Geophys. J. Int. (2001) 146, 801–812
The relative locations of multiplets in the vicinity of the Western
Almerı́a (southern Spain) earthquake series of 1993–1994
Daniel Stich,1 Gerardo Alguacil1,2 and José Morales1,2
1
2
Instituto Andaluz de Geofı́sica, Universidad de Granada, E-18071 Granada, Spain. E-mail: daniel@iag.ugr.es
Departamento de Fı́sica Teórica y del Cosmos, Universidad de Granada, E-18071 Granada, Spain
Accepted 2001 May 1. Received 2001 March 30; in original form 2000 July 24
SUMMARY
We have analysed 721 earthquakes (1.5jmbj5.0) of the 1993–1994 Western Almerı́a
(southern Spain) series and the following seismicity in the area until 1998. Among the
data there are several multiplets, events characterized by very similar seismograms at the
short-period stations of the local network. We detected similar seismograms using crosscorrelation analysis of the P and S arrivals and classified similar events into families, or
clusters. We found 39 multiplet clusters of 3–33 events. Within each cluster, relocations
relative to a master event have been calculated by using the interpolated cross-correlation
maxima for the precise relative timing of P and S phases at each station. Relative arrival
times have been compared for all the possible selections of the master event, and adjusted
by forming the mean value after removing the outliers. The distribution of the stations
does not permit a satisfactory resolution of focal depths, but relative epicentres have
been determined with an accuracy of a few tens of metres. Typically they draw well-defined
lineaments and show two dominant strike directions: N120u–130uE and N60u–70uE.
These directions are coincident with known fault systems in the area and with the source
parameters of three of the largest events (Mw=4.8, 3.6 and 4.9), which were estimated
from waveform modelling of near-field acceleration records at a single station.
Consistent with previous studies, distances within multiplets (typically several tens
of metres) are for the most part clearly smaller than the fracture radii of these events.
This indicates repeated slip on the same fault segment. It was possible to obtain precise
relative locations between several nearby clusters, thereby imaging a very heterogeneous
seismotectonic fine structure of the source area, i.e. the positions of adjacent active fault
segments and the fragmentation of the crust into small (approximately 1 km) tectonic
blocks.
Key words: earthquake location, earthquake-source mechanism, fault tectonics, waveform
analysis.
INTRODUCTION
Earthquakes with nearby locations and similar source mechanisms radiate similar wavefields and generate similar ground
motions at the recording stations. Those events with seismograms
showing nearly identical waveform character are commonly
referred to as doublets (for a pair of events) or multiplets
(for larger sequences). Geller & Mueller (1980) suggested that
doublets and multiplets represent repeated rupture at the same
fault segment. The hypocentres of a multiplet sequence are
tightly clustered, and usually a standard location procedure is
not sufficiently accurate to resolve their spatial distribution.
The relative locations of multiplets can be determined very
precisely by making use of the seismograms’ similarity to obtain
an accurate relative timing of phase arrivals by cross-correlation
# 2001
RAS
or cross-spectral techniques. Several authors have described
these methods:
A bibliography of the first decade of multiplet relocation
can be found in Deichmann & Garcia-Fernandez (1992); more
recent work includes Nadeau et al. (1994), Maurer & Deichmann
(1995), Cattaneo et al. (1997), Phillips et al. (1997) and Lees
(1998). The resulting relative locations give an image of the
distribution of points of maximum energy release on the rupture
surface rather than giving conventional hypocentres as points
where rupture started, because a finite window of signal is
evaluated instead of just the first-break onset (Frémont &
Malone 1987, Nadeau et al. 1994).
In the following we describe the application of multiplet
relocation to data from southernmost Spain, where a seismic
series occurred precisely in the epicentral area of the severe
801
802
D. Stich, G. Alguacil and J. Morales
earthquakes of 1910 (I0 = VIII MSK, mb = 6.3) and 1804
(I0 = IX MSK): see Kárnı́k (1969) and Vidal (1986). We
collected information on the tectonic fine structure of the
source area of the seismic series using two kinds of relative
location: (1) the relocations within the multiplets, to image the
orientation of the active fault segments; and (2) the relative
locations between individual multiplet clusters, to image the
relative positions of the corresponding fault segments. The source
parameters of three of the largest events (which do not belong
to any multiplet cluster) were estimated by modelling their
waveforms. We used near-field strong-motion recordings, for
which the short epicentral distances lead to well-constrained
Green’s functions.
The tectonic setting of these earthquakes is at the transition
between the Betic Cordilleras and the Alborán Basin. The Betic
Cordilleras are, together with the Rif in Morocco, the westernmost part of the alpine mountain belt. The Betic Cordilleras
fold-and-thrust belt was formed both by the approximately
NW–SE-directed convergence of the African and Eurasian plates
since the Late Cretaceous and by the relative westward drift of
the Alborán domain (Betic–Rif mountains and Alborán Sea).
From the Early Miocene, extensional tectonics affected simultaneously the inner part of the Betic–Rif mountain chain and
crustal thinning formed the Alboran Basin (Watts et al. 1993;
Comas et al. 1997). Geodynamic models often try to explain
the coeval development of compressional and extensional
features in this area since the Miocene with some loss of lithosphere, for example slab break off (Blanco & Spakman 1993;
Zeck 1996), delamination of thickened lithosphere (Seber et al.
1996), or the convective removal of a thickened lithosphere
(Platt & Vissers 1989; Calvert et al. 2000). In the study area
itself, a regional ENEWSW extensional stress field is currently
dominant (Rodriguez-Fernandez & Martin-Penela 1993; Herraiz
et al. 2000).
EARTHQUAKE DATA
Two moderate earthquakes occurring within 12 days marked
the beginning of a period of increased seismic activity in the
study area (longitude 3.2u–2.5uW, latitude 36.4u–37.0uN). The
earthquakes occurred on 1993 December 23 (14:22:35, mb=4.9)
and 1994 January 4 (8:03:14, mb=5.0) near the town of Adra,
separated by a distance of about 25 km. They were felt with
maximum intensity of I0 = VII MSK. The earthquakes were
followed by a large number of smaller events (mbj4.1). During a
five-year period (from 1993 December to 1998 November), 721
events were recorded in the area by up to 18 fixed and portable
short-period vertical-component stations of the local seismic
network of the Instituto Andaluz de Geofı́sica (Fig. 1). About
half of the events followed the two major events during the winter
of 1993/94 and the spring of 1994, and further relative maxima
of the seismic activity were observed in the autumn of 1995
and the summer of 1996. Most of the events were located in the
upper crust at depths between 0 and 12 km. Only one permanent
and one portable short-period station (ADRA and PENX,
respectively) were deployed in the study area itself, enabling
recording at fairly short epicentral distances for events in the
northern and central part of the study area.
