| 1. |
- Dineva, Savka, et al.
(author)
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Seismicity of south-western Bulgaria and border regions
- 1998
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In: Journal of Geodynamics. - 0264-3707 .- 1879-1670. ; 26:2-4, s. 309-325
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Journal article (peer-reviewed)abstract
- Earthquakes that occurred between 1985-1991 in SW Bulgaria and border regions with Greece and the Former Yugoslav Republic of Macedonia (bounded by 22.0°E-22.4°E and 40.8°N-42.4°N) are located to define the characteristics of the present seismicity. The hypocenters of 2657 earthquakes with magnitudes 0.3 < ML < 5.1 are determined using the program HYPOLOC (modification of the HYPO71 program) and a velocity model is obtained for the local area of SW Bulgaria. Earthquake tend to cluster in seven areas. The seismic activity in the clusters changes with time and correlates mainly with the occurrence of the strongest events in each cluster. The clusters are classified by the density of earthquake epicenters. Following this classification, those with the 'highest' seismicity are characterised as the Krupnik cluster, and those with the 'lowest' seismicity, the Butkovsko (Kerkini) Lake cluster. The location and shape of the cluster areas are related to boundaries between areas of subsidence (grabens, depressions) and areas of uplift (horsts, swells). A few areas without seismicity, which are related to horst structures are outlined. Historical data exists for each of the obtained cluster areas (from 52) for earthquakes of magnitude larger than 5.0.
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| 2. |
- Pan, M., et al.
(author)
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An analysis of the Ethiopian Rift Valley GPS campaigns in 1994 and 1999
- 2002
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In: Journal of Geodynamics. - 0264-3707 .- 1879-1670. ; 33:3, s. 333-343
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Journal article (peer-reviewed)abstract
- In cooperation with the Geophysical Observatory, Addis Ababa University, the Department of Geodesy and Photogrammetry of the Royal Institute of Technology carried out GPS measurements at three deformation networks in the Ethiopian Rift Valley in 1994 and 1999. For these campaigns the session-to-session repeatabilities were about 5 and 4 mm in the horizontal components and 50 and 10 mm in the vertical components using the IGS precise orbits, respectively. The results show, that the stations of the E3 network in the Rift Valley moved with a magnitude of 2.5 +/- 1.3 mm/a to the SE, and a magnitude of 21.3 +/- 1.4 mm/a to the ENE in the northeast (close to Assab of the Red Sea) relative to the station Addis Ababa on the African plate. The direction of movement is consistent with the local spreading vector of the Red Sea Rift with the N84degreesE slip direction estimated from seismic data. The sites of the E1 network moved by a rate of about 1-2 mm/a to the SE relative also to the station Addis Ababa. Further to the south there is an indication from our GPS data that the stations on the Somalian plate of the E2 network move by a rate of about 2.5 mm/a to the E or SE relative to the African plate. This motion can also be confirmed by geological and geodynamic data.
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| 3. |
- Pan, M., et al.
(author)
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Crustal movements in Skane, Sweden, between 1992 and 1998 as observed by GPS
- 2001
-
In: Journal of Geodynamics. - 0264-3707 .- 1879-1670. ; 31:3, s. 311-322
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Journal article (peer-reviewed)abstract
- We estimate a network of crustal deformations in Skane, southern Sweden, using observations of the Global Positioning System (GPS) from 1992, 1996 and 1998. The network straddles the Tornquist zone, potentially one of the most active fault zones in Sweden. In addition to two stations of the Swedish permanent GPS network (Onsala and Hassleholm), it includes seven GPS sites spaced approximately 80 km apart. The precisions of the relative horizontal components for these stations are described by repeatabilities with approximately 3 mm in the north-south direction and about 2 mm in the east-west direction in the campaigns in the reference frame ITRF96. About 70% of the GPS integer carrier phase ambiguities were resolved for the three campaigns. Three stations south of the Tornquist zone moved with a rate of 5+/-0.2 mm/a toward the SW-SE, and two other stations further to the east with a rate of 2+/-0.2 mm/a toward the SE, relative to station Onsala. The displacements mean that a lateral strike-slip transtensional fault exists within the Tornquist zone, and the relative motion between the two sides along the zone was about 2 mm/a. The old fault is still active, which is consistent with geological results. The station Stavershult closest to Onsala moved NE with about 1.5+/-0.8 mm/a relative to the station Onsala. This may imply a transtensional fault between Stavershult and Onsala. If we assume that Stavershult represents the Baltic shield, Onsala has moved 1.5 mm/a toward the southwest with respect to the Baltic shield, which was confirmed by data of Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) from 1976 to 1997 (Argus, D.F., Peltier, W.R., Watkins, M.M. Glacial isostatic adjustment observed by Very Long Baseline Interferometry and Satellite Laser Ranging geodesy. JGR 1999;104(B12);29077-93).
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| 4. |
- Sjöberg, Lars Erik, et al.
(author)
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Glacial rebound near Vatnajokull, Iceland, studied by GPS campaigns in 1992 and 1996
- 2000
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In: Journal of Geodynamics. - 0264-3707 .- 1879-1670. ; 29:2-Jan, s. 63-70
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Journal article (peer-reviewed)abstract
- Since about 1920 the Vatnajokull ice cap in Iceland has experienced a significant retreat, corresponding to a volume reduction of more than 180 km(3). With two GPS campaigns in 1992 and 1996 along the southern border of the glacier preliminary results reveal land uplift rates of 1-6 mm/yr, after a one-parameter (bias) fit with recent earth rheology models. The best fit model suggests that the lithosphere in the area is about 30 km thick and the viscosity of the asthenosphere 5 x 10(18) Pa s. The rms fit of uplift rate at all GPS sites is +/-1.4 mm/yr. As the GPS data alone cannot provide the absolute uplift rates, the one-parameter fit to the theoretical modelling implies that the absolute rates were estimated by the matching of the GPS data and model. The resulting uplift rate at station Hofn (1 mm/yr) is not consistent with two independent sources, and we therefore conclude that further GPS epoch and permanent GPS site data are needed to confirm the present geodynamic processes near Vatnajokull.
