Postseismic deformation at El Asnam (Algeria)

16 Pages·2006·3.87 MB·English

Geophys. J. Int. (1997) 129, 597-612
Postseismic deformation at El Asnam (Algeria) in the seismotectonic
context of northwestern Algeria
K. Lammali,' M. Bezzeghoud,'.2.t F. Oussadou,' D. Dimitrovl**a nd H. Benhallou'
'CRAAG, DCpt. ESS, BP 63,16340 BouzarCah, Algiers, Algeria
'
Ecole des Mines de Paris, CG, 35 rue St. Honor&, F-77305 Fontainebleau Cedex, France
Accepted 1997 January 29. Received 1997 January 2; in original form 1996 March 27
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SUMMARY lo
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We use a combination of seismicity, tectonic features, focal mechanisms, seismic strain e
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and postseismic movement to study the western part of North Algeria, the El Asnam fro
region and its surrounding area in particular. A seismotectonic map of this part of m
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Algeria, delimited by the Mediterranean Sea in the north and the Tellian mountains in ttp
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the south, was built from available geological and seismological data. An examination ://a
of this map shows that the most significant earthquakes are concentrated along tectonic ca
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features and quaternary basins elongated in an east-west direction, suggesting e
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NNW-SSE compressional movements. During the large El Asnam earthquake of ic
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1980 October 10, M,=7.1, vertical movement was measured along a 40 km northeast- u
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southwest thrust fault. These movements were determined geodetically in 1981 with .co
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reference to a basic network previously measured in 1976. In order to control /g
postseismic movement and to ensure the monitoring of the seismic area, a dense ji/a
geodetic network has been regularly measured since 1986, both in planemetry and rtic
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altimetry. The results of the altimetric remeasurements show significant vertical -a
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movements. The elevation changes of the benchmarks have been deduced from precise s
levelling measurements: a remarkable uplift (5.1 5 1.9 mm yr-') of the northwestern trac
block, during the 1986-91 period has been observed, whereas the southeastern block t/12
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is seen to be relatively stable. The Sar El Maiirouf anticline, situated along the central /3
/5
segment of the El Asnam surface breaks, appears to be growing with a maximum 9
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postseismic slip rate of (9.6+ 1.4 mm yr-I). The mean uplift rates computed for the /6
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northwestern block support the view that the 1954 earthquake did not occur on the 6
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same reverse fault as the 1980 event. 6 b
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Key words: El Asnam, focal mechanism, postseismic deformation, seismic strain, gu
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tectonics. st o
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important system of faults. The instrumental seismicity of the rch
INTRODUCTION contact between Iberia and Africa for the period 1928-94 20
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The geology, seismicity and seismotectonics of the western (Fig. 1) is the result of the seismic strain released by the activity 9
part of North Algeria have been the subject of numerous of this system of faults. The seismicity of the studied area is
studies: for example Rotht (1950); Perrodon (1957); Benhallou characterized by the continuous activity of moderate-
& Roussel (1971); McKenzie (1972); Tapponier (1977); (5 < M 56.5) and low-magnitude earthquakes (Mi5 ). In
Girardin et al. (1977); Philip (1983); Thomas (1985); Ouyed examining the seismic map shown in Fig. 1, it can be clearly
(1981), Meghraoui (1988); Buforn, San de Galdeano & Udias seen that most of the seismic activity in the Ibero-Maghrebian
(1995). The seismic activity in this area is directly associated region is concentrated along the Atlas mountains, the gulf of
with the plate boundary between Europe and Africa. Formed Cadiz and southern Iberia. The large El Asnam (Algeria,
basically by the Alpine domain, the Atlas is located on an ,1980 October 10, M,=7.1) and Cape St Vicente (offshore of
Portugal, 1968 February 2, M,=7.8) earthquakes are the most
important events to occur in the western Mediterranean area
*Now at: Acadernie des Sciences de Bulgarie, Laboratoire de Geodesie,
Sofia, Bulgaria. during this century.
?Now at: Dept de Geofisica y Meteorologia, Facultad de Ciencias, The Cheliff region (formerly El Asnam), in the north of
Universidad Complutense, 28040 Madrid, Spain. Algeria, is interesting for the study of regional deformation,
