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**and H. Benhallou'
' C R A A G , DCpt. ESS, BP 63,16340 BouzarCah, Algiers, Algeria
'Ecole des Mines de Paris, CG, 35 rue S t . Honor&,F-77305 Fontainebleau Cedex, France
Accepted 1997 January 29. Received 1997 January 2; in original form 1996 March 27
SUMMARY
We use a combination of seismicity, tectonic features, focal mechanisms, seismic strain
and postseismic movement to study the western part of North Algeria, the El Asnam
region and its surrounding area in particular. A seismotectonic map of this part of
Algeria, delimited by the Mediterranean Sea in the north and the Tellian mountains in
the south, was built from available geological and seismological data. An examination
of this map shows that the most significant earthquakes are concentrated along tectonic
features and quaternary basins elongated in an east-west direction, suggesting
NNW-SSE compressional movements. During the large El Asnam earthquake of
1980 October 10, M,=7.1, vertical movement was measured along a 40 km northeastsouthwest thrust fault. These movements were determined geodetically in 1981 with
reference to a basic network previously measured in 1976. In order to control
postseismic movement and to ensure the monitoring of the seismic area, a dense
geodetic network has been regularly measured since 1986, both in planemetry and
altimetry. The results of the altimetric remeasurements show significant vertical
movements. The elevation changes of the benchmarks have been deduced from precise
levelling measurements: a remarkable uplift (5.1 5 1.9 mm yr-') of the northwestern
block, during the 1986-91 period has been observed, whereas the southeastern block
is seen to be relatively stable. The Sar El Maiirouf anticline, situated along the central
segment of the El Asnam surface breaks, appears to be growing with a maximum
postseismic slip rate of (9.6+ 1.4 mm yr-I). The mean uplift rates computed for the
northwestern block support the view that the 1954 earthquake did not occur on the
same reverse fault as the 1980 event.
Key words:
tectonics.
El Asnam, focal mechanism, postseismic deformation, seismic strain,
INTRODUCTION
The geology, seismicity and seismotectonics of the western
part of North Algeria have been the subject of numerous
studies: for example Rotht (1950); Perrodon (1957); Benhallou
& Roussel (1971); McKenzie (1972); Tapponier (1977);
Girardin et al. (1977); Philip (1983); Thomas (1985); Ouyed
(1981), Meghraoui (1988); Buforn, San de Galdeano & Udias
(1995). The seismic activity in this area is directly associated
with the plate boundary between Europe and Africa. Formed
basically by the Alpine domain, the Atlas is located on an
* N o w at: Acadernie des Sciences de Bulgarie, Laboratoire de Geodesie,
Sofia, Bulgaria.
?Now at: Dept de Geofisica y Meteorologia, Facultad de Ciencias,
Universidad Complutense, 28040 Madrid, Spain.
0 1997 RAS
important system of faults. The instrumental seismicity of the
contact between Iberia and Africa for the period 1928-94
(Fig. 1) is the result of the seismic strain released by the activity
of this system of faults. The seismicity of the studied area is
characterized by the continuous activity of moderate(5 < M 56.5) and low-magnitude earthquakes ( M i 5 ) . In
examining the seismic map shown in Fig. 1, it can be clearly
seen that most of the seismic activity in the Ibero-Maghrebian
region is concentrated along the Atlas mountains, the gulf of
Cadiz and southern Iberia. The large El Asnam (Algeria,
,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
during this century.
The Cheliff region (formerly El Asnam), in the north of
Algeria, is interesting for the study of regional deformation,
597
598
K . Lammali et a].
-5"
0'
40"
35"
Figure 1. Instrumental seismicity of the contact between Iberia and Africa for the period 1928-94 (NEIC data file) and location of the studied
area shown in Fig. 2. Figs 1, 2, 3, 8 and 10 are plotted with GMT software (see Wessel & Smith 1991j.
linked with seismotectonic activity. Numerous geological and
geophysical investigations of this region indicate that this is a
very active area, where several seismic events have occurred
since the beginning of the century (see Bezzeghoud et al. 1995).
Bezzeghoud et al. (1995) showed, from historical levelling data,
a significant vertical uplift (around 1.34 m) near the Sar el
Magrouf anticline, probably associated with a blind shallow
thrust fault parallel to the 1980 main thrust fault with an offset
of 6 km towards the west.
In this paper, we first of all present an overview of the
seismicity and seismotectonics of northwestern Algeria and the
El Asnam region in particular, based on geological features,
earthquake distribution and focal mechanisms. Second, we
present the coseismic movements associated with the 1980 El
Asnam earthquake and the postseismic movements on the
entire El Asnam fault zone obtained from periodical surveys
from 1986 to 1991 by a precise levelling method. The results,
given as mean displacement and uplift rates, are interpreted in
relation to the pattern of major faults inferred from studies of
the 1980 surface breaks, the dislocation model of Bezzeghoud
et al. (1995) and the seismic strain computed from 22 events
that occurred in the western part of North Algeria.
SEISMOTECTONIC FRAMEWORK
Tectonics and seismicity
The global tectonic activity of the western Tellian Atlas is
located in a broad zone (Fig. 2) of fractures and deformation
in a wedge-like area that extends from the limit of the Tellian
Atlas, in the south, to the coast line. This tectonic zone (Fig. 2 ) ,
contained in Neogone and Quaternary deposits, extends to
the Messetta basin, in the western part of the Tellian Atlas,
and to the Mitidja basin, close to the Blidean Atlas. These
tectonic features (Fig. 2) are consistent with the map of maximum observed intensities (Mokrane et al. 1994; Bezzeghoud
et al. 1996), for the period 1716-1989 (Fig.2) and the new
aeromagnetic map of northern Algeria (Asfirane & Galdeano
1995).
The analysis of the distribution of epicentres, during the last
three centuries, leads to the conclusion that the seismogenic
zones are located around the following regions: Oran, the axes
of Mascara-Relizane-El Asnam, Medea-Blida-Algiers and
Sour el Ghozlane city. The location of recent earthquakes
(event numbers 13, 15, 16 and 21, Table 1, Fig. 2) is consistent
with this analysis. In previous centuries, a number of moderate
earthquakes affected the region of Oran; the most significant
of them occurred in 1790 with an intensity of X. During this
century, on the other hand, no significant earthquake (A425.0)
has occurred in this part of Algeria. This seems to indicate
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
et al. (1994) and Benouar (1994).
The history of earthquakes in the El Asnam region during
the last 288 years (Mokrane et al. 1994) does not suggest the
presence of great shocks before 1980 along the El Asnam
thrust fault, except for the event of 1954 September 9 ( M , =
0 1997 RAS, G J l 129, 597-612
2"
2"
3"
' 37"
L.
Figure 2. Seismotectonic map associated with a topographic shaded relief generated using a high-resolution Digital Elevation Model ( D E M ) showing the western Tellian Atlas of Algeria. EA Rev.:
El Asnam reverse fault, Rev.: reverse fault, QF: Quaternary fault N F normal fault, 10 =X: maximal observed intensity for 10 = X, Quat.-basin: quaternary basin (Messeta, Habra, Ghriss, lower
Cheliff, middle Cheliff, Mitidja), Lake: induced 1980 lake. Tectonic features come from Glangeaud (1932), Perrodon (1957), Philip & Thomas ( 977), Philip & Meghraoui (1983), Meghraoui et ul.
(1986), Meghraoui (1988), Bounif et a / . (in preparation). The focal mechanisms (see Table 1 ) are shown on lower-hemisphere projections, where dark quadrants indicate compressional arrivals. In
the upper left corner there is a lower-hemisphere projection of P-axes (close circles) and T-axes (open circles) from 22 events occurring between 1954 and 1994 (Table 1). Note the roughly NNE-SSE
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 yr-') evaluated from the cumulative
seismic moment for the period 1716-1994 (see text for details). The El Asnam area studied in this paper is framed (see Fig. 3).
