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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.

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.

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.

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.

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 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.

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.

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

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

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

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.

[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.

This paper describes and reviews the progress of a three year (11/97–11/00) European Commission FP4 (Climate and Natural Hazards) funded project entitled GPS Seismic Hazard in Greece (SING), A major international interdisciplinary consortium is investigating and comparing strain derived using both geodetic and seismic methods.