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Geodetic measurements on the Asal Rift (Djibouti): Twelve years of observations
JOURNAL OF GEODYNAMICS7, 221 226 (1987)
221
G E O D E T I C M E A S U R E M E N T S ON THE ASAL R I F T ( D J I B O U T I ) : TWELVE YEARS O F O B S E R V A T I O N S
M. KASSER j, J. C. R U E G G 2 and J. C. LEPINE 2 J 1 G N - 2 avenue Pasteur, 94160 Saint-MandO, France IPG - T o u r 14 24, 4Ome ('rage, 4 place Jussieu, 75230 Paris COde.'( 05, France (Accepted January 3, 1986)
Kasser, M., Ruegg, J. C. and LOpine, J. C., 1987. Geodetic measurements on the Asal Rift (Djibouti): twelve years of observations. In: A. M. Wassef, A. Boud, P. Vyskocil, Recent Crustal Movements in Africa. Journal q/" Geodynamics, 7: 221-226.
A seismo-tectonic and volcanic event occurred in November 1978 in the Asal Ghoubbet rift, first subaerial section of the accreting plate boundary between the African and Arabian plates. The activity was located in the center of a geodetic network set up in the winter 1972-1973 by the Institut GOographique National in collaboration with the Institut de Physique du Globe de Paris. (see fig. 1). Simultaneously, a precise leveling line of about 100 km was established across the area. The resurveying of both the geodetic network and the leveling line was carried out after the event, between November 1978 and March 1979. Extensions up to 2.4m and vertical displacements up to 0 . 7 m were measured. After this period, geodetic measurements regularly conducted from 1979 to 1983 across this area in a medium aperture network set up in 1979 (see fig. 2), have shown an accumulation of extension in the inner floor of the rift valley with a rate of about 60 mm/yr perpendicular to the normal faults that bound the rift. (see fig. 3). Periodic levelling surveys across the major faults in the rift show vertical creep movements of about 5 to 10 mm/yr. (see fig. 4). Although the observed deformations could be contaminated by postseismic relaxation after the rifting episode in November 1978, the regularity and the magnitude of these movements during the last two years argue for a steady state mechanism of deformation of the rift valley. The deformation is uniformly distributed in time and in space inside the inner floor of the rift. The necking of the lithosphere in the central part of 0264-3707/87/$3.00
; ( 1987 Geophysical Press Ltd.
222
•
KASSER, R U E G G A N D LEPINE
/
"Red " ~/
/
/ /
I
/
/
/
/
t B
Fig. 1. Tectonic sketch map of the Asal Ghoubbet rift and location of the initial large aperture geodetic network. The heavy lines show the main faults which were reactivated during the seismovolcanic episode in November 1978. The Ardoukoba volcano is shown by a solid star and the main earthquake by a circled open star.The heavy dashed line indicates the initial precise leveling line of 1973. The inset location map shows the geodynamical environment. Solid areas are spreading segments in the Red sea and Gulf of Aden and axial w)lcanic ranges in the Afar depression (Tazieff et al. 1972).
the rift and the high temperature gradient imply a ductile rheology in the inner floor which is stretched in a steady state process. Such an interesting tectonically-active zone is ideal for testing all sorts of topographic methodologies. The main criteria appear to be the precision and the speed of observation: - - T h e precision because the movements have to be very accurately known to be helpful for the geophysical modeling of the rifting process. - - T h e speed because the time and money available for these investigations are quite limited in comparison with the high interest of the results, so that a lot of work has to be made in a very short time.
GEODETIC MEASUREMENTS ON THE ASAL RIFT
223
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Fig. 2. Tectonic map of the Asal~Ghoubbet rift and location of the medium aperture trilateration network which has been used in this study. The rectangular areas show the location of the three smallaperture networks and of the test sites for monitoring the faults; the numbers in these areas refer to the benchmarks used for vertical fault movement evaluation.
224
KASSER, R U E G G A N D LEPINE
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Fig. 3. Accumulation rate across the inner floor for different segments of the geodetic network which have a direction approximately perpendicular to the general trend of the rift.
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Fig. 4. Vertical movements during the period 1979 1983 across three faults monitoring test sites. The site of Daloale is across the northern normal fault delimiting the inner floor of the rift (see figure 2.) and the other two sites are across one of the southern faults. The error of each plotted point is approximately the size of the symbol (i.e. 0,5 mm); in the three sites the distance between benchmarks is less than 100 m.