.35"
3.
io
1"
35"
0-
,36"
-1"
36"
37"
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
9
6
22
4
5
12
10
3
18
2
8
11
17
1
20
19
14
13
15
16
21
09/09/54
10/09/54
05/06/55
13/07/67
lO/lO/SO
101I 0/80
13/10/80
30/10/80
0811 1/80
05/12/80
07112/80
15/01/81
0 1/02/8 1
14/02/81
19/04/8 1
15/11/82
03/05/85
3 1/10/88
29/10/89
09/02/90
19/01/92
18/08/94
6.5
6.0
5.2
5.1
7.3
6.1
4.0
4.8
5.0
5.0
5.8
4.7
5.5
4.9
4.2
5.0
4.5
5.7
5.8
4.5
4.7
5.5
253
44
172
30
225
58
63
209
270
112
277
181
210
26
198
274
225
103
242
49
277
255
61
90
56
40
54
43
42
46
45
61
40
53
43
67
57
70
54
55
55
18
85
55
104
-8
-32
132
83
81
69
115
126
-179
140
29
64
-18
-16
-169
83
167
87
95
-169
149
46
134
281
260
51
250
27 1
64
44
21
39
72
64
124
297
180
57
20 1
71
225
186
146
32
82
64
61
36
47
51
49
55
89
66
67
52
73
76
80
36
79
34
72
79
65
76
172
- 141
60
80
98
108
66
59
- 29
51
139
112
- 156
- 146
- 20
80
36
94
88
175
39
194
89
45
216
106
227
239
42
257
70
266
31
33
254
64
228
106
68
132
132
23 1
106
71
0
5
61
80
84
75
72
65
19
56
44
72
4
12
6
80
33
11
63
6
45
333
179
140
329
320
334
348
137
156
333
152
130
138
346
162
136
320
328
336
316
322
202
15
0
45
12
9
2
5
2
5
21
15
9
5
29
33
21
9
16
80
27
10
7
Espinoza & Lopez-Arroyo (1984)
Dewey (1990)
Shirokova (1967)
Mc Kenzie (1972)
Deschamps et ul. (1982)
Harvard
Harvard
Cisternas et ul (1982)
Harvard
Harvard
Harvard
Harvard
Harvard
Harvard
Coca & Bufod (1994)
Harvard
Jimenez (1991)
Harvard
Bounif et al in prep
Harvard
Bezzeghoud et al. (1994)
Bezzeghoud & Buforn (1996)
-
6.6), which we suspect did not take place on the El Asnam
thrust fault (Bezzeghoud et al. 1995). However, numerous
earthquakes were located northwest of the El Asnam fault
system (Fig. 3) with a continuing occurrence of small to
moderate events ( M < 6.0) and large to major shocks ( M 2 6.0)
separated by long time intervals (see the Discussion section).
The most important earthquakes to have occurred in the El
Asnam region and its surrounding area are those of
1858 March 9 (Kherba, lo=IX, M=6.5), 1891 January 15
(Gouraya, l o = X ) , 1922 August 25 (Bord Abou el Hassen, lo=
IX, M=5.1), 1934 September 7 (El Abadia-El Attaf, I o = I X ,
M = 5.0) and 1954 September 9 (El Asnam, l o =X-XI, M =
6.5). In the instrumental period (1950-80, Mokrane et al.
1994) events of such magnitudes (3.5 I
M < 5.0) occurred
predominantly between El Asnam and Tenes (near the coast).
Focal mechanisms
The focal mechanisms of the El Asnam region shown in Fig. 3
can be divided into two groups: (1) events with thrust mechanisms located on the NW block of the El Asnam thrust fault
and particularly near the 1980 surface breaks; (2) events with
strike-slip mechanisms distributed on the SE block. The earthquakes in the first group included the 1954 (M,"=6.6) and
1980 (M,=7.1) events. These events are distributed on the
main N W-dipping fault plane and have generally higher magnitudes than those of the SE block. Besides, the NW block has
a higher seismic activity. No M 2 6.0 earthquake has struck
the SE block during the twentieth century (Mokrane et a!.
1994; Bezzeghoud et ul. 1995). These observations suggest two
alternatives: (1) the most important tectonic activity of the El
Asnam region affects the NW block; ( 2 ) there is a high seismic
risk southeast of the El Asnam thrust fault. However, the
second hypothesis is unlikely because the seismicity is very
/
diffuse, no active Quaternary folds are identified in this zone
and no evidence of surface displacement was found in
reconnaissance performed during several field studies (Philip
& Meghraoui 1983).
The strike-slip earthquakes with a normal component in the
second group (nos 16, 17, 18, 20 in Fig. 3 and Table 1) appear
to be distinct from those in group 1, which are on the uplifted
block. Events in group 2 are shallow (5 5 h 2 10 km), like the
Rouina earthquake (no. 16 in Fig. 3 and Table l), and probably
ruptured into the relatively unconsolidated sediments. This
could also be explained by the fact that the 'basement' under
the SE block (3-6 km) is deeper than that situated below the
NW block (2-3 km), as shown by Asfirane (1993) from
aeromagnetic data of the El Asnam region. The limit between
the NW and SE blocks is marked by a large decrease of
seismic activity, showing strain and stress accumulation on the
uplifted block (NW block). As shown by these compressional
focal mechanisms perpendicular to the fold axes, most of the
deformation is taken up by thrusting and folding distributed
in the area along the El Asnam surface breaks and other
possible active faults striking NNE-SSW (Fig. 3).
The spatial distribution of the thrust mechanisms agrees
both with the zone of intense seismic activity (Fig. 3) and with
the postseismic movements recorded in the NW block, which
are the second subject of this study. Here, the El Asnam region
which carries the majority of the thrusting in north Algeria is
divided into two principal blocks. The first is a northwest part
of the El Asnam thrust fault which is dominated by
NE-trending thrust faults and active seismicity characterized
by medium and large magnitudes. The second, southeast, part
is characterized by earthquakes with small magnitude and
strike-slip mechanisms with a large normal component
resulting in roughly E-W crustal extension, particularly the
event numbers 16, 17 and 18 (Fig. 3, Table 1).
0 1997 RAS, G J I 129, 597-612
Postseismic deformation at El Asnam
1.2"
m
1.4"
1.6"
1.8"
60
2"
36.6"
2000
1200
1100
36.4"
1000
900
e36.2"
800
700
L
e
V
600
e
-36"
1
500
400
-35.8"
300
200
- 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
j
K . Lammdi et al.
The mechanisms of 22 events with 4 . 0 5 M , I7.3 distributed
in the western part of North Algeria (Fig. 2, Table l ) , dominated by thrust faulting, show that the P-axes are nearly
normal to the Africa-Europe plate boundary, with a
NNW-SSE direction. This is in agreement with the view of
several authors (e.g. Udias & Buforn 1991; Buforn et al. 1995).
network to monitor vertical displacements contains 32 benchmarks forming a dozen closure loops within a total distance
of 80 km. This network crosses the faulted area in four places,
and the observation field is 10 km from each part of the fault.
The benchmarks and levelling lines are shown in Fig.4, on
top of a geological map.
COSEISMIC D E F O R M A T I O N
P R O C E D U R E S A N D PRECISION
Most of the benchmarks used for measuring vertical strain are
on the railway, near Oued Fodda village, where the most
important offset was observed during the El Asnam earthquake
(Ouyed et a/. 1981). The others are situated in the El-Karimia
region and on the Oued Fodda dam. The profile cuts the fault
perpendicularly 4 km west of Oued Fodda village, and has a
NW-SE trend with 40 km length. The northern end of the
profile is near the Ouled bou Zina village (West of Beni
Rached).
