Active crustal deformation in two seismogenic zones of the Pannonian region — GPS versus seismological observations

The seismicity and the associated seismic hazard in the central part of the Pannonian region is moderate, however the vulnerability is high, as three capital cities are located near the most active seismic zones. In our analysis two seismically active areas, the Central Pannonian and Mur-Mürz zones, have been considered in order to assess the style and rate of crustal deformation using Global Positioning System (GPS) and earthquake data.We processed data of continuous and campaign GPS measurements obtained during the years 1991–2007. Velocities relative to the stable Eurasia have been computed at HGRN, CEGRN and EPN GPS sites in and around the Pannonian basin. Uniform strain rates and relative displacements were calculated for the investigated regions. GPS data confirm the mostly left lateral strike slip character of the Mur-Mürz–Vienna basin fault system and suggest a contraction between the eastward moving Alpine-North Pannonian unit and the Carpathians.The computation of the seismic strain rate was based on the Kostrov summation. The averaged unit norm seismic moment tensor, which describes the characteristic style of deformation, has been obtained from the available focal mechanism solutions, whereas the annual seismic moment release showing the rate of the deformation was estimated using the catalogues of historical and recent earthquakes.Our analysis reveals that in the Central Pannonian zone the geodetic strain rate is significantly larger than the seismic strain rate. Based on the weakness of the lithosphere, the stress magnitudes and the regional features of seismicity, we suggest that the low value of the seismic/geodetic strain rate ratio can be attributed to the aseismic release of the prevailing compressive stress and not to an overdue major earthquake. In the Mur-Mürz zone, although the uncertainty of the seismic/geodetic strain rate ratio is high, the seismic part of the deformation seems to be notably larger than in the case of the Central Pannonian zone. These results reflect the different deformation mechanism, rheology and tectonic style of the investigated zones.     

Contributions to the Deformation Analysis in Germany Based on Precise and Continuous GPS Measurements

Modern space geodetic techniques enable deformation monitoring of continental plate interiors with high spatial and temporal coverage. Resolving data and results are currently evaluated for their application for the integrated assessment of seismic hazard and risk in Germany. This goes especially for regions where earthquakes are generally rare but high magnitudes are still not unrealistic while vulnerability of today’s society is steadily growing. The present contribution deals with the continuous monitoring of tectonic fracture systems in Germany using the GPS. The estimation of the station velocities with GPS and the resulting geodetic strain is supposed to provide additional input to the earthquake hazard assessment. Unfortunately, the low expected and currently seen velocities (<1–2 mm/year) make it extremely difficult to distinguish between noise and a tectonic signal. Because of the short observation interval the velocity uncertainties are about 2 mm/year in the horizontal components. The essential goal of this program is to provide and model highly precise deformation data and to discuss its needs for a better assessment of geological hazard, especially for the most active tectonic regions in Germany, the Rhine-Graben, the Swabian Alb, the Alpine foreland, and the Vogtland. Here we present preliminary results from 2 years of measurements at currently 150 GPS stations throughout Germany. The time span of this program has proven to be too short and the density of the station network to be not dense enough yet for reliable significant horizontal station velocities and supporting the earthquake hazard assessment.     

Local seismic hazard assessment in areas of weak to moderate seismicity—case study from Eastern Germany

Generally the seismic hazard of an area of interest is considered independent of time. However, its seismic risk or vulnerability, respectively, increases with the population and developing state of economy of the area. Therefore, many areas of moderate seismic hazard gain increasing importance with respect to seismic hazard and risk analysis. However, these areas mostly have a weak earthquake database, i.e., they are characterised by relative low seismicity and uncertain information concerning historical earthquakes. In a case study for Eastern Thuringia (Germany), acting as example for similar places in the world, seismic hazard is estimated using the probabilistic approach. Because of the lack of earthquakes occurring in the recent past, mainly historical earthquakes have to be used. But for these the actual earthquake sources or active faults, needed for the analysis, are imprecisely known. Therefore, the earthquake locations are represented by areal sources, a common practice. The definition of these sources is performed carefully, because their geometrical shape and size (apart from the earthquake occurrence model) influence the results significantly. Using analysis tools such as density maps of earthquake epicentres, seismic strain and energy release support this. Oversizing of areal sources leads to underestimation of seismic hazard and should therefore be avoided. Large location errors of historical earthquakes on the other hand are represented by several alternative areal sources with final superimposition of the different results. In a very similar way information known from macroseismic observations interpreted as source rather than as site effects are taken into account in order to achieve a seismic hazard assessment as realistic as possible. In very local cases the meaning of source effects exceeds those of site effects very likely. The influence of attenuation parameter variations on the result of estimated local seismic hazard is relatively low. Generally, the results obtained by the seismic hazard assessment coincide well with macroseismic observations from the thoroughly investigated largest earthquake in the region.     