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| 5. |
- Sun, W. K., et al.
(author)
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Permanent components of the crust, geoid and ocean depth tides
- 2001
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In: Journal of Geodynamics. - 0264-3707 .- 1879-1670. ; 31:3, s. 323-339
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Journal article (peer-reviewed)abstract
- The tidal deformation caused by the luni-solar potential includes not only a periodic part, but also a time-independent part, called the permanent tide. How to deal with the tidal correction in gravimetric observations, especially the treatment of the permanent tide, has been discussed for a long time, since some practical and physical problems exist anyhow. A resolution adopted by IAG (1983) was that the permanent tidal attraction of the Moon and the Sun should be eliminated, but the permanent tidal deformation of the Earth be maintained. This is called zero gravity, and the geoid associated with it is the zero geoid. As to the crust deformation, Poutanen et al. (Poutanen, M., Vermeer, M., Makinen, J., 1996. The permanent tide in GPS positioning. Journal of Geodesy 70, 499-504.) suggested that co-ordinates should be reduced to the zero crust, i.e. the crust that includes the effect of the permanent tide. This research shows that horizontal components of the permanent earth tides, which are not considered in recent studies, are also important in GPS positioning and geoid determination. Since the tide-generating potential can be expanded into harmonics and divided into two parts (geodetic coefficients and the group of harmonic waves), the permanent earth tides can be easily obtained by multiplying the amplitude of the zero-frequency wavelength by the corresponding geoid geodetic coefficient. Formulas for both elastic and fluid cases are presented. Numerical results for the elastic case show that he vertical permanent crust (zero crust), geoid and ocean depth tides reach -12.0, -5.8 and 6.1 cm at the poles, and 5.9, 2.9 and -3.0 cm at the equator, respectively. The horizontal permanent crust, geoid and ocean depth tide components reach as much as 2.5, 8.7 and 6.3 cm, respectively. According to the solution of IAG (1983), the permanent vertical components are kept in GPS positioning and geoid computation. Thus, it is natural to include the horizontal components correspondingly.
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| 6. |
- Bagherbandi, Mohammad, et al.
(author)
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A study on the relationship between isostatic equilibrium and seismicity : A case study in Africa
-
In: Journal of Geodynamics. - 0264-3707 .- 1879-1670.
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Journal article (other academic/artistic)abstract
- The principle of isostasy plays an important role in understanding the relationship between the geodynamic processes. It is difficult to find an exact method that delivers a complete image of the Earth structure, gravimetric and seismic methods can provide images of the interior of the Earth. The Earth’s crust parameters, i.e. crustal depth and crust-mantle density contrast, can reveal information about the solid Earth system e.g. earthquakes and continental rifting processes. In this study, a combined Moho model using seismic and gravity data is determined to investigate the relationship between the isostatic state of the crust and seismic activities in our study area in Africa. To do this we determine the isostatic gravity disturbance and the density contrast. Our results show that isostatic equilibrium and compensation states are closely correlated to the seismicity patterns in the study area. This paper presents a method to determine the crustal thickness and crust-mantle density contrast and consequently one can detect low-density contrasts (about 200 kg/m3) and thin crust (about 30 km) near the Afar Triangle, which confirms the state of over-compensation. Furthermore, the density structure of the mantle lithosphere shows a large correlation with the earthquake activity, sub-crustal stress and volcanic distribution.
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| 7. |
- Bagherbandi, Mohammad, et al.
(author)
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Improved global crustal thickness modeling based on the VMM isostatic model and non-isostatic gravity correction
- 2013
-
In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707 .- 1879-1670. ; 66, s. 25-37
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Journal article (peer-reviewed)abstract
- In classical isostatic models for a gravimetric recovery of the Moho parameters (i.e., Moho depths and density contrast) the isostatic gravity anomalies are usually defined based on the assumption that the topographic mass surplus and the ocean mass deficiency are compensated within the Earth's crust. As acquired in this study, this assumption yields large disagreements between isostatic and seismic Moho models. To assess the effects not accounted for in classical isostatic models, we conduct a number of numerical experiments using available global gravity and crustal structure models. First, we compute the gravitational contributions of mass density contrasts due to ice and sediments, and subsequently evaluate respective changes in the Moho geometry. Residual differences between the gravimetric and seismic Moho models are then used to predict a remaining non-isostatic gravity signal, which is mainly attributed to unmodeled density structures and other geophysical phenomena. We utilize three recently developed computational schemes in our numerical studies. The apparatus of spherical harmonic analysis and synthesis is applied in forward modeling of the isostatic gravity disturbances. The Moho depths are estimated globally on a 1 arc-deg equiangular grid by solving the Vening-Meinesz Moritz inverse problem of isostasy. The same estimation model is applied to evaluate the differences between the isostatic and seismic models. We demonstrate that the application of the ice and sediment density contrasts stripping gravity corrections is essential for a more accurate determination of the Moho geometry. We also show that the application of the additional non-isostatic correction further improves the agreement between the Moho models derived based on gravity and seismic data. Our conclusions are based on comparing the gravimetric results with the CRUST2.0 global crustal model compiled using results of seismic surveys.
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| 8. |
- Bagherbandi, Mohammad, et al.