Eight months after the 1980 quake, a French-Algerian crew
carried out levelling measurements through the rupture area
along a SE-NW profile crossing the fault trace. The profile
was tied to a levelling line (first order) along the Algiers-Oran
railway, installed at the beginning of the century (1905, see
Bezzeghoud et al. 1995) by the French administration and
completed in 1976 by the National Institute of Cartography
of Algiers (INC). The 1976 elevations have been taken as the
reference for the coseismic slip measurements of the 1954 and
1980 events. Relative vertical positions were determined by
trigonometric levelling (Ruegg et al. 1982), observing simultaneously reciprocal vertical angles and distances. The precision was estimated to be about 1 cm km-'. The reference
point is the Oued Fodda dam benchmark, far from the
southeast end of the active zone. Ruegg et al. (1982) observed
a vertical uplift of 5.15 m of the overthrusting block with a
progressive reduction of this displacement towards the northwest; they also noticed a 0.76 m depression on the SE block,
within 5 km of the fault, but this movement decreases rapidly
towards the southeast. This vertical movement, measured in
1981 along the railway levelling route, was confirmed by an
independent method of classical levelling (Dimitrov et al. 1987)
in 1986. A comparison of the altitudes determined in 1976,
1981, 1983 and 1986 corroborates the important deformation
caused by the 1980 October 10 earthquake, measured in June
1981 (Ruegg et al. 1982). Our reference point (the benchmark
Rn12 located near El Attaf village, Fig. 4) is situated on the
SE block of the fault, which is relatively stable.
Field procedures, data processing and error estimates for the
1986-91 levelling surveys are described here and summarized
in Tables 2 and 3. Although the Wild N3 spirit levels are
immune to the systematic magnetic errors that plague all the
compensator levels, the Wild NA2 level associated with one
pair of Wild 3 m wooden rods were used during all the surveys.
Tests have shown that its magnetic error is relatively small
(Rumpf & Meurish 1981). The geometric second-ofder doublerun segments method, with specific considerations, in the flat
region was carried out.
Because levelling operations can be contaminated by both
systematic and random errors, we developed a specific measurement procedure with regard to the instrumentation employed
in the field. Random errors are caused by several aspects of
the surveying process (Marshall, Stein & Thatcher 1991):
inaccurate readings of the levelling instrument caused by
atmospheric scintillation and ground vibrations, inadequate
rods, etc. The wooden rods used are subject to an undetermined
dilatation factor due to the fluctuation of the temperature and
humidity. Random error is also caused by unpredictable
variations in instruments, environmental factors (topography)
and procedures; it cannot be eliminated but can be minimized
by proper procedures (Dzurisin & Yamashita 1987). To discriminate among these disturbances, the observations were
carried out during the periods of the lowest temperature
changes: spring and autumn. Times of observations were
chosen to be during periods with lower amounts of sunshine
(7-10 am and 4-8 pm).
The systematic forward and backward observations (40 m
of maximum distance between the level and the rods, 50 cm
above ground level), with a strict stadimetric reading tolerance
(< 1 mm), minimize much of the random error. However, the
precision of a levelling survey is degraded by systematic error
as well as by random error. The principal random errors
(Bomford 1971) are associated with rod miscalibration,
unequal refraction, and rod settlement. The former two errors
are correlated with topography, and the latter error depends
upon soil conditions (Dzurisin & Yamashita 1987). Systematic
errors have received considerable attention and are the subject
of continuing debate (e.g. Rumpf & Meurish 1981; Strange
1981; Stein 1981; Hodahl 1982; Packard & MacNeil 1983).
The random error which accumulates with the square root of
distance (L),expressed as
ALTIMETRIC NETWORK F O R
MONITORING
In 1985, INC measured a 175 km levelling route which belongs
to the General Levelling Network of Algeria (second order),
cutting the seismogenic area of Cheliff along the No. 4 National
Road, between the towns of El-Khemis and Relizane (measurements carried out with a Wild N3 level and invar rods, secondorder double run). In 1986, a new levelling network was
installed on the basis of 13 old benchmarks of the Algiers-Oran
railway, and 13 new ones o n the No. 4 National Road. Thus,
26 benchmarks form a 40 km initial network, which cuts the
faulted area twice within an observation field of 10 km.
After the data had been obtained from three measurement
campaigns (Dimitrov et a/. 1987), in 1988, 1989 and 1990,
complementary levelling routes were installed. The levelling
r=aL"' m m k m - ' ,
(1)
where a is the instrumental precision, was assumed to be
1.5 mm km-'. Operator change after each run allows us to
avoid systematic reading errors. Then, the accidental kilometric
error q can be computed from the random error (General
Direction of Geodesy and Cartography 1980):
ql = [ 1/4n z(d2/l)]1'2
mm km-' ,
where d = (!I~,~- h2.1)is the discrepancy between the backward
and forward measurements of a section, n the total number of
0 1997 RAS, GJ1 129, 597-612
Figure 4. Schematic map of the vertical network. Benchmarks and levelling lines on a detailed geological map (from Jacob & Ficher 1906). Geological reference: I, conglomerates and red sands;
11, blue clays; 111, marine Pliocene; IV, ancient alluvial deposits; V, Ostrea sandstones; VI, limestones and Lithothamniurn sandstones; VII. yellow mark (Tortonian); VIII, Recent alluvial deposits;
XIV, shales (Medjanian). Symbols used in the legend: 1, Algiers-Oran National Road levelling section; 2, Algiers-Oran railway levelling section; 3, Old National Road levelling section; 4, Sar el
Mairouf anticline levelling section; 5, El Karimia levellinn section; RP, Reference Point ( R n 12 reference altitude): TF, El Asnam thrust fault.
604
K . Lammali et al.
Table 2. Error estimates (mm/km)
Period
5
‘11
1986
1987.4
1987.9
1988.5
1989
1991
1992
0.08
0.05
0.05
0.06
0.08
0.08
0.04
0.51
0.34
1.19
0.83
0.50
0.77
0.58
PKE
‘12
ylz = [ 1/N
-
-
2.66
-
-
~ ( W ~ / Lmm
) ] km-’
”~ ,
(3)
where w, N and L are, respectively, the observed misclosures,
the number of levelling closed circuits and the total length of
the levelling network. The systematic error z is estimated
by the following formula:
0.51
0.34
1.19
0.83
0.50
0.77
0.58
-
double-run sections, and I the distance in kilometres. The
observed circuit misclosures, given as follow, are used to
control the results of formula (2) when it is possible:
z= [ 1/4CL C(SZ/L)l1’’mm km-’,
(4)
where S is the cumulative error. The Probable Kilometric
Table 3. Benchmark elevation and vertical displacements, 1986-91, Cheliff region.