Maximum Expected Magnitude Assessment in a Geo Computer Environment: Case Study

Different approaches to seismic hazard assessment are compared. Each of them could be applied more or less successfully for territories which are stable in time and have a high level of seismic activity. A long-term seismic catalogue, not only including historical but also paleoearthquake data, is an essential requirement. But, in practice, such an ideal situation is very rare. Initial data is usually poor and short-term. Seismic hazard assessment could be more complicated for regions which are transient between relatively stable platforms and active mountain massifs. A new step in geoinformation technology for seismic hazard assessment based on a GEO computer environment is presented, its application is illustrated by the real case hazard evaluation for the territory of the Stavropol region, which is situated between the Russian platform and the Great Caucasus. The regional catalogue covers a time period of about 150 years. Though the region under consideration is not large, seismic activity is variable in space, from almost aseismic zones to rather active areas. GEO allows us to incorporate different techniques and all available information in the analysis, including those which are very difficult to formalize. The space distribution of the maximum expected earthquake magnitude is determined as a function of geological and geophysical data. An important feature of GEO is that it makes it possible to control the result of complicated algorithms through some relatively simple physical reasons.     

Crustal deformation in northern Central America

Evaluation of the seismic moment tensor for earthquakes on plate boundary is a standard procedure to determine the relative velocity of plates, which controls the seismic deformation rate predicted from the slip on a single fault. The moment tensor is also decomposed into an isotropic and a deviatoric part to discover the relationship between the average strain rate and the relative velocity between two plates. We utilize this procedure to estimate the rates of deformation in northern Central America where plate boundaries are seismically well defined. Four different tectonic environments are considered for modelling of the plate motions. The deformation rates obtained here compare well with those predicted from the plate motions models and are in good agreement with actual observations. Deformation rates on faults are increasingly being used to estimate earthquake recurrence from information on fault slip rate and more on how we can incorporate our current understanding into seismic hazard analyses.     

Tectonic stress,seismicity, and seismic hazard in the southeastern Carpathians

Intermediate-depth earthquakes in the Vrancea region occur in response to stress generation due to descending lithosphere beneath the southeastern Carpathians. In this article, tectonic stress and seismicity are analyzed in the region on the basis of a vast body of observations. We show a correlation between the location of intermediate-depth earthquakes and the predicted localization of maximum shear stress in the lithosphere. A probabilistic seismic hazard assessment (PSHA) for the region is presented in terms of various ground motion parameters on the utilization of Fourier amplitude spectra used in engineering practice and risk assessment (peak ground acceleration, response spectra amplitude, and seismic intensity). We review the PSHA carried out in the region, and present new PSHA results for the eastern and southern parts of Romania. Our seismic hazard assessment is based on the information about the features of earthquake ground motion excitation, seismic wave propagation (attenuation), and site effect in the region. Spectral models and characteristics of site-response on earthquake ground motions are obtained from the regional ground motion data including several hundred records of small and large earthquakes. Results of the probabilistic seismic hazard assessment are consistent with the features of observed earthquake effects in the southeastern Carpathians and show that geological factors play an important part in the distribution of the earthquake ground motion parameters.     

利用GPS数据反演青藏高原东北缘主要活动断裂滑动速率

GPS测量可以提供高精度的时空图像, 但前任对比研究尚未深入.通过使用线性球面块体模型理论, 在前人针对活动块体研究的基础上, 建立青藏高原东北缘地区三维块体几何模型.以1999—2007年的GPS水平速度场数据作为约束, 反演得到青藏高原东北缘的主要活动断裂长期滑动速率和广义海原断裂带的空间亏损滑动分布及其耦合变化.上述结果为青藏高原东北缘现今运动变形的动力学和强震中-长期危险性预测研究提供了约束和参考.      

