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The European VLBI Network activity in geodesy: crustal deformation in Europe
New Astronomy Reviews 43 (1999) 603–607 www.elsevier.nl / locate / newar
The European VLBI Network activity in geodesy: crustal deformation in Europe P. Tomasi a ,b , M.J. Rioja a , P. Sarti c a
Istituto di Radioastronomia del C.N.R., Via Gobetti N.101, 40129 Bologna, Italy b Istituto di Tecnologia Informatica Spaziale del C.N.R., Matera, Italy c DISTART, Topografia e Geodesia, Universita´ di Bologna, V.le Risorgimento N.2, 40126 Bologna, Italy
Abstract The European VLBI Network has been performing a regular series of geodetic observations (EUROPE) whose main aim is to detect and monitor deformations in the European part of the Eurasian plate with high accuracy. The interaction between the African and Eurasian plates drives the tectonic motions detected in the Southern part of Europe that, with the spreading of the Middle Atlantic ridge and other local geophysical (e.g., post-glacial rebound) and geological phenomena, determine the overall crustal deformation of the region. We present the latest results obtained from the analysis of the EUROPE campaigns of observations; for certain stations, we have integrated other data acquired during different campaigns that involved at least three antennae of the network. In particular, for Ny Aalesund, we had to increase further the amount of data by using 144 experiments providing a more continuous and redundant data span to ensure a stronger statistical solution and a network with better spatial distribution. We present the velocities estimated for the antennas in the European geodetic network, along with an interpretation within their local geophysical frame. 1999 Elsevier Science B.V. All rights reserved. PACS: 91.10.-v; 91.10.Kg; 93.30.Ge; 93.55.1z Keywords: Instrumentation: interferometer; Methods: data analysis
1. Introduction Since 1990 a series of purely European VLBI geodetic experiments (EUROPE) have been regularly performed and can now be used to monitor the Earth’s surface movements. EUROPE experiments involve almost half of the antennas (Fig. 1) that constitute the EVN (Simeiz, Effelsberg, MadridDSS65, Medicina, Noto, Yebes, Onsala, Wettzell), and they form a set of more then 40 experiments that can be analysed to obtain the position and velocity of each VLBI station. Combining their data with those of other selected experiments that included at least three European stations (and a number of non-European stations) improves the reliability of the results for those stations which were not scheduled in all
experiments. Furthermore, we have included all experiments with Ny Aalesund station. Our principal aims in creating a special schedule for European experiments were to determine co-ordinates and velocities of stations, and to harmonize technology and performance at different observatories to maximize the quality of geodetic results within Europe.
2. Data analysis A set of about 140 experiments has been analysed with CALC / SOLVE software package (Caprette et al., 1990) running on the HP 282 work station of I.T.I.S. in Matera in order to produce relative and total velocities of the antennae that form the network
1387-6473 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S1387-6473( 99 )00062-7
604
P. Tomasi et al. / New Astronomy Reviews 43 (1999) 603 – 607
Fig. 1. European VLBI geodetic stations.
(Fig. 1). GLOBL is the batch programme in CALC / SOLVE that allows, using a least square procedure, the computation of velocities using stations positions as global or arc parameters. In the first case, the data from all individual 24-h experiments are considered as a whole in producing the estimated velocities (Figs. 2 and 3, Table 1), while in the second case the positions are estimated independently for each experiment and the velocity is obtained interpolating the single results. Rates of change for baseline lengths obtained with this latter method are shown in Table 2. The atmospheric and short period clock variations have been estimated using a piecewiselinear function with segments of one hour. We have used Wettzell as the reference station for position and clock parameters, and we have tied its velocity
to the Nuvel-1A-NNR velocity model (Argus & Gordon, 1991).
3. Results The estimates for total and relative velocities are depicted in Fig. 2 and Fig. 3, respectively, and are summarised in Table 1. Estimated total velocities (Table 1) are consistent with those calculated from geological models on time scales of millions of years, and express mostly the spreading rate of the Middle-Atlantic Ridge. The results for the total velocity of Ny Aalesund station shows a remarkable difference from the prediction of Nuvel-1A model. It should be pointed out that Ny Aalesund came into
P. Tomasi et al. / New Astronomy Reviews 43 (1999) 603 – 607
605
Fig. 2. Total horizontal velocities.
