Alain Geiger

GPS‐derived strain rate field within the boundary zones of the Eurasian, African, and Arabian Plates

We use the GPS velocity field (1988–1998) for eastern Mediterranean and Asia Minor to determine the crustal deformation strain rate field in an area bounded by 35°N and 43°N, and 20°E and 48°E. We calculate the normal and shear strain rate components associated with the major faults and compare these qualitatively with seismological data. Uncertainties in the calculation of the strain rates reach 50 nstrain yr−1 in sparsely observed parts of Anatolia, whereas we estimate errors <20 nstrain yr−1 in the Aegean and Marmara regions. The largest compressional strain rate components in the eastern part of the study area occur along the Greater Caucasus mountain front reaching 70 nstrain yr−1. (1 nstrain yr−1 = 0.0317 × 10−15 s−1). The North Anatolian Fault Zone is the clearest feature in the shear strain rate field. It is expressed as a pronounced dextral strike-slip fault zone, reaching rates of up to 170 nstrain yr−1. This holds true also for the Izmit area, where the August 17, 1999, earthquake occurred. Central Anatolia is almost strain-free, whereas extension prevails in western Anatolia. The principal axes of extension vary around the N-S direction with strain rates of up to 85 nstrain yr−1. These extensional areas coincide with graben features and normal faulting earthquakes. The central and southwestern Aegean Sea is strain-free with values far below 40 nstrain yr−1. The seismic cluster around the Dodekanissa islands, southeastern Aegean Sea, coincides with NW-SE oriented extension, attaining strain rates of up to 90 nstrain yr−1. This area of extension also exhibits recent active volcanism. The entire Hellenic arc shows compressional strain rates perpendicular to the arc. The Pliny-Strabo troughs along the eastern segment of the arc show left-lateral shear strain rates reaching 80 nstrain yr−1. Significant extension is found in central Greece, with a NNE-SSW oriented maximum of 120 nstrain yr−1 centered around the Gulf of Corinthos. The Kephalonia Fault Zone in NW Greece is a distinct dextral fault zone, separating Apulia from the rapidly moving Aegean microplate. Right-lateral shear strain rates reach 150 nstrain yr−1.

THE STRAIN FIELD IN NORTHWESTERN GREECE AND THE IONIAN ISLANDS : RESULTS INFERRED FROM GPS MEASUREMENTS

Abstract 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.

The GPS strain rate field in the Aegean Sea and western Anatolia

The GPS velocity field of the Aegean Sea and western Anatolia is used to determine crustal deformation strain rates for the period 1988 to 1996. The zone from the Marmara Sea to the North Aegean Trough is associated with strong right-lateral shear motion, with maximum strain rates of 170 nstrain/a. In the northern Aegean Sea the extensional deviatoric axes are oriented NNE-SSW, associated with strain rates reaching 150 nstrain/a, and in western Anatolia varying around N-S with rates of up to 95 nstrain/a. The extensional areas mostly coincide with active graben features. The south-western Aegean Sea is almost strain-free. In general, the areas of high geodetic strain rates are accompanied by distinct seismic clusters whereas the strain-free regions are nearly aseismic.

Evaluating the potential of an airborne laser-scanning system for measuring volume changes of glaciers

Airborne laser scanning (ALS) is well suited for the production of digital elevation models (DEM), and can, in contrast to photographic methods, be used to acquire a DEM independently of surface texture and external light sources. ALS thus serves as a tool to generate DEMs of firn areas where photogram- metric methods often fail. The potential of an integrated ALS system – comprising a laser scanner, precise differential global positioning system, and a gyro platform – for DEM generation of firn areas is currently being assessed. The Unteraargletscher, Bernese Alps, Switzerland, has been chosen as a test site. As part of a pilot project aimed at determining the mass balance distribution of that glacier without the use of in situ information, ALS measurements were conducted in autumn 1997. The DEM derived from laser measurements is extremely sensitive to the position and attitude of the aircraft. Currently the main work focuses on assessing and improving the systems accuracy by error modelling and by the development of error-correction algorithms. Preliminary results from Unteraargletscher are presented, and the potential of this method for the generation of DEMs of firn areas is discussed.

