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Geophysical Journal International, 168 (1), ISSN:0956-540X, ISSN:1365-246X

Geophysical Journal International, 157 (3), ISSN:0956-540X, ISSN:1365-246X

GPS rates based on data of an extended continuous and campaign-type GPS network in the North Aegean domain are presented. The data processed for the time period 1993–2009 is used to analyze the complex kinematic and deformation fields in the North Aegean Sea and adjacent regions. The presence of slowly deforming areas is investigated. Southern Bulgaria, eastern Macedonia and Thrace move uniformly southward relative to Eurasia (1.5–3.5 mm/yr). Western Macedonia, Epirus, Thessaly and Central Greece rotate rather coherently clockwise. The region comprising the islands of Limnos, Agios Efstratios and Alonnisos moves like a counterclockwise rotating slowly deforming block. The new GPS rates allow a quantification of the spatial change of strike-slip motion and locking depth along the North Aegean trough. Dextral strike-slip motion diminishes from east toward west amounting to 21.2 mm/yr along the Saros basin and 12.5 mm/yr south of the Chalkidiki peninsula. Less than 5 mm/yr (50 nstrain/yr shear strain rate) is transferred from the Sporades islands/Pelion toward Northern Evia. The locking depth is shallow for the Ganos fault and the western Saros basin (5.6–8.9 km). It is deeper between the Sporades islands and Pelion (~ 17.7 km) corresponding to a more diffuse shear zone. An elementary finite element model is applied to derive slip rates of the three main ENE–WSW to NE–SW trending dextral strike-slip faults in the North Aegean. Large-scale N–S to NNE–SSW extension in the North Aegean domain is analyzed by employing finite element and GPS based strain rate analyses. Pronounced extension (> 100 nstrain) is associated with known tectonic structures (e.g., Mygdonian graben, northern Gulf of Evia). In offshore areas such as the Sporades basin the correspondence between GPS derived extension rates and active fault structures is not entirely evident. However, important constraints are provided for seismotectonic interpretations.

Greece is characterized by high seismotectonic activity. Extended Global Positioning System (GPS) reoccupation and continuous networks have been measured in order to study the crustal motion and deformation in this region, with particular interest in earthquake-related effects. In this article we present GPS-derived coseismic displacements in Greece, caused by the M s 6.6 2001 Skyros and M s 6.2 2003 Lefkada earthquakes. The coseismic effects are presented in terms of time series, displacement vectors, differences of velocity vectors, and changes of accumulated strain. Furthermore, we compare the GPS results with fault plane solutions and slip models of the corresponding earthquakes. We found discontinuous changes of position of about 70 mm for the island of Skyros, which can be attributed to the 2001 Skyros earthquake (Aegean Sea). A detailed displacement field was obtained for the 2003 Lefkada earthquake (Ionian Sea, Greece), which induced coseismic southwestward displacements of up to 72 mm. It is in agreement with the focal mechanism of the mainshock, which caused a dextral strike slip of maximum 34 cm on a northeast striking plane. The cumulative strain after the Lefkada earthquake indicates an increased seismic hazard for the island of Kefalonia, Ionian Sea, Greece. The 25 March 2007 Kefalonia earthquake was located in this region.

To improve estimates of the long-term average seismic potential of the slowly straining South Central Mediterranean plate boundary zone, we integrate constraints on tectonic style and deformation rates from geodetic and geologic data with the traditional constraints from seismicity catalogs. We express seismic potential (long-term average earthquake recurrence rates as a function of magnitude) in the form of truncated Gutenberg–Richter distributions for seven seismotectonic source zones. Seismic coupling seems to be large or even complete in most zones. An exception is the southern Tyrrhenian thrust zone, where most of the African–European convergence is accommodated. Here aseismic deformation is estimated to range from at least 25% along the western part to almost 100% aseismic slip around the Aeolian Islands. Even so, seismic potential of this zone has previously been significantly underestimated, due to the low levels of recorded past seismicity. By contrast, the series of 19 M6–7 earthquakes that hit Calabria in the 18th and 19th century released tectonic strain rates accumulated over time spans up to several times the catalog duration, and seismic potential is revised downward. The southern Tyrrhenian thrust zone and the extensional Calabrian faults, as well as the northeastern Sicilian transtensional zone between them (which includes the Messina Straits, where a destructive M7 event occurred in 1908), all have a similar seismic potential with minimum recurrence times of M ≥ 6.5 of 150–220 years. This potential is lower than that of the Southern Apennines ( M ≥ 6.5 recurring every 60 to 140 years), but higher than that of southeastern Sicily (minimum M ≥ 6.5 recurrence times of 400 years). The high seismicity levels recorded in southeastern Sicily indicate some clustering and are most compatible with a tectonic scenario where the Ionian deforms internally, and motions at the Calabrian Trench are small. The estimated seismic potential for the Calabrian Trench and Central and Western Sicily are the lowest (minimum M ≥ 6.5 recurrence times of 550–800 years). Most zones are probably capable of generating earthquakes up to magnitudes 7–7.5, with the exception of Central and Western Sicily where maximum events sizes most likely do not exceed 7.