0 1997 RAS 597
598 K. Lammali et a].
-5" 0'
40"
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Figure 1. Instrumental seismicity of the contact between Iberia and Africa for the period 1928-94 (NEIC data file) and location of the studied rtic
area shown in Fig. 2. Figs 1, 2, 3, 8 and 10 are plotted with GMT software (see Wessel & Smith 1991j . le-a
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s
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linked with seismotectonic activity. Numerous geological and in a wedge-like area that extends from the limit of the Tellian ct/1
geophysical investigations of this region indicate that this is a Atlas, in the south, to the coast line. This tectonic zone (Fig. 2), 2
9
very active area, where several seismic events have occurred contained in Neogone and Quaternary deposits, extends to /3
/5
since the beginning of the century (see Bezzeghoud et al. 1995). the Messetta basin, in the western part of the Tellian Atlas, 9
7
Bezzeghoud et al. (1995) showed, from historical levelling data, and to the Mitidja basin, close to the Blidean Atlas. These /6
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a significant vertical uplift (around 1.34 m) near the Sar el tectonic features (Fig. 2) are consistent with the map of maxi- 16
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Magrouf anticline, probably associated with a blind shallow mum observed intensities (Mokrane et al. 1994; Bezzeghoud 6
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thrust fault parallel to the 1980 main thrust fault with an offset et al. 1996), for the period 1716-1989 (Fig.2) and the new y
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of 6 km towards the west. aeromagnetic map of northern Algeria (Asfirane & Galdeano u
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In this paper, we first of all present an overview of the 1995). st o
seismicity and seismotectonics of northwestern Algeria and the The analysis of the distribution of epicentres, during the last n
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El Asnam region in particular, based on geological features, three centuries, leads to the conclusion that the seismogenic 0
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earthquake distribution and focal mechanisms. Second, we zones are located around the following regions: Oran, the axes
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present the coseismic movements associated with the 1980 El of Mascara-Relizane-El Asnam, Medea-Blida-Algiers and rc
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Asnam earthquake and the postseismic movements on the Sour el Ghozlane city. The location of recent earthquakes 2
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entire El Asnam fault zone obtained from periodical surveys (event numbers 13, 15, 16 and 21, Table 1, Fig. 2) is consistent 19
from 1986 to 1991 by a precise levelling method. The results, with this analysis. In previous centuries, a number of moderate
given as mean displacement and uplift rates, are interpreted in earthquakes affected the region of Oran; the most significant
relation to the pattern of major faults inferred from studies of of them occurred in 1790 with an intensity of X. During this
the 1980 surface breaks, the dislocation model of Bezzeghoud century, on the other hand, no significant earthquake (A425 .0)
et al. (1995) and the seismic strain computed from 22 events has occurred in this part of Algeria. This seems to indicate
that occurred in the western part of North Algeria. that a significant seismic gap exists in the vicinity of Oran city
(Bezzeghoud et al. 1996). Information concerning the seismic
history of Algeria can be found in the recent works of Mokrane
SEISMOTECTONIC FRAMEWORK
et al. (1994) and Benouar (1994).
The history of earthquakes in the El Asnam region during
Tectonics and seismicity
the last 288 years (Mokrane et al. 1994) does not suggest the
The global tectonic activity of the western Tellian Atlas is presence of great shocks before 1980 along the El Asnam
located in a broad zone (Fig. 2) of fractures and deformation thrust fault, except for the event of 1954 September 9 (M,=
0 1997 RAS, GJl 129, 597-612
0- 1" 2" 3" -1" ' 37" 37" ,36" 36" .35" 35" io 3. 2" 2. Figure Seismotectonic map associated with a topographic shaded relief generated using a high-resolution Digital Elevation Model (DEM) showing the western Tellian Atlas of Algeria. EA Rev.: =X: QF: El Asnam reverse fault, Rev.: reverse fault, Quaternary fault NF normal fault, maximal observed intensity for X, Quat.-basin: quaternary basin (Messeta, Habra, Ghriss, lower 10 10 = ul. Thomas 977), Philip Meghraoui (1983), Meghraoui Cheliff, middle Cheliff, Mitidja), Lake: induced 1980 lake. Tectonic features come from Glangeaud (1932), Perrodon (1957), Philip & & et ( L. on et 1) (1986), Meghraoui (1988), Bounif (in preparation). The focal mechanisms (see Table are shown lower-hemisphere projections, where dark quadrants indicate compressional arrivals. In a/. of the upper left corner there is a lower-hemisphere projection P-axes circles) and T-axes (open circles) from events occurring between 1954 and 1994 (Table Note the roughly NNE-SSE 22 (close 1). yr-') evaluated from the cumulative orientation of the P-axes. The large open arrow show the average direction given by the P-axes associated with the horizontal shortening slip rate (7.6 mm El seismic moment for the period 1716-1994 (see text for details). The Asnam area studied in this paper is framed (see Fig. 3). Downloaded from https://academic.oup.com/gji/article-abstract/129/3/597/601626 by guest on 30 March 2019
600 K. Lummali et al.
Table 1. Focal parameters of earthquakes from western Algeria (Figs 2, 3).
