Vol.5, No.8A1, 18-26 (2013) Natural Science
http://dx.doi.org/10.4236/ns.2013.58A1003
Stripe of normal mechanisms for crustal earthquakes
with M 3.5 flanking the western side of the thrust
front zone in the Andes backarc
Silvina Nacif1*, Enrique Triep2, Renzo Furlani2, Silvana Spagnotto3
1Fondo Argentino Sectorial, Agencia de Promoción Científica y Tecnológica, Godoy Cruz, Argentina;
*Corresponding Author: nacif.silvina@gmail.com
2Instituto Geofísico Sismológico F. Volponi (IGSV), Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San
Juan, San Juan, Argentina
3Consejo Nacional de Investigaciones Científicas y Técnicas, Universidad Nacional de San Luis, San Luis, Argentina
Received 13 June 2013; revised 13 July 2013; accepted 20 July 2013
Copyright © 2013 Silvina Nacif et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
Earthquakes with magnitudes M 3.5 were re-
gistered in the Andes backarc between ~32.5˚S -
33.75˚S within a temporary experiment from
November 2002 to March 2003. Data were col-
lected from 15 seismological broad band sta-
tions, deployed above flat subduction section
and also above the transitional to normal sec-
tion of the Nazca plate. Seismic events were lo-
cated mostly in part of the Southern Precordil-
lera and Cerrilladas Pedemontanas of Mendoza
Province. Focal mechanism solutions were ob-
tained, for selected data between 15 km and 35
km depths, from P wave first motion using
FOCMEC software. A band trending NW-NNW of
normal focal mechanism earthquakes is located
just by the thrust front towards West, and cov-
ers the Southern tip of the Southern Precordil-
lera and the Western side of the Cerrilladas
Pedemontanas, Mendoza Province, Argentina.
Thrust focal mechanism solutions obtained in
the present work for events with magnitude less
than or equal to 3.5 also show that the thrust
front South of ~33.1˚S is located in the same
place as Mw 3.6 earthquakes. The most sig-
nificant findings in this work are these signals of
extensional regimen which appear in a com-
pressional subduction tectonic regimen. Nev-
ertheless the short temporal experiment pre-
cludes firm interpretations about this particular
phenomenon. Probably it is related to a tempo-
rary stress relaxation. The normal earthquakes
are likely associated to normal faults, and/or to
their subsidiaries, that were inverted by con-
traction and now re-inverted at least transitorily
as normal. These faults, which are near and to
the East of the suture between Chilenia and
Cuyania Paleozoic terranes, were originally in-
volved in the Cuyo Triassic basin formation.
Keywords: Small Magnitudes Seismicity; Normal
Focal Mechanism Solut ions; Norm al Fault Activation;
Regional Compressional Regime
1. INTRODUCTION
Seismological data from CHARSME (CHile ARgen-
tina Seismological Measurement Experiment) Experi-
ment (Figure 1), allowed us characterize the crustal
seismicity with magnitude less than or equal to 3.5.
Crustal seismicity from earthquakes with Mw > 3.5 is
widely known by INPRES (Instituto Nacional de PRe-
vención Sísmica) and NEIC (National Earthquake In-
formation Center). The major percent of focal mecha-
nisms show normal solutions. In the region, focal
mechanisms from earthquakes with magnitude greater
than 3.6 were obtained from Harvard CMT Catalog,
[1-5]. In Figure 2 some of them were plotted. Forward
wave modeling [1] and Moment Tensor Inversion Tech-
niques [3-5] returned in most cases, according to geo-
logical evidence [6,7], thrust focal mechanism solutions
(in some cases with a strike component).
Subduction process supplied compressive stress to the
crust, principally in the East-West direction [8]. In fact,
this explains the almost total dominance of observed
thrust focal mechanisms (Mw 3.6).
The notable distinction in this work is the presence of
small events with normal focal mechanism solutions.
Clearly we have to inquire about the relationship of those
Copyright © 2013 SciRes. OPEN ACCESS
S. Nacif et al. / Natural Science 5 (2013) 18-26 19
Figure 1. (a) Location map. (b) Contours of Wadati Benioff zone
from [9], blue box: shows the study region. (c) CHARSME
Experiment map. Blue frame: study region, white triangles:
broad band stations, black triangles: active volcanoes, dotted
thin black line: boundary between Chilenia and Cuyania ter-
ranes [16].
different size earthquakes to the compressional dominant
stress pattern.
2. TECTONIC SETTING
The region under study is situated above the Nazca flat
subduction section and also above the transitional section
to normal subduction section South of ~33.5˚S [9].