International audience; Western Greece is one of the seismotectonically most active regions in Europe. The main tectonic structures are the West Hellenic Arc (WHA) and the Kephalonia Fault Zone. In order to monitor and understand the crustal movements in space and time, a continuous GPS networkwas installed. In this paperwe present results of 6 yr (1995–2001) of measurements. To ensure a consistent reference frame, 54 mainly European IGS and EUREF sites were included in the processing. A selected subset was used to estimate an Euler pole for the rotation of Eurasia. In order to obtain coordinate time-series of high precision that are representative for crustal deformation, special emphasiswas given to the elimination of non-tectonic effects. Four steps of improvement were pursued, including a reprocessing after exclusion of poor data, the removal of remaining outliers, the correction of unknown phase centre offsets after antenna changes and weighted common-mode filtering.With this procedure, non-tectonic irregularities were reduced significantly, and the precision was improved by an average of 40 per cent. The final time-series are used as a base for depicting trajectories of crustal motion, interpreting the temporal behaviour of the sites and for estimating velocities. For the first time, height changes in the WHA area were detected and quantified by GPS. Sites that are located near the epicentres of the 1997 Strofades (Mw = 6.6) and the 1999 Athens (Mw = 6.0) earthquakes are particularly considered.

With the advent of the GPS navigation system, a promising ground based technique has been introduced which makes it possible to estimate the amount of water vapor in the troposphere from operational GPS networks at relatively low additional costs. While the estimation of the integrated amount is currently well established, the determination of the spatial water vapor distribution and its temporal variation are still a major challenge. To account for the vertical resolution, several tomographic approaches were pursued. We developed the software package AWATOS (atmospheric water vapor tomography software) which is based on the assimilation of double differenced GPS observations. Applying a least-squares inversion, the inhomogeneous spatial distribution of water vapor is determined. An extensive investigation has been carried out in Switzerland. GPS measurements are performed by the dense permanent Swiss national GPS network AGNES of the Swiss Federal Office of Topography (swisstopo). A total of 40 equally distributed water vapor profiles have been estimated on an hourly basis. For the purpose of validation, 22 radiosonde profiles were used at the GPS and meteorological station Payerne. Furthermore, data of the numerical weather model aLMo (alpine model in Switzerland, MeteoSwiss) were compared with the tomographic results. An overall good agreement of the three methods with an rms of better than 1.6 g/m3 absolute humidity was achieved. The results show that AGNES can be used as a dedicated network for the purpose of GPS-tomography, using a horizontal resolution of approximately 50 km and height layers of 300–500 m thickness in the lower troposphere.

The southern end of the Upper Rhine Graben is one of the zones in Switzerland where recent crustal movements can be expected because of ongoing seismotectonic processes as witnessed by seismicity clusters occurring in this region. Therefore, in 1973 a control network with levelling profiles across the eastern Rhine Graben fault was installed and measured in the vicinity of the city of Basel in order to measure relative vertical movements and investigate their relationship with seismic events. As a contribution to EUCOR-URGENT, the profiles were observed a third time in the years 2002 and 2003 and connected to the Swiss national levelling network. The results of these local measurements are discussed in terms of accuracy and significance. Furthermore, they are combined and interpreted together with the extensive data set of recent vertical movements in Switzerland (Jura Mountains, Central Plateau and the Alps). In order to be able to prove height changes with precise levelling, their values should amount to at least 3–4 mm (1σ). The present investigations, however, have not shown any significant vertical movements over the past 30 years.

New geodetic data combined with seismicity, geologic and geochemical information document a major tectonic reorganization in the central Mediterranean around 0.8–0.5 Ma, when rapid trench migration and consequent Tyrrhenian back-arc extension which dominated the region's evolution since the Tortonian (10–8 Ma) essentially stopped. In response, the African convergence in Sicily was transferred to a back-thrust in the Southern Tyrrhenian, characterized by frequent M5–6 earthquakes in the last 20 years. A diffuse transform boundary formed across northeastern Sicily to connect the Sicilian and Calabrian plate boundaries, causing opening of the Messina Straits—home to large historical earthquakes—and unusual volcanism in the Aeolian Island arc and the intraplate Mount Etna. A divergence of the Ionian region from the main part of Africa, with northeastward motions that reflect the joint influence of African push and Hellenic slab pull, and are transmitted to Calabria and Adria, could explain motions observed in the Sicily Channel, Calabria and Apulia, as well as the high level of southeastern Sicilian seismic activity.

Airborne Laser Altimetry (ALA) has experienced a rapid increase in popularity as a method serving a wide range of applications in Remote Sensing, Geodesy, Geophysics and Geodynamics. Besides the ‘traditional’ approach of using laser scanning solely as a supplement for photogrammetry in acquiring digital terrain models, ALA has also been applied to various geoscience research problems. After a short overview of airborne laser altimetry activities in Europe, an application of airborne laser scanning dedicated to Alpine Glaciology is presented.

Airborne laser techniques have evolved during the last years and have been tested in several pilot projects which cover a wide range of geodetic applications. In this project, airborne laser altimetry was used to determine the sea level in coastal areas of Greece, and to connect satellite radar altimetry results over the deep sea with tide gauge stations at the coast. Because airborne laser altimetry is capable to provide sea surface heights at the decimeter to centimeter level, it allows for an independent validation of spaceborne radar altimetry results. Airborne laser data acquired along densely spaced tracks of a total of 30-h flight time were used to determine instantaneous sea surface heights of the Ionian Sea, Greece. Differential GPS and inertial platform data were utilized as ancillary information for the purpose of ensuring a precise trajectography of the aircraft. Emphasis was put on the assessment of errors and the reduction of the raw data to mean sea level by crossover analysis and the incorporation of tidal predictions. The airborne laser data yield a high-resolution sea surface over the coastal areas of the Ionian Sea. The most prominent feature is a steep gradient of the sea surface amounting to 15 m over a distance of 150 km. This slope can be followed all along the bathymetric relief between the Hellenic Arc and Hellenic Trench.