operation only three years ago and that this time span is quite short in order to have a good determination of the station’s velocity. Moreover, the geometry of the European network including Ny Aalesund is very elongated in the north-south direction, and a large uncertainty can be expected for this station, particularly in the East component, even including all the available experiments. We also cannot exclude some contribution to this result from the Earth Orientation Parameters we used, which were computed at Goddard Space Flight Center also using Ny Aalesund. On the other hand, the model predictions for the velocity of stations located so far from the rotation axis of the tectonic plate, so close to plate boundaries (i.e., the Middle Atlantic Ridge), and in such remote areas could be quite unreliable and some differences could be expected. On the regional scale
of the network, we can compare the movements of the different sites with Wettzell, which is the antenna that should be most representative of the Eurasian plate motion. Relative horizontal velocity vectors (Fig. 3, Table 1) therefore express the regional deformation of the crust, and are strictly coupled with the interaction between the major and minor plates in the area (Mueller & Kahle, 1993). Topocentric vertical velocities (Table 1) express the movement of each station along the local zenith direction, with respect to that of Wettzell. For some stations, the movements obtained in the vertical direction are expected. Medicina shows a downward movement most probably related to the subsidence of the particular geological area where the antenna is located. Onsala, with a movement of about the same magnitude but in the opposite direction, shows the
606
P. Tomasi et al. / New Astronomy Reviews 43 (1999) 603 – 607
Fig. 3. Vertical topocentric and relative horizontal velocities.
expected crustal uplift related to post glacial rebound in that area. Ny Aalesund does not show the same trend, and more investigation into the thickness and distribution of the ice sheet that used to cover that area during the latest glacial era is required (Lambeck, 1998). Results for Simeiz station are not shown in Table 1 because data collected during the 3–4 years preceding 1998 are seriously influenced by
local oscillator instabilities (due to interruptions of electrical power supply and other problems), and they cannot be used for precise geodetic measurements. The results obtained for Noto are believed to be representative of the real tectonic motion pertinent to that area. Effelsberg shows a vertical rate with respect to Wettzell that still must be understood in the framework of the local area.
Table 1 Estimated velocities
Vertical topocentric vel. (mm / yr) Total hor. velocity (mm / yr) Relative hor. velocity (mm / yr) Azimuth of rel. hor. vec. (deg)
Medicina
Onsala
Madrid
Noto
Matera
Effelsberg
Ny Aalesund
2 2.960.4 27.560.1 2.660.1 53.362.5
2.660.3 22.160.1 1.060.1 247.464.5
2.860.3 25.260.1 2.560.1 123.362.2
2 1.760.4 29.860.1 5.460.1 16.560.9
2 0.560.4 31.760.1 6.660.1 36.661.0
3.860.7 24.060.2 1.560.2 13768
2 1.060.6 20.760.2 6.960.2 320.161.4
P. Tomasi et al. / New Astronomy Reviews 43 (1999) 603 – 607
607
Table 2 Rate of change of baseline lengths
Effelsberg Simeiz Madrid Matera Medicina Noto Onsala Ny Aalesund Wettzell
Madrid (mm / yr)
Matera (mm / yr)
Medicina (mm / yr)
Noto (mm / yr)
Onsala (mm / yr)
0.760.8 2 1.065.8 – – – – – – –
2 3.360.9 0.562.6 1.460.3 – – – – – –
2 3.060.6 1.662.2 2.260.2 2 1.660.2 – – – – –
2 4.962.2 2.063.3 2 2.360.4 0.660.2 2 3.160.3 – – – –
1.560.8 3.062.1 0.360.3 2 4.060.4 2 2.660.3 2 4.660.3 – – –
4. Conclusions Results summarised in Tables 1 and 2 generally confirm those obtained by earlier solutions, although some differences have been observed, and for lessfrequently observed stations results do not yet provide an ideal statistical base. The EUROPE experiments do not contain enough data for certain stations (e.g. Ny Aalesund) to estimate their movements reliably. Therefore, use of a longer and more dense data span from some EVN stations should help reach a better determination of the phenomena affecting the European area. Since the first EUROPE experiment of 1998, Simeiz has supplied high quality data
Ny Aalesund (mm / yr) 5.862.7 2.262.9 1.361.3 2 2.361.5 2 2.461.1 2 3.561.5 0.860.8 – –
Wettzell (mm / yr)
Yebes (mm / yr)
2 1.060.4 1.862.1 0.460.2 2 4.060.2 2 2.660.2 2 5.160.2 2 0.460.2 2 0.161.0 –
– – 0.062.9 2 2.562.5 0.363.1 2 9.863.5 2 6.762.9 2 11.567.5 2 3.863.0
and we believe that in the near future good results will be obtained for this station.
References Argus, D. & Gordon, R., 1991, Geophys. Res. Lett., 18, 2039. Caprette, D., Ma, C., & Ryan, J.W., 1990, Crustal dynamics project data analysis, NASA Tech. Mem. 100765, Goddard Space Flight Center, Greenbelt, Md. Lambeck, K., 1998, in: Proc. of International Symposium of IAG, Section II, October 1998, Munich, Germany, in press. Mueller, S. & Kahle, H.G., 1993, in: Smith, D.E., Turcotte, D.L., (Eds.), Contributions of Space Geodesy to Geodynamics: Crustal Dynamics, Geodynamics Series, Vol. 23, p. 249.