Airborne gravity measurements over mountainous areas by using a LaCoste & Romberg air‐sea gravity meter

This paper introduces a new approach to airborne gravity data reduction well-suited for surveys flown at high altitude with respect to gravity sources (mountainous areas). Classical technique is reviewed and illustrated in taking advantage of airborne gravity measurements performed over the western French Alps by using a LaCoste & Romberg air-sea gravity meter. The part of nongravitational vertical accelerations correlated with gravity meter measurements are investigated with the help of coherence spectra. Beam velocity has proved to be strikingly correlated with vertical acceleration of the aircraft. This finding is theoretically argued by solving the equation of the gravimetric system (gravity meter and stabilized platform). The transfer function of the system is derived, and a new formulation of airborne gravity data reduction, which takes care of the sensitive response of spring tension to observable gravity field wavelengths, is given. The resulting gravity signal exhibits a residual noise caused by electronic devices and short-wavelength Eotvos effects. The use of dedicated exponential filters gives us a way to eliminate these high-frequency effects. Examples of the resulting free-air anomaly at 5100-m altitude along one particular profile are given and compared with free-air anomaly deduced from the classical method for processing airborne gravity data, and with upward-continued ground gravity data. The well-known trade-off between accuracy and resolution is discussed in the context of a mountainous area.

Crustal Deformation along the Caribbean — South American Plate Boundary Derived From The Casa GPS Project

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.

Hydrostatic levelling systems: Measuring at the system limits

Abstract Three hydrostatic displacement monitoring system applications in Switzerland are discussed; the first concerns experience gained monitoring the foundation of the Albigna dam, the second relating to the underground stability of the Swiss Light Source synchrotron and the third concerning the deformation of a bridge near the city of Lucerne. Two different principles were applied, the Hydrostatic Levelling System (HLS) using the “half-filled pipe principle” developed by the Paul Scherrer Institute and the Large Area Settlement System (LAS) using the “differential pressure principle”. With both systems ground deformations induced by tidal forces can be seen. However, high accuracy of single sensors is not sufficient. A well-designed configuration of the complete system is equally important. On the other hand there are also limits imposed by installation logistics and by the environmental conditions. An example is the bridge monitoring application, where the acceleration along the bridge due to the passage of heavy trucks limits the feasibility of using hydrostatic levelling measurements.

Mitigation of Tropospheric Effects in Local and Regional GPS Networks

Although modern technologies, especially space borne techniques, provoked a revolutionary advance in geodetic sciences, atmospheric refraction still remains a persistent problem. Modelling the atmospheric effects and estimation of tropospheric parameters are two methods to reduce the tropospheric biases. In this paper the development and testing of both methods, separately and in combination, are presented for applications in local and regional GPS-networks. The atmospheric state variables are modelled in four dimensions (space and time) based on operational ground measurements. Parameter estimation and collocation techniques are applied for modelling. By integration along the actual microwave paths the corresponding correction values are obtained. Compared to standard corrections the modelling method considerably reduces the rms of the GPS solution. An additional estimation of zenith path delays further reduces the rms. In a local network with considerable height differences (Swiss testnet ‘Turtmann’) the repeatability in height is clearly enhanced (up to a factor of 2) by introducing modelled path delays. For high precision applications the combination of modelled path delays and the estimation of tropospheric parameters is very promising.

Grundlagen und Anwendungen des Navigationssystems GPS

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.

Characteristics and limitations of GPS L1 observations from submerged antennas

Observations from a submerged GNSS antenna underneath a snowpack need to be analyzed to investigate its potential for snowpack characterization. The magnitude of the main interaction processes involved in the GPS L1 signal propagation through different layers of snow, ice, or freshwater is examined theoretically in the present paper. For this purpose, the GPS signal penetration depth, attenuation, reflection, refraction as well as the excess path length are theoretically investigated. Liquid water exerts the largest influence on GPS signal propagation through a snowpack. An experiment is thus set up with a submerged geodetic GPS antenna to investigate the influence of liquid water on the GPS observations. The experimental results correspond well with theory and show that the GPS signal penetrates the liquid water up to three centimeters. The error in the height component due to the signal propagation delay in water can be corrected with a newly derived model. The water level above the submerged antenna could also be estimated.