In this paper we present the most recent observations of crustal motion across the entire West Hellenic Arc (WHA). These are based on repeated GPS measurements carried out in the period from 1993 to 1998. The results are presented in terms of trajectories and rates, relative to Eurasia. Within these five years southwestern Greece has moved to the southwest by an average rate of 30 mm/a, increasing from 10 mm/a at the island of Lefkada, in the center of the Ionian islands, to nearly 40 mm/a along the southwest part of the Peloponnesus and to 35 mm/a on the islands of Crete and Gavdhos. The data provide strong evidence that distributed shear strain starts at the Kephalonia Fault Zone (KFZ), were an anomalously high earthquake activity is also observed. A striking interruption of the motion is seen at the island of Strofades, at the southwest leading edge of the WHA, where a south-oriented displacement of 12 cm was detected, coincident with the M = 6.4 Strofades earthquake of Nov. 18, 1997.

The WEGENER group has promoted the development of scientific space-geodetic activities in the Mediterranean and in the European area for the last fifteen years and has contributed to the establishment of geodetic networks designed particularly for earth science research. WEGENER currently has three scientific objectives which are related to plate-boundary processes, sea-level and height changes, and postglacial rebound. In a full exploitation of the space-geodetic techniques, namely SLR, VLBI and GPS, the individual scientific projects do not only pursue these objectives but also contribute to improving and developing the observation techniques as well as the modelling theories. In the past, particularly SLR observations within WEGENER-MEDLAS have provided a fundamental contribution to determine the regional kinematics of the tectonic plates in the Mediterranean with high precision. With GPS, spatially denser site distributions are feasible, and in several WEGENER projects detailed studies of tectonically active areas were possible on the basis of repeated episodic GPS observations. Current projects associated with WEGENER are successful in separating crustal movements and absolute sea-level variations as well as in monitoring postglacial rebound. These tasks require high-precision height determinations, a problem central to all of the present WEGENER activities. In these projects, continuously occupied GPS sites are of increasing importance. Time series of heights observed with continuous GPS can be determined with a few centimeters RMS error thus enabling the reliable estimates of vertical rates over relatively short time intervals. Regional networks of continuous GPS sites are already providing results relevant, for example, for the study of postglacial rebound. The Mediterranean area is an extraordinary natural laboratory for the study of seismotectonic processes, and the wealth of observations acquired in previous WEGENER projects together with new space-geodetic observations will allow the test of geophysical hypotheses linking three-dimensional deformations of the Earth's surface to the dynamics of the Earth's interior. In particular, it is anticipated that WEGENER projects will aim at a test of the slab-detachment hypothesis. The complex investigations on sea-level fluctuations presently carried out at basin scale from the Strait of Gibraltar to the Black Sea make it possible to study the present and recent past interactions of ocean, atmosphere and solid Earth, as well as to develop appropriate models to assess future aspects.

We report on the establishment of a continuous GPS network across the Kephalonia Fault Zone (KFZ), central Ionian islands, Greece. The KFZ is an important segment of the boundary zone between the rapidly moving Aegean microplate and the Apulian platform, Italy. Repeated GPS measurements carried out between the northern Ionian islands and the island of Crete have shown rates of crustal motion of the Aegean reaching 35 mm/a, oriented SW relative to southern Italy. The KFZ coincides with a maximum shear strain rate (180 nano-strain/a) and a very high level of seismicity. In order to monitor the ongoing deformation across the KFZ and correlate the strain accumulation with earthquakes, a continuous GPS network has been installed, operating at a sampling rate of 5 s. This paper describes the instrumental set-up, the data-logging strategy and first preliminary results.

We use the combined GPS velocity field of the eastern Mediterranean for the period 1988 to 1996 to determine crustal deformation strain rates in a region comprising the Hellenic arc, the Aegean Sea, and western Anatolia. We interpret the velocity field and determine the strain rate tensor by the spatial derivatives of the collocated motion vectors. The region following the line Marmara Sea, North Aegean Trough, northern central Greece, and the central Ionian islands is associated with strong right-lateral shear motion, with maximum shear strain rates of 180 nano-strain/a (180×10−9/a). In the central Aegean Sea, N–S-oriented extensional processes prevail, reaching 100 nano-strain/a. The southern Aegean is characterized by relatively small strain rates. Maximum extensional components of the strain rate tensor, reaching 150 nano-strain/a in a N–S direction, are found in central Greece. The Hellenic arc is associated with moderate arc-parallel extension and strong compression perpendicular to it. Projections of the strain rates parallel to the major fault zones reveal that the northern Aegean is governed by the westward continuation of the North Anatolian Fault Zone which is associated with strong dextral shearing (maximum 220 nano-strain/a), accompanied by numerous large earthquakes in this century.

The Mediterranean-Alpine region marks the broad transition zone between the African/Arabian and Eurasian plates. In a generalized scheme, the recent major tectonic processes there can be understood as a direct consequence of active sea-floor spreading in the Atlantic Ocean, the Red Sea/Gulf of Aden and NW Indian Ocean spreading centres. The higher spreading rate in the South Atlantic compared to that in the North Atlantic leads to a gradual counterclockwise rotation of the African plate resulting in a north-northwestward directed push against Eurasia, which in turn leads to a lithospheric shortening of about 5 mm/a increasing to 40 mm/a in active subduction zones. With northwest-southeastward oriented spreading in the North Atlantic the zone of Eurasian/African plate contact is, therefore, to a large extent under compression thus providing the framework for mountain building processes accompanied by active rifting in preconditioned zones of lithospheric weakness.