The European VLBI Network activity in geodesy: crustal deformation in Europe P. Tomasi a ,b , M.J. Rioja a , P. Sarti c a
Istituto di Radioastronomia del C.N.R., Via Gobetti N.101, 40129 Bologna, Italy b Istituto di Tecnologia Informatica Spaziale del C.N.R., Matera, Italy c DISTART, Topografia e Geodesia, Universita´ di Bologna, V.le Risorgimento N.2, 40126 Bologna, Italy
Abstract The European VLBI Network has been performing a regular series of geodetic observations (EUROPE) whose main aim is to detect and monitor deformations in the European part of the Eurasian plate with high accuracy. The interaction between the African and Eurasian plates drives the tectonic motions detected in the Southern part of Europe that, with the spreading of the Middle Atlantic ridge and other local geophysical (e.g., post-glacial rebound) and geological phenomena, determine the overall crustal deformation of the region. We present the latest results obtained from the analysis of the EUROPE campaigns of observations; for certain stations, we have integrated other data acquired during different campaigns that involved at least three antennae of the network. In particular, for Ny Aalesund, we had to increase further the amount of data by using 144 experiments providing a more continuous and redundant data span to ensure a stronger statistical solution and a network with better spatial distribution. We present the velocities estimated for the antennas in the European geodetic network, along with an interpretation within their local geophysical frame. 1999 Elsevier Science B.V. All rights reserved. PACS: 91.10.-v; 91.10.Kg; 93.30.Ge; 93.55.1z Keywords: Instrumentation: interferometer; Methods: data analysis
1. Introduction Since 1990 a series of purely European VLBI geodetic experiments (EUROPE) have been regularly performed and can now be used to monitor the Earth’s surface movements. EUROPE experiments involve almost half of the antennas (Fig. 1) that constitute the EVN (Simeiz, Effelsberg, MadridDSS65, Medicina, Noto, Yebes, Onsala, Wettzell), and they form a set of more then 40 experiments that can be analysed to obtain the position and velocity of each VLBI station. Combining their data with those of other selected experiments that included at least three European stations (and a number of non-European stations) improves the reliability of the results for those stations which were not scheduled in all
experiments. Furthermore, we have included all experiments with Ny Aalesund station. Our principal aims in creating a special schedule for European experiments were to determine co-ordinates and velocities of stations, and to harmonize technology and performance at different observatories to maximize the quality of geodetic results within Europe.
2. Data analysis A set of about 140 experiments has been analysed with CALC / SOLVE software package (Caprette et al., 1990) running on the HP 282 work station of I.T.I.S. in Matera in order to produce relative and total velocities of the antennae that form the network
1387-6473 / 99 / $ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S1387-6473( 99 )00062-7
604
P. Tomasi et al. / New Astronomy Reviews 43 (1999) 603 – 607
Fig. 1. European VLBI geodetic stations.
(Fig. 1). GLOBL is the batch programme in CALC / SOLVE that allows, using a least square procedure, the computation of velocities using stations positions as global or arc parameters. In the first case, the data from all individual 24-h experiments are considered as a whole in producing the estimated velocities (Figs. 2 and 3, Table 1), while in the second case the positions are estimated independently for each experiment and the velocity is obtained interpolating the single results. Rates of change for baseline lengths obtained with this latter method are shown in Table 2. The atmospheric and short period clock variations have been estimated using a piecewiselinear function with segments of one hour. We have used Wettzell as the reference station for position and clock parameters, and we have tied its velocity
to the Nuvel-1A-NNR velocity model (Argus & Gordon, 1991).
3. Results The estimates for total and relative velocities are depicted in Fig. 2 and Fig. 3, respectively, and are summarised in Table 1. Estimated total velocities (Table 1) are consistent with those calculated from geological models on time scales of millions of years, and express mostly the spreading rate of the Middle-Atlantic Ridge. The results for the total velocity of Ny Aalesund station shows a remarkable difference from the prediction of Nuvel-1A model. It should be pointed out that Ny Aalesund came into
P. Tomasi et al. / New Astronomy Reviews 43 (1999) 603 – 607
605
Fig. 2. Total horizontal velocities.