GEODETIC MEASUREMENTS ON THE ASAL RIFT
225
Our experiments have, in our mind, shown that: - - I t is more interesting to observe the geodetic networks (wide and medium aperture ones) using EDM instruments than with classical triangulation. First, the accuracy is often better, especially for long ranges. Second, the programming of the fieldwork is easier, so that the speed of observation is quite high; the difficulty of checking the measurements is no longer a problem, as portable computers become cheaper every year. For example, the large 22-points network took 4 months of work to set up in 1972-73. Now, in one week with half this number of people, we do the whole reobservation of this network. - - T h e vertical movements have to be observed in different ways, depending on the topography: when the slopes are important (the surroundings of the inner floor for example), high precision trigonometric levelling is used, with observation of reciprocal zenith angles with good theodolites and EDM for the distance. This is quite precise (as precise as classical levelling in these zones, perhaps better) and extremely quick. For flat areas, it may appear that the classical high precision levelling is more suitable, because its lower speed is compensated by its good precision; and the high density of benchmarks (1 every 500 m) is not very adapted to trigonometric levelling. - - I t is necessary to develop specifical instrumentation for the continuous recording of the movements. Tiltmeters are often used, but they do not measure the movements passing in the faults. The same for extensometers. So we plan to build some special instruments which, with the Argos satellite data teletransmission systems, will enable us to measure quasi-continuously the movements of a fault system. Such data appear to be necessary for scientific studies of these phenomena. BIBLIOGRAPHY Allard, P., Tazieff, H. and Dajlevic, D., 1979. Observation of sea floor in Afar during the November, 1978 fissure eruption. Nature, 279, 30-33. Anis, A., Courtillot, V., Kasser, M., Le Dain, A. Y., Lepine, J. C., Robineau, B., Ruegg, J. C., Tapponier, P. and Tarantola, A., 1979. The Afar November, 1978 seismic and volcanic events and their relevance to the mechanics of accreting plate boundaries. Nature, 282, 17-23. Chahbert, C., 1975. Geodetic measurements across part of the Afar Depression, East Africa. Tectonophysics, 29, 471-477. Delibrias, Marinelli, G. and Stieljes, L., 1975. Asal rift spreading rate: a geological approach in: Afar depression of Ethiopia, edited by A. Pilger and A. Rosler, 214-221. Schweizerbart, Stuttgart, 1975. Harrizon, C. G., Bonatti, E. and Stieltjes, L., 1975. Tectonism of axial valleys in spreading centers: Data of the Afar rift. In: Afar Depression of Ethiopia, edited by A. Pilger and A. Rosier, 178 198. Schweizerbart, Stuttgart, 1975. I.G.N., 1973. Implantation d'un r6seau g6od6sique pour la mesure directe de l'expansion d'un rift oc6anique. IGN, Paris.
226
KASSER, R U E G G AND LEPINE
Kasser, M., L6pine J. C. and Ruegg, J. C., 1979. Acad. Sci, Paris, 228 serie D., 469-472. Kasser, M., Ruegg, J. C., L6pine, J. C. and Tarantola, A. Annales de G6ophysique, 35, n ° 4, 171-176, 1979. D/fformations continues du rift 6merg6 d'Asal (Djbouti) depuis la crise sismovolcanique de 1978. C.R. Acad. ci. Paris t 297, s6rie II, 131--136, 1983. Laughton, A. S., Whitmarsh, R. B. and Jones, M. T., 1979. The evolution of the Gulf of aden. Philos. Trans. Roy. Soc. London, ser. A, 267, 227-266. L6pine, J. C., Ruegg, J. C. et Anis, A. M., 1980. Sismicit6 du rift d'Asal Ghoubbet pendant la crise sismo-volcanique de novembre 1978. Bull. Soc. Geol. Fr. Ser. 7, 22 (6), 809-816. Needham, H. D., Choukroune, P., Chemin+e, J. L., Le