Nodal planes T-axis P-axis
N Date Ms str dp rake str dp Rake az pl az pl References
-
7 09/09/54 6.5 253 61 104 46 32 76 194 71 333 15 Espinoza & Lopez-Arroyo (1984)
9 10/09/54 6.0 44 90 -8 134 82 172 89 0 179 0 Dewey (1990)
6 05/06/55 5.2 172 56 -32 281 64 - 141 45 5 140 45 Shirokova (1967)
22 13/07/67 5.1 30 40 132 260 61 60 216 61 329 12 Mc Kenzie (1972)
4 lO/lO/SO 7.3 225 54 83 51 36 80 106 80 320 9 Deschamps et ul. (1982)
5 101I 0/80 6.1 58 43 81 250 47 98 227 84 334 2 Harvard
12 13/10/80 4.0 63 42 69 27 1 51 108 239 75 348 5 Harvard
10 30/10/80 4.8 209 46 115 64 49 66 42 72 137 2 Cisternas et ul (1982)
3 0811 1/80 5.0 270 45 126 44 55 59 257 65 156 5 Harvard
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18 05/12/80 5.0 112 61 -179 21 89 - 29 70 19 333 21 Harvard o
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2 071 12 /80 5.8 277 40 140 39 66 51 266 56 152 15 Harvard n
181 01 15/ /0021//88 11 45..75 211801 4533 2694 7624 6572 113192 3313 4742 113308 95 HHaarrvvaarrdd / loade
d
171 1194//0042//88 11 44..92 12968 6577 --1186 219274 7736 -- 115466 25644 142 314662 2393 HCoarcvaa &rd Bufod (1994) from
20 15/11/82 5.0 2 74 70 -169 180 80 - 20 228 6 136 21 Harvard h
19 03/05/85 4.5 225 54 83 57 36 80 106 80 320 9 Jimenez (1991) ttp
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14 3 1/ 10/88 5.7 103 55 167 20 1 79 36 68 33 328 16 Harvard ://a
13 29/10/89 5.8 242 55 87 71 34 94 132 11 336 80 Bounif et al in prep ca
15 09/02/90 4.5 49 18 95 225 72 88 132 63 316 27 Harvard de
16 19/01/92 4.7 277 85 -169 186 79 175 23 1 6 322 10 Bezzeghoud et al. (1994) m
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21 18/08/94 5.5 255 55 149 146 65 39 106 45 202 7 Bezzeghoud & Buforn (1996) .o
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6.6), which we suspect did not take place on the El Asnam diffuse, no active Quaternary folds are identified in this zone ji/a
thrust fault (Bezzeghoud et al. 1995). However, numerous and no evidence of surface displacement was found in rtic
earthquakes were located northwest of the El Asnam fault reconnaissance performed during several field studies (Philip le
system (Fig. 3) with a continuing occurrence of small to & Meghraoui 1983). -ab
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moderate events (M < 6.0) and large to major shocks (M 2 6.0) The strike-slip earthquakes with a normal component in the tra
separated by long time intervals (see the Discussion section). second group (nos 16, 17, 18, 20 in Fig. 3 and Table 1) appear ct/1
The most important earthquakes to have occurred in the El to be distinct from those in group 1, which are on the uplifted 2
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Asnam region and its surrounding area are those of block. Events in group 2 are shallow (55 h 2 10 km), like the /3
/5
1858 March 9 (Kherba, lo=IX, M=6.5), 1891 January 15 Rouina earthquake (no. 16 in Fig. 3 and Table l), and probably 9
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(Gouraya, lo=X), 1922 August 25 (Bord Abou el Hassen, lo= ruptured into the relatively unconsolidated sediments. This /6
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IX, M=5.1), 1934 September 7 (El Abadia-El Attaf, Io=IX, could also be explained by the fact that the 'basement' under 16
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M = 5.0) and 1954 September 9 (El Asnam, lo =X-XI, M = the SE block (3-6 km) is deeper than that situated below the 6
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6.5). In the instrumental period (1950-80, Mokrane et al. NW block (2-3 km), as shown by Asfirane (1993) from y
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1994) events of such magnitudes (3.5 IM < 5.0) occurred aeromagnetic data of the El Asnam region. The limit between u
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predominantly between El Asnam and Tenes (near the coast). the NW and SE blocks is marked by a large decrease of st o
seismic activity, showing strain and stress accumulation on the n
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uplifted block (NW block). As shown by these compressional 0
Focal mechanisms M
focal mechanisms perpendicular to the fold axes, most of the a
The focal mechanisms of the El Asnam region shown in Fig. 3 deformation is taken up by thrusting and folding distributed rch
can be divided into two groups: (1)e vents with thrust mechan- in the area along the El Asnam surface breaks and other 20
1
isms located on the NW block of the El Asnam thrust fault possible active faults striking NNE-SSW (Fig. 3). 9
and particularly near the 1980 surface breaks; (2) events with The spatial distribution of the thrust mechanisms agrees
strike-slip mechanisms distributed on the SE block. The earth- both with the zone of intense seismic activity (Fig. 3) and with
quakes in the first group included the 1954 (M,"=6.6) and the postseismic movements recorded in the NW block, which
1980 (M,=7.1) events. These events are distributed on the are the second subject of this study. Here, the El Asnam region
main N W-dipping fault plane and have generally higher magni- which carries the majority of the thrusting in north Algeria is
tudes than those of the SE block. Besides, the NW block has divided into two principal blocks. The first is a northwest part
a higher seismic activity. No M 2 6.0 earthquake has struck of the El Asnam thrust fault which is dominated by
the SE block during the twentieth century (Mokrane et a!. NE-trending thrust faults and active seismicity characterized
1994; Bezzeghoud et ul. 1995). These observations suggest two by medium and large magnitudes. The second, southeast, part
alternatives: (1) the most important tectonic activity of the El is characterized by earthquakes with small magnitude and
Asnam region affects the NW block; (2)t here is a high seismic strike-slip mechanisms with a large normal component
risk southeast of the El Asnam thrust fault. However, the resulting in roughly E-W crustal extension, particularly the
second hypothesis is unlikely because the seismicity is very event numbers 16, 17 and 18 (Fig. 3, Table 1).
