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Recent crustal deformation in the Ethiopian rift valley
Tectonophysics, 29 (1975) 461-469 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
RECENT CRUSTAL VALLEY
DEFORMATION
P.A. MOHR, A. GIRNIUS, J.R. CHERN~CK, Smithsonian Astrophysical (Accepted
IN THE ETHIOPIAN
E.M. G~OSCHKIN
461
RIFT
and J. LATTER
Observatory, Cambridge, Mass. (U.S.A.)
for publication May 28, 1975)
ABSTRACT Mohr, P.A., Girnius, A., Cherniack, J.R., Gaposchkin, E.M. and Latimer, J., 1975. Recent crustal deformation in the Ethiopian rift, valley. In: N. Pavoni and R. Green (Editors), Recent Crustal Movements. Tectonophysics, 29 (l-4): 461-469. Three geodimeter networks have been established in the Ethiopian rift: at latitude 8$“N, in 1969; at latitude 7$“N, in 1970; and at latitude 6”N, in 1971. All three networks have been remeasured annually since their inception. Line-length changes, previously analyzed on a semi-quantitative basis, have now been obtained from a least-squares adjustment program. A rather complex but hopefully realistic weighting scheme has been applied, and the overall network adjustment sigmas (dimensionless) are close to unity. With reference to an arbitrarily selected datum for each network, station vectors have been derived, each with its appropriate foot point curve. The results confirm a significant motion of station RABBIT in the Wolen~hiti quadrilateral, a region of episodic ground cracking and subsidence. Significant motions of stations in the Adama region (8:“N) form a complex pattern, but a component of longitudinal motion along the rift seems to be a common feature. In the Langana network (7$“N), motions are perpendicular to the rift faults, at rates of up to 12 mm/yr.
INTRODUCTION
Geodimeter networks have been set up in the Ethiopian rift valley to attempt to detect and measure the rate and manner of rifting. The largest net, comprising over one hundred lines, traverses the northern end of the rift. Two smaller nets, centred on the axial Wonji fault belt of the rift floor, are sited in the Lake Langana and Lake Chamo regions. Discussion of the techniques employed, the work accomplished, and the reduction of the data (including problems of geodimeter calibration and correction for atmospheric density) has been given previously (Mohr, 1973, 1974). METHOD OF ANALYSIS
The earlier analysis of apparent line-length changes in the Ethiopian geodimeter nets was performed semi-quantitatively. Each triangle within a net-
462
work was analyzed separately, and the assemblage of triangles then examined for consistency (Mohr, 1974). In this presentation, a weighted least-squares best-fit analysis of the networks has been applied, based on two independent programs. One, devised especially for this work by E.M.G. and J.R.C., is performed on a Nova 1200 minicomputer and can manage up to eight stations with thirteen unknowns. The other program, developed at S.A.O. for reduction of data from simultaneous laser-ranging to satellites, has been adapted by J.L. and can manage a net of up to 42 stations using a CDC 6400 computer. We hold one station fixed as the origin, and use a second station to define the x (and y) axis along which that station is constrained to move in the adjustment. The station z values (elevations) are held fixed in the network adjustments: because the nets are close to being planar (the maximum line slope is ca. 3g), the effects of any real vertical movements of stations will not affect the adjustment unless such movements were catastrophic (tens of metres). The elevations have been derived from triangulation and altimetry. Weighting is a problem that pervades the entire approach to reduction of the Ethiopian geodimeter data. The line-length means used in the computations have themselves resulted from averaging of individual, observed linelengths, weighted according to inst~mental internal consistency (Mohr, 1973). In the network adjustments, the line-length means are weighted according to the scheme:
where zu is the applied weight, n’ is related to IZ (number of measurements of a given line) by n’ = ($ + f + $ + . . . + *), s is the number of set-ups for a given line in a given survey, and u is the standard deviation about the line-mean (a minimum of 53 mm is accepted, and for single observations the value is taken from table 2 in Mohr, 1974). Where a network lacks sufficient observations to give a unique solution, estimated values for non-observed lines are included with a weight of lo- 3 -1 O- 4 that of the observed lines. If the line was measured, but in a different year, then the weight is lo- 2 for a one-year gap and lo- 2 .3 for a two-year gap. This subjective weighting system is tested in the adjustment computation, from a dimensionless sigma:
where (o - c) is the residual between observed and computed line-lengths, (r is the standard deviation for each line, and f is the number of degrees of freedom in the network. If the observations are mutually consistent and the weightings are reasonably applied, then the value of 5’ is close to unity, as indeed is the case with the finai reductions made in this work.