Pichon, S., Francheteau, J. and Taponnier, P., 1976. The accreting plate boundary: Ardoukoba rift and oceanic rift valley. Earth Planet Sci. Lett., 28, 439-453. Ruegg, J. C., 1975. Structure profonde de la crofite et d'un manteau sup6rieur du Sud-Est de I'Afar d'apr6s les donn6es sismiques, Ann. G6ophys., 31, 329 360. Ruegg, J.C., Kasser, M., L6pine, J. C. and Tarantola, A., 1979. Geodetic measurements of rifting associated with a seismo-volcanic crisis in Afar. Geophys. Res. Lett., 6, (11), 817 820. Ruegg, J. C., L6pine, J. C. et Vincent, C., 1980. Sismicit6 et microsismicite de la dorsale de Tadjoura, tectonique et fronti6res de plaques. Bull. Soc. Geolog. Fr. Ser. 7, 22 (6) 917-924. Ruegg, J. C., Tarantola, A., Kasser, M., L6pine, J. C. et Lev6que, J. J., 1982. Mouvements actuels de d6formation dans le Rift d'Asal (Djibouti) reajustement post-sismique ou nouvelle phase d'6tirement tectonique? Ann. G6ophys., t. 38, fas. 3, 391-403. Stieltjes, L., 1973. L'axe tectono-volcanique d'Asal (Afar Central), doctoral thesis, Orsay Univ., France, 1973. Tarantola, A., Ruegg, J. C. and L6pine, J. C., 1979. Geodetic evidence for rifting in Afar. Earth Planet. Sci. Lett., 45, 435-444. Tarantola, A., Ruegg, J. C. and L6pine, J. C., 1980. Geodetic evidence for rifting in Afar; 2 vertical displacements. Earth Planet. Sci. Lett., 48, 363 370. Tazieff, H., Varet, J., Barberi, F. and Giglia, G., 1972. Tectonic significiance of the Afar Depression. Nature, 235, 144-147.
221
G E O D E T I C M E A S U R E M E N T S ON THE ASAL R I F T ( D J I B O U T I ) : TWELVE YEARS O F O B S E R V A T I O N S
M. KASSER j, J. C. R U E G G 2 and J. C. LEPINE 2 J 1 G N - 2 avenue Pasteur, 94160 Saint-MandO, France IPG - T o u r 14 24, 4Ome ('rage, 4 place Jussieu, 75230 Paris COde.'( 05, France (Accepted January 3, 1986)
Kasser, M., Ruegg, J. C. and LOpine, J. C., 1987. Geodetic measurements on the Asal Rift (Djibouti): twelve years of observations. In: A. M. Wassef, A. Boud, P. Vyskocil, Recent Crustal Movements in Africa. Journal q/" Geodynamics, 7: 221-226.
A seismo-tectonic and volcanic event occurred in November 1978 in the Asal Ghoubbet rift, first subaerial section of the accreting plate boundary between the African and Arabian plates. The activity was located in the center of a geodetic network set up in the winter 1972-1973 by the Institut GOographique National in collaboration with the Institut de Physique du Globe de Paris. (see fig. 1). Simultaneously, a precise leveling line of about 100 km was established across the area. The resurveying of both the geodetic network and the leveling line was carried out after the event, between November 1978 and March 1979. Extensions up to 2.4m and vertical displacements up to 0 . 7 m were measured. After this period, geodetic measurements regularly conducted from 1979 to 1983 across this area in a medium aperture network set up in 1979 (see fig. 2), have shown an accumulation of extension in the inner floor of the rift valley with a rate of about 60 mm/yr perpendicular to the normal faults that bound the rift. (see fig. 3). Periodic levelling surveys across the major faults in the rift show vertical creep movements of about 5 to 10 mm/yr. (see fig. 4). Although the observed deformations could be contaminated by postseismic relaxation after the rifting episode in November 1978, the regularity and the magnitude of these movements during the last two years argue for a steady state mechanism of deformation of the rift valley. The deformation is uniformly distributed in time and in space inside the inner floor of the rift. The necking of the lithosphere in the central part of 0264-3707/87/$3.00
; ( 1987 Geophysical Press Ltd.