DETECTION OF MULTIPLETS
37.5
The tight spatial clustering of events in the study area
corresponds to the occurrence of several multiplets. In order
to detect similar waveforms, a cross-correlation analysis was
performed (Deichmann & Garcia-Fernandez 1992; Maurer
& Deichmann 1995; Cattaneo et al. 1997). The similarity of
two waveforms (bandpass-filtered, 1–16Hz) was quantified as
the maximum value of the normalized correlation coefficient
function, calculated in moving windows around the P and S
onsets of the two recordings. On the picked phase arrival,
the left border of the zero-lag cross-correlation window was
anchored. Window lengths as well as maximal shifts between
the windows were 2 s for P and 3 s for S arrivals. Where only a
P reading was available, or the S-reading was attributed a low
ASMO
PARA
CRT Granada
ALOJ
ACHM
ACLR
ASNV/FUE
PALB
37
AAPN
ASCB
PSAL
APHE
ATEJ
PENX
RESI
93/12/23
ADRA Adra
Almería
AALM
ACBG
36.5
94/01/04
0 10 20 30 40 50km
-4
-3.5
-3
-2.5
-2
Figure 1. Epicentres of events (circles) in the study area (box) and the distribution of the short-period stations of the Instituto Andaluz de Geofı́sica
(triangles) around the active zone. Axis labels are degrees latitude/longitude. Stars mark the epicentres of the two major events. Places referred to in
the text are annotated in italics.
#
2001 RAS, GJI 146, 801–812
Multiplets of the Western Almerı́a earthquake series
02
36.8
01
15/17/20
23
100
200
300
400
500
catalogue-number of event
600
700
39
36
33
30
27
24
21
18
15
12
9
6
3
07 05
13
33/34/36
26
11/18
25
16/19/21/28/38
10
27
22/29
36.6
multiplet-cluster
37.0
39
36
33
30
27
24
21
18
15
12
9
6
3
0
12
03/06/09/14/24
04/08
37
35
39
30/31/32
0
1994
1995
1996
1997
1998
calendar date of event
Figure 2. Classification of 289 events (40 per cent of the initial catalogue)
into 39 multiplet sequences with at least three members. A further 34
doublets have been detected. In the top plot, the assignment is plotted
against the order of occurrence; in the bottom, against the actual
occurrence time of the events.
#
clusters. Using thresholds of 0.5 for the P similarity, 0.75 for
the S similarity, and 0.5 for the cluster separation, we detected
39 multiplet clusters, each with 3–33 members (Fig. 2). They
contain 40 per cent of the initial data set. For comparison: using
a two-threshold approach (P and S, Aster & Scott 1993) and
the same trial-and-error procedure to optimize the thresholds,
we detected 23 clusters containing 31 per cent of the initial data
set.
On several occasions the cross-correlations between seismograms of two different clusters are in the range of 0.6–0.75,
as compared with average values of about 0.4 for arbitrarily
selected seismograms within this data set. Such intermediate
cross-correlations, although not sufficient to classify these events
into one common cluster, again correspond to fairly similar
waveforms, and the clusters involved can be assumed to have
nearby locations and similar focal mechanisms. We will use this
information for precise relative locations between these clusters,
as described in the last section. Groups of similar clusters have
been identified by inspection of the waveforms and the crosscorrelation matrix. We found eight groups of similar clusters,
each containing 2–5 individual clusters.
The absolute locations of the clusters (Fig. 3) were calculated
as the mean of all well-constrained locations of the individual
events of the cluster, assuming that the cluster extensions are
small compared with the single-event location errors (quality
criteria: phase readings at 10 or more stations; rms error <0.25 s).
The majority of the detected multiplets occurred around the
site of the mb=5, 1994 January 4 event. All detected multiplets
are located between depths of 3 and 8 km. For some of the
clusters, however, the depth estimate is uncertain due to the large
distance to the closest recording stations (see Fig. 1).
2001 RAS, GJI 146, 801–812
-3.2
5
10
15
-3.0
20 [km]
-2.8
-2.6
36.4
multiplet-cluster
quality, an appropriate zero-lag position of the S-wave crosscorrelation window was estimated from the origin time T0
[TS=VP /VS(TPxT0)+T0], using an average VP /VS ratio for
this region of 1.73 (Serrano 1999). Considering the length and
the maximal shift (3 s each) of the S-wave correlation window,
this approach is not sensitive to local anomalies of the VP /VS
ratio. After calculating the cross-correlation maxima at all the
individual stations in this way, the overall similarity of the P
and S phases of the two events was defined as a mean value of
the cross-correlations at the individual stations. Prior to forming
the mean, the lowest cross-correlations (25 per cent) were rejected
because they were considered to suffer from data insufficiencies
(see Maurer & Deichmann 1995).
In order to classify similar events into clusters, Maurer &
Deichmann’s (1995) algorithm was used. Two events are determined to belong to the same multiplet sequence (cluster) if they
exceed three threshold values applied to the P-wave similarity,
the S-wave similarity, and the normalized scalar product of the
corresponding rows of the S-wave cross-correlation matrix (the
latter is termed the cluster separation threshold). Application
of the cluster separation threshold rejects those pairs of events
that show somehow similar waveforms but do not coincide in
their behaviour towards the other events of the catalogue; the
reliability of the cluster assignment is thus increased compared
with an algorithm that evaluates P and S similarities only. the
influence and appropriate values of the individual thresholds
are discussed in detail in Maurer & Deichmann (1995). We
optimized the thresholds for this data set by trial and error,
with the aim of obtaining a large percentage of clustered events
but keeping a high waveform similarity within all the individual
803
Figure 3. Locations of multiplets; cluster numbers according to Fig. 2
are labelled. Axis labels are in degrees latitude/longitude. For similar,
nearby clusters (see text), one common location is given. The depths of
the multiplets vary between 3 and 8 km. Stars mark the epicentres of the
two major events (cf. Fig. 1). Most multiplets occurred around the site
of the 1994 January 4 event.
804
D. Stich, G. Alguacil and J. Morales
PRECISE RELATIVE TIMING OF P AND S
PHASES OF MULTIPLETS
The information on the relative location of two nearby events
comes packaged in the slight variations of their relative arrival
times among the network stations. For a precise timing, the
relative arrival times of the picked or estimated wave onsets
were adjusted by the time lags corresponding to the maxima
of the cross-correlation functions. Window lengths and maximal shifts of the cross-correlation analysis remain 2 s for P and
3 s for S arrivals (except for recordings of clusters 01 and 02
at ADRA, where the short P–S times do not permit long
windows). We overcame the resolution limit of the sampling
rate by polynomial interpolation of the cross-correlation peak,
thereby increasing the precision of relative timing by one order
of magnitude (10 msp1 ms).