463
The adjustment results in corrections to station coordinates, with variante-covariance matrices from which the foot-point curves are derived, correlation matrices and adjusted line-lengths each with an a-posteriori standard deviation. Computations of given nets have been made using different stations to define the x (and y) axis. Where any station is composed of a main and one or more auxiliary points, computations have been made both for independent adjustment of these points, and for the geologically more realistic tying of these points. RESULTS
The Wolenchiti quadrilateraE The Wolen~hiti quadrila~ral forms the no~heastern part of the northern network of the rift (Mohr, 1973, fig. 18), and was established in 1970 on the advice of J. Rolff after the appearance once more of surface cracks and subsidence pits in the Wolenchiti region (Gouin and Mohr, 1967). Figure 1 shows the adjusted motion vectors, relative to TABLE taken as origin, of the other three stations during the two periods 1970-1971 and 1971-1973. Station MENDENO defines the x-axis here, and is constrained to move in that direction only. The vectors reveal that the apparent motions of stations MENDENO and AYGU are hardly significant, even though the foot-point curves are small, and this is a reassuring justification of the field techniques and method of data reduction. The fourth station, RABBIT, has moved si~~ic~tly by 22 mm at 2069 in 1970-1971, and 23 mm at 320g in 1971-1973. Weighting is not critical here, as its removal changes these respective values to 19 mm at 21gg and 22 mm at 315g, but the application of the ground-radiation correction (Mohr, 1973, 1974) to the atmosphericdensity factor is more important. In the extreme case, if no correction is applied, then the RABBIT vectors are 15 mm at about 200g and 5 mm reversal, in 1970-1971 and 1971-1973 respectively. Whatever their precise vectors, the annual motions of RABBIT are significant, to the south in 1970-1971 and to the west in 1971-1973. What might be their geological significance? The 1970 survey was conducted three months after the appearance of ground cracks and subsidences some 2 km southeast of station AYGU. The 1973 survey was conducted some three months prior to resumed ground cracking. The cracks occurred in two linear sets, oriented NNE and E, and it is possible though by no means certain that they are a conjugate pair resulting from rift extension. The horizontal displacements at the cracks are only a few millimetres, though in toto they could add up to a centimetre or more. On the other hand, motions of 2 emlyr might be expected to be associated with a certain seismicity, but even micro~ismicity in the region is extremely infrequent (Molnar et al., 1970; Gouin and Mohr, 1967). An alternative explanation is that RABBIT is a floating station. The rock
464
\
'i,
/
Fig. 1. Plan of the Wolenchiti quadrilateral, showing station vectors for the periods 1979-1971 and 1971-1973 relative to station TABLE (TA) held fixed. Two paths are shown for RABBIT (RA), according to the presence or absence of weighting in the adjustment (the former employs MENDENO (ME) as origin). The line TABLE-MENDENO defines the x-axis. The foot-point curves mark a ca. 45% confidence limit.
outcrop bearing the ground-marker occurs on an old lava front of 15-20g slope, but similar slopes are sites for stations such as AYGU and MENDENO not subject to significant motions. Furthermore, the 19’70-1971 vector would require an uphill movement of a floating RABBIT, followed by a traverse in 1971-1973. Nevertheless, the singularity of RABBIT’s motion in the Wolenchiti quadrilateral does suggest local ground consolidation: surface cracking close to AYGU might be expected to shift that station also, relative to TABLE and MENDENO. The problem is the classic one of geodetic deferment: further observations over a longer timespan are required before definite conclusions can be made. It is hoped that the establishment of auxiliary points at all four stations in 1973 will hasten such conclusions.
