SEISMOTECTONIC OF THE AZORES-ALBORAN - UPStrat-MAFA

Tectonophysics,
31 (1976) 259-289
Scientific Publishing Company,
@ Elsevier
259
Amsterdam
- Printed
SEISMOTECTONIC
OF THE AZORES-ALBORAN
A. UDIAS’ , A. LdPEZ
ARROYO’
r Departamento
(Spai?
Secch
(Spain)
de Fisica
de Sismologia
(Submitted
REGION
and J. MEZCUA’
de la Tierra
e Ingenieria
July 4, 1975; revised
in The Netherlands
y de1 Cosmos,
Sismica,
version
Uniuersidad
Inst. Geogrbfico
accepted
January
de Barcelona,
y Catastral,
Barcelona
Madrid
15, 1976)
ABSTRACT
Udias, A., Lopez Arroyo, A. and Mezcua, J., 1976. Seismotectonic
Alboran region. Tectonophysics,
31: 259-289.
of the Azores-
New seismicity and focal-mechanism
data from the area of the Azores Islands, in the
Mid-Atlantic
Ridge, to the Alboran Sea and the southern part of Spain are presented.
As a consequence
of the different characters in the focal-mechanism
solutions and
b-values associated,
the area has been divided in four different parts, namely, Mid-Atlantic
and Terceira Ridge, Azores-Gibraltar
fault, Gulf of Cadiz, Alboran Sea and Betica. The
last two form the interaction
between the Eurasian and African continental
plates.
The fracture zone is the locus of very large earthquakes
with mechanisms
showing a
predominant
right-lateral
horizontal
motion. Seismic foci in the continental
interaction
zone are spread over the whole region with mechanisms
changing in character from west
to east. It is suggested that this may be consequence
of the behaviour of the Spanish
Peninsula as a partly independent
subplate. In the eastern part of the studied zone, the
so-called Alboran plate may be considered
as a buffer plate.
INTRODUCTION
The area covered by this study extends from 5”E to 4O”W and from 30”N
to 50”N. This region includes at its west side a section of the Mid-Atlantic
Ridge near the Azores Islands, in its middle part the oceanic region between
the Azores and the Strait of Gibraltar, and at its east side the Iberian Peninsula, the Alboran Sea and the northern coast of Morocco and Algeria.
The ocean-bottom
topography
of the area is shown in Fig. 1. The MidAtlantic Ridge follows a north-south
direction down to latitude 41”N. At
that point it bifurcates into two branches; one follows a S45”W trend and
constitutes the continuation
of the main ridge, and the other, a short branch
with a S45”E trend, stops abruptly at 25”W. The Azores, with the exception
of Flores and Corro islands, are located on this last branch, sometimes called
the Terceira ridge. To the west there is a fairly flat rise (average depth 1000
m), while to the east, towards the Strait of Gibraltar, the bottom slopes
slowly to depths of 3000 m. At about 15” W a rugged region begins with a
E
e
261
series of seamounts rising up to depths of 500 m. An elongated basin (Horseshoe plain) trending E-W separates the Gorringe ridge from Ampere and
Coral banks. The Alboran Sea east of the Strait of Gibraltar is formed by
two basins of about 1000 m depth, separated by a narrow neck called the
Alboran ridge where the Alboran island is situated. The eastern boundary of
the Alboran Sea may be located at about 1”W.
With regard to seismicity, the region is quite active and heterogeneous.
It
contains a zone of shallow earthquakes with moderate magnitude at the
Mid-Atlantic Ridge, sporadic but large shocks along the line from the Azores
to the Strait of Gibraltar, and a continuous seismic activity spread over
southern Spain and North Africa. Worthy of mention also are the very deep
(h - 650 km) earthquakes with epicenters near Granada.
Tectonically
this region is of great interest. From the point of view of
plate tectonics it represents the interaction of the American, Eurasian and
African plates, which results in processes at the plates of different nature,
such as ridge formation, transverse faulting and oceanic-crust consumption.
The area is a key to understanding
the transition from the tectonics of the
opening of the North Atlantic Ocean to the processes of formation of the
Mediterranean
Sea.
Among the first studies of this region are those of Rothe (1951, 1954) in
which he calls attention to the importance of the line of earthquakes from
the Azores to Gibraltar, and the complexities
of the structure under the
Alboran Sea. More recently, with the general acceptance of the fundamentals
of the theory of plate tectonics, the region has received wider attention.
Many authors have interpreted the geophysical and geological data of the
region in the light of this theory, among others Isacks et al. (1968), Ritsema
(1969), Smith (1971), and McKenzie (1972). The Alboran Sea and the Betica and Rift mountain ranges have been studied in particular by many authors, among them by Arabia and Vegas (1974), Andrieux et al. (1971),
Egeler and Simon (1960) Galdeano et al. (1974), Le Pichon et al. (1972),
and Olivet et al. (1973). Some seismological aspects have already been
treated by the present authors (Udias, 1967; Udias and Lopez Arroyo, 1969;
Lopez Arroyo and Udias, 1972). A preliminary interpretation
of the region
as a whole was given in Udias and Lopez Arroyo (1972).
New seismicity and focal-mechanism
data have now been evaluated and
are presented in this paper making possible more definite conclusions about
the tectonic conditions. As a conclusion, a dynamic model consistent with
all the available information
is attempted.
SEISMICITY
To be meaningful over a wide range of magnitude, a study of the seismicity of the region must extend to historical dates, long before the installation
of seismographic stations, because of the long time intervals separating the
occurrence of large earthquakes. In such a study, the problem of lack of
262
homogeneity
of the data becomes very important. It is obvious that the
minimum size of detected earthquakes has changed considerably with time,
depending on the geographical distribution of the population and, in the
later years, on the sensitivity and coverage of seismic-station networks. These
two factors set differences in the magnitude threshold of listed earthquakes,
for the same time interval, in different subzones of the region. Differences
are specially large between ocean and inland areas. Such inhomogeneity
of
the basic data precludes their treatment by many of the current mathematical procedures, and forces one to a more descriptive analysis, as presented in
the following paragraphs.
As the time of occurrence and location of earthquakes of moderate to
small magnitudes are known with sufficient accuracy only since recent times,
we divide the seismicity data into three classes which have been plotted and
studied separately, namely: large earthquakes (1500-1972),
moderate and
small earthquakes (1962-1972)
for the total region and moderate to small
earthquakes (1961-1972)
for the area near the Iberian Peninsula. In this
way each class consists of approximately
homogeneous
and complete data
for the region and time interval considered. The very deep earthquakes with
epicenters in southern Spain are also treated separately.
