gravity station was established in S. Miguel Island and the Faial station was re- ... Ribeiro, 1990, 1992) with the axial zone between Pico and S. Jorge islands, ...
Presented at the Workshop: High Precision Gravity Measurements with Application to Geodynamics, and Second GGP Workshop. March 24th to 26th, 1999, Munsbach Castle, Grand Duchy of Luxembourg
PRECISE GRAVITY MEASUREMENTS IN AZORES ISLANDS LÁZARO, C.(1), MÄKINEN, J.(2), OSÓRIO, J. (3), BASTOS, L.(3), BAPTISTA, P.(3), HEIN, G.(4)
ABSTRACT The Azores Archipelago is a region of well-known seismic and volcanic activity. It has been the object of scientific interest since it is located in the junction of three main tectonic plates: Eurasian, North American and African. Nine islands, two of them lying in the North American plate and the others at the African-Eurasian plate boundary, form the archipelago. The gravimetric surveys started in 1992 with the establishment of two absolute gravity stations, Flores and Faial, and relative measurements on all islands. These gravity measurements were intended to complement the GPS data obtained, since 1988, in the TANGO (TransAtlantic Network for Geodynamics and Oceanography) project. Therefore, the relative stations include all GPS sites and other stations of geophysical, seismological and geodetic interest. Later, in 1994, a new absolute gravity station was established in S. Miguel Island and the Faial station was reobserved. The three absolute stations were re-observed in 1997. The absolute measurements have been made using the JILAg-5 gravimeter, belonging to the Finnish Geodetic Institute (FGI). The relative gravity stations were measured by using LaCoste & Romberg gravimeters – 1992, 1994 and 1997. In the paper, it is intended to present details of the three gravity surveys, the methods used for data processing and the results derived so far. Possible directions for future work are also presented.
1.
INTRODUCTION
The TANGO (TransAtlantic Network for Geodynamics and Oceanography) project was established in 1988 with 12 GPS (Global Positioning System) stations located in four of the main tectonic plates: Eurasian, North American, African and Caribbean (Bastos et al., 1990). A high precision geodetic network was fundamental to validate the different theories developed for the dynamics of the region. The study of the kinematics of the Azores triple junction and also the geodynamic evolution of the African-Eurasian plate boundary through the Azores-Gibraltar section, with the use of GPS and other geodetic techniques (absolute and relative gravity measurements) were the main purposes of this project. The TANGO project is also a contribution to WEGENER, by yielding a geodetic database for the support of geodynamics and oceanographic studies in the Azores-Gibraltar area (Bastos et al., 1998). It was intended to be a long-term project with observations every three years, developed by the Astronomical Observatory (University of Porto, Portugal) and the Institute for Geodesy and Navigation (University of Federal Armed Forces, Germany) with international co-operation (like the Finnish Geodetic Institute, Finland). The network was enlarged in 1991 to include more stations on the African plate to monitor also the Azores-Gibraltar boundary (Bastos et al., 1998). _____________________________________________________ (1) (2) (3) (4)
Instituto de Investigação Científica Tropical, Lisboa, Portugal Finnish Geodetic Institute, Masala, Finland Observatório Astronómico, University of Porto, Portugal Institute for Geodesy and Navigation, University of FAF, Munich, Germany
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The gravimetric network was established in 1992 with two absolute stations and relative measurements on all the islands. It was established as a complement of the GPS observations, since it is known that gravimetric information can be useful to monitor vertical displacements. More observation campaigns took place in 1994 and 1997 with GPS and gravity measurements. In 1994, a new absolute gravity station was established and has been re-observed since then. In the last campaign a geological study of GPS station conditions was also performed.