BM*
Dist. (km)”
1986.9
Elev. (m)
Elevation change (mm), Rn 4 levelling route (Rn)
VfdV
(mm/y)
86.9-87.4
87.4-87.9
87.9-88.5
88.5-89.5
04
-01
06
04
07
00
- 07
-01
-01
00
00
- 03
09
- 02
00
- 01
00
04
01
- 01
00
-02
- 02
- 02
00
02
08
- 09
Rn21
Rn20‘
Rn20
Rn18
Rn17”
Rn17’
Rn17
Rn16
Rn15
Rn14
Rn13
Rn12b
00.000
01.150
02.455
06.436
07.673
10.034
11.230
12.470
13.778
14.957
16.797
17.997
123.496
134.617
178.157
237.484
216.757
181.664
172.242
162.202
156.922
156.970
158.999
150.098
-02
-01
03
06
03
BM *
Dist. (km)”
1986.9
Elev. (m)
Elevation change (mm), Railway levelling route (R)
-
- 04
00
00
-01
00
00
86.9-87.4
R75
R74
R73
R72
R71B
R71A
R71
R70
R69“’
R69
R68
R67A
BM*
00.0
1.25
2.55
3.85
4.90
6.10
8.04
9.68
11.22
12.9
14.5
16.08
Dist. (km)’
120.078
133.992
147.214
159.891
164.653
172.645
191.617
207.616
185.606
169.087
157.971
158.952
1987.4
Elev. (m)
0.0
0.62
1.92
2.37
3.07
3.57
4.47
5.05
5.80
6.72
191.614
182.929
236.442
262.178
296.534
238.428
201.121
171.766
154.798
157.970
87.4-87.9
-
03
- 1.5
00
-
-
00
00
87.9-88.5
02
04
06
00
05
08
03
01
03
00
11
00
00
-01
02
- 03
- 03
- 03
05
- 04
-05
-01
- 05
03
05
-01
00
02
05
05
03
05.5
-01
01
- 05
06
06
00
02
-01
11
04
- 03
02
02
02
03
02
-
01
06
- 06
-
-
89.5-91.5
00
04
-01
-02
-
-11
03
01
00
88.5-89.5
11
16
07
05
05
07
05
20
15
02
06
11
-01
-
3.4k 1.9
3.8 f1.6
4.8 k 2.3
0.7k1.6
1.7f 1.8
3.3 f3.4
5.8 f2.9
5.4 f2.7
- 2.5 f2.5
- 7.2 f2.5
0.8 f2.3
2.6 & 2.2
v+6v
(mmiy)
87.9-88.5
02
01
02
-0.6f 1.6
2.2k 1.9
5.2f 1.7
1.0f 1.9
2.7f 1.9
- 1.7f 2.3
- 0.4 k 2.0
-2.5f1.2
0.5 k 1.8
0.7 f3.2
00
Vk6V
(mmiu)
88.5-89.5
-
00
E.C. (mm), SEM anticline levelling route
87.4-87.9
R71
S8
s7
S6
s5
s4
s3
s2
s1
R68
-07
-05
-02
04
03
04
04
00
89.5-91.5
-
89.5-91.5
-
-
23
11
-
06
-02
- 08
01
03
9.8f1.2
16.9 f 2.5
13.3k0.7
4.1 f1.4
9.5+ 1.4
4.0 f0.9
-
-0.6+ 1.4
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
Table 4. Global 1954 and 1980 dislocation model.
(a)
1954 DISLOCATION MODEL
Width
km
Length
km
d
deg.
U
m
d,
km
M o x loz6
dynecm
Mw
5.15
21
67.5
3
1
0.98
6.6
U
m
d,
km
M o x 10''
dynecm
Mw
5
3.7
0
3.5
0
0
3.3
2.2
1.1
0
2.85
0.044
0.049
0.145
0.082
0.442
0.877
0.194
0.116
0.077
4.9
1980 DISLOCATION MODEL
Subfault
1
2
3
4
5
6
7
8
9
10
Total Mo
Width
km
Length
km
d
deg.
5.15
21
67.5
8
1.35
2.5
67.5
4
4
2.5
67.5
1.5
4
1.78
6.2
60
4
6.2
60
1
6.2
2.7
53.5
8
9.5
30
8
3.5
1.24
9.5
60
5
1.24
9.5
60
3
1.24
9.5
60
2
and associated Mw magnitude
Vertical measurements
7.1
Error (PKE) is then:
PKE=(q4+r2)''2 m m k m - '
We can see the PKE of the total network, for the 1986-91
measurements period, in Table 1. The accidental errors calculated by formula (2) are always smaller than those calculated
by formula (3). In our case q1 1/3 qz, which represents the
normal error distribution (Marshall et al. 1991). The systematic
error values (T) range between 0.04 and 0.08 mm km-l
(Table 2).
The velocity (Vi)is derived by dividing the elevation change
by the time interval between two different surveys, and the
uplift rate for the period 1986-91 is given by the formula
(6)
where N is the number of reiterations.
The velocity error 6V is given by the elevation change error
6(h), divided by the time interval between constituent surveys
(Dzurisin & Yamashita 1987). The velocity error rate (Table 3)
for the same period is given by the formula
6V= 1/N ~ { [ ~ ( A h i ) Z + ~ ( A h i + , ) 2 ] 1 ' 2 / t i + l - tmm
i J yr-',
(7)
The elevation-change error &Ah) is equal to the square root
of the sum of the squares of the Probable Kilometric Error
(PKE), of the constituent surveys.
The instrumentation should be improved to conduct the
highest standards of a first-order double-run levelling. Until
now, we have used a specific methodology to discriminate
between the maximums of random and systematic errors and
to have the highest precision possible. However, in spite of all
the precautions undertaken in the field to limit the various
0 1997 RAS, G J I 129, 597-612
errors, the miscalibrated wooden rods could not be taken as
a sure reference for the readings. To improve the data it is
necessary to use calibrated levelling rods, together with a
control of the thermal extension of the rod tapes. Furthermore,
the automatic level (Wild NA2) used for all the campaigns
should be equipped with a micrometer to take more precise
readings.
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
observed movements reach 5-10 times the PKE, we can
consider these movements as tectonic activity.
DATA A N D RESULTS
d, U, d,, Mo and Mw are, respectively, dip, displacement at the source,
depth of fault upper edge, seismic moment and magnitude. The
subfaults numbered from 1 to 10 are computed with a fixed mean
strike of 217".
V=c ~ijN
mm y r - l ,
605
Three levelling routes (Algiers-Oran railway, No. 4 National
Road, and Sar el Maiirouf anticline) that have been measured
five or six times were taken as a reference, in order to form an
idea about elevation changes of the benchmarks. The comparison of altimetric remeasurements of 1987-89 with those of
1986 for the Rn4 levelling route, and 1986 for the two others
(Algiers-Oran railway and Sar el Maiirouf levelling routes)
taken as reference, permitted us to determine altitude changes.
We can see in Fig. 5, which shows these three profiles, that
there are very small movements near the fault. It can be seen
that an uplift of 2 cm on average is seen on the northwestern
block; meanwhile, on the SE block, a slight subsidence occurred
near the fault trace.
These vertical displacements are moderate, but in good
agreement with the coseismic movements: uplift of the zone
situated northwest of the fault, subsidence on the southeast.
We should take into account, however, that the benchmark
Rn12, the reference for the altimetric variation, is relatively
close to the rupture area (3 km perpendicular to the fault
trace, Fig. 4). This benchmark subsided 38 cm during the 1980
earthquake and could be subject to postseismic displacement.
This led us to extend the network on the uplifted block and
to install the El Karimia levelling section towards the south
(Dimitrov & Lammali 1989).
In two cases, the observed differences of deformation for
two adjacent benchmarks (Rn18-Rn17" and S6-S5, see Figs
6, 7 and 8) are probably due to heterogeneity shown on the
detailed geological map (Fig. 4) described by Jacob & Ficher
(1906). We can observe this benchmark behaviour particularly
for Rn18 and S6 which are on the same geological structure
(ostrea sandstone). They behave with a lower uplift rate with
regard to the others (Figs 7b and c). This superficial heterogeneity is also identified from the structural analysis of the surface
deformation (Philip & Meghraoui 1983), showing hangingwall flexure and normal faulting inside the NW block.
Benchmark Rn19' is not taken into account in our interpretation because it has been damaged several times. However,
the behaviour of the others benchmarks shows a regular
movement during the 5 years of measurements (Fig. 5 ) , particularly benchmark R72, and could be explained by the existence
of a smaller 1954 thrust blind fault, suggested by Bezzeghoud
et al. (1995). This result is discussed in the following sections.
606
K . Lummuli et al.
20
,-
10
0
.................