Although the relative location makes use of the master-event
technique (see below), the cross-correlation analysis was performed for all the pairs of events within a cluster, at all active
stations. For recordings with P readings only, the zero-lag
position of the S-wave correlation window was calculated from
the origin time and VP /VS ratio (see above). At stations with no
phase reading at all, the zero-lag positions of the windows for P
and S arrivals were obtained from the location and origin time
of the event, using ray-tracing in a layered velocity model. The
model reduces to a one-layer model for direct arrivals of the
multiplets, all of them located in the uppermost layer (0–12 km,
VP=5.9 km sx1, VS=3.4 km sx1; Serrano 1999). Actually,
the lithosphere in the area shows significant lateral variations
corresponding to the very different characteristics of the Betic
Cordilleras’ fold-and-thrust belt and the extensional basin of
the Alboran Sea; however, striking lateral variations are observed
mainly below 12 km depth. In the upper crust, seismic velocities
(Banda et al. 1992; Dañobeitia et al. 1998; Carbonell et al.
1998; Serrano et al. 1998) are fairly constant over a wide area
on- and offshore, except for surface low-velocity anomalies in
Neogene deposits.
Considering the large epicentral distances of some of the
recording stations, the similarity of waveforms will frequently
be obscured by noise, and consequently the cross-correlation
might pick a maximum that does not give an accurate relative
timing of the arrivals. A criterion to test relative arrival times is
that the ‘direct’ relative timing of P and S arrivals between
master A and slave B (DtBA) should be virtually identical to the
sum of the relative timings via a third event C at the same
station (DtBC+DtCA). Forming this sum simulates the replacement of the former master event A by the new master C. Hence
relative arrival times have been compared for all possible
selections of the master event, the erroneous pickings (outliers)
have been removed, and the mean and standard deviation of
the remaining values define the relative timing and its standard
error for the following inversion. For our data, this control
and adjustment of relative arrival times was essential for a
successful relocation procedure.
The precise relative timing would be pointless in the presence
of unresolved timing inaccuracies among the instruments.
Two subnetworks provided data for this research, with central
recording sites in Granada and Almerı́a, respectively. Time
differences between the two independent clocks are unknown
for some epochs of synchronization malfunction and cannot
be corrected. This problem will be addressed during the inversion. Within the subnetworks, signals are telemetered to the
central recording sites and all stations have a common time
base. Usually, the stations are digitized in a fixed order, and
digitization delays do not affect the relative timing. Exceptions
occurred due to changes in the pattern of portable stations, and
in consequence stations have been digitized through different
channels over different periods of time. This error, termed
the digitization skew error by Poupinet et al. (1984), can be
corrected by subtracting the digitization delay between the
involved channels for those event pairs that were affected by
a change. A correction of similar form is necessary to account
for a modification in the equipment in 1996, because the
digitization delays afterwards are considerably smaller.
RELATIVE LOCATION OF MULTIPLETS
All events of a cluster were located relative to a master event.
The small cluster extensions permit a linear approximation,
based on the assumption of constant velocities in the source
volume and parallel ray paths towards a given recording station.
The aforementioned timing inaccuracies between the two subnetworks mean we cannot assign common origin times to all
observations of an event pair. In general, the relative origin
times will differ with respect to the clocks of the two subnetworks. Therefore the inversion has to treat relative origin
times of observations at the Almerı́a and observations at
the Granada stations as two independent model parameters
(DT0,Gra, DT0,Alm). The relative timing of P and S phases
between master and slave event at a station k depends on the
model parameters (DT0,Gra, DT0,Alm and the relative location
vector d=[Dx; Dy; Dz], pointing from the master to the slave
event):
*tkP,S ¼
8
d . nk
>
>
>
*T0,Gra ,
>
<
VP,S
for stations digitized in Granada
>
>
d . nk
>
>
,
: *T0,Alm VP,S
for stations digitized in Almera
(1)
where nk is the unit-length normal vector in the direction
of the emergent ray to the kth recording station, and VP,S are
the velocities of P,S-wave propagation at the hypocentre. The
temporal variations of the velocity field, concerning, for example,
anisotropy or VP /VS ratio, are assumed to be insignificant
(velocity variations and multiplets are treated in Poupinet et al.
1984 and Haase et al. 1995).
All available relative timings of P and S phases at all network
stations lead to a system of linear equations. The elements of
the forward matrix depend on the velocity distribution, cluster
location and station locations, and were calculated for the previously described one-layer velocity model (VP=5.9 km sx1,
VS=3.4 km sx1). The model parameters were obtained by a
least-squares inversion using singular value decomposition
(Press et al. 1989). No weighting of individual data values was
introduced into the inversion owing to the rather arbitrary
definition of data errors.
The standard errors of the model parameters are described
by the model covariance matrix. The largest model standard
errors, reaching hundreds of metres, correspond to the principal
error axes pointing in more or less a vertical direction: the depth
#
2001 RAS, GJI 146, 801–812
Multiplets of the Western Almerı́a earthquake series
resolution is very low as a result of the lack of observations at
short epicentral distances. The horizontal errors of the relative
locations are typically a few tens of metres. Average residuals
for the relative locations are about 5 ms, approximately 50 times
less than for the absolute locations. Three examples of multiplet relocations are plotted in Fig. 4 and given numerically
in Table 1. The model standard errors do not include the
uncertainties of the forward matrix, introduced by errors of
the velocity model or absolute cluster location. Errors are likely
to be caused by wrong estimates of cluster depths and by disregarding vertical velocity gradients. Both affect the take-off angle
of the emergent ray, further reducing the resolution of relative
depths. Consequently, relative depths were not interpreted.
805
SPATIAL AND TEMPORAL
DISTRIBUTION OF THE RELOCATED
MULTIPLETS
The epicentres of relocated multiplets are tightly grouped,
typically within a few hundred metres. Often the epicentres
produce well-defined lineaments. The distances between the epicentres are usually small in comparison with the fracture size of
the earthquakes; histograms of estimated fracture diameter and
closest distance are given in Fig. 5, and some numerical values
are given in Table 1. Fracture areas for several events of the
multiplet sequences have been estimated from their displacement amplitude spectra (Garcı́a-Garcı́a 1995; Garcı́a-Garcı́a
Table 1. Relative locations (Dx, Dy, Dz), minor (dmin) and major (dmax) axes of the 68 per cent confidence ellipsoids and rms errors of the residuals
(rmsrel, rms) within three multiplet sequences (cf. Fig. 4). The rupture diameter (2r, Garcı́a-Garcı́a 1995) and the epicentre distance to the nearest
neighbour (dmin) are given for comparison, see text.