0 1997 RAS, GJI 129, 597-612
Postseismic deformation at El Asnam 60
1.2" 1.4" 1.6" 1.8" 2"
m
36.6"
2000
1200 D
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1100 d
36.4" fro
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9
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500 0
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400 gu
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-35.8" t o
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300 0 M
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200 1
9
- 35.6"
Figure 3. Seismotectonic map of the El Asnam region with the instrumental (circle) and historical (square) seismicity of Cheliff basin from 1858
to 1994: solid circles, NEIC data file; open circles CRAAG data file associated with relocated epicentres given by Dewey (1991); squares, CRAAG
data file. Size is proportional to maximum intensity or magnitude. The Sar el Mairouf area is shown in the box (see Fig. 8). Other symbols are as
in Fig. 2.
0 1997 RAS, GJI 129, 597-612
602 K. Lammdi et al.
The mechanisms of 22 events with 4.05 M, I 7.3 distributed network to monitor vertical displacements contains 32 bench-
in the western part of North Algeria (Fig. 2, Table l), domi- marks forming a dozen closure loops within a total distance
nated by thrust faulting, show that the P-axes are nearly of 80 km. This network crosses the faulted area in four places,
normal to the Africa-Europe plate boundary, with a and the observation field is 10 km from each part of the fault.
NNW-SSE direction. This is in agreement with the view of The benchmarks and levelling lines are shown in Fig.4, on
several authors (e.g. Udias & Buforn 1991; Buforn et al. 1995). top of a geological map.
COSEISMIC DEFORMATION PROCEDURES AND PRECISION
Most of the benchmarks used for measuring vertical strain are Field procedures, data processing and error estimates for the
on the railway, near Oued Fodda village, where the most 1986-91 levelling surveys are described here and summarized
important offset was observed during the El Asnam earthquake in Tables 2 and 3. Although the Wild N3 spirit levels are
(Ouyed et a/. 1981). The others are situated in the El-Karimia immune to the systematic magnetic errors that plague all the
region and on the Oued Fodda dam. The profile cuts the fault compensator levels, the Wild NA2 level associated with one
D
perpendicularly 4 km west of Oued Fodda village, and has a pair of Wild 3 m wooden rods were used during all the surveys. o
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NW-SE trend with 40 km length. The northern end of the Tests have shown that its magnetic error is relatively small n
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profile is near the Ouled bou Zina village (West of Beni (Rumpf & Meurish 1981). The geometric second-ofder double- ad
Rached). run segments method, with specific considerations, in the flat ed
Eight months after the 1980 quake, a French-Algerian crew region was carried out. fro
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carried out levelling measurements through the rupture area Because levelling operations can be contaminated by both h
along a SE-NW profile crossing the fault trace. The profile systematic and random errors, we developed a specific measure- ttp
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was tied to a levelling line (first order) along the Algiers-Oran ment procedure with regard to the instrumentation employed ://a
railway, installed at the beginning of the century (1905, see in the field. Random errors are caused by several aspects of c
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Bezzeghoud et al. 1995) by the French administration and the surveying process (Marshall, Stein & Thatcher 1991): de
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completed in 1976 by the National Institute of Cartography inaccurate readings of the levelling instrument caused by ic
of Algiers (INC). The 1976 elevations have been taken as the atmospheric scintillation and ground vibrations, inadequate .o
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reference for the coseismic slip measurements of the 1954 and rods, etc. The wooden rods used are subject to an undetermined p.c
1980 events. Relative vertical positions were determined by dilatation factor due to the fluctuation of the temperature and om
trigonometric levelling (Ruegg et al. 1982), observing simul- humidity. Random error is also caused by unpredictable /g
taneously reciprocal vertical angles and distances. The pre- variations in instruments, environmental factors (topography) ji/a
cision was estimated to be about 1 cm km-'. The reference and procedures; it cannot be eliminated but can be minimized rtic
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point is the Oued Fodda dam benchmark, far from the by proper procedures (Dzurisin & Yamashita 1987). To dis- -a
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southeast end of the active zone. Ruegg et al. (1982) observed criminate among these disturbances, the observations were s