Flat subduction in the North of 33˚S relates with the
absence of Cenozoic volcanism [10] and with the mor-
pho-structure units of Principal, Frontal and Pre-Cordil-
lera, and with Pampean Sierras. This flat subduction is
explained by Juan Fernandez Ridge subduction [11]
supported by results in [12], which control Quaternary
volcanism absence and high contractional backarc tec-
tonics [13]. South of 33˚S, with deepening subduction,
the reduction of maximum heights of Principal and
Frontal Cordillera is observable.
Another feature at this latitude is the progressive dis-
appearance of Precordillera and Sierras Pampeanas. The
active arc volcanism appears in this same latitude (volca-
noes: Tupungatito, Marmolejo San Jose and Maipo) [14].
The basement of the study region suffered a complex
history of collisions and accretions during Late Protero-
zoic and Early Paleozoic [15]. The boundaries between the
different terranes exert a high control in the Andean Tec-
tonic [15]. Chilenia terrane constitutes Andes basement
and Cuyania composite terrane makes up Precordillera
and Pie de Palo basement [16]. The region studied includes
Frontal Cordillera, Precordillera, and Piedmont Hills.
3. DATA AND METHODOLOGY
The CHARSME Experiment financed by Chilean
CONICYT was a project of scientific cooperation be-
tween the DRO team UMR Géoazur (France), UR082
the IRD (France), Department of Geophysics of the Uni-
versity of Chile (Santiago, Chile) and the Geophysics
Seismological F. Volponi Institute, University of San
Juan (Argentina). The experiment deployed seismologi-
cal stations from 31˚S to 34.5˚S and 67˚W to 72˚W from
November 2002 to March 2003. Data used in this work
came from the 15 broad band stations located in Argen-
tina (Figure 1). Even though data are from an Experiment
conducted 10 years ago, there is no work which refers to
these seismic events. On the other hand, results show a
phenomenon not observed before in the study region.
Hypocenter [17] was used to locate seismicity (Fig-
ures 3 and 4), it is iterative software written in Fortran
77. The software finds location parameters for single
events from a set of arrival times. In most cases P and S
arrival times were available. A Flowchart showing the
main steps is available in [17]. Confidence regions were
computed for events of Table 1, in all cases the se-
mi-major axis of the 90% confidence region was less
than ~6 km (Figure 4). For the crustal structure we used
the one-dimensional velocity model from [18], obtained
for the study region.
We found focal mechanisms solutions from P first mo-
tion using FOCMEC [19]. This traditional methodology
was used instead of Moment Tensor Inversion techniques
e.g. [20], considering seismic records from small earth-
quakes are highly influenced by the cortical structure
below the stations. Unless the velocity model has well
known is very difficult find focal mechanism solutions
from Moment Tensor Inversion for events with magni-
tude less than 4.0 [20]. We used the Aki and Richards
convention [21] for nodal planes parameters (strike, dip,
rake). These parameters were varied at a constant angle
of 5˚ to perform the search of focal sphere consistent
Copyright © 2013 SciRes. OPEN ACCESS
S. Nacif et al. / Natural Science 5 (2013) 18-26
Copyright © 2013 SciRes.
20
Figure 2. Seismicity of NEIC Catalog (USGS) from May-1973 to June-2013. Focal
Mechanism Solutions (Double Cupla part of Moment Tensor) from: CMT-Harvard in
blue color (4.8 Mw 5.9) and from [3] in red color (3.6 Mw 4.7). Orange line:
boundary between Chilenia and Cuyania terranes [16].
with the P first motion distribution. The excellent station
coverage (most cases Azimuthal Gap <90˚) yields a reli-
able estimate of fault plane solution. The major source of
errors came from uncertainties in first movement reading.
We considered impulsive and emergent movement when
the first motion was read. Acceptable solutions were
those which have at least 8 readings and only one incon-
sistency in the polarity.
4. RESULTS
Seismicity registered in ~4 months shows similar pat-
tern to the NEIC USGS Catalog (for comparation see
Figures 2-4). Seismicity is mainly located in South Pre-
cordillera [22], East of Uco Valley and on the foothill of
Frontal Cordillera (Figures 3 and 4). The magnitude
duration scale was not calibrated for the study region
so we did not obtain realistic magnitudes for these
events. Nevertheless comparison of four events (studied
here) registered from NEIC Catalog (USGS) permit to
infer that all magnitudes are less equal than 3.5. At least
readings from 8 stations were necessary for acceptable
focal mechanism solutions (Ta b le 1). A quality classifi-
cation was applied to the focal mechanism solutions
Figure 3. Located seismicity using hypocenter [17] from the
experiment period. This figure does no show the epicenters
from seismicity used to obtain focal mechanism solutions.