Large earthquakes
(1500-l
972)
By large earthquakes it is meant those with surface magnitudes equal to or
larger than M = 7, and, for historical shocks without instrumental magnitude
determination,
those whose damage was very significant and extended over a
wide region.
A list of earthquakes with estimated magnitudes M > 7 is given in Table I.
The last five earthquakes in the table occurred within the last 50 years and
have instrumental
epicenter and magnitude determinations.
Magnitudes for
historical earthquakes can be approximated
from estimates of maximum
intensity derived from descriptions of damage included in historical records.
For shocks with epicenters on land, it is possible to obtain information
from references to effects on towns and buildings nearby and, from these, to
get a rough estimate of the maximum intensity and probable magnitude.
Assigned epicentral coordinates of historical earthquakes are those of the
town where damage was greatest. Epicenters at sea (shown in parentheses in
Table I) are even more uncertain and are based on comparison of the distribution of earthquake effects with those of more recent times and whose
epicenters have been instrumentally
determined.
Epicenters are plotted in
Fig. 2; those of historical shocks are represented by open squares.
Regarding historical earthquakes in the ridge zone, estimations of location
and magnitude are difficult and necessarily of poor accuracy. The population
of the Azores islands was very sparse from the XVI to XIX century and
written accounts of estimates of damage from earthquakes inspire very little
confidence. Therefore, the data for these shocks, provided by Dr. Trepa
263
TABLE I
List of large shocks for the period 1500-1972
Date
Origin time
Lat. (ON)
Long. (OW)
M
5 Apr 1504
26 Jan 1531
9 Ott 1680
27 Dee 1722
-
37.0
37.0
36.0
37.0
5.0
12.5
4.0
10.0
-
-
1
31
21
25
Nov
Mar
Mar
Dee
1755
1761
1829
1884
-
37.0
37.0
38.0
37.0
10.0
13.0
1.0
4.0
20
8
25
29
28
May
May
Nov
Mar
Feb
1931
1939
1941
1954
1969
02-22-56
01-46-48
18-03-54
06-17-05
02-40-32.5
37.4
37.0
37.4
37.0
36.1
15.9
23.9
19.0
3.3
10.6
(8.91
7.1
7.1
8.114
7.0
8.0
(Sercico Meteorologico
National de Portugal), have not been included in
Table I.
Shocks with epicenters at sea, away from the coast, do not usually produce very large damage on land. Thus, the historical catalog for the region
may be incomplete. Plotted epicenters (Fig. 2) roughly delineate the fracture
from the ridge to the southern Spain-northern
Morocco area.
A brief description of shocks of the historical period is presented below.
30
40
20
I
0
l y
IO
0
I
HISTORICAL
INSTRUMENTAL
,93*
0
1500-
,9&’
0
1973
M>7
I
I
Fig. 2. Location and dates of large earthquakes for the period 1500-1972
tude M > 7. Open squares represent location of the historical data.
with magni-
264
As general references we have used Perrey (1847),
Galbis (1932) and Fontsere and Iglesias (1971).
Navarro-Neumann
(1920),
April 5, 1504 - Epicenter near Carmona (Sevilla). Extensive damage and loss
of lives in Carmona and nearby towns. A fault ruptured at surface with a
vertical offset of 1.8 m extending up to 3 km length. It was felt in Medina
de1 Campo (at 500 km from Carmona), Algarve (Portugal), in the province of
Murcia and northern Morocco.
January 26, 1531 - Epicenter probably at sea, SW Cape San Vicente. Over
one fourth of the houses in Lisbon were totally destroyed with many persons killed. Damage was also reported from towns in Spain and northern
Morocco and the shock was felt as far away as Holland and Switzerland.
There are descriptions of tsunamis along the southwest coast of Portugal.
The intensity is believed by some authors to have been as large as that of the
earthquake of 1755.
October 9, 1680 - Epicenter near Malaga. Felt area was limited mainly to
southern Spain and northern Morocco. In Malaga alone more than 850
houses were totally destroyed and 1250 seriously damaged, In Granada most
houses suffered severe damage. There are reports of ground ruptures.
December 27, 1722 - Epicenter at sea, near the southern coast of Portugal
with an area of destruction
including towns from Cape S. Vicente to Faro
for a distance of more than 100 km. Many people were killed either by the
shock itself or by the large tsunami which swept the coast.
November 1, 1755 - The epicenter has been estimated at about 300 km
southwest of Cape San Vicente (Machado, 1966). Known as the Lisbon
earthquake, it is considered as one of the largest shocks of which we have
record. The shock itself and the subsequent large tsunami produced destruction and loss of lives especially in Lisbon (estimates vary from 20,000 to
40,000 deaths) and Fez, as well as extensive damage in other towns and
cities of Portugal, Spain and Morocco. There exist very detailed accounts of
the damage in Portugal and Spain from the commissions established in these
two countries. The bibliography on all aspects of this earthquake is very
large; one of the most complete references is that of Pereira de Sousa (1919),
for the damage in Portugal.
March 31, 1761 - The epicenter was also probably offshore in the Atlantic.
Destructive in the Azores and Canary Islands, it was felt in Madrid, Bordeaux
and as far as Holland and Ireland. This was the first of a sequence of large
shocks; a total of fourteen were felt in the interval between 31 March and 14
June.
265
March 21, 1829 - Epicenter near Torrevieja (Alicante). Navarro-Neumann
reports a total of 839 persons killed and 375 seriously injured. Property
damage is estimated at more than 5,000 houses totally or partially destroyed. According to Sieberg (1904), in Murcia alone more than 3,500
houses were destroyed.
December 25, 1884 - Epicenter between Granada and Malaga. A commission that studied the effects in the provinces of Granada and Malaga gives
745 persons dead and 1,253 seriously injured, although the number of victims was probably much higher; more than 17,000 houses suffered from
total to serious damage. This shock aroused great interest in the learned
circles of Europe, and the Academic des Sciences (Parr’s) and the Accademia
dei Lincei (Rome) sent commissions to Spain to study the damage. There
exists a large bibliography.
Earthquakes
for the period 1962-l
972
To obtain the best possible picture of the distribution of moderate to
small earthquakes,
we have considered the lo-year period 1962-1972.
During that interval the WWSSN stations were already in operation and the reliability of the epicentral and magnitude determination
is greater than before.
Data for the period 1962-1972
have been obtained from the Hypocentral
Data File of the National Geophysical and Terrestrial Data Center, NOAA,
.
.