Tropospheric water vapor is of central interest in a large variety of geoscientific fields, such as geodesy, geodynamics, climate research and meteorology. A new instrumental approach to ground-based mapping of tropospheric water vapor has been developed. It is based on high-resolution absorption measurements in the near infrared region by means of a solar spectrometer (SSM). To prove the feasability and accuracy potential of the new technique, a 30 day field experiment was carried out, performing SSM measurements simultaneously with two independent methods. One of them uses the Global Positioning System (GPS). This technique, called GPS meteorology, exploits the high sensitivity of the satellite signals to atmospheric delay for a determination of tropospheric parameters. As a third technique two ground-based microwave water vapor radiometers (WVR) were operated. A comparison of the three different techniques, exploiting absorption-, refraction-, and emission properties of water vapor, respectively, demonstrated the potential of solar spectrometry for precise and absolute determination of PW without meteorological a priori information.

A dense network of 52 Global Positioning System (GPS) sites is used to determine the velocity field and strain rate pattern at the western end of the North Anatolian Fault Zone (NAFZ) in the Marmara Sea region, NW Anatolia. Based on four GPS campaigns carried out biannually between 1990 and 1996, the detailed kinematic field of crustal motion is calculated. The southernmost station shows an average rate of 22±3 mm/yr oriented westward relative to Istanbul (Eurasia). This value is indicative of the dextral strike-slip motion of NW Anatolia relative to the Black Sea. The GPS velocity field reveals that the sites located on the Thrace-Black Sea block show negligible motion relative to Istanbul. The GPS-based deformation pattern of three sections crossing the westward prolongation of the NAFZ is compared with neotectonic data. Both the GPS and the neotectonic data give clear evidence that most of the deformation occurs along a relatively narrow E-W oriented zone. This belt extends from the single fault trace of the NAFZ through the Gulf of Izmit, the Marmara Sea, the Sarkoy region, and the Gulf of Saros into the North Aegean Trough (NAT). In addition, the cross sections show the increasing influence of the N-S oriented extension in the Aegean Sea region. From the comparison of the directions and rates of shear and normal strain deduced from GPS data with geologic and seismic observations we conclude that the most active zone of dextral shear follows a line along Mudurnu Valley, the northern Marmara Sea and the Saros Trough where it joins the NAT.

This paper describes the longest record of gravity measurements in the area of the Yellowstone caldera, Wyoming. The temporal gravity changes, at the ±12 µGal (10−8 ms−2) precision level, are compared with changes in heights from leveling and GPS. The gravity field decreased across the caldera from 1977 to 1983 during the uplift and attained a maximum decrease of up to −60± 2 µGal along the Caldera axis. The gravity field then reversed polarity to increasing values, of up to 60±12 µGal between 1986 and 1993. The ratio between height and gravity changes varied during the entire time, but converged over the latter period following the free-air gravity gradient. General ground deformation deduced from leveling showed caldera-wide uplift of ∼15 mm/a during the period of gravity decrease, then from leveling and GPS, subsidence of ∼25 mm/a during the gravity increase.

Tropospheric water vapor is of central interest in a large variety of geoscientific fields, such as geodesy, geodynamics, climate research, and meteorology. A new instrumental approach to ground-based mapping of tropospheric water vapor has been developed. It utilizes high-resolution absorption measurements in the near-infrared region by means of a solar spectrometer (SSM). The processing algorithm for retrieval of the precipitable water vapor (PW) is based on a line-by-line calculation of the observed solar spectrum in a narrow wavelength interval (1 nm) using a simple absorption model of the troposphere. To prove the feasibility and accuracy potential of the new technique, we carried out a 30-day field experiment. Simultaneous measurements of colocated SSM, water vapor radiometers (WVR) and Global Positioning System (GPS) receivers were performed, exploiting absorption, emission and refraction properties of water vapor, respectively. A comparison of the three different techniques demonstrated the potential of solar spectrometry for precise and absolute determinaton of PW without meteorological a priori information. Apart from apparent systematic errors of the GPS measurements, a good agreement between the SSM and WVR results within their individual accuracy limits was observed. The PW standard deviations of the techniques were determined to 0.37 mm for the WVR, 0.75 mm for the SSM, and 1.40 mm for the GPS retrievals. The independence of SSM from external calibration by radiosondes and the high potential for further development may qualify this new technique to contribute to developing an error budget for other techniques, such as GPS meteorology.