(author)
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On the residual isostatic topography effect in the gravimetric Moho determination
- 2015
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In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707 .- 1879-1670. ; 83, s. 28-36
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Journal article (peer-reviewed)abstract
- In classical isostatic models, a uniform crustal density is typically assumed, while disregarding the crustal density heterogeneities. This assumption, however, yields large errors in the Moho geometry determined from gravity data, because the actual topography is not fully isostatically compensated. Moreover, the sub-crustal density structures and additional geodynamic processes contribute to the overall isostatic balance. In this study we investigate the effects of unmodelled density structures and geodynamic processes on the gravity anomaly and the Moho geometry. For this purpose, we define the residual isostatic topography as the difference between actual topography and isostatic topography, which is computed based on utilizing the Vening Meinesz-Moritz isostatic theory. We show that the isostatic gravity bias due to disagreement between the actual and isostatically compensated topography varies between -382 and 596 mGal. This gravity bias corresponds to the Moho correction term of -16 to 25 km. Numerical results reveal that the application of this Moho correction to the gravimetrically determined Moho depths significantly improves the RMS fit of our result with some published global seismic and gravimetric Moho models. We also demonstrate that the isostatic equilibrium at long-to-medium wavelengths (up to degree of about 40) is mainly controlled by a variable Moho depth, while the topographic mass balance at a higher-frequency spectrum is mainly attained by a variable crustal density.
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| 9. |
- Eshagh, Mehdi
(author)
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An integral approach to regional gravity field refinement using earth gravity models
- 2013
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In: Journal of Geodynamics. - : Pergamon Press. - 0264-3707 .- 1879-1670. ; 68, s. 18-28
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Journal article (peer-reviewed)abstract
- The idea of this paper is to refine the terrestrial gravimetric data with the Earth's gravity models (EGMs) and produce a high quality source of gravity data. For this purpose, biased and unbiased integral estimators are presented. These estimators are used to refine gravimetric data over Fennoscandia with the ITG-GRACE2010s and GO_CONS_GCF_2_DIR_R2 EGMs, which are the recent products of the gravity field and climate experiment (GRACE) and the gravity field and steady-state ocean circulation explorer (GOCE) satellite missions. Numerical results show that the biased integral estimator has smaller global root mean square error (RMSE) than the unbiased one. Also a simple strategy is presented to down-weight the low-frequencies the terrestrial data in spectral combination. The gravity anomalies, computed by EGM08, are compared to the refined anomalies for evaluation purpose. In the case of using a cap size of 1 degrees for integration the EGM08 gravity anomalies are more correlated with the refined ones. Also the band-limited kernels can simply be generated to maximum degree of the used EGMs for both estimators. Comparisons of the combined anomalies and those of EGM08 show insignificant differences between the biased and unbiased estimators in practice. However, the biased estimator seems to be proper one for gravity data refinement due to its smaller global RMSE.
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| 10. |
- Eshagh, Mehdi, et al.
(author)
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Atmospheric effects on satellite gravity gradiometry data
- 2009
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In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707 .- 1879-1670. ; 47:1, s. 9-19
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Journal article (peer-reviewed)abstract
- Atmospheric masses play an important role in precise downward continuation and validation of satellite gravity gradiometry data. In this paper we present two alternative ways to formulate the atmospheric potential. Two density models for the atmosphere are proposed and used to formulate the external and internal atmospheric potentials in spherical harmonics. Based on the derived harmonic coefficients, the direct atmospheric effects oil the satellite gravity gradiometry data are investigated and presented ill the orbital frame over Fennoscandia. The formulas of the indirect atmospheric effects oil gravity anomaly and geoid (downward continued quantities) are also derived using the proposed density models. The numerical results show that the atmospheric effect can only be significant for precise validation or inversion of the GOCE gradiometric data at the mE level.
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| 11. |
- Eshagh, Mehdi, et al.
(author)
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Determination of gravity anomaly at sea level from inversion of satellite gravity gradiometric data
- 2011
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In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707 .- 1879-1670. ; 51:5, s. 366-377
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Journal article (peer-reviewed)abstract
- Gravity gradients can be used to determine the local gravity field of the Earth. This paper investigates downward continuation of all elements of the disturbing gravitational tensor at satellite level using the second-order partial derivatives of the extended Stokes formula in the local-north oriented frame to determine the gravity anomaly at sea level. It considers the inversion of each gradient separately as well as their joint inversion. Numerical studies show that the gradients T-zz, T-xx, T-yy and T-xz have similar capability of being continued downward to sea level in the presence of white noise, while the gradient T-yz is considerably worse than the others. The bias-corrected joint inversion process shows the possibility of recovering the gravity anomaly with 1 mGal accuracy. Variance component estimation is also tested to update the observation weights in the joint inversion.
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| 12. |
- Eshagh, Mehdi
(author)
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Least-squares modification of extended Stokes' formula and its second-order radial derivative for validation of satellite gravity gradiometry data
- 2010
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In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707 .- 1879-1670. ; 49:2, s. 92-104
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Journal article (peer-reviewed)abstract
- The gravity anomalies at sea level can be used to validate the satellite gravity gradiometry data. Validation of such a data is important prior to downward continuation because of amplification of the data errors through this process. In this paper the second-order radial derivative of the extended Stokes' formula is employed and the emphasis is on least-squares modification of this formula to generate the second-order radial gradient at satellite level. Two methods in this respect are proposed: (a) modifying the second-order radial derivative of extended Stokes' formula directly, and (b) modifying extended Stokes' formula prior to taking the second-order radial derivative. Numerical studies show that the former method works well but the latter is very sensitive to the proper choice of the cap size of integration and degree of modification.
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| 13. |
- Kiamehr, Ramin, et al.