Fig. 3. Seismicity
map for the period
1962-1972
with magnitude
rn~~s
> 4.
266
Boulder, Colorado. Epicenters have been plotted in Fig. 3. The number of
earthquakes is 302 and the lower limit of magnitude mCG s = 4, but the list is
probably complete only to magnitude m CGS = 4.5. On the western side,
epicenters are concentrated
on the axis of the ridge. The line from the
Azores to Gibraltar is interrupted
between longitudes 23” W and 16” W. This
zone coincides with the location of the large shocks of 1931 and 1941 (see
Fig. 2). Absence of low-level seismicity at this section indicates the sudden
release of the strain by the two quoted large shocks. Perhaps there is a locking mechanism that prevents release of accumulating strain by means of
small shocks, making this gap zone a potentially dangerous area (Sykes,
1971). This suggestion has been partly confirmed by the large earthquake of
26 May, 1975 of M = 8.2 with epicenter (35” .8N, 17”.6W). For the region
east of longitude 15” W the epicenters are spread over a wider area covering
the Alboran Sea, southern Spain and northern Morocco. The southern limit
of the seismicity continues eastwards through northern Algeria and Tunisia,
to join there with the seismicity of the Sicily-Calabrian
arc.
Seismic activity
near the Spanish Peninsula,
1961-l
972
Seismicity in the region of the Spanish Peninsula has been studied by
Munuera (1963, 1964). Here, we will limit our study to data starting in
1961, when the “Boletin de Sismos Proximos” was first published by the
Instituto Geogrifico y Catastral, giving more accurate epicenter determinations for shocks in this area. Epicenters taken from this bulletin for the period 1961-1972
are plotted in Fig. 4. Most epicenters lie south of the socalled Guadalquivir fault, which marks the southern limit of the stable Spanish plateau. On land, earthquakes are concentrated
in three groups: to the
north of Granada, near Huescar and Orce, and in the vicinity of Murcia. At
sea, shocks of magnitude between 3.5 and 5.0 are spread over a wide area
east and west of the Strait of Gibraltar. Seismicity, when studied in detail,
shows a marked difference in character at both sides of the strait. The percentage of shocks with m > 5 is considerably larger to the west of the strait.
Shocks are spread through the whole extent of the Alboran Sea, but the
activity stops on the east, on the Spanish side, at about l”E, while on the
North African side shocks continue along the coast to Tunisia.
Another active area of relatively small earthquakes is found in the
Pyrenees and Catalunya. On tectonic grounds it seems that this zone should
be linked to that of the Guadalquivir through the Balearic Islands, but seismicity does not support this relation, because the islands are almost completely free of seismic activity.
Deep earthquakes
The problem concerning the depth of earthquakes of this region is worth
treating here. Shocks at the ridge are shallow, average depth being around 20
267
km. Shocks at the Gulf of Cidiz and southern Spain are mostly also shallow,
but there may be some intermediate-depth
shocks; some 22 in the period
1951-1960
are listed by Munuera (1963) as having depths between 60 and
250 km and from 1961 to 1972 another 22 with depths over 60 km. These
depth determinations,
however, are not very reliable because of the asymmetrical distribution of stations and lack of information regarding the crustal
and upper-mantle
velocities. The question of the existence of intermediatedepth shocks in the zone cannot be solved unless new stations are installed
and the velocity distribution
with depth is known. The solution of the last
problem may benefit from the results of the deep seismic profiles made
recently in the area.
Of particular importance is the occurrence of shocks of great depth, over
600 km, in the region near Granada. The first of these which is well documented is the so-called “deep Spanish earthquake”
of 29 March 1954, depth
650 km and magnitude 7. Another deep shock in the same area and at approximately
the same depth occurred on 30 January 1973 with magnitude 4.
This shows that the earthquake of 1954 was not a completely isolated event.
A focus of seismic activity exists at that depth, which must be accounted for
in any tectonic interpretation
of the region.
-
.
.
.?
*
.
---%.(
l
m-1on
.
.
. ozo..
art.
.
.
ALa”
.
.
l
l
.
d
.
.
:
.
AI
.
.
“!
c
.
.
Fig. 4. Seismicity
map of the Iberian Peninsula
are those determined
by LCSS (Madrid).
for the period
1961-1972.
Magnitudes
Fig. 5. A. Return period vs. magnitude;
full circles (Lcipez Arroyo and Stepp,
1971). B.~Maximum yearly magnitude
for-1 850-19j3.
open circles (Khik,
1973) and
As indicated in the Introduction,
earthquakes do not follow the same time
distribution
through the whole region under study. The strongest contrast is
presented between the middle part of the fracture, locus of very strong
shocks separated by long intervals of quiescence, and the Peninsula and
Alboran Sea, where small shocks occur fairly frequently.
Obviously, historical data on earthquakes with epicenters in the Atlantic
Ocean are not complete except for the very large magnitudes and, therefore,
are not adequate for a quantitative treatment. As for those with epicenters
on land and in the Alboran Sea and Gulf of Cidiz, Lopez Arroyo and Mezcua (1972) studied their time distribution
for the years 1962-1972,
and
their relation to the structure and motion of tectonic plates as derived from
other investigations; they found some grounds for suspecting that earthquakes in the Gulf of Cidiz induce seismicity which propagates towards the
east to the borders of the assumed Alboran subplate.
An extreme-value
analysis of almost the same region (see insert in Fig. 5)
with data from the catalogue of the Spanish Seismological Survey, has been
made by Kirnik (1971) and Lopez Arroyo and Stepp (1973). The return
periods for different magnitudes found in these investigations are presented
in the same figure. The lower curve of the two given by K&nik was obtained
by correcting to an infinite time interval by a hyperbolic law to the data for
three finite intervals of 30, 55 and 67 years, respectively. The upper and
lower curve by Lopez Arroyo and Stepp correspond, respectively, to data
from 1901 to 1967 and 1850 to 1972. These later values are much smaller
than those by Karnik, because data previous to 1900, including the large
Andaluciaearthquake
of 1884, did not enter into his analysis. Karnik gives
6.3 as maximum magnitude observed in the region, while the largest shock of
1884 reached a value M - 7.
Return periods for magnitudes above 6.5 may be in error as a type-1
Gumbel distribution
has been fitted to the data and such a law may not be
appropriate.
The lowest graph in Fig. 5 represents the time variation of the largest
magnitude observed every year, according to the same catalogue. It shows
another feature of the long-term variation of seismicity of the region, i.e.,
the jump around 1890 in the mean maximum magnitude from about 5.0 to
5.5.