An airborne gravity survey of Switzerland was performed in a joint project between the Swiss Federal Institute of Technology, Zurich and LaCoste and Romberg Gravity Meters, Inc. The survey was flown in a Twin-Otter aircraft equipped by the Swiss Federal Directorate of Cadastral Surveying. The aircraft was outfitted with three Global Positioning System receivers, one for navigational purpose and two as sources of positioning, velocity, and vertical acceleration for airborne gravity reduction. Four receivers were installed on the ground as reference stations. The gravity data were recorded with a modified LaCoste and Romberg marine gravimeter at a sampling rate of 1 s. Modifications included anti-alias filters and an absolute encoder for the measuring screw. The flights were performed at a barometric altitude of 5100 m above sea level. An airborne Bouguer anomaly map, computed at flight altitude with topographic corrections of up to 167 km and a density of 2670 kg/m3, is presented. Bouguer anomaly values at crossing points between lines flown at comparable altitudes (ΔH

The SELF (SEa Level Fluctuations: geophysical interpretation and environmental impact) project has been developed and realized in the framework of the Environment Programme designed by the Commission of the European Communities. The SELF project was aimed at providing a reliable base for the determination, in the Mediterranean area, of sea-level variations which could then be used as a possible indicator of climate changes and to study the interactions taking place among the ocean, the atmosphere: and the solid Earth. The project has made it possible to define a consistent network of well-established tide gauges encompassing the Mediterranean Basin as far as the Black Sea and to determine to centimeter accuracy the tide gauge benchmark heights in a global well-defined reference system such as the one provided by the SLR/VLBI space techniques. The SELF network constitutes, for the Mediterranean, the necessary prerequisite towards achieving the actual capability to separate vertical crustal movements from true sea-level variations. This has been accomplished through the use of space techniques namely SLR, VLBI and GPS in conjunction with Water Vapor Radiometer observations and absolute gravity measurements. The analysis of the available tide gauge records has shown a high spatial coherence of the annual to multidecadal sea-level variability. Sea-level fluctuations at periods longer than two months were found to be strongly correlated with air pressure. The seasonal cycle was found to be variable in time. Relative sea-level trends determined from records longer than 30 years are less than 1.5 mm/yr. Crustal movement rates as determined from the tide gauge records are in general of the order ± 1.0 mm/yr. The geological observations have shed light on the fact that a marked variability of crustal movements occurs on both the temporal and spatial scale, and it represents a major contribution to relative sea-level fluctuations. This fact has been verified for the selected sector which belongs to one of the more geodynamically active areas of the Central Mediterranean (Aeolian Archipelago). However, this work has shown that, at least at the tide gauges included in the present study, crustal movements are small compared to the decadal to multidecadal sea-level variability but of the same order as the long-term trend in sea level, thus necessitating a careful monitoring if crustal movement is to be separated from the oceanographic contribution to relative sea-level changes.

The geodynamic processes in the Alpine-Mediterranean area are reflected in the recent crustal movements determined by satellite geodesy. There is counterclockwise rotation of Africa relative to Eurasia, with complex deformation in between. We report repeated GPS measurements carried out between SE Italy and the island of Crete in the period 1989 to 1994. The data reveal distinct crustal motion of the central Ionian Islands and SW Greece: Over the time span of five years Western Greece has moved 200mm±10mm to the SW relative to SE Italy and Epiros. The trajectories constructed from the repeated observations show that the boundary zone against the sites of no significant motion relative to South Italy coincides with the Kephalonia fault zone (KFZ). This intense deformation accounts in part for the earthquakes that have frequently devastated the central Ionian Islands.

The results of the 1993 GPS campaign in the Central and South America (CASA) project are presented. 21 stations have been observed along the Bocono-El Pilar fault system in Venezuela. By simultaneously occupying a fiducial site at Richmond/Florida the regional Venezuelan network is related to the International Terrestrial Reference Frame (ITRF). Several alternatives in the evaluation procedure are discussed. The accuracy of the final solution is about ± 5 mm in latitudinal and in longitudinal position components, and ± 10 mm in vertical direction. A comparison with the 1988 survey of five identical stations shows coordinate shifts of stations on both sides of the Bocono fault in rough accordance with predictions of the Caribbean and South American plate motions.

Deformation in the boundary zone between the Anatolian and Eurasian plates has been determined by means of repeated Global Positioning System (GPS) measurements. Three campaigns (1990, 1992, 1994) were carried out around the Marmara Sea. The network lies within the active earthquake belt of NW Anatolia, extending from the western part of the North Anatolian Fault Zone (NAFZ) to the Aegean extensional province. Results of the dense network consisting of 52 sites clearly reveal the westward motion of Anatolia. The rates amount to 17 mm/a ±3 mm/a on average relative to Istanbul, located at the northeastern edge of the Sea of Marmara. Based on the trajectories for the displacements of the observation sites it is concluded that the Marmara Sea forms a transition zone between the Europe fixed region of Thrace/Black Sea (Eurasia) and NW Anatolia. At the western extremity of NW Anatolia the motion bends from W to WSW(247°). The strain field calculated yields an E-W oriented right-lateral strike-slip movement (average 0.12 ppm/a, maximum 0.41 ppm/a) accompanied by NE-SW extension (average −0.12 ppm/a, maximum −0.52 ppm/a) at the eastern end of the Marmara Sea and N-S extension at the Aegean shore line. Comparisons with the directions of T axes derived from 65 fault plane solutions of earthquakes show remarkable agreement.

Vertical components of lithospheric deformations are key elements in assessing geodynamic processes such as mountain building, precursors of volcanic eruption or subduction of lithospheric plates. The recent success of modern geodetic space techniques such as the satellite positioning system GPS relates mainly to the “horizontal” components of lithospheric movements. One of the limiting factors for the height component is the refraction caused by the non-ionized atmosphere. This part is non-dispersive and the refractive effect cannot be determined by using two carrier waves. Modeling of the content of water vapor is particularly difficult because of its high variability both spatially as well as temporally. The objective of this paper is to present results from an experiment recently carried out in the frame of the European Sea Level Fluctuation program, SELF. Transportable water vapor radiometers were used to determine the refraction effect of the wet component of the troposphere between several tide gauge sites and neighboring SLR/VLBI stations in Italy and France. The corresponding differential path delays of the GPS radio signals are on the order of several cm. The example shown in this paper is a baseline measured between the French SLR station Grasse, France, and the tide gauge site Genoa, Italy. Apparent temporal changes of height differences of up to 8 cm have been observed over a time span of 12 h. The consequence for assessing vertical movements of GPS based stations is discussed.