465
The Lungana network A polyhedral closed network has been established on the southwestern side of Lake Langana, crossing the Wonji fault belt. The northern part of this network was initiated in 19’70, and the southern part added in 1971. It now comprises six stations, one of which, EUPHORBIA, includes an auxiliary point. Figure 2 shows the computed vectors for all the stations relative to EUPHORBIA held fixed, and with the x-axis defined by line EUPHORBIATERMITE along which any motion of TERMITE is therefore constrained. EUPHORBIA AUX. has been tied to EUPHORBIA using the average of the adjusted coordinates obtained from the 1970, 1971 and 1973 surveys, lacking as yet any direct measurements between the two. The main and auxiliary
-
1971-1973
Fig. 2. Plan of the Langana network, showing station vectors for the periods 1970-1971 (pointing into stations) and 1971-1973 (pointing away from stations), relative to station EUPHORBIA (EU) held fixed. Station TERMITE is constrained to move along the x-axis, and in this particular adjustment EUPHORBIA AUX. is not tied to the main point. The foot point curves mark a ca. 45% confidence limit. I’
points at EUPHORBIA are separated by only 28 m on the flat summit of a small horst, and neither tectonic motion nor soil creep is expected to affect this inter-point distance. This is borne out in adjustments in which EUPHORBIA and EUPHORBIA AUX. (EUA) are not tied; their relative apparent motions do not exceed 3 mm/yr (Fig. 2). Relative to station EUPHORBIA (and EUA) held fixed, station HOTEL has apparently moved 6 mm at 146g in 1970--1971, and a further 15 mm at 087g in 1971.--1973. This motion is significant (Fig. 2), and has been related to possible outward tilt of the rim of the steep, east-facing fault scarp upon which HOTEL is situated. This interpretation is reinforced by the apparent motion of station TERMITE, constrained in the present analysis to move only along the defined x-axis (137g). TERMITE shows motions of 3$ mm at 337’ in 1970-1971, and 4& mm at 337’ in 1971-1973, which form a consistent pair but are barely significant statistically. Since TERMITE lies on the rim of the same fault scarp as HOTEL, this suggests that the motions of HOTEL are indeed due to ground consolidation rather than tectonism on any of the circa ten faults intervening between EUPHORBIA and HOTELTERMITE. Station GALLA is sited on the downfaulted block below HOTEL and TERMITE. Relative to EUPHORBIA it shows an almost exact back-tracking from 1970-1971 to 1971-1973: 12 mm at 283g, then 12 mm at 100g. There is no cause to question the stability of the ground-marker at GALLA, and the apparent reversal of its motion is either tectonic, or the result of instrumental measuring errors. The problem is exacerbated by the almost identical 1971--1973 vectors for HOTEL and GALLA: as ground consolidation at GALLA is ruled out, we have here either an unfortunate statistical coincidence, or there is a tectonic element which, in addition to shortening, implies a component of dextral shear along the rift faults. Dextral shear is also implied in the apparent motion of station ARJO, but 3’, mm at O08g during 1971-1973 is barely significant (Fig. 2). ARJO is situated on the downthrown, western side of the same fault as forms the western boundary of the EUPHORBIA hoist. Station LANGANA shows a significant apparent motion of 12 mm at 359” during 1971-1973. The suggestion is of crustal extension perpendicular to the faults passing between LANGANA and EUPHORBIA. As LANGANA lies immediately west of the west-upthrown fault that can be traced passing east of GALLA, there appears to be an inconsistency in the LANGANA-GALLA pair that will have to be resolved by future remeasurements. The Adama network The northern network in the rift comprises an extensive system of lines connecting 44 stations. The full analysis of this network cannot be given here; rather, an analysis is made for that part of the network covering the
467
-
O1::4
km
-
,970
-1971
197,
- 1973
Fig. 3. Plan of part of the Adama graben network, 1969-1970 (lines with dots), 1970-1971 (single relative to station RIDGE (RI) held fixed. Station along the x-axis. The foot-point curves mark a ca.
showing station vectors for the periods lines) and 1971-1973 (double lines), ADAMA (AD) is constrained to move 45% confidence limit.