-111
300 I
200
Unl9
D-I
100
- 10
'
I
200
(m>
100
1988-1986
1989-1986
0
-10
s5
300
Elevation
I
(c)
4n
H-HO
(mm>
I
-5
I
0
I
5
Distance (km)
I
10
' -10
Figure 5 . Postseismic movements along (a) the Algiers-Oran railway levelling route, (b) the Algiers-Oran National Road levelling route and (c)
the Sar el Malrouf anticline levelling route. Two periods of measurements are compared with the 1986 (a and b) and 1987 (c) altitudes.
Mean displacement and uplift rates
Figs 5(a), (b) and (c) indicate the displacement rates of the
benchmarks situated along the Algiers-Oran railway (R),
National Road (Rn) and Sar El Madrouf (S) (1986-91 ) levelling
sections described in the previous section. In this Figure we
can see that the benchmarks situated in the western and
eastern zones show, respectively, positive mean displacement
rates and stable or negative mean displacement rates. We can
interpret this observation as differential movement of the two
blocks separated by the El Asnam thrust fault. The uplift of
the northwestern zone could be considered as an overthrust
of the NW block (Blocks A and B, Fig. 8) on the SE one
(Block C, Fig. 8). This movement has been observed by Ruegg
et al. (1982) and Cisternas, Dorel & Gaulon (1982) from,
respectively, coseismic measurements and a dislocation model.
Figs 7(a), (b) and (c) show the mean uplift rates of each
levelling section (R, Rn and S). This representation reflects the
tectonic activity of the fault: uplift of the N W block and slight
subsidence of the SE block near the fault. The benchmarks
Rn18, and S6, which present a particular mean uplift rate (slow
with respect to the others benchmarks situated on the same
block), were discussed in the previous section. Our study,
based on 1986-91 data, gives an average uplift rate of about
5.1 1.9 mm yr-' of the N W block (Blocks A and B, Fig. 8);
meanwhile, the SE block (Block C, Fig. 8) is relatively stable.
Very close to the thrust fault, the Sar el Madrouf levelling
section gives an average uplift rate of 9.6+ 1.4 mm yr-'. This
value represents the mean uplift rate of the Sar el Madrouf
anticline, where Ruegg et al. ( 1982) observed a vertical uplift
of 5.15 m of the overthrusting block due to the 1980 El Asnam
earthquake. O n the other hand, we confirm the first results
given by Dimitrov et al. (1991) based on the 1985-88 levelling
surveys. We note, particularly, that the Sar el MaCrouf anticline
profile shows a larger uplift rate; this uplift rate is also observed
on the benchmark R71 situated near the fault trace and on
the Sar el Madrouf anticline (Fig. 7). This could be explained
by tectonic activity of the Sar el Madrouf anticline. Mean
uplift rate values for the three levelling sections (Fig. 7) are
given in Table 3. Finally, we note that the errors diminished
to acceptably low levels near unity for the SEM anticline levelling route, indicating that the deduced uplift rate
within the smaller area is more credible than those for the
whole network. The SEM anticline levelling route presents,
particularly, a significant uplift rate with regard to the errors.
1954 and 1980 dislocation models
To verify the postseismic movements presented in the previous
section, by means of displacement and uplift rates, we calculated the displacement field by modelling fault configurations
given recently by Bezzeghoud et al. (1995). The models proposed by these authors are based o n coseismic vertical displacement, measured along the Algiers-Oran railway, induced by
the 1954 September 9 (M,=6.6) and 1980 October 10 (M,=
7.1) earthquakes. We used the dislocation models based on
analytic expressions given by Okada ( 1985).
We combined the two fault models (1954 and 1980) proposed
by Bezzeghoud et al. (1995) to obtain the global model shown
in Fig. 9. The 1954 fault resulting from their model is located
between the 1954 earthquake epicentre (near Beni Rached
surface breaks, N E ) and El Asnam city (SW), parallel to the
1980 main thrust fault, with an offset of 6 km towards the
west (Fig. 8). The modelling derived from this dislocation
model is shown in Fig. 9 and Table 3. We can see the similarity
between the calculated vertical displacement (Fig. 9) and the
0 1997 RAS, G J I 129, 597-612
Postseismic deformation at El Asnam
607
\ T
Figure 6. Mean displacement rates (6hlbt) of the benchmarks situated on (a) Algiers-Oran levelling section; (b) Algiers-Oran National Road
levelling section; (c) Sar el MaLrouf Anticline levelling section.
mean uplift rates of the benchmarks situated along the
Algiers-Oran railway levelling section (Fig. 7a). The tectonic
activity of the 1954 and 1980 thrust faults, given by the model
proposed by Bezzeghoud et ul. ( 1995), explains the mean uplift
rates discussed previously rather well. On the other hand, the
stability of benchmark R72 situated on the Algiers-Oran
railway (on Block A) could be explained by a compensation
of the movement generated by the two thrust faults. However,
this benchmark does not constrain the shape of this relative
stability well enough to justify modelling. Our results, summarized in Figs 9 and 10, together with the arguments developed
in Bezzeghoud et al. (1995), and particularly the vertical
displacement induced by the 1954 event, show that the 1954
earthquake did not take place along the 1980 El Asnam thrust
fault, in contrast to the model suggested by Avouac, Meyer &
Tapponier (1992). The 1954 blind thrust fault is, probably,
situated at the boundary of the two blocks (A and B), shown
0 1997 RAS, G J I 129, 597-612
in Fig. 8, characterized by an opposite motion and gives,
alternatively, an uplift and subsidence movement. These
ascending and descending movements are probably associated,
respectively, with the seismic and/or aseismic activity of the
thrust faults responsible for the 1954 and 1980 earthquakes.
We can note the similarity of the behaviour of the two blocks
(A and B, Fig. 8): both show uplift. However, the mean uplift
rate of block A is lower (2.7k1.9 mm yr-') than that of
block B (5.1 _+ 1.9 mm yr-').
DISCUSSION
Results and interpretation
This report shows that the Sar El Mafirouf (SEM) anticline
grows rapidly (about 10 mm yr-') with respect to the others
geological structures situated along the fault trace. This result
608
K . Lammali et al.
25
20
c
E
W
-4
$8
I
0
I
Distince (kg)
I
1s
Figure7. Uplift rates for the benchmarks that have been calculated
for the 1986-91 period of measurements: (a) Algiers-Oran Railway
(R) section with an error of 1.15 mm; (b) Algiers-Oran National Road
(Rn) section with an error of 0.7 mm; (c) Sar el Magrouf anticline (S)
section with an error of 0.5 mm. The longest dotted line shows the
crossing place of the El Asnam thrust fault for each section. The other
dotted line on the railway section locates the 1954 buried fault.
confirms the geological evidence presented by King & Yielding
( 1984), who remarked that this anticline is more developed
than the other geological structures situated along the southwest fault segment. The total geological throw is greater than
250 m on the central segment (along the SEM anticline) and
decreases rapidly to a few tens of metres (< 50 m) along the
southwest segment. King & Yielding (1984) suggest that in the
past the SEM segment has experienced more earthquakes than
the southwestern part of the El Asnam thrust fault. We observe
in Fig. 10(b) that the 1980 aftershock distribution is much
wider (10-30 km) iu the central and northeast than in the
southwest zone (only a few kilometres). This distribution
provides evidence that the northeast and the central segments
are more active than the southwest segment. The topography
of the SEM anticline may be the result of cumulative motion
due to several earthquakes similar to the 1980 event that have
occurred in the last several thousand to tens of thousand years
(King & Vita Finzi 1981; King & Yielding 1984; Swan 1988;
Meghraoui et al. 1988a,b). The postseismic movement that we
recorded on this anticline indicates that the cumulative slip on
the central segment can also be generated by fault creep or
volume creep. Thus, we conclude that the postseismic movement recorded on the Sar El Mairouf anticline agrees very
well with the geomorphology observed on the field as discussed
previously.