Recent crustal movements detected by the analysis of repeated satellite geodetic measurements reflect the ongoing geodynamic processes in the Alpine-Mediterranean area. Superimposed on the large-scale counterclockwise rotation of the African plate, complex dynamic processes are affecting the lithospheric fragments between the African and Eurasian plates. Key features to better understand the driving forces and associated seismic activity in the Africa/Eurasia collision zone are the Calabrian and Hellenic arcs. In this paper geodynamic investigations along the West Hellenic arc are discussed. They are based on two epochs (1989 and 1993) of satellite geodetic measurements carried out using the US Global Positioning System (GPS). The results are presented in terms of relative displacements and strain rates. Within the time span of 4 years southwestern Greece has moved to the southwest relative to southeastern Italy by an average of 120 mm, increasing from 80 mm at Lefkada, in the center of the Ionian Islands, to 160 mm at the Peloponnesus. The maximum strain rate is 0.18 μstrain/a located in the vicinity of Lefkada, where anomalously high earthquake activity is observed. The data provide strong evidence for dextral strike-slip motion on the order of 25 mm/a along the Kephalonia Fault Zone (KFZ). The deformation field of the KFZ is interpreted as a transition zone between the kinematics of the Apulian platform and the West Hellenic fold and thrust belts.

In the Marmara Sea region the relative motion between the Anatolian and Eurasian lithosphere has been measured by means of the modern techniques of space geodesy using the Global Positioning System (GPS). In order to resolve in detail the kinematic field within the active earthquake belts of Northwestern Anatolia, two GPS campaigns were carried out across a dense network consisting of 45 sites. Preliminary results reveal horizontal westward motion of Anatolia relative to Eurasia of 2.4 cm/y on average that runs along the western end of the North Anatolian Fault Zone. E-W oriented right-lateral strike-slip movement (average 0.06 ppm/y, maximum 0.13 ppm/y) (1ppm/y= 1 μstrain/y and NE-SW extension (average 0.06 ppm/y, maximum 0.21 ppm/y) can be obtained from the calculated strain field. In the west strike-slip motion trending WSW-ENE is observed. These findings are compared to the stress pattern derived from seismology and neotectonics.

A GNSS water vapour tomography system developed to reconstruct spatially resolved humidity fields in the troposphere is described. The tomography system was designed to process the slant path delays of about 270 German GNSS stations in near real-time with a temporal resolution of 30 min, a horizontal resolution of 40 km and a vertical resolution of 500 m or better. After a short introduction to the GPS slant delay processing the framework of the GNSS tomography is described in detail. Different implementations of the iterative algebraic reconstruction techniques (ART) used to invert the linear inverse problem are discussed. It was found that the multiplicative techniques (MART) provide the best results with least processing time, i.e., a tomographic reconstruction of about 26,000 slant delays on a 8280 cell grid can be obtained in less than 10 min. Different iterative reconstruction techniques are compared with respect to their convergence behaviour and some numerical parameters. The inversion can be considerably stabilized by using additional non-GNSS observations and implementing various constraints. Different strategies for initialising the tomography and utilizing extra information are discussed. At last an example of a reconstructed field of the wet refractivity is presented and compared to the corresponding distribution of the integrated water vapour, an analysis of a numerical weather model (COSMO-DE) and some radiosonde profiles.

This paper describes a collaborative GPS-Project to determine recent crustal movements and deformations across the NE Ionian Sea. This region is characterized by rifting in the African Foreland (Pelagian Sea), subduction along the Calabrian and West Hellenic Arcs, rotation of the Italian peninsula and distributed normal faulting in Central Greece. From GPS-campaigns on the Ionian Islands and in NW Greece performed in 1989 and 1991 a kinematic field is deduced which is controlled by the deep trough following the NW coast of Kephalonia island. It is concluded that this prominent bathymetric feature resembles an active fault system (Kephalonia fault) which has the signature of a right-lateral transform fault.

[1] Atmospheric water vapor is a crucial factor in achieving highest accuracies in the field of space geodesy, particularly in the determination of height. A new instrumental approach to high-precision determination of tropospheric water vapor has been investigated and realized in the construction of a mobile solar spectrometer. The optical approach as applied to the spectrometer allows the simultaneous measurement of single vibrational-rotational absorption lines in the wide wavelength range between 728 nm and 915 nm. Dedicated field experiments have been carried out in the frame of an EU project for calibrating the Jason onboard microwave radiometer. A comparison of the spectrometer results with a conventional ground-based water vapor radiometer and radiosondes revealed a fit on the order of 10 mm for the wet path delay, which corresponds to 1.6 kg/m2 integrated water vapor content (IWV). The comparison with the measurements from the radiometer of Jason reveals an even better agreement for the wet path delay. Long time series are foreseen for a detailed statistical analysis. The mobile solar spectrometer can be considered as a novel portable tool for determining tropospheric water vapor and as an excellent space-borne radiometer calibration and validation system.