Magnitude
distribution
Certain characteristic
of
the magnitude-frequency
the NOAA data from 1963
region, and separately, for
(east of 20” W). The values
total
ridge
fracture
the seismicity of a region can be obtained from
relation. This relation has been determined from
to 1972. Figure 6 shows the curves for the whole
the ridge (west of 2O”W) and the fracture zone
found by a least-squares fit for the slope are:
b = 1.19 * 0.04
b = 1.34 f 0.06
b = 0.87 * 0.04
The linearity of the relation holds for shocks larger than mcos = 4.5
which indicates that for smaller shocks the set is probably not complete.
The difference in the values of b for the ridge and fracture zone is significant, since magnitude determination
of the set is fairly uniform for the
whole region. Considering as a reference the overall value of b (1.19) for the
whole region, b is higher for the ridge and lower for the fracture. A similar
difference in the value of b for several ridge and fracture zones of the MidAtlantic region has been obtained by Sykes (1967) who attributes it to the
difference in faulting mechanism; normal faults in the ridge and strike-slip in
the fracture. Francis (1968) has also found a consistent difference in the
values of b for ridge and fracture regions (b = 0.65, fracture; b = 1.33, ridge),
that he interprets in terms of the conditions in the crust and upper mantle
under the two regions which, according to ocean-floor spreading, favor brittle fracture in the fractures and cataclastic faulting in the ridge. According to
270
Fig. 6. Cumulative
fracture zones.
frequency
vs. magnitude
for earthquakes
in the whole
area, ridge anti
6
t
5-
3I
3
4
6
5
Magnitude
Fig. 7. Comparison
spond to data after
of magnitudes
1970.
determined
LCSS
by USCGS
(Madrid1
and LCSS.
Open
circles
271
Wyss (1973) low values of b can be considered as indicative of high stresses
and large source dimensions, while high b-values correspond to low stresses
which in the ridge result from a weaker crust and presence of high temperatures.
Using only data of the Spanish region, as taken from the “Boletin de Sismos Proximos” from 1961 to 1972 and magnitudes above mLCSS = 4, a
value of b = 1.11 has been obtained. This value is higher than the one obtained from the NOAA data. There are two reasons for this; one is that data
come predominantly
from east of the strait, the other that the magnitudes
calculated by Laboratorio
Central de la Section de Sismologia (LCSS),
Madrid, are biased toward higher values and, thus, shocks of magnitudes
actually under 4 are being assigned magnitudes between 4 and 5. The relation between the NOAA (or USCGS) magnitudes and those of the LCSS, is
shown in Fig. 7; open circles correspond to data since 1970, when the
method for determination
of magnitudes was modified.
TABLE
II
List of earthquakes
with calculated focal
taken from ISS bulletins and magnitudes
GS and magnitude rn~~ S.
No.
Date
13
11
12
19
20
18
17
1
6
8
5
3
15
7
9
10
16
14
2
4
22
21
20
8
25
19
29
5
15
17
11
6
17
18
29
29
4
5
28
6
18
13
7
30
May 1931
May 1939
Nov 1941
May 1951
Mar 1954
Dee 1960
Mar 1964
May 1964
Jul 1964
Sep 1964
Sep 1964
Sep 1964
Jun 1965
Sep 1965
Jul 1966
Jul 1966
Feb 1969
Sep 1969
Nov 1970
May 1972
Nov 1972
Jan 1973
Origin time
___02-22-56
01-46-48
18-03-54
15-54-24
06-17-05
21-21-47
22.30-26.0
19-26-20.6
22-34-43.8
18-55-47.4
15-02-00.9
13-12-42.3
04-27-58.3
23-20-17.9
12-15-26.5
05-09-04.7
02-40-32.5
14-30-39.5
12-23-18.
16-40-22.
12-05-13.8
02-36-11.6
mechanism.
Before 1962 epicentral data are
are M. After 1962 data from USCGS, NERL and
Lat. (ON)
Long.
37.4
37.4
37.4
38.1
37.0
35.6
36.2
35.2
41.7
38.3
44.5
39.8
36.6
45.2
37.5
37.6
36.1
36.9
35.1
45.0
49.0
37.0
15.9
23.9
19.0
3.7
3.3
6.5
7.6
35.9
29.9
26.6
31.3
29.7
12.3
28.2
24.7
24.7
10.6
11.9
35.7
28.2
39.4
3.6
(“W)
h (km)
Magnitude
N
N
N
N
(6)
650
34
27
33
33
33
24
20
33
24
20
18
22
33
33
33
33
634
(Z.5,
6.2
5.6
4.8
4.9
5.6
5.5
4.8
5.4
5.4
5.1
7.3
5.7
5.4
5.0
5.2
4.0
7.1
7.1
8.3
272
EARTHQUAKE
FOCAL
MECHANISM
Data and solutions
The focal mechanism of 22 earthquakes in this area has been studied using
the direction of motion of the onset of the P-wave. Earthquakes are listed in
Table II, and solutions in Table III. Planes are given by their strike and dip,
axes of tension and pressure by their trend and plunge. Solutions are shown
in Figs. 8-14. Data for all solutions have been read by the authors, except
for shock 9 (6 Sept. 1966) whose solution is that of Sykes (1967) and it is
not shown in the figures. Of the 22 earthquakes, those numbered from 1 to
7 belong to the Mid-Atlantic Ridge. Numbers 8-11 are situated on the Terceira ridge, and, of these 9, 10 and 11 are placed at the end of the ridge and
beginning of the fracture zone. Shocks 12 and 13 occurred at the center of
the fracture zone. Shocks 14 and 15 are at the eastern end of the fracture
and beginning of the more complex region that we designate as Gulf of
Cadiz; to that region belong shocks 16-18. The two deep shocks 20 and 21
TABLE
III
Focal-mechanism
solutions.
trend (p) and plunge (6).
No.