Blue dashed line shows the deformation thrust front pro-
posed in this work. Orange line: Boundary between Chilenia
and Cuyania terranes [16].
OPEN ACCESS
S. Nacif et al. / Natural Science 5 (2013) 18-26 21
Figure 4. Hypocenters were plotted for seismicity with available focal mechanism solutions obtained in
this work these are showed by black circles. Events 17, 18, 23 and 34 located to the North of 32.5˚S
were not plotted for clarity figure (indicated by asterisk in Table 1). Ellipses errors were plotted for
each events (obtained for 90% of confidence region). Inset cross section: events were project on an
East-West plane. Note that in general events are between depths 15 and 35 km.
considering the family solutions behavior, i.e. strike and
dip variation within the family solutions for each event.
Strike and dip maximum variation less than 25˚, 30˚ and
40˚ correspond to Very Well (VW), Well (W) and Re-
gular (R) quality, respectively (Table 1). In all cases,
the errors have not changed the compressional or di-
latational character of the mechanism. Type mechanism
classification (Table 1) was obtained with the same
procedure as in [23]. This method is based on the
plunge calculation of null (B), pressure (P) and ten-
sional (T) axes. The nomenclature are TH, thrust; NO,
normal; SS, strike-slip; TS, thrust with a strike-slip
component; NS, normal with a strike-slip component
strike-slip component; ST, strike-slip with a thrust
component; SN, strike-slip with a normal component.
Focal mechanism solutions from 34 seismic events
(Figure 5 and Ta b le 1) show dominantly normal solu-
tions (20 normal, 11 thrust and 3 strike).
5. DISCUSSION
The seismicity showed in the present work, between
32.5˚S and 33.75˚S in the backarc of the Andes, is
mainly related to two morphotectonic units (Figures 3
and 4), part of the Southern Precordillera [22] and Cer-
rilladas Pedemontanas of Mendoza Province, Argentina.
The area shows focused Quaternary deformation, in [6]
and [24], and most of it pertains to the thrust front (blue
dashed line in Figure 3). A regional NW trending neo-
tectonic structure (see blue line in Figure 5), the Río de
Mendoza—Tupungato strip [6] mainly includes the Cor-
dillera Frontal Eastern part between 32.75˚S and 33.15˚S,
and to the South the Cerrilladas Pedemontanas occidental
branch (Figure 5). The structure has resulted from the
interference of the Late Cenozoic Andean deformation
with NW-trending rift structure of the Triassic Cuyo ba-
sin]. [6
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S. Nacif et al. / Natural Science 5 (2013) 18-26
22
Ta b l e 1 . Focal mechanism solutions obtained from P first motion using FOCMEC [19] software. Equal area projection of inferior
hemisphere was used. We made use of Aki and Richards convention [23] for focal mechanism solutions. Nodal planes were defined
by: 0˚ strike 360˚, 0˚ dip 90˚ and 180˚ rake 180˚ (Thrust Fault: 0˚ < rake < 180˚ and Normal Fault: 180˚ < rake < 0˚).
Values of strike, dip and rake for each event shown in this work was the selected focal mechanism solution between a family of solu-
tions.
ID Date GMT Origin
time Lat. Long. Dep.Dep.
Error
Focal Mechanism
Solution TypeQt.
