Marc Cocard

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 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.

Establishment of a continuous GPS network across the Kephalonia Fault Zone, Ionian islands, Greece

Abstract 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.

GPS‐meteorology: Impact of predicted orbits on precipitable water estimates

Studies of atmospheric effects on Global Positioning System (GPS) signals have proven the possibility of deriving the total water vapor content from estimates of tropospheric path delays. The accuracy of GPS derived Precipitable Water (PW) depends (besides other parameters) on the quality of satellite orbits used in the analysis. High precision orbits provided by the International GPS Service (IGS) yield PW estimates with an accuracy of about 1 mm. While these orbits are provided with a delay of several days, weather forecasting requires near real-time determination of PW. Therefore operational meteorological GPS analysis would have to rely on orbit predictions. We investigate the impact of introducing predicted orbit information on the accuracy of GPS water vapor retrievals. The presented data were acquired during a 14-day field experiment carried out in the north-west region of Madrid, Spain using GPS and a Water Vapor Radiometer (WVR). The comparison of WVR measurements with estimated time series of PW using both 24 and 48 hour predicted orbits and final precise IGS orbits shows that the accuracy of PW decreases by a factor of about 2 from precise to predicted orbit data.

GPS interactive time series analysis software

Time series analysis is an important part of geodetic and geodynamic studies, especially when continuous GPS observations are used to explore areas with a low rate of deformation. In this domain, having precise and robust tools for processing and analyzing position time series is a prerequisite. To meet this requirement, a new software package called GPS Interactive Time Series Analysis was developed using the MATLAB language. Along with calculating basic statistics and quality parameters such as mean and variance, the software is capable of importing and visualizing different time series formats, determining and removing jumps and outliers, interpolating data, and producing numerical and publication quality graphical outputs. Furthermore, bivariate statistical analysis (such as correlation coefficients, curvilinear and nonlinear regression), residual analysis, and spectral analysis (such as auto-spectrum, Lomb–Scargle spectrum, evolutionary power spectrum, and wavelet power spectrum) form the main analysis features of the software.

Kinematic GPS as a source for airborne gravity reduction in the airborne gravity survey of Switzerland

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<10 m) show differences varying between 1.1 and 2.7 mGal (10−5 m/s2). Comparison of the ground Bouguer anomaly map, upward continued to 5100 m, and the one computed from airborne data shows a strong similarity both in amplitude and wavelength.

EPC: Matlab software to estimate Euler pole parameters

The estimation of Euler pole parameters has always been an important issue in global tectonics and geodynamics studies. In addition, the increasing number of permanent GPS stations and the ease of access to their data, along with advances in computers, promise new methods and tools for the estimation and the quantitative analysis of Euler pole parameters. Therefore, we developed the Euler pole calculator software using a set of mathematical algorithms based on the model of tectonic plate motion on a spherical surface. The software is able to calculate the expected velocities for any points located on the earth’s surface given the relevant Euler pole parameters and to estimate the Euler pole parameters given the observed velocities of a set of sites located on the same tectonic plate. Mathematical algorithms and functions of the software are explained in detail.

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.

Noise behavior in CGPS position time series : the eastern North America case study

Abstract We analyzed the noise characteristics of 112 continuously operating GPS stations in eastern North America using the Spectral Analysis and the Maximum Likelihood Estimation (MLE) methods. Results of both methods show that the combination ofwhite plus flicker noise is the best model for describing the stochastic part of the position time series. We explored this further using the MLE in the time domain by testing noise models of (a) powerlaw, (b)white, (c)white plus flicker, (d)white plus randomwalk, and (e) white plus flicker plus random-walk. The results show that amplitudes of all noise models are smallest in the north direction and largest in the vertical direction. While amplitudes of white noise model in (c–e) are almost equal across the study area, they are prevailed by the flicker and Random-walk noise for all directions. Assuming flicker noise model increases uncertainties of the estimated velocities by a factor of 5–38 compared to the white noise model.

GeoStrain: An open source software for calculating crustal strain rates

Abstract We developed an open source software for crustal strain analysis using the least-squares collocation method based on the spherical model of the earth. The software is able to simultaneously determine the signal and noise of the velocities at the observation points with the best possible removal of the observational errors, or at any other position with no velocity observation. Furthermore, the software can calculate strain and rotation rate tensors at any points of interest, including observation points, grid points or points along a fault line. The advantage of the software is that the sphericity of the earth is considered in all the calculations. Moreover, it can optionally consider the effect of the vertical velocities on the strain rates that is principally important for regions where vertical deformation is the major geophysical signal compared to the horizontal deformation.