Adama graben (Mohr, 1972, 1973), to illustrate some problems of adjustment and geological interpretation. Figure 3 shows the net, with RIDGE (and RIDGE AUX.) on the western rim overlooking the graben, GANTI and ELPASO on a cinder cone on the eastern rim, and ADAMA (and ADAMA AUX.) and FARENJI (and FARENJI AUX.) on the upthrown, eastern sides of two faults running along the graben floor. Adjustment computations have used RIDGE as origin, and first GANTI and then ADAMA to define the x-axis: adjustments have been made both with the auxiliary points free from and tied to their respective main points. Observed-minus-computed changes to observed line-lengths are less than 1 ppm, but the geometry of the net is such that the unmeasured 200 m distance between GANTI and ELPASO takes up much of the misfit in the adjustments. The a-posteriori line-length standard deviations lie in the range of l-4 mm, except when main-auxiliary pairs are free; then, some less well determined lines involving auxiliary points can yield standard deviations of up to +8 mm. During the computations, it was found that the network sigma was appreciably reduced by raising the elevation of station FARENJI by 1.5 m above the value obtained from triangulation.
The lines in the Adama graben net have been measured in 1969, 1970, I971 and 1973, except those involving stations GANTI and FARENJI AUX. which were only established in 1971. The results of adjustments for unconstrained main-auxiliary lines show: for ADAMA, the auxiliary point has apparently moved in conjunction with the main point; for FARENJI, the auxiliary point shows a significant and apparently diametrically opposed motion to that of the main point (the lava bedrock is less massive at FARENJI than at ADAMA); for RIDGE, a relative southward motion of RIDGE AUX. (RIA) of nearly 3 cm in 1969-1971 is completely reversed in 1971-1973 (RIDGE is marked in flat outcrop, RIDGE AUX. in an upstanding knob on the same outcrop, 20 m to the south). The station vectors shown in Fig. 3 are obtained when main-auxiliary pairs are tied. Station ADAMA-ADAMA AUX. is constrained along the x-axis, and only shows signific~t motion for the period 1971-1973, 13 mm at 080R (Note: when GANTI is used to define the x-axis, the motion of ADAMA is directed to 100”). This motion is consistent with an episode of rift extension. Station FARENJI shows a southward motion of nearly 2 cm, from an initial, opposite motion in 1969-1970. The foot-point curves are relatively large for stations GANTI and ELPASO, and the apparent motions of these stations are close to parallel to the semi-major axes; thus these motions should be interpreted with caution. ELPASG shows an apparent motion of 1; cm at 275’ during 1969-1970, then a reversal of 5$ cm at 17gg during 1970-1971, and again a turn to 2 cm at 295g during 1971-1973. During this last period, GANTI apparently moved l$ cm at 241g, which is a fair agreement for two stations linked tectonically in the sense of being located on the same cinder cone. It remains to be shown whether the large oscillatory motions of ELPASO (and GANTI) are merely errors resulting from poor network configuration, or are real. This will be revealed when the Adama network is analyzed with surrounding points added in a more extensive adjustment. Simulation tests already made, show that the foot-point curves are doubled in size if the line-length observations are degraded by an additional standard deviation of about +6 mm.
CONCLUSIONS
Despite the limitations imposed by instrumental precision (Mohr, 1974) and existing network configurations, it is considered that real station motions have been detected. Some of these can best be explained in terms of local ground consolidation, but others intimate tectonic displacements. Because the rates of motion are rather slow (of the order of a centimetre per year, or less), further re-surveys are required before firm conclusions can be drawn.
469
ACKNOWLEDGEMENTS
This work has been supported by NASA grant NGR-09-015-002 Smithsonian Research Foundation award 427220.
and
REFERENCES Gouin, P. and Mohr, P.A., 1967. Recent effects possibly due to tensional separation in the Ethiopian rift system. Bull. Geophys. Obs. Addis Ababa, 10: 69-78. Mohr, P.A., 1972. Crustal deformation rate and the evolution of the Ethiopian rift. In: D.H. Tarling and S.K. Runcorn (Editors), Implications of Continental Drift to the Earth Sciences. Academic Press, London, 2: 159-768. Mohr, P.A., 1973. Ethiopian rift geodimeter surveys. Bull. Geophys. Obs. Addis Ababa, 14: 92pp. Mohr, P.A., 1974. 1973 Ethiopian-rift geodimeter survey. Smithsonian Astrophys. Obs. Spec. Rep., 358: 110 pp. Molnar, P., Fitch, T.J. and Asfaw, L.M., 1970. A micro-earthquake survey in the Ethiopian rift. Earthquake Notes, 41: 37-44.