yr/
mon./
day
hh:
mm:
ss
Grade
minute
grade
minute km km Strike
Dip
grade Rake
1
02/
11/
26
21:
24:
50.81
32
48.64
68
56.84 29.5 7.3 258.7
98
75.1
15.8
84.8
108.7 TH VW
2
02/
11/
28
16:
20:
51.54
32
34.99
69
21.34 0.0 8.6 259.3
16.2
65.8
44.8
51
144.5 NS VW
3
02/
12/
06
01:
03:
04.97
32
35.26
69
8.65 15.8 5.5 254.9
28.6
54.4
46
58.7
126 NO W
4
02/
12/
07
21:
02:
53.14
33
28.17
68
51.98 23.8 4.0 337
176.3
15.8
75.1
71.3
95.2 TH R
5
02/
12/
10
05:
08:
00.40
33
16.04
69
10.17 26.4 2.7 220
352
75.5
21.1
74.5
136 NO W
6
02/
12/
12
04:
17:
27.70
33
13.96
68
42.18 19.8 3.8 338.2
236.9
66.1
66.1
26.3
153.6 ST W
7
02/
12/
17
14:
59:
35.44
33
13.88
69
0.70 17.6 4.4 286.6
77.5
22.3
70.3
62.7
100.6 NO R
8
02/
12/
19
02:
13:
44.88
33
9.84
69
4.60 26.3 1.9 205
25
65
25
90
90 NO VW
9
02/
12/
22
01:
42:
15.72
32
50.22
69
8.13 29.5 5.1 239.2
0.8
80.3
18
74.8
147 NO VW
10
02/
12/
23
13:
50:
12.80
32
49.22
69
14.28 21.2 4.0 335.1
77.7
20.6
85.3
13.5
110.1 NO VW
11
02/
12/
24
10:
10:
15.92
33
0.09
69
26.94 10.1 10.8 322.4
79.3
44.8
65.8
35.5
128.9 NS VW
12
02/
12/
25
08:
31:
33.85
32
54.17
69
4.21 20.9 2.4 113.5
278.2
20.6
70.1
75.6
95.3 NO VW
13
02/
12/
30
13:
29:
18.58
33
1.58
68
56.25 20.0 3.0 260.7
22.7
71.9
31.6
63.6
143.7
NO VW
14
02/
12/
31
04:
28:
07.65
33
4.61
68
58.36 24.6 2.9 309.3
167.1
48.4
48.4
62.8
117.2 TH VW
15
03/
01/
02
03:
18:
50.54
33
37.04
68
49.71 22.6 2.5 116.6
258.4
52.8
43.9
64.6
119.5 NO VW
16
03/
01/
02
06:
13:
43.08
33
35.82
68
48.87 21.0 2.5 32.9
201.4
60.1
30.4
84.2
100 NO VW
17*
03/
01/
03
09:
36:
11.02
31
55.82
69
10.37 30.1 6.8 117.1
286.5
35.3
55.1
81.3
96.1 NO W
18*
03/
01/
10
00:
44:
26.05
31
45.33
69
34.23 10.0 15.3 220
40
50
40
90
90 NO W
19
03/
01/
12
08:
20:
58.99
33
25.53
68
54.69 21.3 3.6 285.9
195.8
50
89.9
0
140 ST R
Copyright © 2013 SciRes. OPEN ACCESS
S. Nacif et al. / Natural Science 5 (2013) 18-26 23
Continued
20
03/
01/
13
21:
08:
36.92
32
52.69
68
37.93 26.0 2.9 28.3
150
80.3
18
74.8
147 NO VW
21
03/
01/
14
16:
07:
00.98
33
14.79
68
59.32 28.0 3.0 30
210
45
45
90
90 TH VW
22
03/
01/
15
09:
25:
14.72
32
58.59
69
5.08 33.3 4.5 18
271.7
44
74.8
22.2
131.8 TS VW
23*
03/
01/
16
03:
12:
38.63
31
26.27
69
26.86 2.9 20.3 23.4
137.6
42.1
69.7
31.1
127.7 NS VW
24
03/
01/
17
18:
58:
50.27
33
18.15
68
40.47 28.7 3.8 142.7
35.3
85.2
15.8
74.9
162 TH VW
25
03/
01/
18
05:
34:
06.35
32
48.91
68
42.95 29.9 2.7 35
215
5
85
90
90 TH W
26
03/
01/
19
09:
09:
59.95
33
17.53
68
54.65 30.4 3.3 265.1
37.2
21.1
75.5
44
105.5 NO VW
27
03/
01/
21
00:
57:
18.85
32
49.54
69
14.79 34.9 5.0 175.9
328.7
80
11.2
84.9
116.7 NO VW
28
03/
01/
22
14:
29:
48.14
33
7.82
68
59.66 24.1 2.3 102.9
313.4
56.4
37.7
71.9
115 TH VW
29
03/
01/
23
07:
00:
18.09
33
32.63
68
50.42 28.7 4.5 296.5
80.5
28.9
65.9
57.6
106.5 NO VW
30
03/
01/
29
00:
00:
40.09
32
35.41
68
40.02 19.6 13.2 332
171.3
15.8
75.1
71.3
95.2 TH VW
31
03/
02/
07
00:
51:
57.52
33
20.59
68
44.40 25.3 5.3 333.7
180.9
11.2
80
63.3
95.1 TH W
32
03/
02/
08
16:
20:
41.86
32
48.01
68
54.98 32.5 4.8 274.9
65.7
70.3
22.3
79.4
117.3 NO W
33
03/
02/
10
08:
46:
50.07
32
51.09
69
14.51 17.5 4.5 23
284.2
76
58.7
32.4
163.5 ST R
34*
03/
02/
11
18:
00:
08.90
32
4.48
69
7.58 24.7 2.1 109.2
337.1
75.5
21.1
74.5
136 TH VW
IN: Identification Number for the Events; Location Parameters: (Lat.: Latitude, Long.: Longitude, Dep.: Depth); Mc: Coda Magnitude; Location Depth Error:
Dep. Error; Type of solutions: Type. TH, thrust; NO, normal; SS, strike-slip; TS, thrust with a strike-slip component; NS, normal with a thrust with a strike-slip
component strike-slip component; ST, strike-slip with a thrust component; SN, strike-slip with a normal component. Quality of Solution: Qt. Very Well (VW),
Well (W) and Regular (R).