a vertical uplift of 5.15 m of the overthrusting block with a carried out during the periods of the lowest temperature trac
progressive reduction of this displacement towards the north- changes: spring and autumn. Times of observations were t/1
2
west; they also noticed a 0.76 m depression on the SE block, chosen to be during periods with lower amounts of sunshine 9/3
within 5 km of the fault, but this movement decreases rapidly (7-10 am and 4-8 pm). /5
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j towards the southeast. This vertical movement, measured in The systematic forward and backward observations (40 m 7/6
1981 along the railway levelling route, was confirmed by an of maximum distance between the level and the rods, 50 cm 01
independent method of classical levelling (Dimitrov et al. 1987) above ground level), with a strict stadimetric reading tolerance 62
6
in 1986. A comparison of the altitudes determined in 1976, (< 1 mm), minimize much of the random error. However, the b
y
1981, 1983 and 1986 corroborates the important deformation precision of a levelling survey is degraded by systematic error g
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caused by the 1980 October 10 earthquake, measured in June as well as by random error. The principal random errors e
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1981 (Ruegg et al. 1982). Our reference point (the benchmark (Bomford 1971) are associated with rod miscalibration, t o
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Rn12 located near El Attaf village, Fig. 4) is situated on the unequal refraction, and rod settlement. The former two errors 3
0
SE block of the fault, which is relatively stable. are correlated with topography, and the latter error depends M
upon soil conditions (Dzurisin & Yamashita 1987). Systematic arc
ALTIMETRIC NETWORK FOR errors have received considerable attention and are the subject h 2
MONITORING of continuing debate (e.g. Rumpf & Meurish 1981; Strange 01
9
1981; Stein 1981; Hodahl 1982; Packard & MacNeil 1983).
In 1985, INC measured a 175 km levelling route which belongs
The random error which accumulates with the square root of
to the General Levelling Network of Algeria (second order),
distance (L),e xpressed as
cutting the seismogenic area of Cheliff along the No. 4 National
Road, between the towns of El-Khemis and Relizane (measure- r=aL"' mm km-', (1)
ments carried out with a Wild N3 level and invar rods, second- where a is the instrumental precision, was assumed to be
order double run). In 1986, a new levelling network was 1.5 mm km-'. Operator change after each run allows us to
installed on the basis of 13 old benchmarks of the Algiers-Oran avoid systematic reading errors. Then, the accidental kilometric
railway, and 13 new ones on the No. 4 National Road. Thus, error q can be computed from the random error (General
26 benchmarks form a 40 km initial network, which cuts the Direction of Geodesy and Cartography 1980):
faulted area twice within an observation field of 10 km.
After the data had been obtained from three measurement ql = [ 1/4n z(d2/l)m]m1 'k2m -' ,
campaigns (Dimitrov et a/. 1987), in 1988, 1989 and 1990, where d = (!I~,~- h2.1)i s the discrepancy between the backward
complementary levelling routes were installed. The levelling and forward measurements of a section, n the total number of
0 1997 RAS, GJ1 129, 597-612
sands; posits; Sar el
omerates and red Recent alluvial develling section; 4,
I, ogical reference: conglVIII, mark (Tortonian); 3, Old National Road le Downlo
Figure on Schematic map the vertical network. Benchmarks and levelling lines a detailed geological map (from Jacob Ficher 1906). Geol& of 4. IV, blue clays; marine Pliocene; ancient alluvial deposits; Ostrea sandstones; VI, limestones and Lithothamniurn sandstones; yellow 11, V, VII. 111, 2, XIV, shales (Medjanian). Symbols used in the legend: Algiers-Oran National Road levelling section; Algiers-Oran railway levelling section; 1, 5, 12 TF, El El Karimia levellinn section; RP, Reference Point (Rn reference altitude): Asnam thrust fault. Mairouf anticline levelling section; aded from https://academic.oup.com/gji/article-abstract/129/3/597/601626 by guest on 30 March 2019
604 K. Lammali et al.
Table 2. Error estimates (mm/km) double-run sections, and I the distance in kilometres. The
observed circuit misclosures, given as follow, are used to
Period 5 ‘11 ‘12 PKE control the results of formula (2) when it is possible:
1986 0.08 0.51 - 0.51 ylz = [ 1/N ~(W~/Lm)m] k”m~-’ , (3)
1987.4 0.05 0.34 - 0.34
where w, N and L are, respectively, the observed misclosures,
1987.9 0.05 1.19 - 1.19 the number of levelling closed circuits and the total length of
1988.5 0.06 0.83 - 0.83
the levelling network. The systematic error z is estimated
1989 0.08 0.50 2.66 0.50
by the following formula:
1991 0.08 0.77 - 0.77
1992 0.04 0.58 - 0.58 z = [ 1/4CL C(SZ/L)l1’’m m km-’, (4)
where S is the cumulative error. The Probable Kilometric
Table 3. Benchmark elevation and vertical displacements, 1986-91, Cheliff region.