(author)
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Impact of the precise geoid model in studying tectonic structures- a case study in Iran
- 2006
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In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707 .- 1879-1670. ; 42:1-3, s. 1-11
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Journal article (peer-reviewed)abstract
- Iran is one of the most active regions in the world with respect,to earthquakes and tectonic motions in the lithosphere. In order to study the impact of the geoid model in detecting plate tectonic boundaries and in the establishment of an accurate height datum for future geodynamic observations, a new combined gravimetric geoid model for Iran (IRG04C) was computed by the method of least squares modification of Stokes formula based on the most recent gravity anomaly database, SRTM high resolution Digital Elevation Model (DEM), GRACE GGM02 Global Geopotential Model and GPS/levelling data. The RMS fit of the new geoid model versus GPS/levelling data is 9 cm, which is a 10 times better fit compared to the most recent published gravimetric geoid model in the area. An integrated approach, combining gravity, geoid and seismology data as well as a digital elevation model, was used to find possible correlations between these parameters. Our investigation indicates that all earthquakes with magnitude over 6.0 in the Richter scale are located in areas with a geoid slope exceeding 7.5%. The study shows a significant correlation between the horizontal gradient of the geoid and plate tectonic activities.
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| 14. |
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| 15. |
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| 16. |
- Ma, Delong, et al.
(author)
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The role of deep-seated half-grabens in the evolution of Huoerguosi-Manasi-Tugulu fold-and-thrust belt, northern Tian Shan, China
- 2019
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In: Journal of Geodynamics. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0264-3707 .- 1879-1670. ; 131
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Journal article (peer-reviewed)abstract
- The Huoerguosi-Manasi-Tugulu fold-and-thrust belt, which is located in the southern Junggar Basin, has formed in response to contraction during Late Cenozoic. However, the tectonic environment for its formation before Late Cenozoic is still controversial. In this paper, we use surface data, recently collected and processed subsurface seismic refection data, isopach map of Lower Jurassic and balanced sections to propose pre-existing half-graben system developed in the Lower Jurassic with this fold-and-thrust belt. We also use results of a series of scaled sandbox analogue models, where industrial CT apparatus was used to monitor deformation, to simulate the evolution of this fold-and-thrust belt. We suggest that the segmented shape of the Huoerguosi-Manasi-Tugulu fold-and-thrust belt is a response to the presence of thrust ramps, which were formed during Early Jurassic. During Late Cenozoic shortening, the Lower Jurassic syn-rift sediments served as major detachment horizon, making a pre-existing normal fault act as a stress concentration zone leading to steeping of a thrust-ramp over the normal fault and cover detachment overstep the underlying half-grabens. Modeling results reveal that the presented structural framework has close resemblance with paleostructures especially in the intracontinental environment, which underwent a complex multicycle evolution process, and provide a new prospective for the interpretation of natural examples.
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| 17. |
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| 18. |
- Shephard, G. E., et al.
(author)
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A North Pole thermal anomaly? Evidence from new and existing heat flow measurements from the central Arctic Ocean
- 2018
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In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707 .- 1879-1670. ; 118, s. 166-181
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Journal article (peer-reviewed)abstract
- Constraining the thermal evolution of the Arctic Ocean is hampered by notably sparse heat flow measurements and a complex tectonic history. Previous results from the Lomonosov Ridge in the vicinity of the North Pole, and the adjacent central Amundsen Basin reveal varied values, including those higher than expected considering plate cooling or simple uniform stretching models. Furthermore, in the vicinity of the North Pole an anomalously slow velocity perturbation exists in upper mantle seismic tomography models. However, whether these observations are related to a thermal anomaly in the mantle remains unknown. We present new heat flow results gathered from 17 sediment cores acquired during the Arctic Ocean 2016 and SWERUS-C3 expeditions on the Swedish icebreaker Oden. Three sites located on oceanic lithosphere in the Amundsen Basin between 7 degrees W-71E degrees reveal surface thermal conductivity of 1.07-1.26 W/mK and heat flow in the order of 71-95 mW/m(2), in line-with or slightly higher (1-21 mW/m(2)) than expected from oceanic heat flow curves. These results contrast with published results from further east in the Amundsen Basin, which indicated surface heat flow values up to 2 times higher than predicted from oceanic crustal cooling models. Heat flow of 49-61 mW/m(2) was recovered from the Amerasia Basin. Sites from the submerged continental fragments of the Lomonosov Ridge and Marvin Spur recovered heat flow in the order of 53-76 and 51-69 mW/m(2) respectively. When considering the additional potential surface heat flux from radiogenic heat production in the crust, these variable measurements are broadly in line with predictions from uniform extension models for continental crust. A seismically imaged upper mantle velocity anomaly in the central Arctic Ocean may arise from a combination of compositional and thermal variations but requires additional investigation. Disentangling surface heat flow contributions from crustal, lithospheric and mantle processes, including variable along-ridge rifting rates and timing, density and phase changes, conductive and advective dynamics, and regional tectonics, requires further analysis.
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| 19. |
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| 20. |
- Bock, Dagmar, et al.
(author)
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Atmospheric pressure loading displacement of SLR stations
- 2005
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In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707. ; 39:3, s. 247-266
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Journal article (peer-reviewed)abstract
- This paper addresses the local displacement at ground stations of the world-wide Satellite Laser Ranging (SLR) network induced by atmospheric pressure variations. Since currently available modelling options do not satisfy the requirements for the target application (real-time availability, complete coverage of SLR network), a new representation is developed. In a first step, the 3-dimensional displacements are computed from a 6-hourly grid of 1°×1° global pressure data obtained from the ECMWF, for the period 19972002. After having been converted into pressure anomalies, this pressure grid is propagated into horizontal and vertical station displacements using Greens functions and integrating contributions covering the entire globe; oceans are assumed to follow the inverted barometer (IB) approximation. In the next step, a linear regression model is developed for each station that approximates the time-series of the predicted vertical displacements as well as possible; this regression model relates the vertical displacement of a particular station to the local (and instantaneous) pressure anomaly. It is shown that such a simple model may represent the actual vertical displacements with an accuracy of better than 1 mm; horizontal displacements are shown to be negligible. Finally, the regression model is tested on actual SLR data on the satellites LAGEOS-1 and LAGEOS-2, covering the period January 2002 until April 2003 (inclusive). Also, two model elements are shown to be potential risk factors: the global pressure field representation (for the convolution method) and the local reference pressure (for the regression method). The inclusion of the atmospheric pressure displacement model gives improvements on most of the elements of the computations, although the effects are smaller than expected since the nominal effect is absorbed by solved-for satellite parameters.