Crustal deformation in the northern Andes – A new GPS velocity field

We present a velocity field for northwestern South America and the southwest Caribbean based on GPS Continuously Operating Reference Stations in Colombia, Panama, Ecuador and Venezuela. This paper presents the first comprehensive model of North Andean block (NAB) motion. We estimate that the NAB is moving to the northeast (060°) at a rate of 8.6 mm/yr relative to the South America plate. The NAB vector can be resolved into a margin-parallel (035°) component of 8.1 mm/yr rigid block motion and a margin-normal (125°) component of 4.3 mm/yr. This present-day margin-normal shortening rate across the Eastern Cordillera (EC) of Colombia is surprising in view of paleobotanical, fission-track, and seismic reflection data that suggest rapid uplift (7 km) and shortening (120 km) in the last 10 Ma. We propose a “broken indenter” model for the Panama-Choco arc, in which the Choco arc has been recently accreted to the NAB, resulting in a rapid decrease in shortening in the EC. The Panama arc is colliding eastward with the NAB at approximately 15–18 mm/yr, and the Panama-Choco collision may have been responsible for much of the uplift of the EC. The present on-going collision poses a major earthquake hazard in northwestern Colombia from the Panama border to Medellin area. Since the northeastward margin-parallel motion of the NAB is now greater than the rate of shortening in the EC, northeast trending right-lateral strike-slip faulting is the primary seismic hazard for the 8 million inhabitants of Bogota, the capital city of Colombia. There continues to be a high risk of a great megathrust earthquakes in southern Colombia along the Ecuador-Colombia trench. Trench earthquakes have only released a fraction of the energy accumulated in the Ecuador-Colombia trench since the 1906 Ecuador earthquake, and interseismic strain is accumulating rapidly at least as far north as Tumaco, the rupture area of the 1979 earthquake.     

Estimation of GPS strain rate and its error analysis in the Chinese continent

In this paper we first estimate the strain rate field with 1202 GPS vectors from 1999 to 2005 in the Chinese continent. Then we propose a method to make error analysis of the strain rates computed from GPS vectors based on the Monte Carlo technique. In general, the orientations of compressive principal strain rates are in agreement with those of present tectonic stresses. The strain rates in the paper confirm the tectonic features of Tibetan Plateau such as NE–W compression and shortening, E–W extension, and normal faulting along near N–S faults. At the same time, the eastward extrusion in the east part of Tibet can also be seen from the strain rate field. In the error analysis, independent computation of strain rates is repeated for a large number of times, both the absolute and relative errors as well as the mean value of strain rates are obtained through statistical theory. The error result shows that the errors of strain rates may be associated with the uncertainties of GPS measurement and the distribution of GPS stations in space. The magnitude of the strain rate error is in the range of 3–8 × 10^?9/yr across China in general. However, we found that the relative errors are much larger in East China than those in the west of China. In addition, the relative errors of maximum shear strain rates are smaller than those of surface dilation rates. Also, we found that there may be some correlations between maximum shear strain rates and seismic activities. Where the maximum shear strain rates are higher, there the earthquakes occur more frequently. The computed GPS strain rates reveal the distributed nature of deformation across the whole Chinese continent, and can represent the main present-day tectonic features. The strain rate and its error in the paper may provide important constraints to geodynamical modeling.     

Spatial-temporal variability of seismic hazard in peninsular India

This paper examines the variability of seismic activity observed in the case of different geological zones of peninsular India (10°N–26°N; 68°E–90°E) based on earthquake catalog between the period 1842 and 2002 and estimates earthquake hazard for the region. With compilation of earthquake catalog in terms of moment magnitude and establishing broad completeness criteria, we derive the seismicity parameters for each geologic zone of peninsular India using maximum likelihood procedure. The estimated parameters provide the basis for understanding the historical seismicity associated with different geological zones of peninsular India and also provide important inputs for future seismic hazard estimation studies in the region. Based on present investigation, it is clear that earthquake recurrence activity in various geologic zones of peninsular India is distinct and varies considerably between its cratonic and rifting zones. The study identifies the likely hazards due to the possibility of moderate to large earthquakes in peninsular India and also presents the influence of spatial rate variation in the seismic activity of this region. This paper presents the influence of source zone characterization and recurrence rate variation pattern on the maximum earthquake magnitude estimation. The results presented in the paper provide a useful basis for probabilistic seismic hazard studies and microzonation studies in peninsular India.     