D
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BM* Dist. (km)” 1986.9 Elevation change (mm), Rn 4 levelling route (Rn) VfdV wn
Elev. (m) (mm/y) lo
a
86.9-87.4 87.4-87.9 87.9-88.5 88.5-89.5 89.5-91.5 d
e
d
Rn21 00.000 123.496 - 02 04 - 03 - 02 00 -0.6f 1.6 from
Rn20‘ 01.150 134.617 -01 -01 09 - 01 02 2.2k 1.9 h
Rn20 02.455 178.157 03 06 00 04 08 5.2f 1.7 ttp
s
RRnn1187 ” 0067..463763 223176..478547 0063 0074 -- 0075 - 0011 - 09 21..70ff 11..99 ://ac
Rn17’ 10.034 181.664 - - - 02 00 - - 1.7f 2.3 ade
Rn17 11.230 172.242 - 04 00 04 - 02 03 - 0.4 k 2.0 m
Rn16 12.470 162.202 00 - 07 03 - 02 - 1.5 -2.5f1.2 ic.o
Rn15 13.778 156.922 00 -01 04 - 02 00 0.5 k 1.8 up
Rn14 14.957 156.970 -01 -01 04 00 0.7 f3 .2 .co
Rn13 16.797 158.999 00 00 00 - - - m
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Rn12b 17.997 150.098 00 00 00 00 00 00 ji/a
rtic
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BM * Dist. (km)” 1986.9 Elevation change (mm), Railway levelling route (R) Vk6V -a
b
Elev. (m) 86.9-87.4 87.4-87.9 87.9-88.5 88.5-89.5 89.5-91.5 (mmiu) stra
c
t/1
2
R75 00.0 120.078 00 02 06 03 00 3.4k 1.9 9
R74 1.25 133.992 00 04 - 05 04 3.8 f 1.6 /3/5
R73 2.55 147.214 -01 06 06 -01 4.8 k 2.3 97
R72 3.85 159.891 02 00 00 00 -01 0.7k1.6 /6
0
R71B 4.90 164.653 - 03 05 02 02 - 02 1.7f 1.8 16
R71A 6.10 172.645 - 03 08 -01 05 3.3 f3 .4 26
R71 8.04 191.617 - 03 03 11 05 5.8 f2 .9 by
R70 9.68 207.616 05 01 04 03 5.4 f2 .7 g
u
R69“’ 11.22 18 5.606 - 04 - - 03 05.5 - - 2.5 f2 .5 es
R69 12.9 169.087 - 05 03 02 -01 -11 - 7.2 f2 .5 t o
n
R68 14.5 157.971 -01 00 02 01 03 0.8 f2 .3 3
R67A 16.08 158.952 - 05 11 02 - 05 2.6 & 2.2 0 M
a
rc
h
BM* Dist. (km)’ 1987.4 E.C. (mm), SEM anticline levelling route v+6v 20
1
Elev. (m) (mmiy) 9
87.4-87.9 87.9-88.5 88.5-89.5 89.5-91.5
R71 0.0 191.614 03 11 05 - 9.8f1.2
S8 0.62 182.929 02 16 20 - 16.9 f 2.5
s7 1.92 236.442 - 07 15 - 13.3 k0.7
S6 2.37 262.178 01 05 02 - 4.1 f1.4
s5 3.07 296.534 06 05 06 23 9.5+ 1.4
s4 3.57 238.428 - 06 07 11 11 4.0 f0 .9
s3 4.47 201.121 - - -01 - -
s2 5.05 171.766 - 02 - 06 -
s1 5.80 154.798 01 01 - 02 - 08 -0.6+ 1.4
R68 6.72 157.970 00 02 01 03 1.4f 1.5
*Benchmark number; a distance along levelling route; b assuming no displacement at benchmark Rn12 since 1986. SEM=Sar el Mabrouf.
0 1997 RAS, GJI 129, 597-612
Postseismic deformation at El Asnam 605
Table 4. Global 1954 and 1980 dislocation model. errors, the miscalibrated wooden rods could not be taken as
(a) a sure reference for the readings. To improve the data it is
necessary to use calibrated levelling rods, together with a
1954 DISLOCATION MODEL
control of the thermal extension of the rod tapes. Furthermore,
the automatic level (Wild NA2) used for all the campaigns
Width Length d U d, Mox loz6 Mw
should be equipped with a micrometer to take more precise
km km deg. m km dynecm
readings.
5.15 21 67.5 3 1 0.98 6.6 The benchmarks do not all have the same quality of
implantation. Some of them are installed on concrete piers of
bridges, posts and stone aqueducts. They thus have local
movements which should be taken into account. When the
1980 DISLOCATION MODEL observed movements reach 5-10 times the PKE, we can
consider these movements as tectonic activity.