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| 21. |
- Corti, G, et al.
(author)
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Centrifuge models simulating magma emplacement during oblique rifting
- 2001
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In: JOURNAL OF GEODYNAMICS. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0264-3707. ; 31:5, s. 557-576
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Journal article (peer-reviewed)abstract
- A series of centrifuge analogue experiments have been performed to model the mechanics of continental oblique extension (in the range of 0 degrees to 60 degrees) in the presence of underplated magma at the base of the continental crust. The experiments re
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| 22. |
- Kareinen, Niko Petteri, 1986, et al.
(author)
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Automated ambiguity estimation for VLBI Intensive sessions using L1-norm
- 2016
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In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707. ; 102, s. 39-46
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Journal article (peer-reviewed)abstract
- Very Long Baseline Interferometry (VLBI) is a space-geodetic technique that is uniquely capable of direct observation of the angle of the Earth's rotation about the Celestial Intermediate Pole (CIP) axis, namely UT1. The daily estimates of the difference between UT1 and Coordinated Universal Time (UTC) provided by the 1-h long VLBI Intensive sessions are essential in providing timely UT1 estimates for satellite navigation systems and orbit determination. In order to produce timely UT1 estimates, efforts have been made to completely automate the analysis of VLBI Intensive sessions. This involves the automatic processing of X- and S-band group delays. These data contain an unknown number of integer ambiguities in the observed group delays. They are introduced as a side-effect of the bandwidth synthesis technique, which is used to combine correlator results from the narrow channels that span the individual bands. In an automated analysis with the c5++ software the standard approach in resolving the ambiguities is to perform a simplified parameter estimation using a least-squares adjustment (L2-norm minimisation). We implement L1-norm as an alternative estimation method in c5++. The implemented method is used to automatically estimate the ambiguities in VLBI Intensive sessions on the Kokee–Wettzell baseline. The results are compared to an analysis set-up where the ambiguity estimation is computed using the L2-norm. For both methods three different weighting strategies for the ambiguity estimation are assessed. The results show that the L1-norm is better at automatically resolving the ambiguities than the L2-norm. The use of the L1-norm leads to a significantly higher number of good quality UT1-UTC estimates with each of the three weighting strategies. The increase in the number of sessions is approximately 5% for each weighting strategy. This is accompanied by smaller post-fit residuals in the final UT1-UTC estimation step.
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| 23. |
- Kiamehr, Ramin, et al.
(author)
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Analysis of Surface Deformation Patterns Analysis Using 3D Finite Elements Method: A case study in Skåne area, Sweden
- 2005
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In: J. Geodynamics. - : Elsevier BV. - 0264-3707. ; 39:4, s. 403-412
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Journal article (peer-reviewed)abstract
- The application of geodetic techniques to study crustal deformations associated with the geodynamic activity of a region is considered as a fundamental tool in seismic risk mitigation and in earthquake prediction research. In principle, the crustal deformation analysis is a purely geodetic problem as it concerns alteration of the Earth shape, so that deformations of the crust are directly connected with geodetic observables. The Tornquist zone across Skane in southern Sweden is a classical fault zone that separates the Precambrium gneisses of the Baltic shield in the north from Phanerozoic Europe to the south. In this region, a Global Positioning Network (GPS) was established to study possible crustal motions. The aim of this article is to improve on previous study in to estimate the possible crustal strains and dilation parameters by a finite element analysis. Results show that the areas with maximum shear strain and dilation are located exactly in the active fault zones and their intersections. However, further observations in a dense network as well as integration with geological and geophysical data are needed to fully explore the recent crustal motions over the Tornquist zone.
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| 24. |
- Kierulf, Halfdan Pascal, et al.
(author)
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A GNSS velocity field for geophysical applications in Fennoscandia
- 2021
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In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707. ; 146
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Journal article (peer-reviewed)abstract
- In Fennoscandia, tectonics, Glacial Isostatic Adjustment (GIA), and climatic changes cause ongoing crustal deformation of some millimetres per year, both vertically and horizontally. These displacements of the Earth can be measured to a high degree of precision using a Global Navigation Satellite System (GNSS). Since about three decades, this is the major goal of the Baseline Inferences for Fennoscandian Rebound, Sea-level, and Tectonics (BIFROST) project. We present a new velocity field for an extended BIFROST GNSS network in the ITRF2008 reference frame making use of the GNSS processing package GPS Analysis Software of MIT (GAMIT). Compared to earlier publications, we have almost doubled the number of stations in our analysis and increased the observation time span, thereby avoiding the early years of the network with many instrument changes. We also provide modelled vertical deformation rates from contributing processes, i.e. elastic deformation due to global atmospheric and non-tidal ocean loading, ice mass and hydrological changes as well as GIA. These values for the vertical component can be used for removal of these contributions so that the residual uplift signal can be further analysed, e.g., in the context of local or regional deformation processes or large-scale but low-magnitude geodynamics. The velocity field has an uplift maximum of 10.3 mm/yr in northern Sweden west of the Gulf of Bothnia and subsidence exceeding 1 mm/yr in northern Central Europe. The horizontal velocity field is dominated by plate motion of more than 20.0 mm/yr from south-west to north-east. The elastic uplift signal sums up to 0.7–0.8 mm/yr for most stations in Northern Europe. Hence, the maximum uplift related to the past glaciation is ca. 9.6 mm/yr. The residual uplift signal after removal of the elastic and GIA contribution may point to possible improvements of the GIA model, but may also indicate regional tectonic and erosional processes as well as local deformation effects. We show an example of such residual signal discussing potential areas of interest for further studies.