Fig. 10 shows the spatial and temporal distribution of earthquakes in the El Asnam area and its surrounding region from
1954 to 1994. This period has produced the most destructive
earthquakes of this century in the Ibero-Maghrebian region
and from the standpoint of understanding the rupture process
this area is important because it represents a typical case-study
of temporal seismic clustering that poses a challenge for models
of seismic recurrence on a fault system. The time distribution
of shocks in this area (Fig. 10) shows a clear seismic quiescence
between the two strong earthquakes of 1954 and 1980. No
significant earthquake (A42 4.5) occurred near El Asnam
Uplift rate (mm/y)
0
0
0
0
0
0
4.8
4.0
3.0
2.7
2.2
1.7
<1.0
-7.2
-2.5
-1.7
0 >-1.0
1954 fault
OF Oued Fodda city
MCB Middle Chellif Basin
LCB Lower Chellif Basin
Figure 8. Plot showing the uplift rates in topographic and tectonic context, where one can see the tendency of the three blocks (A, B and C)
separated by the 1954 blind fault and 1980 surface breaks. The other symbols are as in Fig. 2.
0 1997 RAS, GJI 129, 597-612
Postseismic deformation at El Asnam
DISLOCATION
-~
MODEL -
609
~
6
F
a
N108-O
,/\
] motion
--1
4
m
2
0
r~~~
~~
~
-2 -
A
.
~
~
BLOCK A
-
BLOCK B
1
* - *-__-
~~
:
BLOCK C
300
100
-10
-7 5
-5
-2 5
0
25
5
D i \ t a n c e (km)
Figure 9. Bottom, topographic level oriented N108 through the El Asnam fault trace. Top, vertical displacement induced from a global dislocation
(1954 and 1980) model proposed by Bezzeghoud et al. (1995). The parameters for this model are listed in Table4. The dashed and full lines
indicate, respectively, 1954 buried and 1980 thrust faults.
between the two seismic crises (1963-80). We note, also, a
south-north migration from 1985. On the other hand, from
the spatial distribution of seismicity, we observe an extreme
quiescence in the vicinity of the El Asnam thrust fault, and a
background seismicity over the whole region for the period
1983-91 (Fig. 1Oc). These earthquakes cycles, as illustrated in
Fig. 10 (main shock, aftershocks, quiescence, background seismicity), have been well described by numerous authors (e.g.
Scholz 1988; Rikitake 1976, 1982; Mogi 1985). The seismic
gap observed between 1983 and 1991 in the vicinity of the El
Asnam fault trace favours the hypothesis that recent postseismic uplift recorded in the hanging wall of the fault is probably
due to aseismic slip at depth. We suggest that this aseismic
slip occurs on deep buried faults which may break to the
surface in a future earthquake. This deformation process could
be continued for a very long time to produce an M 2 6 . 5
earthquake similar to those of 1954 or 1980. From retrospective
studies on crustal deformation and earthquake prediction
research, Thatcher ( 1981) reported several significant episodes
where postseismic-level changes have been recorded from 1 to
5 years following a main shock.
Bezzeghoud et al. (1995) proposed a dislocation model,
which we have used previously to explain the mean uplift
rates, on the basis of all available data, and particularly those
of the levelling profiles along the Algiers-Oran railway, which
has been remeasured after each event (1954 and 1980 earthquakes). In addition, to constrain and explain their dislocation
model, the authors used numerous seismological and geological
studies. Here, we observe that the mean uplift rates (Fig. 7),
calculated from the elevation changes of the benchmarks
regularly measured between 1986 and 1991, correlate rather
well with the dislocation model (Fig. 9) produced by the fault
system proposed by Bezzeghoud et al. (1995). The most
important coseismic displacements were generated by two of
the most destructive earthquakes to occur in this region:
1954 September 9 (M,=6.6, Bezzeghoud et al. 1995) and
1980 October 10 (M,=7.1, Ruegg et al. 1982).
0 1997 RAS, G J I 129, 597-612
Seismic strain
On the basis of the scalar seismic moment release for earthquakes, we estimate the average seismic slip rate in the western
part of the north Algerian Atlas. This average rate of seismic
slip can be roughly calculated by summing the moment release
of the earthquakes with M,>4.5 that occurred between 1716
and 1994 (see Mokrane et al. 1994). The average rate of slip
U for a time period T=278 years can be estimated directly
from the following relation: U=M,/pLWIT; where p is the
shear modulus (3.3 x 10" dyn cm-'). The area was estimated
using a length L = 500 km as suggested by the fault distribution
reported in Fig. 2, and a width W=20 km as suggested by an
average dip angle of 50" in a 15 km thick layer. The resulting
total rate of seismic slip is 11.9 mm yr-', which corresponds
to a horizontal shortening slip rate of Uh=7.6 mm yr-' and
an uplift slip rate of U,=9.1 mm yr-' in the western part of
North Algeria. These results are to be compared with those
evaluated by McKenzie (1972, Uh=9 mm yr-'), Minster &
Jordan (1978, Uh=11.4 mm yr-') and recently by Buforn
et al. 1988 (uh=11.5 mm yr-') for the rate of convergence
between the African and European plates. Our results explain
about 75 per cent of the magnitude of the values proposed by
the above-mentioned authors. There are two possibilities to
explain this difference: the first one is that the compressional
deformation in western Algeria is accommodated also by
aseismic slip; the second possibility is that the time span of
278 years that we used to estimate the rate of seismic slip is
too short. The first possibility is our preferred interpretation
because ( 1) the most important earthquakes for the historical
and instrumental period used are present and (2) the deficit
observed on our seismic slip rate could be accounted for by
aseismic movement already observed in the El Asnam region
in geodetic measurements.
At El Asnam an average dip angle of 50" with an average
uplift slip rate of 3.9 mm yr-' evaluated from blocks A and B
(Fig. 8) gives a horizontal shortening aseismic slip rate of
610
K . Lammuli et al.
Figure 10. Space-time plot of earthquakes along the El Asnam thrust fault and its surrounding region (the symbols are as in Fig. 2). Bottom,
distribution of epicentres with time (M24.5). Top, maps showing the spatial development of the seismicity from 1954 to 1994: (a) 1954-63; (b)
epicentral locations of aftershocks (1980, 1982) after the 1980 main shock (data given by D. Hatzfeld); (c) seismic gap and background seismicity
over the whole region for the period 1983-91; (d) seismicity from 1954 to 1994.
3.3 mm yr-'. If this value of aseismic slip is added to the
seismic slip computed earlier for western Algeria (7.6 mm yr-I),
the resulting rate of horizontal slip will be 10.9 mm yr-'. This
rate of relative motion is very close to those predicted from
global plate motions by McKenzie (1972, 9 mm yr-'), Minster
& Jordan (1978, 11.4 mm yr-') and Buforn et a/. (1988,
11.5 mm yrf').
CONCLUSION
We studied vertical movements (postseismic phase) in the El
Asnam thrust fault region from repetitive levelling measurements carried out since 1986. Moderate elevation changes of
the benchmarks were observed. (1) Elevation changes are more
intense on the NW block of the fault (5.1 f 1.9 mm yr-') than
on the SE block. (2) The Sar El Magrouf anticline appears to
grow with a local velocity (about 10 mm yr-') in agreement
with geological and seismological data.
Another important observation is that this elevation change
occurs very close to the main thrust fault: less than 10 km
from the rupture area. On the basis of the mean uplift rates
deduced from the levelling profiles along the Algiers-Oran
railway, we can add the other thrust fault situated 6 km
northwest from the El Asnam surface breaks (Fig. 8).