RECENT CRUSTAL VALLEY
DEFORMATION
P.A. MOHR, A. GIRNIUS, J.R. CHERN~CK, Smithsonian Astrophysical (Accepted
IN THE ETHIOPIAN
E.M. G~OSCHKIN
461
RIFT
and J. LATTER
Observatory, Cambridge, Mass. (U.S.A.)
for publication May 28, 1975)
ABSTRACT Mohr, P.A., Girnius, A., Cherniack, J.R., Gaposchkin, E.M. and Latimer, J., 1975. Recent crustal deformation in the Ethiopian rift, valley. In: N. Pavoni and R. Green (Editors), Recent Crustal Movements. Tectonophysics, 29 (l-4): 461-469. Three geodimeter networks have been established in the Ethiopian rift: at latitude 8$“N, in 1969; at latitude 7$“N, in 1970; and at latitude 6”N, in 1971. All three networks have been remeasured annually since their inception. Line-length changes, previously analyzed on a semi-quantitative basis, have now been obtained from a least-squares adjustment program. A rather complex but hopefully realistic weighting scheme has been applied, and the overall network adjustment sigmas (dimensionless) are close to unity. With reference to an arbitrarily selected datum for each network, station vectors have been derived, each with its appropriate foot point curve. The results confirm a significant motion of station RABBIT in the Wolen~hiti quadrilateral, a region of episodic ground cracking and subsidence. Significant motions of stations in the Adama region (8:“N) form a complex pattern, but a component of longitudinal motion along the rift seems to be a common feature. In the Langana network (7$“N), motions are perpendicular to the rift faults, at rates of up to 12 mm/yr.
INTRODUCTION
Geodimeter networks have been set up in the Ethiopian rift valley to attempt to detect and measure the rate and manner of rifting. The largest net, comprising over one hundred lines, traverses the northern end of the rift. Two smaller nets, centred on the axial Wonji fault belt of the rift floor, are sited in the Lake Langana and Lake Chamo regions. Discussion of the techniques employed, the work accomplished, and the reduction of the data (including problems of geodimeter calibration and correction for atmospheric density) has been given previously (Mohr, 1973, 1974). METHOD OF ANALYSIS
The earlier analysis of apparent line-length changes in the Ethiopian geodimeter nets was performed semi-quantitatively. Each triangle within a net-
462
work was analyzed separately, and the assemblage of triangles then examined for consistency (Mohr, 1974). In this presentation, a weighted least-squares best-fit analysis of the networks has been applied, based on two independent programs. One, devised especially for this work by E.M.G. and J.R.C., is performed on a Nova 1200 minicomputer and can manage up to eight stations with thirteen unknowns. The other program, developed at S.A.O. for reduction of data from simultaneous laser-ranging to satellites, has been adapted by J.L. and can manage a net of up to 42 stations using a CDC 6400 computer. We hold one station fixed as the origin, and use a second station to define the x (and y) axis along which that station is constrained to move in the adjustment. The station z values (elevations) are held fixed in the network adjustments: because the nets are close to being planar (the maximum line slope is ca. 3g), the effects of any real vertical movements of stations will not affect the adjustment unless such movements were catastrophic (tens of metres). The elevations have been derived from triangulation and altimetry. Weighting is a problem that pervades the entire approach to reduction of the Ethiopian geodimeter data. The line-length means used in the computations have themselves resulted from averaging of individual, observed linelengths, weighted according to inst~mental internal consistency (Mohr, 1973). In the network adjustments, the line-length means are weighted according to the scheme:
where zu is the applied weight, n’ is related to IZ (number of measurements of a given line) by n’ = ($ + f + $ + . . . + *), s is the number of set-ups for a given line in a given survey, and u is the standard deviation about the line-mean (a minimum of 53 mm is accepted, and for single observations the value is taken from table 2 in Mohr, 1974). Where a network lacks sufficient observations to give a unique solution, estimated values for non-observed lines are included with a weight of lo- 3 -1 O- 4 that of the observed lines. If the line was measured, but in a different year, then the weight is lo- 2 for a one-year gap and lo- 2 .3 for a two-year gap. This subjective weighting system is tested in the adjustment computation, from a dimensionless sigma:
where (o - c) is the residual between observed and computed line-lengths, (r is the standard deviation for each line, and f is the number of degrees of freedom in the network. If the observations are mutually consistent and the weightings are reasonably applied, then the value of 5’ is close to unity, as indeed is the case with the finai reductions made in this work.