Event
Nu
Date
Time
Mag
Relative location
Dx [m]
Dy [m]
Cluster 02, 11 events, absolute location 2.994uW, 36.866uN;
691
93/12/23
19:22:38
1.9
25
697
93/12/24
1:33:50
1.9
0
699
93/12/24
12:42: 8
2.9
26
958
94/ 1/22
7:23:35
2.7
10
1319
94/ 3/12
7:31:31
2.8
41
1321
94/ 3/12
8:25: 8
2.9
master event
1537
94/ 6/19
5:45:58
2.5
26
4335
96/ 4/17
12:26:49
2.2
154
4457
96/ 5/ 9
10: 5:50
2.1
286
5317
96/ 9/21
17:43:20
2.5
118
6686
97/12/12
22:18:56
2.5
125
Dz [m]
dmin [m]
dmax [m]
rmsrel
rms
2r [m]
dmin [m]
6.5 km depth
14
x70
13
130
0
x361
146
x165
56
80
31
18
16
39
7
417
207
236
258
47
0.0017
0.0019
0.0018
0.0054
0.0025
x33
x537
x502
x1139
x389
8
19
27
23
19
52
200
354
383
341
0.0034
0.0054
0.0052
0.0059
0.0039
0.07
0.12
0.29
0.21
0.31
0.40
0.29
0.11
0.08
0.22
0.17
130
130
620
378
422
469
299
203
176
299
299
14
13
14
93
15
13
15
36
138
36
40
30
38
35
32
15
55
15
30
26
14
659
772
604
931
306
1338
270
575
746
281
0.0030
0.0037
0.0069
0.0041
0.0060
0.0028
0.0066
0.0027
0.0019
0.0070
35
52
64
15
33
39
31
715
664
1555
636
687
878
736
0.0032
0.0028
0.0026
0.0042
0.0029
0.0068
0.0051
0.20
0.27
0.15
0.21
0.19
0.22
0.25
0.28
0.31
0.26
0.23
0.21
0.15
0.22
0.23
0.21
0.24
0.21
152
203
130
299
232
130
299
203
176
299
422
130
152
152
422
93
130
469
7
76
12
15
7
15
77
3
28
87
3
28
370
88
81
120
102
76
16
40
8
271
831
228
0.0026
0.0027
0.0027
8
13
14
10
14
209
344
320
240
358
0.0037
0.0004
0.0073
0.0037
0.0036
0.26
0.31
0.23
0.25
0.24
0.15
0.19
0.22
0.23
232
130
299
378
440
77
560
633
440
13
13
38
41
68
31
46
68
38
54
106
148
114
74
Cluster 12, 18 events, absolute location 2.848uW, 36.659uN; 5.9 km depth
770
94/ 1/ 5
1:27:51
2.0
327
x182
319
771
94/ 1/ 5
1:42:14
2.2
493
x211
154
784
94/ 1/ 5
11: 5: 8
1.9
339
x185
1090
790
94/ 1/ 5
17:50:18
2.5
282
x154
516
797
94/ 1/ 5
21:36: 0
2.3
324
x189
x10
807
94/ 1/ 6
8:56: 8
1.9
267
x157
66
809
94/ 1/ 6
12:33:37
2.5
244
x78
464
826
94/ 1/ 7
9:22:38
2.2
1
3
78
832
94/ 1/ 7
21: 5: 3
2.1
167
x84
x94
842
94/ 1/ 8
12:23:19
2.5
x163
45
121
854
94/ 1/ 9
9:56:54
2.8
master event
870
94/ 1/13
1:41: 0
1.9
159
x111
457
913
94/ 1/16
22:54:37
2.0
x404
326
x7
964
94/ 1/22
22:18:42
2.0
62
67
x74
1027
94/ 1/28
1: 6:12
2.8
x79
21
x621
1166
94/ 2/10
5: 4:34
1.7
667
x408
47
1176
94/ 2/10
22:15:17
1.9
178
18
316
4898
96/ 7/12
16:20:58
2.9
434
x259
90
Cluster 22, 9 events, absolute location x2.851uW, 36.670uN; 4.8 km depth
953
94/ 1/21
12:58:18
2.3
66
x17
102
962
94/ 1/22
18:45:49
1.9
73
x28
748
963
94/ 1/22
19:29:48
2.5
121
x41
203
967
94/ 1/23
14:20:26
2.7
master event
977
94/ 1/24
12:24: 5
3.0
x81
12
18
987
94/ 1/24
20:33:22
1.6
35
x23
x294
1001
94/ 1/25
17:40:27
3.0
70
29
143
1015
94/ 1/26
18:36:19
3.2
x111
74
92
1040
94/ 1/29
14:58:14
3.2
134
x77
177
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2001 RAS, GJI 146, 801–812
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D. Stich, G. Alguacil and J. Morales
Figure 4. Relative locations within three multiplet sequences (horizontal planes of the 68 per cent confidence ellipsoids). Catalogue numbers
according to Table 1 are labelled. The master-event location in this and subsequent similar figures is at coordinate (0,0). The selected multiplets show
the two dominant strike directions of N60u–70uE and N120u–130uE (see text). Most multiplet clusters have an extension of a few hundreds of metres,
like clusters 2 and 22.
et al. 1996), using the circular source model of Brune (1970) to
relate the fracture size to the corner frequency of the spectra.
An empirical scaling law between fracture radius and magnitude (Garcı́a-Garcı́a 1995) permits the estimation of fracture
radii for the other events in the study area:
log mb ¼ ð0:33+0:11Þ log r½km þ ð0:67+0:06Þ :
(2)
events
100
75
50
25
0
0
120
240
360
480
600
720
840
distance (black) and diameter (grey) [m]
960
Figure 5. Histograms of the fracture diameters of the multiplets
(according to eq. 2, grey) and the distance between epicentres from the
nearest neighbour within the sequence (black). Average fracture diameters
are a factor of about 5 larger than the distances between events; 74 per
cent of the event distances, but only 4 per cent of the fracture diameters,
are below 80 m.
On the assumption that the vertical cluster extensions do
not exceed some 100 m, this relation indicates that repeated
(typically 3- to 10-fold) rupture of the same source is characteristic of the multiplet sequences. Repeated rupture requires a
short-term temporal variability of shear stresses and/or frictional
resistance along the faults (see, for example, Deichmann &
Garcia-Fernandez 1992). The vast majority of the detected multiplets contain at least one event, usually several, with estimated
fracture diameter clearly exceeding the extension of the whole
epicentre accumulation (see Table 1 and Figs 9 and 10 below).