Geophysical Journal International, 167 (2), ISSN:0956-540X, ISSN:1365-246X

The kinematic field of central western Europe is characterized by relatively small movements (around 1-2 mm/year) and diffuse seismicity with earthquakes occurring mostly in the shallow crust (within 15 km), prevalently concentrated along the Alps and the European Cenozoic Rift System (ECRIS). In order to study and constrain the current crustal kinematic field we reconstructed the velocity and the strain field using permanent GPS stations, belonging to different networks (AGNES, EUREF, REGAL, RGP). The 2D strain rate tensor has been calculated using the method of least-squares collocation. Our results show that the area of maximum compression is located along the Alpine chain, where maximum values of 7 +/- 2 nstrain/year are found, while maximum extension is measured between the Armorican Massif and the Massif Central, where values of 4 +/- 2 nstrain/year are reached. The earthquakes with M > 3.0, have been used to estimate the seismic strain rates, while the style of the seismic deformation was reconstructed from the fault plane solutions (FPS) available from the literature. Relatively high values of seismic strain rates (around 10 nstrain/year) are measured along the Alpine Chain and the ECRIS. Results obtained by geodetic and seismic data are quite in agreement and reflect the different tectonic evolution of the geological features characterizing the area of study. The orientation of the compressional geodetic and seismic strain axes are NW-SE in most of the area of study, on account of the action of plate boundary forces. A rotation of the same axes to N-S direction along the eastern Alps, possibly related to the Adria convergence, is found. (c) 2006 Elsevier Ltd. All rights reserved.

In October 2003 two modernized digital zenith camera systems have been deployed in Switzerland during the campaign CHGeo2003. The mission was carried out under the auspices of the Swiss Geodetic Commission of the Swiss Academy of Sciences (SAS) and coordinated by the Swiss Federal Office of Topography (swisstopo). The goal of the campaign was to provide additional highly accurate deflections of the vertical in order to contribute to an improvement of the presently used Swiss geoid CHGeo98. The observations were carried out at 68 selected stations covering regions with old or inadequate data. Further reasons of the campaign were the proof of the field capability and the comparison of both systems concerning their accuracy by observing at several stations simultaneously. The paper describes the realization of the campaign CHGeo2003 and the results obtained with the digital systems.

International Journal of Earth Sciences, 94 (4), ISSN:1437-3254, ISSN:1437-3262

[1] Southern Italy is a key area for understanding the tectonic processes in the Africa-Eurasia collision zone. We analyze new GPS measurements carried out between 1994 and 2001. The results are presented in terms of time series, trajectories and velocities of crustal motion, as well as a geodetic strain rate field. While central Italy, Corsica, Sardinia and the Tyrrhenian Sea move like the Eurasian plate, the overall motion of the Sicily Rift Zone region matches African plate motion. Unexpected are the north-northwest directed motions of northeastern Sicily and the Eolian islands. Most striking on the deformation field are a north-south oriented compression along the northern Sicilian coast, compression between Apulia and northwestern Greece as well as extension in the Sicily Rift Zone and the interior of Sicily.

Geophysical Journal International, 152 (1), ISSN:0956-540X, ISSN:1365-246X

International audience; During summer of 1995 local seismicity was recorded in the area around the Gulf of Arta in northwestern Greece by a dense temporary seismic network. Of the 441 local events observed at 37 stations, 232 well locatable events with a total of 2776 P-phase readings were selected applying the criteria of a minimum of 6 P-observations and an azimuthal gap less than 180°. This data set is used to compute a minimum 1D velocity model for the region. Several tests are conducted to estimate model stability and hypocenter uncertainties, leading to the conclusion that relative hypocenter location accuracy is about 500 m in latitude and longitude and 1 km in depth. The minimum 1D velocity model serves as initial model in the non-linear inversion for three-dimensional P-velocity crustal structure by iteratively solving the coupled hypocenter–velocity problem in a least-squares sense. Careful analysis of the resolution capability of our data set outlines the well resolved features for interpretation. The resulting 3D velocity model shows generally higher average crustal velocities in the east, and the well resolved area of the eastern Gulf of Arta exhibits a homogeneous velocity around 6 km/s for the whole upper crust. A pronounced north–south trending zone of low velocities in the upper 5–10 km is observed in the area of the Katouna fault zone (KFZ). At greater depths (below 10 km) the KFZ is underlain by high-velocity material. E–W profiles suggest a horst–graben structure associated with the KFZ.

The Central and South America (CASA) GPS project aims at monitoring crustal deformations in the complex tectonic zone of the Caribbean, Cocos, Nazca and South American plate boundaries. Since 1988 several GPS campaigns have been carried out, unfortunately not always covering the whole area simultaneously.

To analyse and test the feasibility of GPS-aided approaches for civil aviation in the Swiss Alps, an internationally co-operating group, leaded by the Swiss Federal Office of Civil Aviation (FOCA) has been set up. During several test flights and dedicated ground missions various problems, such as satellite visibility, multipathing, GPS signal interference and the influence of the topography on the navigation and the GPS system were addressed. The tests showed the potential of satellite based systems for approaches, in particular for use in rugged terrain.

Weltraumunternehmungen der NASA und ESA, wie z.B. die bemannten Mondlandungen, die Fluge der PIONEER- und VOYAGER-Sonden durch unser Planetensystem, die bemannten Shuttle-Missionen oder die ersten Fernerkundungssatelliten ERS1/2 der ESA, haben seit jeher die Aufmerksamkeit der Offentlichkeit auf sich gelenkt. Inzwischen ist auch der Aufbau des neuen U.S. Satellitensystems GPS in der Offentlichkeit bekannt geworden, nachdem sich weitreichende Konsequenzen fur die Navigation und Geodasie aufgezeigt haben.