13
11
12
19
20
18
17
1
6
8
5
3
15
7
9
10
16
14
2
4
22
21
Date
20
8
25
19
29
5
15
17
11
6
17
18
29
29
4
5
28
6
18
13
7
30
May 1931
May 1939
Nov 1941
May 1951
Mar 1954
Dee 1960
Mar 1964
May 1964
Jul 1964
Sep 1964
Sep 1964
Sep 1964
Jun 1965
Sep 1965
Jul 1966
Jul 1966
Feb 1969
Sep 1969
Nov 1970
May 1972
Nov 1972
Jan 1973
Planes are given by their strike and dip and P, T-axis by their
Plane B
Plane A
N88 E
N 79 E
N 84 E
N 68 E
N 3E
N 38 E
N 65 E
N 90 E
N 60 W
N 76 E
N 1W
N 22 E
N 70 W
N 9E
N 68 W
N 84 E
N 64 W
N 84 E
N 31 W
N 16 E
N17W
NO E
80
82
88
66
89
70
80
84
S
s
N
NW
E
NW
SE
N
72
80
44
84
40
80
70
80
68
82
60
50
47
‘70
NE
S
E
SE
s
SE
SW
s
SW
s
NE
w
E
E
N 0’
N 10 W
N 7E
N 7W
N77W
N 54 W
N 64 W
N 1E
N 28 E
N 9W
N 63 E
N 82 W
N 66 E
N 74 E
N 6E
N 1W
N 48 E
N 6W
N 33 E
N81 W
N 52 E
N 48 W
T
P
80 E
84 w
84 E
57 NE
3w
83 SW
16 NE
83 E
75 NW
60 W
68 NW
26 NE
60 NW
22 NW
54 SE
60 W
48 NW
79 w
60 NW
80 s
22 NW
30 SW
cp
6
cp
6
135
304
129
208
89
174
165
225
162
126
314
268
354
256
334
185
178
128
181
159
294
298
10
3
6
40
44
10
34
8
30
14
14
45
11
50
40
13
12
2
45
27
15
58
44
34
39
301
276
80
320
135
10
10
5
4
46
17
54
1
71
28
198
135
108
116
235
37
72
85
83
54
184
75
3
29
51
35
64
31
10
25
47
11
4
8
50
22
Fig. 8. Focal-mechanism
solutions of earthquake.
A. 1; 17 May 1964. B. 2; 18 November
1970, C. 3; 18 September
1964, D. 4; 13 May 1972. Solid circles compressions,
open
circles dilatations,
smaller symbols short-period
data, crossed symbols doubtful data.
are located east of Gibraltar strait, and number 19 is on the Guadalquiv~
fault zone. For earthquakes after 1962, data of first motion have been taken
whenever possible from the WWSSN stations, and, of these, long-period data
have been preferred to short period. When both types of data are used in the
same solution, those from short-period
instruments are shown in the figures
by smaller symbols. Crossed dots are used when data were of uncertain sign.
Six solutions correspond to earthquakes before 1962. Four of these have
also been studied by other authors; the remaining two are the shocks of
1931 and 1939. The two northern quadrants of these solutions are well
Fig. 9. Focal-mechanism solutions of earthquake. A. 5; 17 September 1964. B. 6; 11 July
1964. C. 7; 29 September 1965. D. 8; 6 September 1964. Symbols as in Fig. 8.
covered by North American and European stations, but the southern quadrants have practically no data. One of the few reliable stations operating in
South America at that time was La Paz, Bolivia, whose records have been
studied. For the shocks of 1939 and 1941, data reported by the ISS bulletin
have been used (square symbols), together with data newly read from seismograms of selected stations. For the shock of 1941 Di Filippo’s solutions
(Di Filippo, 1949) have also been replotted in Fig. 30. The only difference
with our solution comes from the data of Tananarive and Johannesburg
which determine the dip of the N-S plane in Di Filippo’s solutions. Data
15 NM 19u
C
D
Fig. 10. Focal-mechanism
solutions of earthquake.
A. 10; 5 July 1966. B. 11; 8 May
1939. C. 12; 25 November 1941. D. 12; 25 November 1941. Squares data taken from ISS
bulletins, other symbols as in Fig. 8.
and solution for the 1954 deep earthquake (Fig. 14) are those of Hodgson
and Cock (1956). For the 5 Dec. 1960 earthquake, data have been newly
read from records of near stations. The solution given in Fig. 11D is that of
Constantinescu
et al. (1966) which, as can be seen in the figure, fits well
with our data. For the earthquake of 19 May 1951 the data have been taken
from Chacon (1955) (Fig. 12B). An error in the original paper concerning
the dip angles of the planes has been corrected.
The solution of March 1964 is taken from Udias and Lopez Arroyo
(1969); that of McKenzie (1972) differs in the orientation
of the low dip-
276
Fig. 11. Focal-mechanism
solutions of earthquake.
ber 1969. C. 15; 29 June 1965. D. 18; 5 December
A. 13; 20 May 1931. B. 14; 6 Septem1960. Symbols as in Fig. 8.
ping plane, which is not well defined by the data, but is similar otherwise.
The solution of the shock of 28 Feb. 1969 (Fig. 12C) is that of Lopez
Arroyo and Udias(1972).
The difference with the one by Fukao (1973)
comes from the dilatations of three short-period records from Central American stations not considered by him which determine the orientation of the
E-W plane. Disregarding these, we can obtain a solution similar to that of
Fukao, which has also been drawn in Fig. 12C (dashed line). McKenzie’s
mechanism (1972) for this earthquake does not take into account the near
stations, two of which belong to the WWSSN (TOL and MAL), and it is not
277
Fig. 12. Focal-mechanism
1951. C. 16; 28 February
solution of earthquake.
A. 17; 15 March
1969. D. 22; 7 November 1972. Symbols
1964. B. 19; 19 May
as in Fig. 8.
consistent with them. McKenzie gives in the same paper a solution for the
shock of 6 Sep. 1969, also inconsistent with the data from near stations. The
strike-slip mechanism solution given in Fig. 11B is more consistent with all
the available data.
TECTONIC
INTERPRETATION
OF THE REGION
A summary of the results from the focal-mechanism
solutions is given in
Fig. 15. As a consequence
of the different character, both in seismicity and
27s
rp
?P
4
I+ *+
. .
+
I,
*‘+-H
__-d
.*’
_--
-_____;_____----
ALEORAW’
CWW
SITE
Fig. 13.
Focal-mechanism
f
SEA
SOLUTION
composite
solution
for the Alboran
region.
Symbols
as in
Fig. 8.
Fig, 14. Focal-mechanism
solution
of earthquake.
March 1954. Symbols
as in Fig. 8.
A, 21; 30 January
1973.
B. 20; 29
Fig.
PLATE
15. Focal-mechanism
AMERICAN
I
solutions
and
outline
units
PLATE
PLATE
of tectonic
I
AFRICAN
EURASIAN
in the
Azores-Alboran
,\
region.
I
30
280
focal mechanism, of various parts of the area; this will be considered in four
subzones, namely, Mid-Atlantic and Terceira Ridge, Azores-Gibraltar
fault,
Gulf of Cadiz, Alboran and Betica.