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| 25. |
- Lidberg, Martin, 1964, et al.
(author)
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Recent results based on continuous GPS observations of the GIA process in Fennoscandia from BIFROST
- 2010
-
In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707. ; 50:1, s. 8-18
-
Journal article (peer-reviewed)abstract
- We present the latest 3D velocity field of the Fennoscandian glacial isostatic adjustment (GIA) processfrom BIFROST. It is derived from more than 4800 days (13 years) of data at more than 80 permanentGPS sites. We use the GAMIT/GLOBK and the GIPSY/OASIS II software packages for GPS analysis andcompare the results. The solution has an internal accuracy at the level of 0.2 mm/year (1 sigma) forhorizontal velocities at the best sites.Wealso present a revised GIA prediction model. At the best sites, theoptimal model agrees with the observations to within 0.4 mm/year. However, the model systematicallyoverpredicts the magnitude of horizontal rates in the north. We discuss limitations in computed andpresented GNSS station velocities, where especially possible instability over time causing non-linearpattern in vertical time series are considered. In extension, preliminary results from an investigationapplying revised analysis strategies on a sparse subset of the database are presented, indicating possibleimprovements for the future.
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| 26. |
- Löfgren, Johan, 1983, et al.
(author)
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Sea level time series and ocean tide analysis from multipath signals at five GPS sites in different parts of the world
- 2014
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In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707. ; 80, s. 66-80
-
Journal article (peer-reviewed)abstract
- We present sea level observations derived from the analysis of signal-to-noise ratio (SNR) data recorded by five coastal GPS stations. These stations are located in different regions around the world, both in the northern and in the southern hemisphere, in different multipath environments, from rural coastal areas to busy harbors, and experience different tidal ranges.The recorded SNR data show periodic variations that originate from multipath, i.e. the interference of direct and reflected signals. The general assumption is that for satellite arcs facing the open sea, the rapid SNR variations are due to reflections off the sea surface. The SNR data recorded from these azimuth intervals were analyzed by spectral analysis with two methods: a standard analysis method assuming a static sea level during a satellite arc and an extended analysis method assuming a time dependent sea level during a satellite arc.The GPS-derived sea level results are compared to sea level records from co-located traditional tide gauges, both in the time and in the frequency domain. The sea level time series are highly correlated with correlation coefficients to the order of 0.89–0.99. The root-mean-square (RMS) difference is 6.2 cm for the station with the lowest tidal range of 165 cm and 43 cm for the station with the highest tidal range of 772 cm. The relative accuracy, defined as the ratio of RMS and tidal range, is between 2.4% and 10.0% for all stations.Comparing the standard analysis method and the extended analysis method, the results based on theextended analysis method agree better with the independent tide gauge records for the stations with a high tidal range. For the station with the highest tidal range (772 cm), the RMS is reduced by 47% when using the extended analysis method. Furthermore, the results also indicate that the standard analysis method, assuming a static sea level, can be used for stations with a tidal range of up to about 270 cm, without performing significantly worse than the extended analysis method.Tidal amplitudes and phases are derived by harmonic analysis of the sea level records. Again, a high level of agreement is observed between the tide gauge and the GPS-derived results. Comparing the GPS-derived results, the results based on the extended analysis method show a higher degree of agreement with the traditional tide gauge results for stations with larger tidal ranges. Spectral analysis of the residuals after the harmonic analysis reveals remaining signal power at multiples of the draconitic day. This indicates that the observed SNR data are to some level disturbed by additional multipath signals, in particular for GPS stations that are located in harbors.
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| 27. |
- Mouyen, Maxime, 1982, et al.
(author)
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Expected temporal absolute gravity change across the Taiwanese Orogen, a modeling approach
- 2009
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In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707. ; 48:3-5
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Journal article (peer-reviewed)abstract
- The island of Taiwan is located on the convergent boundary between the Philippine Sea plate and the Chinese continental margin. It offers very active mountain building and collapsing processes well illustrated by the rugged topography, rapid uplift and denudation, young tectonic landforms, active faulting and numerous earthquakes. In this paper, using simple models, we have estimated vertical movements and associated absolute gravity variations which can be expected along a profile crossing the southern part of the island and probably suffering the highest rates of rising. The two different tectonic styles proposed for the island, thin-skinned and thick-skinned, were taken into account. Horizontal and vertical movements were modeled by an elastic deformation code. Gravity variations due to these deformations are then modeled at a second step. They are dominated by plate and free-air effects, i.e. elevation of the topography, with several μGal yr-1. By comparison, gravity changes generated by mass transfers are weak: maximum 0.1 μGal yr-1 with the thin-skinned tectonic and 0.3 μGal yr-1 with the thick-skinned tectonic. Though elastic rheology has limitations, this modeling offers interesting results on what gravity signal can be expected from the AGTO project (Absolute Gravity in the Taiwanese Orogen), which proposes to study the dynamic of these mountain ranges using absolute gravimetry (AG) and also including relative gravimetry (RG) and GPS measurements.
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| 28. |
- Nordman, M., et al.