operation only three years ago and that this time span is quite short in order to have a good determination of the station’s velocity. Moreover, the geometry of the European network including Ny Aalesund is very elongated in the north-south direction, and a large uncertainty can be expected for this station, particularly in the East component, even including all the available experiments. We also cannot exclude some contribution to this result from the Earth Orientation Parameters we used, which were computed at Goddard Space Flight Center also using Ny Aalesund. On the other hand, the model predictions for the velocity of stations located so far from the rotation axis of the tectonic plate, so close to plate boundaries (i.e., the Middle Atlantic Ridge), and in such remote areas could be quite unreliable and some differences could be expected. On the regional scale
of the network, we can compare the movements of the different sites with Wettzell, which is the antenna that should be most representative of the Eurasian plate motion. Relative horizontal velocity vectors (Fig. 3, Table 1) therefore express the regional deformation of the crust, and are strictly coupled with the interaction between the major and minor plates in the area (Mueller & Kahle, 1993). Topocentric vertical velocities (Table 1) express the movement of each station along the local zenith direction, with respect to that of Wettzell. For some stations, the movements obtained in the vertical direction are expected. Medicina shows a downward movement most probably related to the subsidence of the particular geological area where the antenna is located. Onsala, with a movement of about the same magnitude but in the opposite direction, shows the
606
P. Tomasi et al. / New Astronomy Reviews 43 (1999) 603 – 607
Fig. 3. Vertical topocentric and relative horizontal velocities.
expected crustal uplift related to post glacial rebound in that area. Ny Aalesund does not show the same trend, and more investigation into the thickness and distribution of the ice sheet that used to cover that area during the latest glacial era is required (Lambeck, 1998). Results for Simeiz station are not shown in Table 1 because data collected during the 3–4 years preceding 1998 are seriously influenced by
local oscillator instabilities (due to interruptions of electrical power supply and other problems), and they cannot be used for precise geodetic measurements. The results obtained for Noto are believed to be representative of the real tectonic motion pertinent to that area. Effelsberg shows a vertical rate with respect to Wettzell that still must be understood in the framework of the local area.
Table 1 Estimated velocities
Vertical topocentric vel. (mm / yr) Total hor. velocity (mm / yr) Relative hor. velocity (mm / yr) Azimuth of rel. hor. vec. (deg)
Medicina
Onsala
Madrid
Noto
Matera
Effelsberg
Ny Aalesund
2 2.960.4 27.560.1 2.660.1 53.362.5
2.660.3 22.160.1 1.060.1 247.464.5
2.860.3 25.260.1 2.560.1 123.362.2
2 1.760.4 29.860.1 5.460.1 16.560.9
2 0.560.4 31.760.1 6.660.1 36.661.0
3.860.7 24.060.2 1.560.2 13768
2 1.060.6 20.760.2 6.960.2 320.161.4
P. Tomasi et al. / New Astronomy Reviews 43 (1999) 603 – 607
607
Table 2 Rate of change of baseline lengths
Effelsberg Simeiz Madrid Matera Medicina Noto Onsala Ny Aalesund Wettzell
Madrid (mm / yr)
Matera (mm / yr)
Medicina (mm / yr)
Noto (mm / yr)
Onsala (mm / yr)
0.760.8 2 1.065.8 – – – – – – –
2 3.360.9 0.562.6 1.460.3 – – – – – –
2 3.060.6 1.662.2 2.260.2 2 1.660.2 – – – – –
2 4.962.2 2.063.3 2 2.360.4 0.660.2 2 3.160.3 – – – –
1.560.8 3.062.1 0.360.3 2 4.060.4 2 2.660.3 2 4.660.3 – – –
4. Conclusions Results summarised in Tables 1 and 2 generally confirm those obtained by earlier solutions, although some differences have been observed, and for lessfrequently observed stations results do not yet provide an ideal statistical base. The EUROPE experiments do not contain enough data for certain stations (e.g. Ny Aalesund) to estimate their movements reliably. Therefore, use of a longer and more dense data span from some EVN stations should help reach a better determination of the phenomena affecting the European area. Since the first EUROPE experiment of 1998, Simeiz has supplied high quality data
Ny Aalesund (mm / yr) 5.862.7 2.262.9 1.361.3 2 2.361.5 2 2.461.1 2 3.561.5 0.860.8 – –
Wettzell (mm / yr)
Yebes (mm / yr)
2 1.060.4 1.862.1 0.460.2 2 4.060.2 2 2.660.2 2 5.160.2 2 0.460.2 2 0.161.0 –
– – 0.062.9 2 2.562.5 0.363.1 2 9.863.5 2 6.762.9 2 11.567.5 2 3.863.0
and we believe that in the near future good results will be obtained for this station.
References Argus, D. & Gordon, R., 1991, Geophys. Res. Lett., 18, 2039. Caprette, D., Ma, C., & Ryan, J.W., 1990, Crustal dynamics project data analysis, NASA Tech. Mem. 100765, Goddard Space Flight Center, Greenbelt, Md. Lambeck, K., 1998, in: Proc. of International Symposium of IAG, Section II, October 1998, Munich, Germany, in press. Mueller, S. & Kahle, H.G., 1993, in: Smith, D.E., Turcotte, D.L., (Eds.), Contributions of Space Geodesy to Geodynamics: Crustal Dynamics, Geodynamics Series, Vol. 23, p. 249.