463
The adjustment results in corrections to station coordinates, with variante-covariance matrices from which the foot-point curves are derived, correlation matrices and adjusted line-lengths each with an a-posteriori standard deviation. Computations of given nets have been made using different stations to define the x (and y) axis. Where any station is composed of a main and one or more auxiliary points, computations have been made both for independent adjustment of these points, and for the geologically more realistic tying of these points. RESULTS
The Wolenchiti quadrilateraE The Wolen~hiti quadrila~ral forms the no~heastern part of the northern network of the rift (Mohr, 1973, fig. 18), and was established in 1970 on the advice of J. Rolff after the appearance once more of surface cracks and subsidence pits in the Wolenchiti region (Gouin and Mohr, 1967). Figure 1 shows the adjusted motion vectors, relative to TABLE taken as origin, of the other three stations during the two periods 1970-1971 and 1971-1973. Station MENDENO defines the x-axis here, and is constrained to move in that direction only. The vectors reveal that the apparent motions of stations MENDENO and AYGU are hardly significant, even though the foot-point curves are small, and this is a reassuring justification of the field techniques and method of data reduction. The fourth station, RABBIT, has moved si~~ic~tly by 22 mm at 2069 in 1970-1971, and 23 mm at 320g in 1971-1973. Weighting is not critical here, as its removal changes these respective values to 19 mm at 21gg and 22 mm at 315g, but the application of the ground-radiation correction (Mohr, 1973, 1974) to the atmosphericdensity factor is more important. In the extreme case, if no correction is applied, then the RABBIT vectors are 15 mm at about 200g and 5 mm reversal, in 1970-1971 and 1971-1973 respectively. Whatever their precise vectors, the annual motions of RABBIT are significant, to the south in 1970-1971 and to the west in 1971-1973. What might be their geological significance? The 1970 survey was conducted three months after the appearance of ground cracks and subsidences some 2 km southeast of station AYGU. The 1973 survey was conducted some three months prior to resumed ground cracking. The cracks occurred in two linear sets, oriented NNE and E, and it is possible though by no means certain that they are a conjugate pair resulting from rift extension. The horizontal displacements at the cracks are only a few millimetres, though in toto they could add up to a centimetre or more. On the other hand, motions of 2 emlyr might be expected to be associated with a certain seismicity, but even micro~ismicity in the region is extremely infrequent (Molnar et al., 1970; Gouin and Mohr, 1967). An alternative explanation is that RABBIT is a floating station. The rock
464
\
'i,
/
Fig. 1. Plan of the Wolenchiti quadrilateral, showing station vectors for the periods 1979-1971 and 1971-1973 relative to station TABLE (TA) held fixed. Two paths are shown for RABBIT (RA), according to the presence or absence of weighting in the adjustment (the former employs MENDENO (ME) as origin). The line TABLE-MENDENO defines the x-axis. The foot-point curves mark a ca. 45% confidence limit.
outcrop bearing the ground-marker occurs on an old lava front of 15-20g slope, but similar slopes are sites for stations such as AYGU and MENDENO not subject to significant motions. Furthermore, the 19’70-1971 vector would require an uphill movement of a floating RABBIT, followed by a traverse in 1971-1973. Nevertheless, the singularity of RABBIT’s motion in the Wolenchiti quadrilateral does suggest local ground consolidation: surface cracking close to AYGU might be expected to shift that station also, relative to TABLE and MENDENO. The problem is the classic one of geodetic deferment: further observations over a longer timespan are required before definite conclusions can be made. It is hoped that the establishment of auxiliary points at all four stations in 1973 will hasten such conclusions.