Bezzeghoud et a/. (1995) associate this blind fault with the
1954 El Asnam earthquake and interpret it as the secondary
fault with respect to the main fault. These two faults are
probably re-activated to accommodate relative plate motions
in a transfer zone which separates the lower and the middle
Cheliff basin, as suggested by Dewey (1991) and reported in
Bezzeghoud et al. (1995) to argue in favour of their final 1954
and 1980 dislocation models (Fig. 9). In this scenario, the
deformation produced by these faults is apparently absorbed
by thrusting and uplift, crustal shortening and the subsequent
elevation of the Sar el Magrouf anticline. Based on detailed
topographic levelling and morphological study, Avouac et a/.
(1992) proposed that the 1954 earthquake activated only the
central deep segment of the 1980 fault system, including the
'basement' thrust and the normal faults at Beni Rached. This
does not agree with the modelling of vertical coseismic movements and seismological and geological arguments resulting
from Bezzeghoud et al. (1995), nor with the postseismic analysis
0 1997 RAS, G J I 129, 597-612
Postseismic deformation at El Asnam
presented in this report. Several others studies (Yielding et ul.
1981; King & Yielding 1984; Dewey 1991) have also proposed
that the 1954 earthquake was located further north and very
probably on a different fault from that of the 1980 earthquake.
Since 1986, the 'Centre de Recherche en Astronomie,
Astrophysique et Geophysique' (CRAAG) has organized the
systematic surveying of the horizontal deformation around the
El Asnam thrust fault with a horizontal geodetic network tied
to the National Triangulation Network. The first results were
published by Dimitrov et ul. (1991) and showed clearly that
the relative horizontal deformation was oriented perpendicular
to the fault with a shortening oriented NNW-SSE. This
compression of the two blocks correlates rather well with the
uplift of the northwestern zone evaluated from the vertical
movements given in this study.
The western part of North Algeria is formed by a wide area
of deformation and fractures, due to plate convergence between
Africa and Iberia, limited to the south by the Tellian Atlas and
to the north by the Mediterranean Sea. This deformation,
apparently absorbed by thrusting and uplift, is represented by
several mountains (Beni Chougrane, Dahra, Bou Ma&d,Mont
ChCnoua, Atlas Blideen), and particularly by the Sar el
Maiirouf anticline. The compressional deformation of northwestern Algeria is accommodated by seismic strain and aseismic slip with a horizontal shortening rate of about
10.9 mm yr-', in a NNW-SSE direction as deduced from the
P-axes of 22 significant events.
ACKNOWLEDGMENTS
The authors are grateful to R. Madariaga for his careful
reading of the manuscript, and his valuable suggestions. We
greatly appreciate the critical reading of the preliminary manuscript by J. C. Ruegg. Thanks are due to A. Mahsas for
computing the preliminary data. We are grateful to those who
helped in the field work, including F. Chaoui, A. Mahsas and
A. Merbah. The authors thank the referees for their constructive comments and recommendations. MB and KL acknowledge the support of the seismological department of the Institut
de Physique du Globe de Paris. This research was supported
by the Centre de Recherche en Astronomie, Astrophysique et
Geophysique (CRAAG, Algiers) with a contribution from the
Institut National de Cartographie (INC, Algiers) for the field
operations. Special thanks go to the local collectivities,
including Cheliff, Beni Rached and Oued Fodda.
REFERENCES
Asfirane, F., 1993.Traitement et interpretation des donntes atromagnetiques acquises au dessus de l'Algerie du nord, impact sur la geologie
de la chaine atlasique, PhD thesis, 1'Universite d'Orsay, Paris XI.
Asfirane, F. & Galdeano, 1995. The aeromagnetic map of northern
Algeria: processing, and interpretation, Earth planet Sci. Lett.,
136, 61-78.
Avouac, J.P., Meyer, B. & Tapponier, P., 1992. On the growth of
normal faults and the existence of flats and ramps along the El
Asnam active fold and thrust system, Tectonics, 11, 1-11.
Benhallou, H. & Roussel, J., 1971. Sur les seismes et la seismicite de
1'Algerie de 1951 a 1970, Note IMPGA, No. 4, Alger.
Benouar, D., 1994. Materials for investigation of the seismicity of
Algeria and adjacent regions during the twentieth century, Ann.
Geojs., XXXVII, 860.
Bezzeghoud, M. & Buforn, E., 1996. Study of the Alhucemas (26/05/94)
0 1997 RAS, GJI 129, 597-612
611
and Mascara (18/08/94) earthquakes and seismotectonics of the
region, Ann. Geophys., EGS, the Hague 6-10 May 1996, Part I,
14, 88.
Bezzeghoud, M., Ayadi, A,, Sebai, A. & Benhallou, H., 1994.
Seismogenic zone survey by Algerian Telemetred Seismological
Network. Case-study of Rouina earthquake, January 19th, 1992 M =
5.2, Phys. Earth planet. Inter., 84, 235-246.
Bezzeghoud, M., Dimitrov, D., Ruegg, J.C. & Lammali, K., 1995.
Faulting mechanism of the 1954 and 1980 earthquakes from vertical
movements modelling, Tectonophysics, 249, 249-266.
Bezzeghoud, M., Ayadi, A,, Sebai, A,, Kit Messaoud, M., Mokrane, A.
& Benhallou, H., 1996. Seismicity of Algeria between 1365 and 1989:
Map of Maximum Observed Intensities (MOI), Auances en Geofisica
y Geodesia, I, 107-114, IGN, Madrid.
Bomford, G., 1971. Geodesy, Oxford University Press, New York, NY.
Buforn, E., San de Galdeano, C. & Udias, A,, 1995, Seismotectonics
of the Ibero-Maghrebian region, Tectonophysics, 248, 247-261.
Cisternas, A,, Dorel, J.& Gaulon, R., 1982. Mod Is of the complex
source of El Asnam earthquake, Bull. seism. Soc. Am., 72,2245-2266.
Coca P. & Buforn, E., 1994. Mecanismos foc*s en el sur de Espafia:
period0 1965-85, Esrudios Geologicos, Museo National de Ciencias
Naturales, Madrid, Vol. 50 (1-2), 33-45.
Deschamps, A., Gaudemer, Y . & Cisternas, A,, 1982. The El Asnam,
Algeria, earthquake of 10 October 1980: multiple source mechanism
determined from long period records, Bull. seism. Soc. Am., 72,
1111-1 128.
Dewey, J.W., 1991. The 1954 and 1980 algerian earthquakes: implications for the characteristic displacement model of fault behaviour,
Bull. seism. Soc. Am., 81, 446-467.
Dimitrov, D. & Lammali, K., 1989. Mesures de deformations autour
de la faille d' El Asnam, Bull. Geol. SOC. Gr2ce, XXIIU1, 243-257.
Dimitrov, D., Benhallou, H., Khalfoun A. & Lammali, K., 1987.
Mesures geodesiques liees aux deformations de l'kcorce terrestre
dans la region d'Ech Cheliff (ex El Asnam), Algerie du Nord, C.R.
Acad. Sci. Paris, 305, Strie 11, 1365-1368.
Dimitrov, D., Lammali, K., Khalfoun, A. & Benhallou, H., 1991. Postseismic movements from 1981 to 1988 on the El Asnam thrust fault
(Algeria), in Seismicity, Seismotectonic and Seismic Risk of the IberoMaghrebian Region, pp. 241-254, eds Mezcua, J. & Udias, A,,
Monografia N 8, IGN., Madrid.
Dzurisin, D. & Yamashita, K.M., 1987. Vertical surface displacements
at Yellowstone Caldera, Wyoming, 1976-8, Geophys. Res., 92 (B13),
13 758-13 766.
Espinoza, A.F. & Lopez-Arroyo, A., 1984. Inconsistencies in the
Modified Mercally Intensities scale, El Asnam, Algeria, earthquake,
Proc Conf. Int. sur la Microzonation sismique, Ech Chelifi Algeria,
10-12 October 1984, 51-65.
General Direction of Geodesy and Cartography, 1980. 1st and 2nd
order levelling instructions, Sofia.