Focal mechanism, in spite of its uncertainties
and limitations, is a powerful tool in the study of the tectonics of a region. As is the case with all seismic data, this refers to actual tectonics and does not reflect directly the
geological processes by which this situation was reached.
Mid-Atlantic
and Terceira Ridge
In the context of plate tectonics, this region is formed by the triple junction of the Eurasian, American and African plates. Thus the ridge system
has, at this point, an inverted Y shape. On the main ridge, south of the junction point, the solution for shock 1 (Fig. 8A) is a good example of the mechanism of transform faulting; the motion is strike-slip left-lateral on nearly
vertical planes. The east-west
plane must be selected as the fault plane on
the basis of transform faulting originated from the expansion of the ridge.
The solution of shock 2 (Fig. 8B) represents a different kind of mechanism,
with E-W horizontal tensions resulting in normal faulting along inclined
planes; the strike of one of the planes agrees with the trend of the ridge. The
mechanism of this shock is, then, consistent with the stresses expected at an
expanding ridge, although the tension axis is not exactly normal to the axis
of the ridge.
Among the shocks situated on the main ridge to the north of the junction
point, number 3 (Fig. 8C) and 7 (Fig. 9C) have very similar mechanisms,
corresponding
to normal faulting on steep planes striking nearly northsouth, and tension axes normal to the ridge. This is again the type of mechanism expected at an expanding ridge. Solution of shock 6 (Fig. 9B) is less
reliable; it has a larger strike-slip component of motion, but the tension axis
is also nearly horizontal and normal to the ridge. With regard to shock 4
(Fig. 8D) its solution is also poorly defined; it represents right-lateral horizontal motion on a nearly vertical plane striking east-west,
which would
correspond to transform faulting in the same direction as the large fracture.
Shock 5 is located to the west of the ridge and can be considered as an intraplate shock. The corresponding
solution (Fig. 9A), which is well defined
from the long-period data, represents reverse dip-slip motion on a plane
striking north--south.
This type of motion may be associated with a mechanism of focal failure of the crust under horizontal pressure from the expanding ridge. The crust fails to act as stress guide at this point. Another intraplate shock with magnitude 5.2 occurred on Nov. 7, 1972 to the northwest
of this one (see Table II). Data for this shock is scarce and of low amplitudes. Using short-period data, a solution is obtained (Fig. 12D) which is also
consistent with reverse faulting, similar to that of shock 5.
Ocean-bottom
topography
and the magnetic anomaly map (Pitman and
Talwani, 1972) delineate the extension of the eastern short branch, or Ter-
281
ceira ridge. There is only one earthquake on this ridge, number 8 (Fig. 9D),
for which we have a focal-mechanism
solution, and this is poorly defined.
Earthquakes 9-11, located near the end of the ridge, already belong to the
fracture zone. The solution given for shock 8 is consistent ,with right-lateral
transcurrent
motion, the type of motion present in the Azores%ibraltar
fault. It is possible, then, that the ridge itself may be fractured laterally by
an en-echelon fault system striking east-west.
This hypothesis seems quite
plausible, since this part of the ridge is caught between the east-west
spreading motion of the two main branches of the Mid-Atlantic Ridge, which
creates a differential right-lateral motion along the boundary of the Eurasian
and African plates. The presence of these shear stresses at the Terceira ridge
will favour an accumulation
of material ascending from the mantle, that may
be the origin of the volcanism of the islands.
The nature of the triple junction is, then, using the terminology
of McKenzie and Morgan (1969), a ridge-ridge-ridge
junction, developed from a
ridge-fracture-fracture
junction without any change in the motion direction between the plates, as explained by McKenzie (1972), assuming the
occurrence of oblique spreading. This proposed evolution of the triple junction will explain also the presence of the two transform faults at the sites of
shocks 1 and 9 with motion in opposite directions, one left lateral and the
other right lateral. It would also eliminate the complexities of the interpretation given by Krause and Watkins (1970) in terms of differential velocities of
spreading and the creation of secondary spread centers. The bend of the
principal part of the ridge could also have been formed at the time of the
breakaway of Europe and Africa from America. There would be, then, no
need of any mechanism to explain this change in ridge direction, because it
simply preserves the outline of the original break. As new sea floor is generated at the ridge in a direction normal to the axis, because of the bend, a
zone of tension would be created at its convex side, facilitating the creation
of a secondary ridge. Further out from the ridge the difference in spreading
directions is compensated
solely by the shear horizontal motion along the
Azores-Gibraltar
fracture.
Azores-Gibraltar
fracture
The fracture zone extends from the edge of the Terceira ridge to the proximity of the Strait of Gibraltar. This zone was interpreted by Heezen et al.
(1959) as a branch of the Mid-Atlantic Ridge and named the Azores-Gibraltar ridge. The name has been kept in the literature, although Menard (1965)
concluded that it is a fracture zone. Seismically, as has been already shown,
it is a region characterized
by the occurrence of large earthquakes, of which
four with magnitudes larger than seven have occurred in the last 50 years.
Mechanism solutions of shocks along the fracture show predominant
rightlateral horizontal motion. The first three shocks, numbers 9, 10 and 11 are
situated at the western end of the fault, at the edge of the Terceira ridge.
282
Data for shocks 10 and 11 (Figs. 10A and 10B) are not sufficient to define
their mechanism with accuracy, but the two northern quadrants consistently
give compressions for European stations and dilatations in the North American ones. Shocks 12 (Figs. 1OC and 10D) and 13 (Fig. 11A) are situated at
the central part of the fault. Data for shock 13 are very scarce but still consistent with the general trend. Shock 14 (Fig. 11B) is located at the eastern
end of this section of fracture and has the same type of mechanism. Mechanisms from shock 11 to 14 show a remarkably constant strike-slip rightlateral motion along a plane striking east-west,
which coincides with the
trend of the inferred fault. The western part of this fracture zone has been
mapped by side-scan sonar for a distance of 400 km (Laughton et al., 1972).
If this fault is extended all the way to the Strait of Gibraltar its total
length would be about 1500 km, which would make it one of the largest
faults of this type. The right-lateral character of the motion is the most striking feature of this fracture system, which, according to Ritsema (1969),
continues further east in the North African shocks and may be related to the
same type of motion in the North Anatolian fault.
Gulf of C&Ii2
This zone marks the beginning of the interaction of the continental blocks
of the Iberian Peninsula and Africa which extends from near 12”W to the
Strait of Gibraltar. In this region the fracture that starts at the Azores
changes in character, with epicenters spread out over a wide area and sporadic very large earthquakes. In spite of this change in character, the occurrence of large earthquakes lined up in the direction of the fracture is sufficient evidence that this must be extended to the Strait of Gibraltar itself.