Planes have been fitted to the hypocentre distributions of
each cluster to reveal the orientation of the active fault segments. All these planes dip nearly vertically, an artefact caused
by the low depth resolution and the large vertical scatter of
the relocated multiplets. Consequently, computed strike values
represent apparent (2-D) strikes rather than actual strikes. A
pronounced horizontal elongation of the epicentre distribution,
however, suggests that the apparent strike represents the actual
strike well, and events occurred at similar depths and/or on a
steep dipping fault plane.
The multiplet strike values with standard deviations (estimated
from random dislocations of the relocated hypocentres) less then
15u show two dominant directions: N120u–130uE and N60u–70uE
(Fig. 6). Both directions are consistent with the strike of major
Neogene fault systems in the study area (Rodriguez-Fernandez
& Martin-Penela 1993). Most of the multiplets represent the
#
2001 RAS, GJI 146, 801–812
36.6
36.8
37.0
Multiplets of the Western Almerı́a earthquake series
5
10
-3.2
15
20 km
-3.0
-2.8
36.4
0
-2.6
Figure 6. Comparison of the apparent strikes of the multiplet
relocations (grey lines) and the strike of the fault planes of the two
major events (see Table 3) with positions and directions of Neogene
faults in the study area (black lines). Faults redrawn from RodriguezFernandez & Martin-Penela (1993). Axis labels are degrees latitude/
longitude.
N120u–130uE direction, corresponding to a fault system with
important recent displacement in the ENE–WSW extensional
stress field. The N60u–70uE faults were formed by a Pliocene
stress field and have an unfavourable orientation to the current
regional field. The diffuse distribution of seismicity in the central
part of the study area suggests a volume of fractured material,
containing parallel branches of the N120u–130uE fault system,
rather than just one single fault at the site of the second major
event. We will confirm this suspicion later when treating the
relative location between clusters in this zone.
Multiplet sequences may continue over a long period of
time, like clusters 1 and 2 (the latter over almost 4 years), or
may contain a short burst of activity only, like cluster 30
(33 events over 11 days) or cluster 38 (11 events over 4 days).
Any interpretation of temporal characteristics or chronological
order within multiplet sequences is intrinsically limited, because
probably most of the detected clusters do not represent com-
807
plete sequences. (There are at least two reasons for this: the
catalogue is not complete down to small magnitudes, and the
multiplet detection procedure might fail for noisy recordings or
an event with just a slightly different mechanism.) Nevertheless,
we report some observations.
(1) The interevent times of repeated rupture within the multiplet sequences vary greatly between minutes and years, and one
might expect some scaling between event size and interevent
time due to both a continuous accumulation of shear stress
and fault healing processes (see Marone et al. 1995). However,
there seems to be no relation between magnitude and interevent
time.
(2) Magnitudes showed no chronological characteristics
over a sequence; that is, the largest event may occur at the
beginning, the end, or somewhere in the middle of the multiplet
sequence.
(3) In general, no lateral migration of the epicentres over
a multiplet sequence occurred, the only exception might be
cluster 2 (Fig. 4), where the later events tend to be situated
farther to the northeast. Sometimes, the last events of the clusters
tend to be farther off the cluster midpoint, compared with the
initial events.
MODELLING OF NEAR-FIELD STRONGMOTION RECORDINGS AT A SINGLE
STATION
For the two principal earthquakes of the series, estimations
of source parameters are available from two previous studies,
one evaluating the first-motion polarities of P waves (Rueda
et al. 1996), and one using waveform modelling of broad-band
recordings at regional distances (>300 km, Thio et al. 1999).
A Harvard centroid moment tensor (CMT) solution exists for
the first event (Dziewonski et al. 1994). At least for the 1994
January 4 event, available data are not consistent (Table 2),
and we decided to estimate source parameters independently by
analysing waveforms of strong-motion recordings at station
ADRA at short (<20 km) epicentral distances. The modelling
of near-source seismograms benefits from a well-constrained
velocity model and hence well-constrained Green’s functions.
Source-parameter estimation from a single, near-field station is
treated in, for example, Kanamori et al. (1990) and Singh et al.
(1997).
The two major events generated near-source acceleration
traces with good signal-to-noise ratio, and one major aftershock
(1993 December 23, 18:00:08, mb=4.0) was also evaluated.
The acceleration traces were rotated to separate radial and
transverse components of the horizontal seismograms, and
Table 2. Source parameters of the two major events of the series according to previous studies (see text).
1993 December 23
Harvard CMT solution
Rueda et al. (1996)
Thio et al. (1999)
Plane A
335
300
326
1994 January 4
Rueda et al. (1996)
Thio et al. (1999)
#
2001 RAS, GJI 146, 801–812
43
70
38
Plane B
x88
x130
x94
152
188
151
Plane A
170
220
65
25
47
44
52
Seismic Moment
x92
x29
x87
Plane B
x31
90
274
40
63
65
8.5r1023 dyn.cm, MW=5.2
MW=5.1
Seismic Moment
x152
90
MW=4.7
808
D. Stich, G. Alguacil and J. Morales
double-integrated to obtain displacement. Green’s functions
for the layered earth model were computed using Bouchon’s
(1981) algorithm; synthetic seismograms were generated for
a set of focal mechanisms and compared with the observed
displacement. A surface low-velocity layer (thickness 1.5 km,
VP=4.0 km sx1, VS=2.3 km sx1) was added to the previously
described velocity model to account for the Neogene deposits in
the vicinity of the station.
The displacement records (Fig. 7) of the two major earthquakes appear rather complex, indicating several subevents,
while the smaller event shows a single pulse. For the closer
events (those on 1993 December 23), the near-field displacement is clearly visible between the P and S arrivals. Most of the
main features of the displacement waveforms can be matched
with the source parameters in Table 3. The source radii were
estimated for a circular fault after Boatwright (1980), and the
static stress drops after Keilis-Borok (1959).
For the first major event, showing three distinct pulses, we
obtained a total moment release of 2.0r1023dyn cm (MW=4.8),
a total duration of 0.75 s, and a focal mechanism similar to
the result of Rueda et al. (1996): a steep plane striking 300u
with a normal and a right-lateral component of slip. Differences
in the ratio of SV and SH amplitudes between the two initial
pulses and the third pulse were matched by a minor rotation
of the slip vector. The mechanisms agree with the regional stress
field (ENE–WSW extension, Rodriguez-Fernandez & MartinPenela 1993). For the mb=4 aftershock, at the same location,
a similar mechanism was modelled, the seismic moment was
3.0r1022 dyn cm (MW=3.6), and the duration 0.24 s. These focal
mechanisms are different from those obtained by Dziewonski et al.
(1994) or Thio et al. (1999), both of which indicate smaller dip,
pure normal displacement, and a strike direction of about 335u.