Subfault Width Length d U d, Mox 10'' Mw
km km deg. m km dynecm D
o
w
n
1 5.15 21 67.5 8 5 2.85 DATA AND RESULTS lo
a
2 1.35 2.5 67.5 4 3.7 0.044 d
e
43 41 .78 26..52 6670. 5 41 .5 03 .5 00..014495 Vertical measurements d fro
m
5 4 6.2 60 1 0 0.082 Three levelling routes (Algiers-Oran railway, No. 4 National h
6 6.2 2.7 53.5 8 0 0.442 Road, and Sar el Maiirouf anticline) that have been measured ttp
78 31..52 4 99..55 3600 85 23..23 00..817974 fidiveea oarb soiuxt teimleevsa twioenr ec thaaknegne sa so fa t hreef ebreenncchem, ianr kosr.d Terh eto c ofomrmpa arin- s://aca
9 1.24 9.5 60 3 1.1 0.116 d
son of altimetric remeasurements of 1987-89 with those of e
10 1.24 9.5 60 2 0 0.077 m
Total Mo and associated Mw magnitude 4.9 7.1 (1A9l8g6i efrosr- Othrea nR nr4a illwevaeyll inagn dr oSuater, ealn dM a1i9ir8o6u ffo rl etvheel litnwgo rootuhteerss) ic.ou
p
d, U, d,, Mo and Mw are, respectively, dip, displacement at the source, taken as reference, permitted us to determine altitude changes. .c
o
depth of fault upper edge, seismic moment and magnitude. The We can see in Fig. 5, which shows these three profiles, that m
ssutrbikfaeu oltfs 2n1u7"m. bered from 1 to 10 are computed with a fixed mean tthhearte a anr eu pvleifrty osf m2a lclm m oonve amveenratsg en eisa rs etehne ofanu ltth.e I tn ocratnh wbee ssteeernn /gji/artic
block; meanwhile, on the SE block, a slight subsidence occurred le
Error (PKE) is then: -a
near the fault trace. b
s
PKE=(q4+r2)''2 mmkm-' These vertical displacements are moderate, but in good tra
c
We can see the PKE of the total network, for the 1986-91 asigtrueaetmede nnto rwthitwh etsht e ofc otsheei smfaiucl t,m souvbesmideenntcse: uopnl iftth eo f sothueth zeoasnte. t/129
lmateeads ubrye mfoernmtsu plae r(i2od) ,a rien aTlawbalye s1 s. mThalel earc tchidaenn tthaol seer rcoarlsc uclaaltceud- We should take into account, however, that the benchmark /3/5
Rn12, the reference for the altimetric variation, is relatively 97
by formula (3). In our case q1 1/3 qz, which represents the close to the rupture area (3 km perpendicular to the fault /60
normal error distribution (Marshall et al. 1991). The systematic 1
trace, Fig. 4). This benchmark subsided 38 cm during the 1980 6
(eTrraobrl e v2a)l.u es (T) range between 0.04 and 0.08 mm km-l earthquake and could be subject to postseismic displacement. 26 b
The velocity (Vi)is derived by dividing the elevation change This led us to extend the network on the uplifted block and y g
to install the El Karimia levelling section towards the south ue
by the time interval between two different surveys, and the s
uplift rate for the period 1986-91 is given by the formula (Dimitrov & Lammali 1989). t o
n
In two cases, the observed differences of deformation for 3
0
V=c ~ i j Nmm yr-l, (6) two adjacent benchmarks (Rn18-Rn17" and S6-S5, see Figs M
where N is the number of reiterations. 6, 7 and 8) are probably due to heterogeneity shown on the arc
The velocity error 6V is given by the elevation change error detailed geological map (Fig. 4) described by Jacob & Ficher h 2
(1906). We can observe this benchmark behaviour particularly 0
6(h), divided by the time interval between constituent surveys 1
9
for Rn18 and S6 which are on the same geological structure
(Dzurisin & Yamashita 1987). The velocity error rate (Table 3)
for the same period is given by the formula (ostrea sandstone). They behave with a lower uplift rate with
regard to the others (Figs 7b and c). This superficial heterogen-
6V= 1/N ~{[~(Ahi)Z+~(Ahi+,)2]1'2/ti+l-tmiJm yr-', (7) eity is also identified from the structural analysis of the surface
The elevation-change error &Ah) is equal to the square root deformation (Philip & Meghraoui 1983), showing hanging-
of the sum of the squares of the Probable Kilometric Error wall flexure and normal faulting inside the NW block.