(author)
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Crustal loading in vertical GPS time series in Fennoscandia
- 2009
-
In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707. ; 48:3-5, s. 144-150
-
Journal article (peer-reviewed)abstract
- We compare time series of vertical position from GPS with modelled vertical deformation caused by variation in continental water storage, variation in the level of the Baltic Sea, and variation in atmospheric pressure. Monthly time series are used. The effect of continental water storage was calculated from three different global models. The effect of non-tidal variation in Baltic Sea level was calculated using tide gauge observations along the coasts. Atmospheric loading was computed from a numerical weather model. The loading time series are then compared with three different GPS time series at seven stations in Fennoscandia. A more detailed analysis is computed at three coastal stations. When the monthly GPS time series are corrected using the load models, their root-mean-square scatter shows an improvement between 40 and 0%, depending on the site and on the GPS solution. The modelled load effect shows a markedly seasonal pattern of 15 mm peak-to-peak, of which the uncorrected GPS time series reproduce between 60 and 0%.
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| 29. |
- Olsson, Per-Anders, 1971, et al.
(author)
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Effects on gravity from non-tidal sea level variations in the Baltic Sea
- 2009
-
In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707. ; 48:3-5, s. 151-156
-
Journal article (peer-reviewed)abstract
- The main purpose of this paper is to investigate numerically the effects of non-tidal sea level variations in the Baltic Sea on gravity with special emphasis on the Swedish stations in the Nordic Absolute Gravity Project.To calculate the ocean loading effect on gravity the method described by Farrell (1972) is widely used. This method is based on convolution of a Green's function for gravity with the ocean load, but does not include the direct attraction depending on the height of the observation point. It is described how this effect is included in the Green's functions and how numerical integration is performed over a dense grid bounded by a very high resolution coastline. The importance of this high resolution is shown. The major part of the direct attraction for stations close to the coast comes from relatively small water masses close to the station. The total effect from the Baltic Sea, crustal loading and direct attraction, is calculated for 12 Swedish and one Finnish absolute gravity stations. The distance from the coast for these stations varies from 10 m to 150 km. It is shown that the total non-tidal gravity effect is significant, easily reaching values of 2–3 μgal for stations with high elevation close to the coast.In modelling the Glacial Isostatic Adjustment (GIA), the relation between the change of gravity and the absolute land uplift ((dg/dt) / (dh/dt)) contains information about the viscoelastic properties of the upper mantle. The Baltic Sea is located in the Fennoscandian postglacial land uplift area and experiences therefore a long-term sea level decrease. It is also shown that the magnitude of this long-term effect is not negligible for determination of the unknown part of (dg/dt) / (dh/dt).
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| 30. |
- Olsson, Per-Anders, 1971, et al.
(author)
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Modelling of the GIA-induced surface gravity change over Fennoscandia
- 2012
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In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707. ; 61, s. 12-22
-
Journal article (peer-reviewed)abstract
- This paper deals with the modelling of surface gravity change in Fennoscandia, induced by postglacial rebound or Glacial Isostatic Adjustment (GIA). The theoretical foundation is based on the theory introduced by [Peltier, 1974] and [Peltier, 1976] for a spherical, non-rotating, laterally homogenous, viscoelastic, Maxwell Earth and the solution of the Sea Level Equation, originally introduced by Farrel and Clark (1976), with time-dependent coastline geometry. The ice history is defined by the ice model ICE-5G. Rotational feedback is not included.The sensitivity of predictions of present day gravity rates View the MathML source, with respect to a selection of assumptions and approximations, is investigated numerically. Six model variants are defined: (i) linear relation between View the MathML source and the vertical deformation rate View the MathML source, (ii) direct attraction expressed in terms of internal and (iii) external harmonic series expansions as well as by (iv) analytical integration over rectangular prisms. For the most rigorous treatment of the direct attraction, the effect of simplified modelling of the sea level is also investigated. These modelling approximations of the sea level change include (v) fixed shorelines and (vi) eustatic sea level change. Predictions of View the MathML source for the model variants are plotted, compared and discussed.The most rigorous model (iv) and the linear model (i) differ less than 0.03 μGal yr−1 over land and close to, or over, the ocean the difference reaches maximally ∼0.5 μGal yr−1. Due to truncation at 180°, the high frequent nature of the direct attraction is not properly described by models (ii) and (iii). The two simplified sea level modelling approximations (v) and (vi) induce differences, compared to the most rigorous model exceeding 0.2 μGal yr−1 over land, i.e. about one order of magnitude worse than the linear model.
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| 31. |
- Olsson, Per-Anders, 1971, et al.
(author)
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The relation between gravity rate of change and vertical displacement in previously glaciated areas
- 2015
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In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707. ; 83, s. 76-84
-
Journal article (peer-reviewed)abstract
- The rate of change of surface gravity, dg/dt, and vertical deformation rate of the solid surface, du/dt, are two observables of glacial isostatic adjustment (GIA). They contribute with different information on the same phenomenon. Their relation contains information of the underlying physics and a trustworthy relation allows to combine these observations to strengthen the overall observational accuracy of the phenomenon. In this paper we investigate the predicted relation between dg/dt and du/dt in previously glaciated areas. We use the normal mode approach for one dimensional earth models and solutions of the sea level equation with time-dependent coastline geometry. Numerical predictions of dg/dt and du/dt are computed for Laurentia, Fennoscandia and the British Isles respectively, using six different earth models. Within each region a linear trend is then fitted using the relation dg/dt = C du/dt + dg_0/dt. The estimated C and dg_0/dt differ more between the regions than between different earth models within each region. For Fennoscandia C ≈ −0.163 μGal/mm and for Laurentia C ≈ −0.152 μGal/mm. Maximum residuals between the linear trend and spatially varying model predictions of dg/dt are 0.04 μGal/yr in Fennoscandia and 0.17 μGal/yr in Laurentia. For the British Isles the results are harder to interpret, mainly since this region is located on the zero uplift isoline of Fennoscandia. In addition, we show temporal variation of the relation since the last glacial maximum till present-day. The temporal and spatial variation of the relation between dg/dt and du/dt can be explained by (i) the elastic respectively viscous proportion of the total signal and (ii) the spectral composition of the regional signal. Additional local effects, such as the Newtonian attraction and elastic deformation from local sea level changes, are examined in a case study for six stations in the Nordic absolute gravity network. The influence of these local effects on the relation between View the dg/dt and du/dt is negligible except for extreme locations close to the sea.