465
The Lungana network A polyhedral closed network has been established on the southwestern side of Lake Langana, crossing the Wonji fault belt. The northern part of this network was initiated in 19’70, and the southern part added in 1971. It now comprises six stations, one of which, EUPHORBIA, includes an auxiliary point. Figure 2 shows the computed vectors for all the stations relative to EUPHORBIA held fixed, and with the x-axis defined by line EUPHORBIATERMITE along which any motion of TERMITE is therefore constrained. EUPHORBIA AUX. has been tied to EUPHORBIA using the average of the adjusted coordinates obtained from the 1970, 1971 and 1973 surveys, lacking as yet any direct measurements between the two. The main and auxiliary
-
1971-1973
Fig. 2. Plan of the Langana network, showing station vectors for the periods 1970-1971 (pointing into stations) and 1971-1973 (pointing away from stations), relative to station EUPHORBIA (EU) held fixed. Station TERMITE is constrained to move along the x-axis, and in this particular adjustment EUPHORBIA AUX. is not tied to the main point. The foot point curves mark a ca. 45% confidence limit. I’
points at EUPHORBIA are separated by only 28 m on the flat summit of a small horst, and neither tectonic motion nor soil creep is expected to affect this inter-point distance. This is borne out in adjustments in which EUPHORBIA and EUPHORBIA AUX. (EUA) are not tied; their relative apparent motions do not exceed 3 mm/yr (Fig. 2). Relative to station EUPHORBIA (and EUA) held fixed, station HOTEL has apparently moved 6 mm at 146g in 1970--1971, and a further 15 mm at 087g in 1971.--1973. This motion is significant (Fig. 2), and has been related to possible outward tilt of the rim of the steep, east-facing fault scarp upon which HOTEL is situated. This interpretation is reinforced by the apparent motion of station TERMITE, constrained in the present analysis to move only along the defined x-axis (137g). TERMITE shows motions of 3$ mm at 337’ in 1970-1971, and 4& mm at 337’ in 1971-1973, which form a consistent pair but are barely significant statistically. Since TERMITE lies on the rim of the same fault scarp as HOTEL, this suggests that the motions of HOTEL are indeed due to ground consolidation rather than tectonism on any of the circa ten faults intervening between EUPHORBIA and HOTELTERMITE. Station GALLA is sited on the downfaulted block below HOTEL and TERMITE. Relative to EUPHORBIA it shows an almost exact back-tracking from 1970-1971 to 1971-1973: 12 mm at 283g, then 12 mm at 100g. There is no cause to question the stability of the ground-marker at GALLA, and the apparent reversal of its motion is either tectonic, or the result of instrumental measuring errors. The problem is exacerbated by the almost identical 1971--1973 vectors for HOTEL and GALLA: as ground consolidation at GALLA is ruled out, we have here either an unfortunate statistical coincidence, or there is a tectonic element which, in addition to shortening, implies a component of dextral shear along the rift faults. Dextral shear is also implied in the apparent motion of station ARJO, but 3’, mm at O08g during 1971-1973 is barely significant (Fig. 2). ARJO is situated on the downthrown, western side of the same fault as forms the western boundary of the EUPHORBIA hoist. Station LANGANA shows a significant apparent motion of 12 mm at 359” during 1971-1973. The suggestion is of crustal extension perpendicular to the faults passing between LANGANA and EUPHORBIA. As LANGANA lies immediately west of the west-upthrown fault that can be traced passing east of GALLA, there appears to be an inconsistency in the LANGANA-GALLA pair that will have to be resolved by future remeasurements. The Adama network The northern network in the rift comprises an extensive system of lines connecting 44 stations. The full analysis of this network cannot be given here; rather, an analysis is made for that part of the network covering the
467
-
O1::4
km
-
,970
-1971
197,
- 1973
Fig. 3. Plan of part of the Adama graben network, 1969-1970 (lines with dots), 1970-1971 (single relative to station RIDGE (RI) held fixed. Station along the x-axis. The foot-point curves mark a ca.
showing station vectors for the periods lines) and 1971-1973 (double lines), ADAMA (AD) is constrained to move 45% confidence limit.