(PKE), of the constituent surveys. Benchmark Rn19' is not taken into account in our interpret-
The instrumentation should be improved to conduct the ation because it has been damaged several times. However,
highest standards of a first-order double-run levelling. Until the behaviour of the others benchmarks shows a regular
now, we have used a specific methodology to discriminate movement during the 5 years of measurements (Fig. 5),p articu-
between the maximums of random and systematic errors and larly benchmark R72, and could be explained by the existence
to have the highest precision possible. However, in spite of all of a smaller 1954 thrust blind fault, suggested by Bezzeghoud
the precautions undertaken in the field to limit the various et al. (1995). This result is discussed in the following sections.
0 1997 RAS, GJI 129, 597-612
606 K. Lummuli et al.
, -
20
10 .. . . . . . .. .... .. . .
0
-111
300
I Unl9 D-I
200
100
I 1988-1 986
- 10 1989-1 986
0
D
I ' -10 ow
n
300 s5 loa
Elev(mat>io n 200 (c) ded fro
100 4n m
H-HO h
ttp
(mm> s://a
' ca
I I I I -10 de
-5 0 5 10 m
Distance (km) ic
.o
u
p
Figure 5. Postseismic movements along (a) the Algiers-Oran railway levelling route, (b) the Algiers-Oran National Road levelling route and (c) .c
o
the Sar el Malrouf anticline levelling route. Two periods of measurements are compared with the 1986 (a and b) and 1987 (c) altitudes. m
/g
ji/a
Mean displacement and uplift rates surveys. We note, particularly, that the Sar el MaCrouf anticline rtic
profile shows a larger uplift rate; this uplift rate is also observed le
Figs 5(a), (b) and (c) indicate the displacement rates of the on the benchmark R71 situated near the fault trace and on -ab
s
benchmarks situated along the Algiers-Oran railway (R), the Sar el Madrouf anticline (Fig. 7). This could be explained tra
National Road (Rn) and Sar El Madrouf (S)( 1986-91 ) levelling by tectonic activity of the Sar el Madrouf anticline. Mean ct/1
sections described in the previous section. In this Figure we uplift rate values for the three levelling sections (Fig. 7) are 2
9
can see that the benchmarks situated in the western and given in Table 3. Finally, we note that the errors diminished /3
/5
eastern zones show, respectively, positive mean displacement to acceptably low levels near unity for the SEM anti- 9
7
rates and stable or negative mean displacement rates. We can cline levelling route, indicating that the deduced uplift rate /6
0
interpret this observation as differential movement of the two within the smaller area is more credible than those for the 1
6
blocks separated by the El Asnam thrust fault. The uplift of whole network. The SEM anticline levelling route presents, 26
the northwestern zone could be considered as an overthrust particularly, a significant uplift rate with regard to the errors. by
of the NW block (Blocks A and B, Fig. 8) on the SE one gu
e
(Block C, Fig. 8). This movement has been observed by Ruegg s
1954 and 1980 dislocation models t o
et al. (1982) and Cisternas, Dorel & Gaulon (1982) from, n
respectively, coseismic measurements and a dislocation model. To verify the postseismic movements presented in the previous 30
Figs 7(a), (b) and (c) show the mean uplift rates of each section, by means of displacement and uplift rates, we calcu- M
a
levelling section (R, Rn and S). This representation reflects the lated the displacement field by modelling fault configurations rc
h
tectonic activity of the fault: uplift of the NW block and slight given recently by Bezzeghoud et al. (1995). The models pro- 2
0
subsidence of the SE block near the fault. The benchmarks posed by these authors are based on coseismic vertical displace- 19
Rn18, and S6, which present a particular mean uplift rate (slow ment, measured along the Algiers-Oran railway, induced by
with respect to the others benchmarks situated on the same the 1954 September 9 (M,=6.6) and 1980 October 10 (M,=
block), were discussed in the previous section. Our study, 7.1) earthquakes. We used the dislocation models based on
based on 1986-91 data, gives an average uplift rate of about analytic expressions given by Okada ( 1985).
5.1 1.9 mm yr-' of the NW block (Blocks A and B, Fig. 8); We combined the two fault models (1954 and 1980) proposed
meanwhile, the SE block (Block C, Fig. 8) is relatively stable. by Bezzeghoud et al. (1995) to obtain the global model shown
Very close to the thrust fault, the Sar el Madrouf levelling in Fig. 9. The 1954 fault resulting from their model is located
section gives an average uplift rate of 9.6+ 1.4 mm yr-'. This between the 1954 earthquake epicentre (near Beni Rached
value represents the mean uplift rate of the Sar el Madrouf surface breaks, NE) and El Asnam city (SW), parallel to the
anticline, where Ruegg et al. ( 1982) observed a vertical uplift 1980 main thrust fault, with an offset of 6 km towards the
of 5.15 m of the overthrusting block due to the 1980 El Asnam west (Fig. 8). The modelling derived from this dislocation
earthquake. On the other hand, we confirm the first results model is shown in Fig. 9 and Table 3. We can see the similarity
given by Dimitrov et al. (1991) based on the 1985-88 levelling between the calculated vertical displacement (Fig. 9) and the
0 1997 RAS, GJI 129, 597-612