论地震数值预报——关于我国地震预报研究发展战略的思考

随着中国工业化和社会现代化进程的加快 ,中国城市化进程必将进一步加速。如何减轻地震灾害的问题正变得日益严峻。尽管地震预测是一个举世公认的国际性科学难题 ,但在强化各种减轻地震灾害措施的同时 ,仍须大力推进地震预测研究。为此 ,需要打破长期徘徊在以地震前兆异常监测为基础的经验性预测局面 ,把注意力尽快转向研究以动力学为基础的数值预报。以GPS为代表的空间对地观测技术 ,岩石圈巨型高分辨率宽频带流动地震台阵观测技术以及数值模拟技术已经为地震数值预报研究提供了前所未有的技术基础。以地震数值预报为目标的GPS阵列地壳形变连续观测 ,高分辨率地壳上地幔结构探测 ,地壳动力学 ,地震孕育和破裂过程的理论、模拟试验和实际观测 ,数据同化和计算软件的开发应成为今后研究的重点。现在的问题是 ,需要积极借助数值天气预报的经验 ,强化多学科 ,多部门的组织协调 ,尽早在有条件的地区开展地震动力学数值预报的科学试验。地震数值预报研究必将极大地促进中国地震科学基础研究和地震预报的进一步发展。     

GPS技术应用于中国地壳运动研究的方法及初步结果

文中主要就中国利用GPS等空间测地资料研究地壳运动、构造变形 ,以及用于地震预测探索方面 ,从方法技术和近年来取得的一些初步结果进行了概要性论述。介绍了利用GPS技术资料研究地壳水平运动速度场、水平应变场、建立地壳运动模型等方法研究的进展。由GPS观测给出的地壳水平运动初步结果表明 :中国大陆现时水平运动在全球参考系中为整体向东 ,并兼有顺时针扭转运动。西部地区构造形变强烈 ,整个青藏块体及其边界带 ,以及新疆西部是应变值最高的区域 ,水平应变场主压应变优势分布方向为近NE向 ,空间差异显著 ,反映了印度板块碰撞推挤和青藏块体强烈构造运动的影响。中国大陆东部水平运动的差异性不显著。强震分布于地壳运动的大小、方向显著变化的区域 ,大地震通常发生在水平剪应变高值区或其边缘 ,尤其是与区域主干断裂的构造活动背景相一致的剪应变率高值区。     

Influence of slow active faults in probabilistic seismic hazard assessment: the northwestern margin of the València trough

Areas of low strain rate are typically characterized by low to moderate seismicity. The earthquake catalogs for these regions do not usually include large earthquakes because of their long recurrence periods. In cases where the recurrence period of large earthquakes is much longer than the catalog time span, probabilistic seismic hazard is underestimated. The information provided by geological and paleo-seismological studies can potentially improve seismic hazard estimation through renewal models, which assume characteristic earthquakes. In this work, we compare the differences produced when active faults in the northwestern margin of the València trough are introduced in hazard analysis. The differences between the models demonstrate that the introduction of faults in zones characterized by low seismic activity can give rise to significant changes in the hazard values and location. The earthquake and fault seismic parameters (recurrence interval, segmentation or fault length that controls the maximum magnitude earthquake and time elapsed since the last event or T_e) were studied to ascertain their effect on the final hazard results. The most critical parameter is the recurrence interval, where shorter recurrences produce higher hazard values. The next most important parameter is the fault segmentation. Higher hazard values are obtained when the fault has segments capable of producing big earthquakes. Finally, the least critical parameter is the time elapsed since the last event (T_e), when longer T_e produces higher hazard values.     