The Azores Archipelago The Azores Archipelago is located on North Atlantic Ocean about 2000 Km away from Iberian Peninsula between latitudes 37ºN and 40ºN and longitudes 32ºW and 25ºW (Figure 1). Nine volcanic islands and the Formigas islets form the Azores Archipelago. The islands are aligned in a WNW-ESE direction and lie at the complex plate boundary (called the Azores Triple Junction) of the North American, the African and the Eurasian plates. Corvo Graciosa
Flores
Western Group
Terceira Faial
S. Jorge Pico
Central Group North-Atlantic Ocean
S. Miguel
S.ta Maria
Eastern Group
Figure 1. The Azores Archipelago: Flores and Corvo islands constitute the western group, Santa Maria and São Miguel islands constitute the eastern group and Faial, Pico, Graciosa, Terceira and São Jorge constitute the central group.
The Mid-Atlantic Ridge (MAR) is the tectonic boundary that separates the North American plate from both the Eurasian and African plate and crosses the archipelago. Two of the islands, Flores and Corvo, lie west of the MAR at the North American plate; the others lie east at the African-Eurasian plate boundary. Eastwards of the MAR there are two important alignments, the East Azores Fracture Zone (EAFZ), along the azimuth 38ºN, actually inactive and another one, further north, in a WNW-ESE direction, which exhibits intense seismic and volcanic activity. Both alignments meet in the Gloria Fault. There are several models proposed for the recent behaviour of this triple junction which is still controversial and they are resumed in Figure 2. The one named Terceira Rift is based on seismotectonic studies. The Terceira Rift is a well-defined structure, known from bathymetric data analysis. The author proposed an actual African-Eurasian plate boundary in the Terceira rift domain (Buforn at al, 1988). Another model, based on neotectonics, assumes a leaky transform for the triple junction (Madeira and Ribeiro, 1990, 1992) with the axial zone between Pico and S. Jorge islands, along S. Jorge channel. It is assumed the existence of the African-Eurasian plate boundary along this axis (called S. Jorge Leaky Transform), that will meet Gloria Fault east of Santa Maria Island.
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Corvo
Terceira Rift Flores
Graciosa Terceira
MAR
Faial
Pico
S. Jorge
Leaky transform
Azores microplate S. Miguel ta
S. Maria
EAFZ
Figure 2. Three of the models proposed for the recent behaviour of the Azores triple junction which is still controversial.
Finally, (Freire et al., 1994) suggested the presence of a micro-plate that evolved independently from the main plates, since 10 My until 2.4 My. During this period, the south limit has been migrating to north, as well as the Azores Triple Junction, explained as a consequence of the African-Eurasian relative motion. This study is based on aeromagnetic data. Unfortunately, there are not enough data to define precisely the north limit of this micro-plate. Nevertheless, the author suggests the existence of a north boundary close to Terceira Rift.
The GPS campaigns Until now five GPS campaigns were performed (1988, 1991, 1993, 1994 and 1997). The GPS network was established with one station per island. All the stations have good logistical conditions and were established in geodetic pillars. The GPS data analysis is not finish yet, since different software programs have been used. Nevertheless, from the provisional analysis a model consistent with the seismic and tectonic data available is suggested. A leaky transform model was adopted for the region, from the Mid-Atlantic Ridge passing through SE until Santa Maria Island. Three domains are suggested: Graciosa and Terceira islands constitute the north domain, the other islands from central group and S. Miguel Island constitute the central domain and finally, a domain further south which contains only Santa Maria Island. It is also suggested that this strip has a western direction consistent with the direction of the strike-slip faults that cut the MAR. Passing through the central group the direction changes to WNW-ESSE and to NW-SE in eastern group. These three domains are separated by right-handed strike-slip faults. Also, the previous analysis suggests left-handed conjugate faults which direction changes from NNW-SSE, close to the MAR, to a N-S direction in eastern group. More details of this analysis can be found in (Baptista et al., 1999).
2.