Adama graben (Mohr, 1972, 1973), to illustrate some problems of adjustment and geological interpretation. Figure 3 shows the net, with RIDGE (and RIDGE AUX.) on the western rim overlooking the graben, GANTI and ELPASO on a cinder cone on the eastern rim, and ADAMA (and ADAMA AUX.) and FARENJI (and FARENJI AUX.) on the upthrown, eastern sides of two faults running along the graben floor. Adjustment computations have used RIDGE as origin, and first GANTI and then ADAMA to define the x-axis: adjustments have been made both with the auxiliary points free from and tied to their respective main points. Observed-minus-computed changes to observed line-lengths are less than 1 ppm, but the geometry of the net is such that the unmeasured 200 m distance between GANTI and ELPASO takes up much of the misfit in the adjustments. The a-posteriori line-length standard deviations lie in the range of l-4 mm, except when main-auxiliary pairs are free; then, some less well determined lines involving auxiliary points can yield standard deviations of up to +8 mm. During the computations, it was found that the network sigma was appreciably reduced by raising the elevation of station FARENJI by 1.5 m above the value obtained from triangulation.
The lines in the Adama graben net have been measured in 1969, 1970, I971 and 1973, except those involving stations GANTI and FARENJI AUX. which were only established in 1971. The results of adjustments for unconstrained main-auxiliary lines show: for ADAMA, the auxiliary point has apparently moved in conjunction with the main point; for FARENJI, the auxiliary point shows a significant and apparently diametrically opposed motion to that of the main point (the lava bedrock is less massive at FARENJI than at ADAMA); for RIDGE, a relative southward motion of RIDGE AUX. (RIA) of nearly 3 cm in 1969-1971 is completely reversed in 1971-1973 (RIDGE is marked in flat outcrop, RIDGE AUX. in an upstanding knob on the same outcrop, 20 m to the south). The station vectors shown in Fig. 3 are obtained when main-auxiliary pairs are tied. Station ADAMA-ADAMA AUX. is constrained along the x-axis, and only shows signific~t motion for the period 1971-1973, 13 mm at 080R (Note: when GANTI is used to define the x-axis, the motion of ADAMA is directed to 100”). This motion is consistent with an episode of rift extension. Station FARENJI shows a southward motion of nearly 2 cm, from an initial, opposite motion in 1969-1970. The foot-point curves are relatively large for stations GANTI and ELPASO, and the apparent motions of these stations are close to parallel to the semi-major axes; thus these motions should be interpreted with caution. ELPASG shows an apparent motion of 1; cm at 275’ during 1969-1970, then a reversal of 5$ cm at 17gg during 1970-1971, and again a turn to 2 cm at 295g during 1971-1973. During this last period, GANTI apparently moved l$ cm at 241g, which is a fair agreement for two stations linked tectonically in the sense of being located on the same cinder cone. It remains to be shown whether the large oscillatory motions of ELPASO (and GANTI) are merely errors resulting from poor network configuration, or are real. This will be revealed when the Adama network is analyzed with surrounding points added in a more extensive adjustment. Simulation tests already made, show that the foot-point curves are doubled in size if the line-length observations are degraded by an additional standard deviation of about +6 mm.
CONCLUSIONS
Despite the limitations imposed by instrumental precision (Mohr, 1974) and existing network configurations, it is considered that real station motions have been detected. Some of these can best be explained in terms of local ground consolidation, but others intimate tectonic displacements. Because the rates of motion are rather slow (of the order of a centimetre per year, or less), further re-surveys are required before firm conclusions can be drawn.
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ACKNOWLEDGEMENTS
This work has been supported by NASA grant NGR-09-015-002 Smithsonian Research Foundation award 427220.
and
REFERENCES Gouin, P. and Mohr, P.A., 1967. Recent effects possibly due to tensional separation in the Ethiopian rift system. Bull. Geophys. Obs. Addis Ababa, 10: 69-78. Mohr, P.A., 1972. Crustal deformation rate and the evolution of the Ethiopian rift. In: D.H. Tarling and S.K. Runcorn (Editors), Implications of Continental Drift to the Earth Sciences. Academic Press, London, 2: 159-768. Mohr, P.A., 1973. Ethiopian rift geodimeter surveys. Bull. Geophys. Obs. Addis Ababa, 14: 92pp. Mohr, P.A., 1974. 1973 Ethiopian-rift geodimeter survey. Smithsonian Astrophys. Obs. Spec. Rep., 358: 110 pp. Molnar, P., Fitch, T.J. and Asfaw, L.M., 1970. A micro-earthquake survey in the Ethiopian rift. Earthquake Notes, 41: 37-44.