222
•
KASSER, R U E G G A N D LEPINE
/
"Red " ~/
/
/ /
I
/
/
/
/
t B
Fig. 1. Tectonic sketch map of the Asal Ghoubbet rift and location of the initial large aperture geodetic network. The heavy lines show the main faults which were reactivated during the seismovolcanic episode in November 1978. The Ardoukoba volcano is shown by a solid star and the main earthquake by a circled open star.The heavy dashed line indicates the initial precise leveling line of 1973. The inset location map shows the geodynamical environment. Solid areas are spreading segments in the Red sea and Gulf of Aden and axial w)lcanic ranges in the Afar depression (Tazieff et al. 1972).
the rift and the high temperature gradient imply a ductile rheology in the inner floor which is stretched in a steady state process. Such an interesting tectonically-active zone is ideal for testing all sorts of topographic methodologies. The main criteria appear to be the precision and the speed of observation: - - T h e precision because the movements have to be very accurately known to be helpful for the geophysical modeling of the rifting process. - - T h e speed because the time and money available for these investigations are quite limited in comparison with the high interest of the results, so that a lot of work has to be made in a very short time.
GEODETIC MEASUREMENTS ON THE ASAL RIFT
223
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I
/ \'~'-
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1; t L t
F15}" , F*9 ~ '
|
• leveling brn lOOm' / ,---geo,Jeti, li,',e'-' IJ
Fig. 2. Tectonic map of the Asal~Ghoubbet rift and location of the medium aperture trilateration network which has been used in this study. The rectangular areas show the location of the three smallaperture networks and of the test sites for monitoring the faults; the numbers in these areas refer to the benchmarks used for vertical fault movement evaluation.
224
KASSER, R U E G G A N D LEPINE
.5
.5 tn
7.,,,
, Ep-DF-i
=-
SO
o BY_LS/Eastern zone o EP-SNJ • FG-CFI, CK_Cp.jwestern zone
40
/Q / /o / / 7
~
40
30
30
IF-OKJ
•
""-
~ / / , "
20
20
10
10
"I
I
1980
l
1981
1982
I 1983
Fig. 3. Accumulation rate across the inner floor for different segments of the geodetic network which have a direction approximately perpendicular to the general trend of the rift.
3
Cm
DALOALE 43b-43
2
.o •
o•
0 00
+
1 a
"i +
I
I
2
OUD
KOMA m i
Ct,
"
I
11 - 15 1 m i
g
m
I
I
b
I
I
c
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I
l
FIEALE F 15 - 3 4
:t_.... -I
80
O
Q
O
I
g
O
Q O
C
I
81
I
8z
83
Fig. 4. Vertical movements during the period 1979 1983 across three faults monitoring test sites. The site of Daloale is across the northern normal fault delimiting the inner floor of the rift (see figure 2.) and the other two sites are across one of the southern faults. The error of each plotted point is approximately the size of the symbol (i.e. 0,5 mm); in the three sites the distance between benchmarks is less than 100 m.
GEODETIC MEASUREMENTS ON THE ASAL RIFT
225
Our experiments have, in our mind, shown that: - - I t is more interesting to observe the geodetic networks (wide and medium aperture ones) using EDM instruments than with classical triangulation. First, the accuracy is often better, especially for long ranges. Second, the programming of the fieldwork is easier, so that the speed of observation is quite high; the difficulty of checking the measurements is no longer a problem, as portable computers become cheaper every year. For example, the large 22-points network took 4 months of work to set up in 1972-73. Now, in one week with half this number of people, we do the whole reobservation of this network. - - T h e vertical movements have to be observed in different ways, depending on the topography: when the slopes are important (the surroundings of the inner floor for example), high precision trigonometric levelling is used, with observation of reciprocal zenith angles with good theodolites and EDM for the distance. This is quite precise (as precise as classical levelling in these zones, perhaps better) and extremely quick. For flat areas, it may appear that the classical high precision levelling is more suitable, because its lower speed is compensated by its good precision; and the high density of benchmarks (1 every 500 m) is not very adapted to trigonometric levelling. - - I t is necessary to develop specifical instrumentation for the continuous recording of the movements. Tiltmeters are often used, but they do not measure the movements passing in the faults. The same for extensometers. So we plan to build some special instruments which, with the Argos satellite data teletransmission systems, will enable us to measure quasi-continuously the movements of a fault system. Such data appear to be necessary for scientific studies of these phenomena. BIBLIOGRAPHY Allard, P., Tazieff, H. and Dajlevic, D., 1979. Observation of sea floor in Afar during the November, 1978 fissure eruption. Nature, 279, 30-33. Anis, A., Courtillot, V., Kasser, M., Le Dain, A. Y., Lepine, J. C., Robineau, B., Ruegg, J. C., Tapponier, P. and Tarantola, A., 1979. The Afar November, 1978 seismic and volcanic events and their relevance to the mechanics of accreting plate boundaries. Nature, 282, 17-23. Chahbert, C., 1975. Geodetic measurements across part of the Afar Depression, East Africa. Tectonophysics, 29, 471-477. Delibrias, Marinelli, G. and Stieljes, L., 1975. Asal rift spreading rate: a geological approach in: Afar depression of Ethiopia, edited by A. Pilger and A. Rosler, 214-221. Schweizerbart, Stuttgart, 1975. Harrizon, C. G., Bonatti, E. and Stieltjes, L., 1975. Tectonism of axial valleys in spreading centers: Data of the Afar rift. In: Afar Depression of Ethiopia, edited by A. Pilger and A. Rosier, 178 198. Schweizerbart, Stuttgart, 1975. I.G.N., 1973. Implantation d'un r6seau g6od6sique pour la mesure directe de l'expansion d'un rift oc6anique. IGN, Paris.