The total moment release of the 1994 January 4 event was
2.3r1023 dyn cm (MW=4.9). The displacement records show
two distinct pulses, with the shape of the first one fitted well by
three overlapping subevents with identical focal mechanism.
This mechanism is similar to the solutions for the other events:
a steep plane striking 310u with a normal and a right-lateral
component of slip. The solution is different from those of
Rueda et al. (1996) and Thio et al. (1999), but consistent with
the strike directions of most multiplet clusters around the
epicentre. The second pulse shows very different ratios between
0.10
December 23, 1993
0.00
-0.10
-0.20
-0.30
Mw=4.8
mb=4.9
-0.40
-0.50
[cm]
1s
0.005
December 23, 1993
0.000
-0.005
-0.010
-0.015
-0.020
-0.025
Mw=3.6
mb=4.0
-0.030
-0.035
[cm]
1s
January 4, 1994
0.10
0.00
-0.10
-0.20
Mw=4.9
mb=5.0
-0.30
-0.40
[cm]
1s
Figure 7. Observed (solid) and modelled (dashed) displacement waveforms of three of the largest events of the series (mb=4.9, 4.0 and 5.0).
Traces from top to bottom: transverse, radial and vertical components
of displacement. Synthetics were calculated for the source parameters
given in Table 3. Double-couple fault-plane solutions are plotted in equalarea projection. The two focal mechanisms for each of the major events
correspond to the different mechanisms of subevents (in chronological
order from left to right, cf. Table 3).
SH, SV and P amplitudes compared to the initial pulse, and the
displacement cannot be matched with a mechanism similar to
the previous ones. A rough fit was obtained for a 240u-striking
fault plane, consistent with the other dominant strike direction
Table 3. Source parameters of the two major events and one aftershock of the series, leading to the fit of the near-source waveforms given in Fig. 7.
The subevents of the two major events are given in chronological order
1993 December 23
subevent 1
subevent 2
subevent 3
Plane A
300
300
300
Dec 23 aftershock
x120
x120
x145
193
193
203
Plane A
300
1994 January 4
subevent 1
subevent 2
subevent 3
2nd source
80
80
80
Plane B
55
70
70
70
70
x19
x19
x12
Plane B
x120
165
Plane A
310
310
310
240
32
32
56
45
x55
Plane B
x130
x130
x130
60
198
198
198
120
44
44
44
36
x29
x29
x29
144
MO [dyn.cm]
r [km]
Ds [bar]
1.0r1023
1.0r1023
1.5
1.5
2.0
12.2
12.2
5.5
MO [dyn.cm]
r [km]
Ds [bar]
3.0r1022
1.0
MO [dyn.cm]
r [km]
Ds [bar]
2.1r1022
4.2r1022
7.7r1022
9.0r1022
1.0
0.5
1.7
0.8
9.2
147.0
6.8
76.9
#
1.2
2001 RAS, GJI 146, 801–812
Multiplets of the Western Almerı́a earthquake series
809
Figure 8. Relative locations of multiplets for the clusters 30/31/32 (labelled as A, B, C for clarity). The individual strike directions of the clusters are
given as dotted lines. The three clusters, containing all the detected multiplet activity in autumn 95, broadly overlap (see text).
of multiplet clusters in the area. This solution gives a reverse
component of fault slip and does not agree with the regional
stress field (Rodriguez-Fernandez & Martin-Penela 1993; Herraiz
et al. 2000). It indicates an abrupt change of the local stresses
for the first and second sources of this earthquake. Pure reverse
faulting for this event was suggested by Thio et al. (1999). A
plausible explanation for the sudden occurrence of a compressive
local stress field will be given in the next section.
relative location - north [m]
200
0
B
B
B
B
B
B
-200
B
-400
A
A
-600
A
A A
AA A
A
-800
-1000
-600
-400
-200
0
200
400 600 800 1000 1200 1400 1600 1800
relative location - east [m]
-400
-200
0
200
400 600 800 1000 1200 1400 1600 1800
relative location - east [m]
200
relative location - north [m]
B
0
-200
-400
-600
-800
-1000
-600
Figure 9. Relative locations of multiplets of the clusters 22/29 (labelled as A, B; upper map) and 33/34/36 (labelled as A, B, C; lower map). The strike
directions of the two or three, respectively, accumulations are given as dotted lines and were obtained by fitting planes to the individual accumulations.
The estimated fracture diameters of the largest events (according to an empirical scaling law, see text) of each accumulation are drawn as solid lines in
the direction of the average strike of the clusters (N117uE, N119uE); they approximate the length of the active fault segment. The fault segments almost
touch but do not overlap, and the accumulations represent the activity of adjacent segments along the fault system.
#
2001 RAS, GJI 146, 801–812
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D. Stich, G. Alguacil and J. Morales
PRECISE RELATIVE LOCATIONS OF
NEARBY CLUSTERS AND THE SMALLSCALE HETEROGENEITY OF THE
SOURCE AREA
The very similar events classified into one cluster usually
represent repeated rupture of the same source, and their spatial
distribution reveals the orientation of one fault segment. We
expected to obtain more comprehensive information on the fine
structure of the source area from the precise relative locations of
different multiplets, with events typically belonging to different
fault segments. Therefore we used the previously detected clusters
with intermediate intercluster cross-correlations. Their waveform
similarity still permits the use of cross-correlation techniques
for relative timing.
However, these lower waveform similarities do not allow
a reliable relative timing at many stations, and an attempt to
relocate all events of the similar clusters relative to one master
event did not lead to very precise results. Instead, only certain
selected event pairs with particularly high similarity and good
data quality were used to relocate two clusters, thereby obtaining
400
relative location - north [m]
200
BB
a shift vector between the master events of the two clusters. The
previously relocated multiplets within the clusters were left
in their places. Usually the master-event shift vectors showed
little scatter when derived from different event pairs (standard
deviations of tens of metres in horizontal directions), and again
the relative epicentres can be determined quite precisely.
We observed three different spatial patterns of the seismicity
of similar clusters and present them with one or two examples
each. The first pattern, for example represented by the clusters
30/31/32 (Fig. 8), shows broad overlapping of the epicentral
distributions of the individual multiplets. The differences in
waveform between the clusters were caused by different focal
depths (in particular the tendency of clusters 30 and 31 to be
slightly shifted perpendicular to the overall strike direction
might indicate different depths on a dipping fault) or a slight
variation of the mechanism.