Four focal-mechanism
solutions belong to this area. The solution of shock
15 represents reverse faulting along planes dipping either to the south or to
the north. The south-dipping plane involves underthrusting
of the northern
block and has been selected as the fault plane according to the general interpretation given to this area. The solution given here for shock 16 (Fig. 12D)
is taken from the author’s previous paper (Lopez Arroyo and Udias, 1972).
Arguments put forth in that paper and later (Udias and Lopez Arroyo,
1972), favour the selection of the east-west
striking plane with some component of right-lateral horizontal motion as the fault plane. The mechanism
of shock 17 (Fig. 12A) has a steep plane with reverse dip-slip motion, striking east-west
and a second plane with low-angle thrust and undefined strike.
There is no intrinsic evidence which will favour the selection of either plane,
so that arguments are actually based on the tectonic interpretation
of the
region as a whole. Closer to the strait is shock 18, whose solution (Fig. 11D)
has predominantly
horizontal motion at planes making a 45-degree angle
with the direction of the fault.
In the interpretation
of the motion in this area we have followed the criterion that major earthquakes are related to major features in the crust. Ac-
283
cordingly, we interpret shocks 14-18 as related to the continuation
of the
Azores-Gibraltar
fault and selected as fault plane, from their two nodal
planes, that striking in the direction of this fault. When this is done, solutions of shocks 15-17 represent reverse faulting with the Spanish block
dipping under Africa along a steep plane. There is, then,’ in this area from
west to east a gradual conversion of the right-lateral strike-slip motion into
reverse dip-slip motion. McKenzie (1972) prefers the alternative choice of
considering that the motion takes place along the low-angle thrust plane of
his solutions of shock 16 and 17, which results in the underthrusting
of
Africa. He bases his argument on the concordance
of the direction of the
slip-vector with the motion resulting from the rotation of the Eurasian plate.
His solutions for shocks 14, 16 and 17 are different from ours as already
discussed, giving an orientation of the low dipping plane consistent with his
interpretation.
A different interpretation
of the mechanism of shock 16 is
that of Fukao (1973). He selects the alternative plane, relating the shock to a
regional feature striking northeast, and responsible for the rise of the Gorringe bank, supporting his selection basically on the distribution of the aftershocks. This argument however is not conclusive, since the distribution of a
larger number of aftershocks using near stations gives an east-west
elongated
area (Lopez Arroyo and Udias, 1972).
Another evidence in support of our interpretation
of the motion along the
reverse fault are the results of seismic refraction studies in southern Portugal
which show a considerable dip of the crust toward the southeast (Mueller et
al., 1973).
All solutions obtained for this area, with the exception of that of shock
14, have pressure axes with a consistent north-south
orientation and are
nearly horizontal (greater inclination is that of shock 17, with 34”). This
result is independent
of the selection of the fault planes. The obtained horizontal compressional
stresses are consistent with the motion of approach of
the lberian Peninsula to the African continent.
The change in character at this part of the fault, from strike-slip to reverse
motion due to horizontal north-south
compression may be a consequence
of the behaviour of the Spanish Peninsula as a partly independent
subplate
(see Fig. 15). This independence
from the Eurasian plate in the past is well
documented
from paleomagnetic
data (Van der Voo and Boessenkool,
1973). The counterclockwise
sense of rotation of the peninsula, if still active, coupled to the right-iater~ horizontal motion along the Azores-Gibraltar fault would result in horizontal north-south
compressions along the
boundary of the peninsula and African plates.
Alboran
Sea and Betica region
East of the Strait of Gibraltar the situation becomes even more complicated (Rothe, 1968). The earthquakes in this area are of too small a magnitude to allow an accurate determination
of their focal mechanism with the
seismic net presently
in operation.
Only one solution
is available
(shock 19;
Fig. 12B) and it represents
normal faulting
with a left-lateral
horizontal
component
of motion.
This does not seem to be related with the regional
motion,
but only to some local structure.
For other shocks composite
solutions have been attempted;
the most consistent
picture was obtained
using
only shocks from the Alboran
Sea. The solution
(Fig. 13) shows a considerable amount
of right-lateral
motion,
indicating
that this is still to some extent present at the other side of the strait.
The tectonic
history and structure
of the Alboran
Sea has been the subject
of a number
of recent geological
and geophysical
studies. There exist several
structural
models and interpretations
of the formation
of its two basins.
(1) Glangeaud
et al. (1970) propose a model which implies compressional
forces since the Jurassic which closed a former wider gap between
the peninsula and Morocco.
These compressional
forces are also responsible
for the
“pseudo-arc”
of Gibraltar
formed by the overthrusting
of older strata.
(2) Andrieux
et al. (1971) and Andrieux
and Mattauer
(1973) propose a
fixed Alboran
plate reacting
with the relative eastern movement
of the European and African plates and resulting
in an overriding
of this plate over the
margins of the other two. This interaction
is the origin of the orogenesis
of
the Betica and Riff mountain
range system and the arcuate structure
at Gibraltar that joins these two systems.
The direction
of the geological
features
(faults and folds) is NE-SW
in the northern
margin and NW-SE
in the
southern
one. No mechanism
is given by which the opening
of the Alboran
basins takes place.
(3) Le Pichon et al. (1972) propose
a mechanism
of opening
of the two
Alboran
basins which basically
is also adopted
by Auzende
et al. (1973) and
Olivet et al. (1973).
This mechanism
consists in a motion
along transverse
or
transform
faults trending
ENE-WSW.
Le Pichon proposes
three of these
faults along which the opening
takes place by rotation
about a single pole.
The southernmost
of them separates
a small subplate
(Riff plate) from the
African plate. The Alboran
ridge which divided the two basins is a marginal
fracture
ridge created by the shear of the two portions
of continental
crust.
Time of opening
is placed at early Middle Miocene.
The two basins opened
separately,
sliding along the fault marked by the Alboran
ridge. Le Pichon et
al. (1972) claim that the model is still compatible
with the Alboran
plate
model if such a plate is divided into two. The ENE-WSW
motion
of the Riff
plate would have required
a zone of crust consumption
west of Gibraltar;
such a zone is difficult
to explain.
The arc-like structure
in Gibraltar
is not
explained
unless Andrieux’s
model of the Alboran
plate is also included.