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| 32. |
- Olsson, R
(author)
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An estimation of the maximum b-value in the Gutenberg-Richter relation
- 1999
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In: JOURNAL OF GEODYNAMICS. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0264-3707. ; 27:4-5, s. 547-552
-
Journal article (other academic/artistic)abstract
- An estimation of the maximum b-value in the Gutenberg-Richter relation is achieved from empirical laws of earthquake occurrence and from dimensional considerations of power law distributions of earthquake size. The deduced maximum b-value, 1.5 (or with er
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| 33. |
- Pan, M, et al.
(author)
-
Crustal movements in Skane, Sweden, between 1992 and 1998 as observed by GPS
- 2001
-
In: JOURNAL OF GEODYNAMICS. - 0264-3707. ; 31:3, s. 311-322
-
Journal article (peer-reviewed)abstract
- We estimate a network of crustal deformations in Skane, southern Sweden, using observations of the Global Positioning System (GPS) from 1992, 1996 and 1998. The network straddles the Tornquist zone, potentially one of the most active fault zones in Sweden
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| 34. |
- Rajner, Marcin, 1982
(author)
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Detection of ice mass variation using GNSS measurements at Svalbard
- 2018
-
In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707. ; 121, s. 20-25
-
Journal article (peer-reviewed)abstract
- We compare observed uplift rates of Global Navigation Satellite Systems (GNSS) with geophysical predictions at sites located on Spitsbergen. We show here that using modern postglacial rebound models, realistic mass balance for Present Day Ice Melting (PDIM) and taking the deglaciation of the Little Ice Age (LIA) into account, we can close the overall uplift budget at a level of a few millimetres per year. For this study we used GNSS data from well-established geodetic sites in Ny-Ålesund and the new data from Hornsund. The significant increase of uplift rates since the beginning of 2011 was observed at both gnss sites. These height changes are attributed to recent increased ice mass loss of 6.0 Gt yr−1 for Svalbard. The total mass loss is estimated at 14.7 Gt yr−1. Ice unloading caused also a noticeable increase of distance between Ny-Ålesund and Hornsund. We show that the specific location of the two sites facilitates inference of differential mass loss within the Svalbard Archipelago.
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| 35. |
- Scherneck, Hans-Georg, 1951, et al.
(author)
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Fennoscandian strain rates from BIFROST GPS: A gravitating, thick-plate approach
- 2010
-
In: Journal of Geodynamics. - : Elsevier BV. - 0264-3707. ; 50:1, s. 19-26
-
Journal article (peer-reviewed)abstract
- The aim of this investigation is to develop a method for the analysis of crustal strain determined bystation networks that continuously measurements of Global Navigation Satellite Systems (GNSS). Themajor new ingredient is that we require a simultaneous minimum of the observation error and theelastic and potential energy implied by the deformation.The observations that we analyse come from eight years worth of daily solutions from continuousBIFROST GPS measurements in the permanent networks of the Nordic countries and their neighbours.Reducing the observations with best fitting predictions for the effects of glacial isostatic adjustment (GIA)we find strain rates of maximum 5 nano/yr in the interior of the rebound area predominantly as arealstrain. The largest strain rates are found in the Finnmarken area, where however the GNSS networkdensity is much lower than in the central and southern parts.The thick-plate adjustment furnishes a simultaneous treatment of 3-D displacements and the ensuingelastic and potential energy due to the deformation. We find that the strain generated by flexure due toGIA is important. The extensional regime seen at the surface turns over into a compressive style alreadyat moderated depth, some 50 km.
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| 36. |
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| 37. |
- Doane, Marie, et al.
(author)
-
Exposure of Farm Laborers and Dairy Cattle to Formaldehyde from Footbath Use at a Dairy Farm in New York State
- 2014
-
In: Science of the Total Environment. - : Elsevier. - 0048-9697 .- 1879-1026. ; 487, s. 65-71
-
Journal article (peer-reviewed)abstract
- Formalin footbaths are commonly used in the dairy industry to prevent cattle hoof diseases. Although formalin is a well-documented disinfectant, it is also a carcinogen and irritant. The aim of this study was to estimate the exposure of farm workers and dairy cattle to formaldehyde from footbaths located in a milking facility and a heifer facility at a dairy farm in western New York, USA. The dairy farm included approximately 3900 dairy cattle including young stock; of these, 1670 cows were milked three times per day in a 60-stall carousel milking parlor, and approximately 800 heifers were located at the heifer facility where footbaths with formalin were in use. The formaldehyde concentration of the air was measured using a Formaldemeter™ htV approximately 50 cm above the 3% formalin footbaths in the milking (one footbath location) and heifer (three footbath locations) facilities on three consecutive days. The measured formaldehyde concentrations varied between 0.00 and 2.28 ppm, falling within the safety guidelines established by the Occupation Safety and Health Administration (OSHA) of the United States. Significant differences were found in the formaldehyde concentrations at the different footbath locations in the heifer facility, potentially due to the varying levels of ventilation at each location. Changes in the ambient temperature during the 3-day sampling period did not significantly affect the concentrations. We believe that the substantial ventilation at both the heifer and milking facilities ensured that the formaldehyde concentrations did not exceed OSHA guidelines, thus permitting the safe use of formalin footbaths in this farm.
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