Among other similar clusters, no spatial overlapping is
observed. Events are accumulated within each multiplet cluster
and separated from other clusters. The relative locations between
clusters are often nearly in-line with the individual strikes
(Fig. 9). These clusters should belong to adjacent segments along
B
B
0
C
A
C
C
C CCB
A A
C
-200
-400
-600
-800
D
-1000
-800
D
D
D
-600
-400
-200
0
1000
200 400 600 800
relative location - east [m]
1000 1200 1400 1600
-400 -200
0
200
relative location - east [m]
400
relative location - north [m]
800
600
400
200
0
-200
-1400 -1200 -1000 -800
-600
600
800
1000
Figure 10. Relative locations of multiplets of the clusters 3/6/9/14 (labelled as A, B, C, D; upper map) and 16/19/21/28/38 (labelled as A, B, C, D, E;
lower map). The strike directions were obtained by fitting planes to the individual accumulations and are given as dotted lines if their standard
deviation is less than 15u (all except clusters 6 and 16). The estimated fracture diameters of the largest event of each accumulation are drawn as solid
lines in the direction of the average strike of the clusters (N70uE, N130uE). The clusters reveal the simultaneous activity of (sub) parallel branches of a
fault system. The two cluster groups have nearly identical locations, and the superposition of the N70uE and N130uE fault systems indicates
fragmentation of the crust into small blocks (see text).
#
2001 RAS, GJI 146, 801–812
Multiplets of the Western Almerı́a earthquake series
a seismic fault system. Using eq. (2) for the estimation of the
fracture diameters of the largest events of each accumulation
(to approximate the length of the active fault segment), it turns
out that the distances of the accumulations are determined
quite exactly by these fracture diameters. This means that the
fracture areas of adjacent clusters approximately touch each
other.
The third pattern of seismicity shows separated clusters with
relative locations obviously not in-line with the individual strikes,
thereby revealing simultaneous activity on (sub) parallel faults
(Fig. 10). We found an example for the N70uE and the N130uE
directions. Since the fault-plane dip and relative depths of the
events are not resolved, only a rough estimation of the distances
between the parallel faults is possible (500 m and 1300 m,
respectively, for the N70uE faults, and 700 m for the N130uE
faults). Besides parallel faults, another example of adjacent
fault segments can be seen in the figure. The relative locations
of the N70uE structures follow an N120u–130uE lineament.
The absolute locations of the two cluster-groups with (sub)
parallel faults in Fig. 10 are almost identical (compare Fig. 3),
and the superposition of the two different (N70uE and N130uE)
fault systems indicates fragmentation of the crust into small
(approximately 1 km) blocks near the site of the major 1994
January 4 event. This scale of fragmentation coincides with
typical extensions of the multiplet fracture areas. This suggests
that the parallel branches of the N70uE and N130uE fault
systems delimit the individual fault segments along each
other. The small-scale fragmentation also explains the complex
displacement records of the two major events; they have
affected several fault segments. The estimated fracture size of
the individual subevents (Table 3) coincides with the scale of
fragmentation.
The small-scale fragmentation is an appropriate scenario for
a complex redistribution of local shear stresses after each event
and will probably cause a very heterogeneous and temporally
variable stress field in the area. This might be the driving
mechanism of repeated rupture within the multiplet sequences.
A heterogeneous stress field in the upper crust was deduced
previously in two other study areas within the Alborán domain,
based on the observations that the small to moderate earthquakes do not necessarily reflect the mean state of stress of the
entire region. (Galindo-Zaldı́var et al. 1999; Medina 1995). In
our study, the heterogeneity of the local stress field is verified
by the reactivation of the Pliocene N60u–70uE fault system by
several multiplet sequences, in disagreement with the presentday regional ENE–WSW extension in the area (RodriguezFernandez & Martin-Penela 1993; Herraiz et al. 2000). In the
present-day regional stress field, the Pliocene faults may act as
oversteps between different branches or en-echelon structures of
the N120ux130uE fault system and release stresses introduced
by dislocations along individual segments of the N120u–130uE
faults. The displacement along both fault systems will result in
rotation and tilting of the small tectonic blocks.
ACKNOWLEDGMENTS
We are very grateful to Hansruedi Maurer from ETH Zürich
for his multiplet cluster detection software. The research
was supported by the European Commission (Marie-Curie
research training contract ERB4001GT980288) and by the
CICYT-Project AMB99-0795-C02-01.
#
2001 RAS, GJI 146, 801–812
811
REFERENCES
Aster, R.C. & Scott, J., 1993. Comprehensive characterization of
waveform similarity in microearthquake data sets, Bull. seism. Soc.
Am., 83, 1307–1314.
Banda, E., Gallart, J., Garcı́a-Dueñas, V., Dañobeitia, J.J. &
Makris, J., 1993. Lateral variation of the crust in the Iberian
Peninsula, New evidence from the Betic Cordillera, Tectonophysics,
221, 53–66.
Blanco, M.J. & Spakman, W., 1993. The P-wave velocity structure of
the mantle below the Iberian Peninsula: evidence for subducted
lithosphere below Spain, Tectonophysics, 221, 13–34.
Boatwright, J., 1980. A spectral theory for circular seismic sources:
simple estimates of source dimension, dynamic stress drops, and
radiated energy, Bull. seism. Soc. Am., 70, 1–28.
Bouchon, M., 1981. A simple method to calculate Green’s functions for
elastic layered media, Bull. seism. Soc. Am., 71, 959–971.
Brune, J.N., 1970. Tectonic stress and the spectra of seismic shear
waves from earthquakes, J. geophys. Res., 75, 4997–5009.
Calvert, A., Sandvol, E., Seber, D., Barazangi, M., Roecker, S.,
Mourabit, T., Vidal, F., Alguacil, G. & Jabour, N., 2000.
Geodynamic evolution of the litosphere and upper mantle beneath
the Alboran region of the western Mediterranean: Constraints from
travel time tomography, J. geophys. Res., 105, 10 871–10 898.
Carbonell, R., Sallarés, V., Pous, J., Dañobeitia, J.J., Queralt, P.,
Ledo, J.J. & Garcı́a-Dueñas, V., 1998. A multidisciplinary geophysical
study in the Betic chain (southern Iberia Peninsula), Tectonophysics,
288, 137–152.
Cattaneo, M., Augliera, P., Spallarossa, D. & Eva, C., 1997.
Reconstruction of Seismogenetic Structures by Multiplet Analysis:
An Example of Western Liguria, Italy, Bull. seism. Soc. Am., 87,
971–986.
Comas, M.C., Dañobeitia, J.J., Alvarez-Marron, J. & Soto, J.I., 1999.
The origin and tectonic history of the Alborán basin: Insights
from LEG 161 results, Proc. Ocean Drill. Program Sci. Res., 161,
555–580.