(4) Galdeano
et al. (1974), on the basis of a new map of aeromagnetic
anomalies,
propose
an opening
of the Alboran
Sea along a direction
normal
to that presented
in the previous paragraph,
that is, in a NNW-SSE
direction. This direction
is normal to the trend of the Alboran
ridge, which according to these authors,
was formed later by a NW-SE
compression,
thus
being a compressive
feature and not a result of transform
faulting.
(5) Loomis (1975) proposes a quite different model. The Alboran Sea is
separated into two parts, west of longitude 4” W the crust is continental and
to the east it is oceanic. The main mechanism is extension of the crust which
determines thinning of the crust toward the east and rising of the mantle
material, The plate motion responsible for this process is a counterclockwise
rotation of the Spanish plate pivoting at Gibraltar that must be dated at the
Oligocene or Miocene. This recent rotation of the peninsula (the main rotation is supposed to have taken place in the Late Jurassi~~retaceous)
may
be related also to further opening of the Bay of Biscay. Actual tectonics in
Alboran is then due to extension with oceanization
of the crust between
Spain and Africa.
With regard to present-day processes for this region, data are insufficient,
because of the poor coverage of present seismological stations that results in
lack of accuracy of epicentral determinations,
focal depths and mecb~ism.
En spite of this, available data are still very useful in the understanding
of the
main structural problems of the region.
Shallow shocks spread throughout
the area makes very probable the existence of the so-called Alboran plate, that can be considered as a buffer-plate,
but it offers no indication of a division of this plate into two parts. This
subplate, as defined from seismicity, would extend from the Guadalquivir
fault to the coast of Morocco. Differences in characteristics
of seismic occurrences at both sides of the Strait of Gibraltar support the presence in Alboran
of a structure different from that of a simple continuation
of the AzoresGibraltar fault. The continuation
of this line through northern Morocco,
Algeria and Tunisia marks the northern boundary of the African plate. Composite solution of Fig. 13 with compression axis in NW-SE direction seems
to indicate the shear conditions in the Alboran plate, the effect of the closing motion at that point of the Eurasian and African units.
An even more complicated
problem is presented by the occurrence of
deep earthquakes in the zone near Granada. The focal-mechanism
solutions
of the two deep earthquakes of the area are given in Fig. 14. Both correspond to stresses acting at about 45 degrees from the vertical and trending
east-west.
This direction is not compatible with a simple model of the oceanic part of the African plate dipping under Spain due to a north-south
compressional
motion. Such a type of motion should result in shocks with
pressure axes trending north-south
(Isacks and Molnar, 1969). The orientation of the stresses obtained from the focal mechanism shows that they are
connected with the arc of Gibraltar and possibly originating at a detached
slab of lithospheric material, relic of a paleo - Benioff - zone related to this
arc-like structure trending north-south.
The fact that the stresses along the
slab are of a compressional
nature and the depth of the foci is about 650 km,
argues in favour of material at the limit depth. This material still preserves
enough strength to be able to support accumulation
of strain and to release
it in the form of earthquakes. However, it is possible that the orientation
of
the stresses of the deep shocks corresponds to a situation of the past and has
286
no relation with present tectonic motion.
To the east, seismic activity in the Alboran Sea stops at about 1”E. Another active seismic zone, the Pyrenees and Cat~unya, limits the area of the
peninsula to the north. Earthquakes are in general small to moderate and not
too numerous (Fontsere and Iglesias, 1971). The only mechanism solution
for the area is that of the earthquake of Arette of 13 August 1967 (Hoang
Trong and Rouland, 1971). Motion is predominantly
horizontal and left
lateral. The plane striking 41”SE agrees with the orientation of the great
faults of the north Pyrenees. Motion along this plane is consistent with the
southeast relative motion of the Spanish Peninsula with respect to Europe,
possible continuation
of the motion which opened the Bay of Biscay between Late Jurassic and Late Cretaceous (Van der Voo and Boessenkool,
1973). This motion will result in right-lateral motion in the southern edge
and left-lateral motion in the northern one. If the peninsula has once had
independent
motion with respect to the Eurasian plate, it is possible that it
may still preserve a certain independence,
which is also supported by the
sporadic seismic activity along the western coast of the peninsula (Beuzart,
1972; IJdias, 1975).
CONCLUSIONS
The analysis of seismic data of the Azores-Alboran
region gives support
to the following conclusions:
(1) The Mid-Atlantic Ridge at the Azores islands forms a triple junction of
ridge-ridge-ridge
nature and is under horizontal extension.
(2) The Terceira ridge stops at 20” W and from there to the Strait of Gibraltar runs a long transcurrent
fault with right-lateral motion and which
forms a first-order feature separating the oceanic part of the African and
Eurasian plates. Accumulated strain on this fracture is released by periodic
large-magnitude
shocks with almost pure strike-slip motion.
(3) Near the Strait of Gibraltar, the situation changes to north-south
compressive forces and reverse faulting. This may indicate that from longitude 11” W a still partly independent
Iberian block enters the interaction.
(4) Seismicity east of the Strait of Gibraltar has a different character from
that to the west and, in the Alboran and Betica region, is compatible with
the hypothesis of the existence of a buffer-plate.
(5) The independence
in the past of the Iberian Peninsula with respect to
the Eurasian plate, may still be preserved to some degree. A paleo-bounds
may, thus, exist surrounding the peninsula. The deep shocks near Granada
can be considered as being located at a relic of a paleo-subduction
zone
somewhat related with the Gibraltar arc.
ACKNOWLEDGMENTS
The authors thank all the directors of those Seismologist
Stations which
mailed copies and original records of some of the earthquakes in this study,
and to D. Murioz Sobrino for helping in preparing the illustrations.
287
REFERENCES
Andrieux,
J. and Mattauer,
M., 1973. Precisions
sur un modele explicatif
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Andrieux,
J., Fontbote,
J.M. and Mattauer,
M., 1971. Sur un mod$le explicatif
de I’arc
de Gibraltar.
Earth Planet. Sci. Lett., 12: 191-198.
Arabia, V. and Vegas, P., 1971. Plate tectonics
and volcanism
in the Gibraltar
arc. Tectonophssics
.
I 24: 197-212.
Auzende, J.M., Bonnin, J. and Olivet, J.L., 1973. The origin of the western Mediterranean
basin. J. Geol. Sot. London,
129: 607-620.
Beuzart,
P., 1972. Seismicit
du basin mediterranien
et des regions environnantes.
XIII
General Assembly,
European
Seismological
Commission,
Brasov.
Bonini, W.E., Loomis, T.P. and Robertson,
J.D., 1973. Gravity anomalies,
ultramafic
intrusions
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of the region around the strait of Gibraltar.
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