THE HIGH PRECISION GRAVITY NETWORK 2.1 Absolute gravity measurements
The absolute measurements were performed with the JILAg-5 (Zumberge et al. 1982, Faller et al. 1983, Niebauer et al. 1986) of the Finnish Geodetic Institute, operated by the second author. Two absolute stations were measured in 1992: one in Flores (western group; North American plate) and the other in Faial (central group; Eurasian plate). In 1994 the Faial station was re-occupied but logistical problems prevented the re-occupation of Flores. A new station was established in S. Miguel (eastern group, Eurasian plate). In 1997 all three stations were re-occupied (Bastos et al., 1998). Table 1 summarizes the results.
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Station Faial
Flores S. Miguel
Date 1992-07-10...11 1994-10-15...16 1997-06-06...08 1992-07-22...24 1997-05-31...06-02 1994-10-20...22 1997-05-22...25
No. of sets 93 155 165 130 148 138 300
No. of drops 2325 3875 4125 3250 3700 3425 7500
S.D. of Mean, µGal 2.1 1.4 1.2 0.8 1.0 0.9 0.6
S.E. of result, µGal 5.0 5.0 5.0 5.0 5.0 5.0 5.0
Result at station mark µGal 980 129 197.1 196.0 217.8 980 190 026.8 046.6 980 110 730.2 736.9
Table 1. Results of the absolute gravity measurement in the Azores 1992–1997. The standard deviation (S.D.) of the mean of all drops is statistical scatter only. The standard errors (S.E.) are empirical repeatability estimates (onesigma) using the history of the instrument on several field sites and as such may be pessimistic.
Drop-to-drop scatter varied between 15 and 300 µGal depending on meteorological conditions (ocean noise). Observations were processed at the FGI according to IAGBN standards (Boedecker, 1988). A sample is shown in Figure 3.
Figure 3. Absolute measurements in S. Miguel in 1997. Each point is the mean of 25 drops and the error bars represent its standard deviation. Ocean tides are not corrected for and are responsible for the large sinusoid. However, since the measurements extend over an integral number of periods, their influence is largely eliminated.
In 1997 a gravity increase was observed at all three stations (Table 1, Figure 4). In Faial the increase was 15 ± 7 µGal, in Flores 20 ± 7 µGal and in S. Miguel 7 ± 7 µGal (one-sigma). Initially, we suspected a shift in the instrument. However, we have reference measurements at the gravimeter’s “home base” Metsähovi (Finland) before and after each campaign and at two stable stations in Madrid (Spain): at Valle de los Caídos (IAGBN-station) and at Facultad de Ciencias (Universidad Complutense), immediately after the measurements in the Azores. On the basis of these reference measurements (Figure 5) a shift in the instrument during the 1997 campaign appears unlikely.
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Figure 4. Results of absolute gravity measurements in the Azores 1992–1997 (from Table 1). Zeroes are arbitrary. See comments in text.
Figure 5. Results of absolute gravity measurements at reference sites before and after the campaigns in the Azores. The 1997 reference results do not explain the apparent increase in Azores in 1997 (Figure 4).
Before attempting a tectonic or volcanological interpretation of the results, variation in gravity due to other environmental factors should be estimated and, if possible, corrected for. In particular, the effect of variation in subsurface water storage may be considerable, due to local geology and the fact that the measurements were done at different times of the year. We do not have direct observations of groundwater level or soil moisture content. But some meteorological records are available, such as precipitation and evaporation. We are investigating possibilities of using them to correct for variations in subsurface water storage. A problem is that the meteorological (Figure 6) and hydrogeological situation in small areas on the same island is quite different. To sum up, we have observed significant gravity increase at the Faial and Flores absolute stations 1992– 1997, but are cautious of interpreting it in any way yet.
