cm is inferred to have taken place near Korinth during the 1981,M _s =6.8, earthquake, which occurred 20 km N of this tide gauge (Posidonia). During 2.5 years before the 1968 Thessaloniki,M _s =6.6, earthquake, sea level was lower than average suggesting possible crustal elevation of 3.6 cm at about 28 km epicentral distance. Because of the small amplitude of this change we are not certain that it represents crustal uplift. At station Myrina (on Limnos) a strong and consistent trend of subsidence accumulated a 15 cm change between 1975 and 1980. Chios showed a trend of emergence (total accumulation about +5 cm), while Volos showed a trend of subsidence (approximately ?5 cm total). Kefalinia appears to have subsided about 10 cm during the data period. The six stations along the Hellenic arc plate boundary showed nearly constant MASL, suggesting that crustal stability existed there during the last 14 years. We conclude that MASL data in Greece can be useful for understanding tectonic processes, especially if these data are gathered carefully and at numerous locations, and are cross-correlated to high precision repeat gravity measurements, and geodetic releveling. Also, MASL data on active volcanic islands have excellent potential for detecting uplift before future eruptions.