Girardin, N., Hatzfeld, D. & Guiraud, R., 1977. La seismicite du nord
de I'Algerie, C.R. somm. Soc. Geol. Fr., 2, 95-100.
Glangeaud, L., 1932. Etude Gtoiogique de la region Littorale de la
Province d'Alger, Pub. Serv. Carte Geol. Algerie, Alger, Bull. No. 8.
Hodahl, S.R., 1982. Recomputation of vertical crustal motions near
Palmdale, California, 1959-7, Geophys. Res., 87 (B1 l), 9374-9388.
Jacob, M. & Ficher, M., 1906. Carte geologique detaillee, lj50000,
Alger.
Jimtnez, E., 1991. Focal mechanisms of some European earthquakes
from the analysis of single station long-periods record, in Seismicity,
Seismotectonic and Seismic Risk of the Ibero-Maghrebian Region,
Monografia No. 8, IGN, pp. 87-96, eds Mezcua, J. & Udias, A,,
Madrid.
King, G.C.P. & Vita-Finzi, C., 1981. Active folding in the Algerian
earthquake of 10 October 1980, Nature, 292, 22-26.
King, G.C.P. & Yielding, G., 1984. The evolution of a thrust fault
system: process of rupture initiation, propagation and termination
in the 1980 El Asnam (Algeria) earthquake, Geophys. J. R. astr. Soc.,
77, 915-933.
f
612
’
K . Lammali et al.
McKenzie, D., 1972. Active tectonics of the Mediterranean region,
Geophys. J . R . astr. Soc., 30, 109-185.
Marshall, G.A., Stein, R.S. & Thatcher, W., 1991. Faulting geometry
and slip from coseismic elevation changes: the 18 October 1989
Loma Prieta, California, earthquake, Bull. seism. Soc. Am., 81,
1660- 1693.
Meghraoui, M., 1988. Geologie des zones sismiques du nord de
I’AlgCrie, PhD thesis, Universite de Paris Sud, Orsay, France.
Meghraoui, M., Jaegy, R., Lammali, K. & Albarede, F., 1988a. Late
holocene earthquake sequences on El Asnam (Algeria) thrust fault,
Earth planet. Sci. Lett., 90, 187-203.
Meghraoui M., Philip, H., Albared, F. & Cisternas, A,, 1988b. Trench
investigation through the trace of the 1980 El Asnam thrust fault:
evidence for paleoseismicity. Bull. seism. Soc. Am., 78, 979-999.
Minster, J.B. & Jordan, T.H., 1978. Present day plate motions,
J. geophys. R e x , 83, 5331-5354.
Mogi, K., 1985. Earthquake Prediction, Academic Press, Tokyo.
Mokrane, A,, Ait-Messaoud, A,, Sebai, A,, Menia, N., Ayadi, A &
Bezzeghoud, M., 1994. Les stismes en Algtrie de 1365 a 1992,
Publication du Centre de Recherche en Astronomie, Astrophysique
et Geophysique, supervised by Bezzeghoud, M. & Benhallou, H.,
Alger-Bouzreah, 277.
Okada, Y., 1985. Surface deformation due to shear and tensile faults
in a half-space, Bull. seism. Soc. Am., 75 (4), 1135-1154.
Ouyed, M., 1981. Le tremblement de terre d’El Asnam du 10 Octobre
1980. Etude des repliques, These de Doctorat de 3emc cycle.
Universite Scientifique et Mtdicale de Grenoble, France.
Ouyed, M., Meghraoui, M., Cisternas, A., Deschamps, A,, Dorel, J.,
Frechet, J., Gaulon, R., Hatzfeld, D. & Philip, H., 1981. The El
Asnam earthquake of October 10 1980: seismotectonics, Nature,
292, 26-31.
Packard, R.F. & MacNeil, J.H., 1983. A direct comparison of spirit
and compensator leveling, Geophys. Res. Lett., 10, 849-851.
Perrodon, A,, 1957. Etude Geologique des Bassins NCogdnes Sublittoraux
de I’AlgCrie Occidentale, Pub. Serv. Carte Geol. Algerie, Alger,
Bull. No. 12.
Philip, H., 1983. La tectonique actuelle et recente dans le domaine
Mediterranten et ses bordures: ses relations avec la sismicitt, PhD
thesis, Universite de Montpellier, France.
Philip, H. & Meghraoui, M., 1983. Structural analysis and interpretation of the surface deformation of the El Asnam earthquake of
October 10, 1980, Tectonics, 2, 17-49.
Rikitake, T., 1976. Eathquake Prediction, Elsevier, Amsterdam.
Rikitake, T., 1982. Earthquake Forecasting and Warning, Tokyo Center
for academic puplication Japan, Reidel, Tokyo.
Rothe, J.P., 1950. Les seismes de Kherrata et la sismicite en Algtrie,
Pub. Serv. Carte GPol. AlgCr., 24.
Ruegg, J.C., Kasser, M., Tarantola, A,, Lepine, J.C. & Chouikrat, B.,
1982. Deformation associated with the El Asnam earthquake of
10 October 1980: Geodetic determination of vertical and horizontal
movements, Bull. seism. SOC.Am., 72, 2227-2244.
Rumpf, W.E. & Meurish, H., 1981. Systematische Anderungen der
Ziellinie eines prazisions Compensator - nivelliers - Insbesondere
des Zeiss Ni 1 - Durch Magnetische Gleich- und Wechselfelder,
Paper presented at X V I International FIG Congress, Montreux,
Switzerland.
Scholz, C.H., 1988. Mechanisms of seismic quiescences, Pageoph,
126, 701-718.
Shirokova, E.J., 1967. General features in the orientation of principal
stresses in earthquake foci in the Mediterranean Asian seismic belt,
lzv. Earth Phys., 1, 12-22.
Stein, R.S., 1981. Discrimination of tectonic displacement, from slopedependent errors in geodetic leveling from southe n California,
1953-79, in Earthquake Prediction, A n International f i e , Maurice
Ewing Ser., Vol. 4, pp. 441-4456, eds Simpson, D.W. & Richards,
P.G., AGU, Washington, DC.
I
Strange, W.E., 1981. The impact of refraction correction on leveling
interpretations in southern California, J. geophys. Res., 86 (B4),
2809-2824.
Swan, F.H., 1988. Temporal clustering of paleoseismic events on the
Oued Fodda fault, Algeria, Geology, 16, 1092-1095.
Tapponnier, P., 1977. Evolution tectonique du systeme alpin en
Mediterranee: poinqonnement et Ccrasement rigide plastique, Bull.
Soc. Gtol. France, 19, 437-460.
Thatcher, W., 1981. Crustal deformation studies and earthquake
prediction research, in 4th Maurice Ewing Symp., Earthquake prediction, A n International Review, Vol. 4, pp. 394-410, eds Simpson, D.W.
& Richards, P.G., AGU, Washington, DC.
Thomas, G., 1985. Geodynamique d’un bassin intramontagneux: le
bassin du bas Cheliff occidental (Algerie) durant le Mio-PlioQuaternaire, PhD thesis, Universitt de Pau, France.
Udias, A. & Buforn, E., 1991. Regional stresses on the Eurasia Africa
plate boundary derived from focal mechanisms of large earthquakes,
Pageoph, 136, 433-448.
Wessel, P. & Smith, W.H.F., 1991. Free software helps map and display
data, EOS Trans. Am. geophys. Un., 72,441.
Yielding, G., Jackson, J.A., King, G.C.P., Sinvhal, H., Vita-Finzi, C. &
Wood, R.M., 1981. Relations between surface deformation, fault
geometry, seismicity and rupture characteristics during the El Asnam
(Algeria) earthquake of 10 October 1980, Earth planet. Sci. Lett.,
56, 287-305.
0 1997 RAS, G J I 129, 597-612