5
400
S.Miguel
350
Faial Flores 1
300
Flores 2 250 Flores 3 200 150 Flores 3 Flores 2 Flores 1 Faial S.Miguel
100 50 0 1
2
3
4
5
1997
6
7
1992
8
9
10
11
12
1994
Figure 6. Mean monthly precipitation in millimetres at five sites in the Azores and the epochs of the absolute campaigns. The figures in S. Miguel and Faial are from the absolute stations (which are in meteorological observatories) and refer to 1951–1980. Flores 1 (Ponta Delgada) is less than 2 km from the absolute station, the numbers refer to 1951–1980. Flores 2 (Santa Cruz) and Flores 3 (Fonte dos Frades) are about 10 km from the absolute station, the numbers refer to 1951–1976 and 1958–1979, respectively. While the seasonal pattern is the similar, there are large differences at short distances. From Instituto Nacional de Meteorologia e Geofisica (1991).
2.1 Relative gravity measurements
Station choice The gravimetric network comprises 114 relative stations on the nine islands. The network was established in 1992 and re-observed in 1994 and 1997. In the three campaigns ties between the islands were also measured. The relative stations include all GPS sites. Other stations (levelling benchmarks, geodetic triangulation pillars and stations at permanent buildings like churches, chapels and lighthouses) were selected from the 1965 gravity survey of the Azores (Instituto Geográfico e Cadastral, 1968). This survey was performed by the Portuguese Instituto Geográfico e Cadastral and the French Institut Géographique National to establish the Bouguer anomaly map. It was expected that some changes of the gravity field would be noticeable after 30 years even with the standard survey accuracy. Figure 7 shows the 18 relative stations established on Terceira (central group) in 1992.
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Measurement methods and data processing The instruments used were LaCoste & Romberg model G and D gravity meters (Table 2). Whenever possible, more than one gravity meter was used. Normal gravity survey procedures were followed. All measurements were performed in closed loops and most stations were occupied twice, providing additional drift control. A single reference station in each island was used to start and close the sequences. Some stations were observed only once due to difficulty of access.
30 Km Figure 7. The relative stations on Terceira in 1997. Figure 8 shows the sequences performed in Terceira in 1997.
18 13
15
8
1
9 7
2
14
17 12 11
6
3
16 4 5
Figure 8. Diagram of the links between gravity stations on Terceira in 1997. Each line represents a tie between the stations.
Drift was corrected by linear interpolation with respect to time. The tidal correction is incomplete as ocean tides are not included yet. The main component is semidiurnal, up to 30 µGal peak-to-peak (Figure 3). Table 2 shows the standard error of one gravity difference measured with a single instrument, each island and each campaign. The difficulty of access to the stations, transportation of the instruments on rough
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roads with 4WD vehicles, inexperienced observers and occasional power problems are reflected in the largest errors. The first campaign has the smallest errors, since only three experienced observers made the measurements. Also, the 1994 and the 1997 campaigns were performed at the same time as the GPS campaigns. In addition to relative measurements within islands, ties were also made between the absolute stations and the reference stations of the other islands. Commercial flights and in some cases ship services were used for transport. Because of time and cost limitations it proved very difficult to get a high enough accuracy to detect gravity changes 1992
1994
1997
Island
LCR
σ (µGal)
LCR
σ (µGal)
LCR
σ (µGal)
CORVO
G600
3.3
G1019
18.0
G1054
40.8
FAIAL
G600
7.6
G883/G688
25.0/14.9
G627
56.5
FLORES
G600
6.8
G1019
10.7
G1054
48.3
GRACIOSA
G688
16.5
G688
16.6
G587
67.8
PICO
G688
11.8
G991
70.7
G883
40.3
S. JORGE
G688
11.4
G86
27.4
G1052
25.3
S. MIGUEL
G587
63.4
G883/D38
67.4/41.3
G688
21.4
S.TA MARIA
G587
14.6
G883
35.0
G1054
70.8
TERCEIRA
G688
12.5
G688/G258
16.6/15.2
G1019
18.7
Table 2. This table shows (a) the gravimeters used and the standard errors obtained, on each island and each campaign. They are commented in the section “Measurement methods and data processing.” (b) The different colours of the gravimeter numbers (green, blue, red) show the calibration status of the gravimeter. This is commented in the section “Calibration problems”. Generally, it would be desirable to have both a small standard error and the green colour.