Geodynamic processes which are ascribed to relative motions between the Eurasian plate and the so-called Adriatic promontory govern the ongoing plate tectonic activity in the central Mediterranean/Alpine region. Superimposed on the underthrusting of the Adriatic block beneath the Southern Alps seems to be a rotational motion caused by the postulated counter-clockwise rotation of the Italian peninsula. Recently established GPS networks as well as terrestrial levelling lines carried out in Switzerland since the last century can be used in the future to assess the strain tensor in the Swiss Alps. A crucial problem relevant for geodynamic research is the interpretation of the detected recent crustal movements in terms of stress and driving forces. This paper presents the results of a stress-strain analysis of the Swiss Alps, using an elementary 2D-model as well as a more complete 3D-model. In the latter case the highly complex structure of the Alpine crust has been simplified to a multilayer model, taking also detailed topography into account by means of digital terrain models. Isostatic forces as well as NNW-SSE compression of the Alpine chain have been introduced in finite element (FEM) calculations. The results reveal a striking agreement with the levelled uplift rates. Furthermore comparisons are made between the calculated strain energy and the regional seismicity.

The ongoing geodynamic processes in the Alpine-Mediterranean area are primarily governed by the relative movement of the African lithospheric plate against the Eurasian plate resulting in a predominantly compressional regime along the zone of contact. A counterclockwise rotation of the African plate caused by a higher spreading rate of the floor of the South Atlantic results in a northward directed push leading to lithospheric shortening in the Mediterranean-Alpine belt of about 5 to 10 mm/y. With a NW-SE oriented spreading in the North Atlantic (about 18 mm/y) the zone of contact is under compression thus providing the framework for mountain building processes accompanied by active rifling in preconditioned zones of lithospheric weakness. Although this framework is generally accepted great uncertainty remains with regard to the exact kinematic and dynamic pattern of the large-scale tectonic collision between these two major lithospheric plates. Satellite based geodetic networks have been established in the past 5 years to elucidate the present-day deformation field in the region of the Calabrian and West Hellenic Arcs. Results of the first analysis are being presented in this paper.

[1] GPS meteorology can be successfully used to model the tropospheric refraction effect on radiowave signals. We describe an approach to estimate and model the spatial distribution of the tropospheric water vapor. We refine the tomographic software package AWATOS which is based on the assimilation of GPS double difference observations. The observations are allocated to a voxel model, which is defined according to the distribution of the GPS stations. Performing a least-squares adjustment, the refractivity of each voxel is determined. Tests of the software are performed, based on simulated and real data. The latter were obtained during a dedicated field campaign on the Big Island of Hawaii. The tomographic profiles are compared with radiosonde data. The results obtained for continuous atmospheric conditions fit well, whereas larger differences occur in discontinuous atmospheric weather conditions. Overall, an accuracy of 5–20 ppm (refractivity units) has been achieved.

A new set of 1 x 1 deg mean free-air anomalies in the Indian Ocean is determined on the basis of previously published free-air anomaly maps (Talwani and Kahle, 1975) and the most recent Lamont surface ship gravity measurements. The data are then used to compute a (total) 1 x 1 deg gravimetric Indian Ocean geoid. The computation is carried out by combining the Goddard Space Flight Center (GSFC) GEM-6 geoid and a difference geoid that corresponds to the differences between the set of 1 x 1 deg surface gravity values and the GEM-6 gravity anomalies. The difference geoid is highest over the Madagascar Ridge (+20 m) and lowest over the Timor Trough (-30 m). The total geoid is compared with GEOS-3 radar altimeter-derived geoid profiles, and geophysical implications are discussed.

The Indian Ocean has been the site of numerous geophysical investigations which commenced during the International Indian Ocean Expedition. In this paper gravity surveys are reviewed which have been undertaken since this international effort to elucidate the structure and evolution of the most complex of the three major oceans. Particular attention is paid to the interrelationship between the plate boundaries and the gravity field. Some selected boundaries are discussed in the framework of global plate tectonics. These areas comprise the Red Sea, Gulf of Aden and the Indonesian deep sea trench. In addition the continental margin structure south of India is included which provides some further constraints on the location of the initial break-up of Gondwanaland and consequently bears on pre-drift reconstructions of India and Antarctica.

Prominent features of the long-wavelength variations of the earth's gravity field are the gravity lows in the wake of drifting continents such as along the east coast of America, south of India and southwest of Australia. It is demonstrated that part of these negative anomalies could be caused by the transient temperature field behind drifting continents. As the continental lithosphere moves at a given spreading rate over the asthenosphere, the radioactive heat sources of the granitic crustal layer supply sufficient heat to the deeper lying asthenosphere to increase the temperature by about 80° C. Thus the continent acts like a “flat-iron” moving on the underlying material. After the continental plate has left the area previously covered, the asthenosphere cools off. The corresponding lateral temperature differences are associated with lateral variations in density. The gravity anomaly caused by the lateral temperature differences shows values at the Indian continental margin which are about 20 mGal lower than over the Central Indian Ocean Basin. These theoretical gravity values were subtracted from the observed Indian Ocean free-air surface gravity anomalies averaged over 8° × 8° squares. It is shown that the remaining residual gravity values are close to the average value of the total Indian Ocean plate anomaly. The negative south—north gravity gradient superimposed on the average anomaly can be explained by the transient temperature anomaly behind the drifting continent.