226
KASSER, R U E G G AND LEPINE
Kasser, M., L6pine J. C. and Ruegg, J. C., 1979. Acad. Sci, Paris, 228 serie D., 469-472. Kasser, M., Ruegg, J. C., L6pine, J. C. and Tarantola, A. Annales de G6ophysique, 35, n ° 4, 171-176, 1979. D/fformations continues du rift 6merg6 d'Asal (Djbouti) depuis la crise sismovolcanique de 1978. C.R. Acad. ci. Paris t 297, s6rie II, 131--136, 1983. Laughton, A. S., Whitmarsh, R. B. and Jones, M. T., 1979. The evolution of the Gulf of aden. Philos. Trans. Roy. Soc. London, ser. A, 267, 227-266. L6pine, J. C., Ruegg, J. C. et Anis, A. M., 1980. Sismicit6 du rift d'Asal Ghoubbet pendant la crise sismo-volcanique de novembre 1978. Bull. Soc. Geol. Fr. Ser. 7, 22 (6), 809-816. Needham, H. D., Choukroune, P., Chemin+e, J. L., Le Pichon, S., Francheteau, J. and Taponnier, P., 1976. The accreting plate boundary: Ardoukoba rift and oceanic rift valley. Earth Planet Sci. Lett., 28, 439-453. Ruegg, J. C., 1975. Structure profonde de la crofite et d'un manteau sup6rieur du Sud-Est de I'Afar d'apr6s les donn6es sismiques, Ann. G6ophys., 31, 329 360. Ruegg, J.C., Kasser, M., L6pine, J. C. and Tarantola, A., 1979. Geodetic measurements of rifting associated with a seismo-volcanic crisis in Afar. Geophys. Res. Lett., 6, (11), 817 820. Ruegg, J. C., L6pine, J. C. et Vincent, C., 1980. Sismicit6 et microsismicite de la dorsale de Tadjoura, tectonique et fronti6res de plaques. Bull. Soc. Geolog. Fr. Ser. 7, 22 (6) 917-924. Ruegg, J. C., Tarantola, A., Kasser, M., L6pine, J. C. et Lev6que, J. J., 1982. Mouvements actuels de d6formation dans le Rift d'Asal (Djibouti) reajustement post-sismique ou nouvelle phase d'6tirement tectonique? Ann. G6ophys., t. 38, fas. 3, 391-403. Stieltjes, L., 1973. L'axe tectono-volcanique d'Asal (Afar Central), doctoral thesis, Orsay Univ., France, 1973. Tarantola, A., Ruegg, J. C. and L6pine, J. C., 1979. Geodetic evidence for rifting in Afar. Earth Planet. Sci. Lett., 45, 435-444. Tarantola, A., Ruegg, J. C. and L6pine, J. C., 1980. Geodetic evidence for rifting in Afar; 2 vertical displacements. Earth Planet. Sci. Lett., 48, 363 370. Tazieff, H., Varet, J., Barberi, F. and Giglia, G., 1972. Tectonic significiance of the Afar Depression. Nature, 235, 144-147.