Calibration problems Gravity differences within the islands are up to 300 mGal. The calibration of the gravimeters is therefore critical. It is well known that the manufacturer’s calibration tables for the LCR meters may be in error of the order of one part in 1000. About the calibration we have three different situations, resumed in Table 2: 1. For two meters (G1052 and G1054) the calibration was determined on the Portuguese calibration line between the absolute stations Porto and Mértola, with a gravity difference of 242 mGal. The gravity range corresponds closely to the Azores. These instruments are marked with green in Table 2. The formal calibration accuracy is better than 10-4 but it is based on the internal consistency of the observations. 2. For the meters marked with blue in Table 2, calibration factors known from previous measurements on calibration lines were provided by the owners. They have formal uncertainties of 10-4 or even better, determined mostly between absolute gravity stations but in a gravity range different from the Azores. In some cases the factor provided does not appear to be valid, perhaps because of the different range. Shifts in time may also be possible (Gerstenecker 1995, Carbone and Rymer, 1999). 3. For the meters marked with red in Table 2 we do not have any prior information on the calibration. We determined the factors from the data as described below. As an example, Figure 9 shows the difference of results in Terceira in 1994 (gravimeter G258) and in 1992 (G688), plotted against the readings of the G688. We have a single station up at the caldeira with a gravity far below others. Either its gravity has changed by about 120 µGal or we have a scale problem. Modifying the scale will also change differences between the other stations. From a number of similar plots we have concluded that in this case (a) it is a scale problem and (b) it is in the G688.
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Adjusted Differences ( Gal)
Terceira 1994 (G258) - 1992 (G688) 150 100 50 0 3450 -50
3500
3550
3600
3650
3700
G688 Reading (mGal) -100 T E01
T E02
T E03
T E04
T E05
T E06
T E07
T E08
T E09
T E13
Figure 9. Difference of results in Terceira in 1994 (gravimeter G258) and in 1992 (G688), plotted against the readings of the G688. The reference station at the tide gauge in Angra do Heroísmo (TE04) was assigned to zero.
It is possible to compare results between stations with relatively small gravity differences (like the group on the right in Figure 9) with imperfect knowledge of the calibration. In addition, we have derived the missing calibration factors from a joint adjustment of all the data, assuming that eventual gravity change is uncorrelated with gravity itself. Effectively, the factor is then fixed by the apparent “gravity change” at the station at the island’s summit. Calibration factors determined in this way have standard errors of some parts in 10-4. Therefore the contribution of scale to the error is about 10 to 40 µGal for a 100 mGal gravity difference. It is not included in Table 2. Moreover, the long-term periodic errors, 35 counter units and upwards, of the LaCoste & Romberg G meters are unknown for most instruments and are not included in Table 2 either. Therefore we estimate that our accuracy is at best 20 µGal for gravity differences of some tens of milligal. For larger differences we must add the error of the linear scale.
Discussion We are still in the process of station-by-station screening of the results. However, regarding future campaigns a certain number of conclusions and recommendations may already be made: - Select only a small number of stations with more interest for further measurements. - Perform more measurements between these stations rather than large multi-station loop measurements. - If possible, use the same gravity meters in the same islands and always calibrate them before and after the campaigns on the Portuguese calibration line. Also, all measurements should be performed using at least two gravity meters simultaneously . - Gravity change between the islands can only be studied with absolute measurements.
3.
ACKNOWLEDGMENTS
This work was partially supported by the Portuguese Foundation for Science and Technology through the TANGO project. Gravimeters for relative work were kindly loaned by Institute for Physical Geodesy (Technical University of Darmstadt), Institute for Astronomy and Physical Geodesy (Technical University of Munich), University of Aveiro, Institute of Geophysics (Lisboa), Institute of Meteorology (Lisboa) and the Portuguese Institute of Cartography and Cadastre (Lisboa). We thank Secretaria Regional de Habitação e Obras Públicas, the Azores University and all municipal councils for help in the field work.