跨断层与GPS地壳形变数据联合反演鲜水河断裂地震危险性

为了探索GPS和跨断层地壳形变数据联合反演效果,本文以鲜水河断裂为研究对象,利用1999~2007期,2009~2013期和2013~2017期中国大陆GPS水平速度场数据,使用贝叶斯反演方法,以跨断层数据为先验条件,估算了川滇菱形块体东边界（主要为鲜水河断裂）的断层运动速率。发现增加跨断层数据后,反演图像的近场和远场速率区别更加显著,不同期次的断层活动速率表现出明显的差异。但是,本方法在震前效果并不明显,尤其是在汶川地震前部分跨断层场地的逆向走滑特征很难表现出来,对于地震预测预报也很难起到优势作用,但从反演效果上来看,可以更直观的反映断层在近场和远场上的速率差异以及地震后断层运动速率的分段特征。最终根据上述研究方法认为鲜水河断裂带在汶川地震后,经过多年的应力调整,目前左旋走滑速率已经基本恢复到震前状态,鲜水河断裂南段持续拉张趋势,川滇菱形块体的顺时针旋转作用持续加强,鲜水河断裂的道孚段和磨西段存均在一定的走滑速率亏损,应注意这两个地区的地震危险性,以及这两个地区地震危险的关联性。     

New data on seismicity of the middle Caspian Basin and their possible tectonic interpretation

Short-term (three months) bottom seismic observations in the area of the Yalam-Samur structure in the Middle Caspian Basin revealed a deep-seated compact zone of mantle-earthquake sources that dips beneath the southeastern Caucasus. To a first approximation, this zone may be interpreted as a seismofocal layer that characterizes thrusting of the Turan Plate under the southeastern Caucasus. However, the obtained spatial distribution of sources of microearthquakes and weak earthquakes is insufficiently reliable owing to the low aperture of the observation network of bottom seismographs. More reliable data on the position and parameters of the seismofocal layer could be obtained by the observation network with a wider spread of bottom seismographs (up to 50–100 km). If this result is confirmed, the current concept of interaction between the Alpine structures of the southeastern Caucasus, Turan, and South Caspian plates should probably be revised. The geotectonics of the Caucasus is preliminarily analyzed in the light of the newly revealed relationships.     

Attenuation laws and seismic hazard assessment

In the present study, the seismic hazard in the city of Patras, central Greece, is estimated. The computations are based on a slightly modified version of the method proposed by Cornell, allowing the introduction of individual attenuation laws for each seismic source. The obtained results emphasize the dependence of hazard on attenuation and the importance of the use of local attenuation laws in seismic hazard assessment.     

Unified scaling law for earthquakes in Crimea and Northern Caucasus

This study continues detailed investigations on the construction of regional charts of the parameters of the generalized Guttenberg–Richter Law, which takes into account the properties of the spatiotemporal seismic energy scaling. We analyzed the parameters of the law in the vicinity of the intersections of morphostructural lineaments in Crimea and Greater Caucasus. It was shown that ignoring the fractal character of the spatial distribution of earthquakes in the southern part of the Russian Federation can lead to significant underestimation of the seismic hazard in the largest cities of the region.     

Recent crustal deformation and seismicity studies at Abu-Dabbab area, Red Sea, Egypt

The Abu-Dabbab area is characterized by high seismicity and complex tectonic setting; for these facts, a local geodetic network consisting of 11 geodetic benchmarks has been established. The crustal deformation data in this area are collected using the GPS techniques. Five campaigns of GPS measurements have been collected, processed, and adjusted to get the more accurate positions of the GPS stations. The horizontal velocity vectors, the dilatational, the maximum shear strains, and the principal strain rates were estimated. The horizontal velocity varies in average between 3 and 6 mm per year across the network. The results of the deformation analysis indicate a significant contraction and extension across the southern central part of the study area which is characterized by high seismic activity represented by the clustering shape of the microearthquakes that trending ENE. The north and northeastern parts are characterized by small strain rates. This study is an attempt to provide valuable information about the present state of the crustal deformation and its relationship to seismic activity and tectonic setting at the Abu-Dabbab area. The present study is the first work demonstrating crustal deformation monitoring at the Abu-Dabbab area. The time interval is relatively short. Actually, these results are preliminary results. So, the continuity of GPS measurements is needed for providing more information about the recent crustal deformation in that area.     

Attenuation laws and seismic hazard assessment

In the present study, the seismic hazard in the city of Patras, central Greece, is estimated. The computations are based on a slightly modified version of the method proposed by Cornell, allowing the introduction of individual attenuation laws for each seismic source.The obtained results emphasize the dependence of hazard on attenuation and the importance of the use of local attenuation laws in seismic hazard assessment.Paper presented at the 21st General Assembly of the European Seismological Commission, held in Sofia, 1988.     

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.