Dañobeitia, J.J., Sallarés, V. & Gallart, J., 1998. Local earthquakes
seismic tomography in the Betic Cordillera (southern Spain), Earth
planet. Sci. Lett., 160, 225–239.
Deichmann, N. & Garcia-Fernandez, M., 1992. Rupture geometry
from high-precision relative hypocenter locations of microearthquake clusters, Geophys. J. Int., 110, 501–517.
Dziewonski, A.M., Ekström, G. & Salganik, M.P., 1994. Centroidmoment tensor solutions for October–December, 1993, Phys. Earth
planet. Inter., 85, 215–225.
Frémont, M.-J. & Malone, S.D., 1987. High precision relative locations
of earthquakes at Mount St. Helens, Washington, J. geophys. Res.,
92, 10 223–10 236.
Galindo-Zaldı́var, J., Jabaloy, A., Serrano, I., Morales, J., GonzalezLodeiro, F. & Torcal, F., 1999. Recent and present-day stresses in the
Granada basin & Betic Cordillera: Example of a late Miocenepresent-day extensional basin in a convergent plate boundary,
Tectonics, 18, 686–702.
Garcı́a-Garcı́a, J.M., 1995. Caracteristicas espectrales y de fuente de
terremotos y microterremotos de andalucı́a oriental, PhD thesis,
University of Granada, Granada.
Garcı́a-Garcı́a, J.M., Vidal, F., Romacho, M.D., Martı́n-Marfil, J.M.,
Posadas, A. & Luzón, F., 1996. Seismic source parameters for microearthquakes of the Granada basin (southern Spain), Tectonophysics,
261, 51–66.
Geller, R.J. & Mueller, C.S., 1980. Four similar earthquakes in Central
California, Geophys. Res. Lett., 7, 821–824.
Haase, J.S., Shearer, P.M. & Aster, R.C., 1995. Constraints on
temporal variations in velocity near Anza, California, from analysis
of similar event pairs, Bull. seism. Soc. Am., 85, 194–206.
Herraiz, M. et al., 2000. The recent (upper Miocene to Quaternary) and
present tectonic stress distributions in the Iberia Peninsula, Tectonics,
19, 762–786.
812
D. Stich, G. Alguacil and J. Morales
Kanamori, H., Mori, J. & Heaton, T.H., 1990. The 3 December,
1988, Pasadena earthquake (ML=4.9) recorded with very broadband
system in Pasadena, Bull. seism. Soc. Am., 80, 483–487.
Kárnı́k, V., 1969. Seismicity of the European Area, Part 1, Reidel,
Dordrecht.
Keilis-Borok, V., 1959. On estimation of displacement in an earthquake
source and of source dimension, Ann. Geofis. (Rome), 12, 205–214.
Lees, J.M., 1998. Multiplet analysis at Coso Geothermal, Bull. seism.
Soc. Am., 88, 1127–1143.
Marone, C., Vidale, J.E. & Ellsworth, W.L., 1995. Fault healing
inferred from time dependent variations in source properties of
repeating earthquakes, Geophys. Res. Lett., 22, 3095–3098.
Maurer, H. & Deichmann, N., 1995. Microearthquake cluster detection
based on waveform similarities with an application to the western
Swiss Alps, Geophys. J. Int., 123, 588–600.
Medina, F., 1995. Present-day state of stress in northern Morocco from
focal mechanism analysis, J. struct. Geol., 17, 1035–1046.
Nadeau, R., Antolik, M., Johnson, P.A., Foxall, W. & McEvilly, T.V.,
1994. Seismological studies at Parkfield III: microearthqake clusters
in the study of fault-zone dynamics, Bull. seism. Soc. Am., 84,
247–263.
Phillips, W.S., House, L.S. & Fehler, M.C., 1997. Detailed joint
structure in a geothermal reservoir from studies of induced microearthquake clusters, J. geophys. Res., 102, 11 745–11 763.
Platt, J.P. & Vissers, R.L.M., 1989. Extensional collapse of thickened
continental lithosphere: a working hypothesis for the Alboran Sea
and Gibraltar arc, Geology, 17, 540–543.
Poupinet, G., Ellsworth, W.L. & Fréchet, J., 1984. Monitoring velocity
variations in the crust using earthquake doublets: an application to
the Calaveras Fault, California, J. geophys. Res., 89, 5719–5731.
Press, W.H., Flannery, B.P., Teukolsky, S.A. & Vetterling, W.T., 1989.
Numerical Recipes, Cambridge University Press, Cambridge.
Rodriguez-Fernandez, J. & Martin-Penela, A.J., 1993. Neogene
evolution of the Campo de Dalias and the surrounding offshore
areas—(Northeastern Alboran Sea), Geodinamica Acta, 6, 255–270.
Rueda, J., Mezcua, J. & Sánchez-Ramos, M., 1996. La serie sı́smica de
Adra (Almeria) de 1993–94 y sus principales consecuencias sismotectónicas, Avances Geofı́s. Geod., 1, 91–98.
Seber, D., Barazangi, M., Ibenbrahim, A. & Demnati, A., 1996.
Geophysical evidence for lithospheric delamination beneath the
Alboran Sea and Rif-Betic mountains, Nature, 379, 785–790.
Serrano, I., 1999. Distribución espacial de la sismicidad en las
Cordilleras Béticas- March de Alborán, PhD thesis, University of
Granada, Granada.
Serrano, I., Morales, J., Zhao, D., Torcal y, F. & Vidal, F., 1998.
P-wave tomographic images in the Central Betics-Alboran sea
(South Spain) using local earthquakes: contribution for a continental
collision, Geophys. Res. Lett., 25, 4031–4034.
Singh, S.K., Pacheco, J., Courboulex, F. & Novelo, D.A., 1997.
Source parameters of the Pinotepa Nacional, Mexico, earthquake of
27 March, 1996 (Mw=5.4) estimated from near-field recordings of a
single station, J. Seismol, 1, 39–45.
Thio, H.K., Song, X., Saikia, C.K., Helmberger, D.V. & Woods, B.B.,
1999. Seismic source and structure estimation in the western
Mediterranean using a sparse broadband network, J. geophys.
Res., 104, 845–861.
Vidal, F., 1986. Sismotectónica de la región Beticas-March de Alborán,
PhD thesis, University of Granada, Granada.
Watts, A.B., Platt, J.P. & Buhl, P., 1993. Tectonic evolution of the
Alborán sea basin, Basin Res., 5, 153–177.
Zeck, H.P., 1996. Betic-Rif orogeny: subduction of Mesozoic Tethys
lithosphere under eastward drifting Iberia, slab detachment shortly
before 22 Ma, and subsequent uplift and extensional tectonics,
Tectonophysics, 254, 1–16.
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2001 RAS, GJI 146, 801–812