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The absolute measurements in S. Miguel were part of the Furnas—European Laboratory Volcano programme and were organised by Azores University, Centre of Volcanology. We would also like to thank the Portuguese Air Force, Observatório Príncipe Alberto de Mónaco, Centre d’Essais des Landes, and Council of Santa Cruz das Flores for invaluable help in the absolute work.
4.
REFERENCES
Baptista, P., Fernandes R., Bastos, L., Borges, F. S. (1999): Aplicação de técnicas geodésicas ao estudo do comportamento geodinâmico actual da Junção Tripla dos Açores. GEOlogos, Geology Department of University of Porto. Portugal (Submitted for publication). Bastos, L., Osório, J., Landau, H., Hein, G. (1990): The Azores Global Positioning System Network. Arquipélago, Life and Earth Sciences, 9:1-9, Angra do Heroísmo. Bastos, L., Osório, J., Barbeito, A., Hein, G. (1998): Results from geodetic measurements in the western part of the African-Eurasian plate boundary. Tectonophysics, 294:261-269. Bastos, L., Osório, J., Lázaro, C., Mäkinen, J., Kakkuri, J., Alves, M., Hein, G., Vieira, R. (1998): Campanhas gravimétricas nos Açores. I Assembleia Luso-Espanhola de Geodesia e Geofísica, Almeria, Espanha. Boedecker, G. (1988): International Absolute Gravity Base Station Network (IAGBN). Absolute gravity observations data processing standards and station documentation. BGI Bull. Inf. 63, 51-57. Buforn, E., Udías, A., Colombás, M. A. (1988): Seismicity source mechanisms and tectonics of the Azores-Gibraltar plate boundary. Tectonophysics, 152:89-118. Carbone, D. and Rymer, H. (1999): Calibration shifts in a LaCoste-and-Romberg gravimeter: comparison with a Scintrex CG-3M. Geophysical Prospecting, 47, 73–83. Faller, J.E., Guo, Y.G., Gschwind, J., Niebauer, T.M., Rinker, R.L., Xue, J. (1983): The JILA portable absolute gravity apparatus. BGI Bull. Inf., 53, 87–97. Freire, J., Miranda, J. M., Galdeano, A., Patriat, P., Rossignol., J. C., Mendes Victor, L. A. (1994): The Azores triple junction evolution since 10 Ma. from an aeromagnetic survey of the Mid-Atlantic Ridge. Earth and Planetary Science Letters, 125: 439-459. Gerstenecker, C. (1995): Zur Genauigkeit der Eichtabellen von LaCoste&Romberg Gravimetern. Festschrift Erwin Groten, University FAF Munich, 34–43. Instituto Geográfico e Cadastral (1968): Trabalhos gravimétricos no arquipélago dos Açores. Cadernos Técnicos e de Informação, 14. Instituto Nacional de Meteorologia e Geofísica (1991): O Clima de Portugal, Fascículo XLIX, Vol. 5, 5ª Região. Madeira, A. and Ribeiro, A. (1990): Geodynamic models for the Azores triple junction: a contribution from tectonics. Tectonophysics, 184:405-415. Madeira, A. and Ribeiro, A. (1992): O regime tectónico nos Açores. Proceedings of "Encontro dos 10 anos após o sismo dos Açores de 1/1/80". Angra do Heroísmo. LNEC. Niebauer, T.M., Hoskins J.K., Faller, J.E. (1986): Absolute gravity: a reconnaissance tool for studying vertical crustal motion. J. Geophys. Res., 91, 9145–9149. Zumberge, M.A., Rinker, R.L., Faller, J.E. (1982): A portable apparatus for absolute measurements of the Earth’s gravity. Metrologia, 18, 145–152.
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