The strip shows several different evidences of Quater-
nary tectonic activity. In the area, the Paleozoic suture
between Chilenia and Cuyania is at the boundary be-
tween Cordillera frontal and Southern Precordillera, and
towards West of the oriental border of the Cerrilladas
Pedemontanas. This section of the suture also coincides
with the Triassic basin oriental boundary [6,7]. The
Southern Precordillera is characterized by high angle
normal faults inverted by contraction and associated to
faults trending NW and NNW and having a strike-slip
component [6,7]. Tectonically inverted normal faults
with the same orientation are also recognized within the
Cerrilladas Pedemontanas [25].
Known focal mechanisms (Mw 3.6) from CMT Har-
vard and from [3] are thrust except two that are strike-
slip (Figure 2). The thrusts are located to the East of the
Río Mendoza—Tupungato structure, except one for
which the location error also permits to be placed to East
of that structure. So, the active thrust front seems to be
located to East of the Cerrilladas Pedemontanas occi-
dental branch, which includes the Barrancas-Lunlunta-
Carrizal anticlines.
Focal mechanisms obtained in the present work for M
3.5 earthquakes (Figure 5) also show that the thrust
front south of ~33.1˚S is located in the same place that
for Mw 3.6 earthquakes. But, what is noticeable for
those M 3.5 earthquakes is that just West of this area
and in the Southern tip of he Southern Precordillera t
Copyright © 2013 SciRes. OPEN ACCESS
S. Nacif et al. / Natural Science 5 (2013) 18-26
24
Figure 5. Focal mechanism solutions from Table 1 obtained from P first motion. Number and Type are
showed at the top of the mechanism. Extensional and compressional quadrants are in white and black
color, respectively. For a clearer figure we did not plot 4 focal mechanism solutions North of 32.5˚,
three of them have normal solutions. Red dashed line marked—the North-Northwest fringe of normal
focal mechanism solutions, this fringe extending to the north of 32.2˚ (if we take account the 3 normal
focal mechanisms not plotted here) and for longitude of 120 km. Blue line: Río Mendoza-Tupungato
Structure [6].
there is a fringe of normal focal mechanisms (from South
to North, earthquakes number 16, 23, 5, 7, 8, 13, 11, 12,
9, 24, 10, 3 and 2, Figure 5 and Table 1) with the excep-
tion of an earthquake with strike-slip mechanism (num-
ber 30).
Since the data come from a temporary experiment of
only four months, we do not know to what extends this
behavior were transitory phenomena. This, because is
inserted within a predominant compressional stress field.
Some kind of regional stress relaxation might have being
acting (at least at the time of the experiment) that pro-
duced those normal earthquakes.
We are not able to propose a consistent hypothesis to
explain that probable relaxation. Weak hypothesis are
some stress variation in the backarc related to subduction
earthquake cycle, or even weaker the occurrence of a
subduction slow earthquake (e.g. in [26]) that has not
been documented to occur in any of the subducted Nazca
plate sections.
6. CONCLUSSIONS
Data from a temporal experiment have revealed a nar-
row area about 120 km long trending NW-NNW with
Mw 3.5 normal earthquakes. The area is located just to
the West of the thrust front in the Cerrilladas Pedemon-
tanas occidental branch and in the southern tip of the
Southern Precordillera. The existence of those normal
earthquakes in the Andes backarc within a predominant
compressional field imposed by the subduction process is
a puzzling phenomenon. Some kind of local and/or re-
gional relaxation process must be acted to permit the
normal mechanisms activation.
Nevertheless, whatever the relaxation process might
be we can propose that the normal earthquakes with
Copyright © 2013 SciRes. OPEN ACCESS
S. Nacif et al. / Natural Science 5 (2013) 18-26 25
depths of 15 to 35 km (Figure 5) are likely to be associ-
ated to the mentioned inverted normal faults within the
Southern Precordillera and the Cerrilladas Pedemontanas
occidental branch. Those faults, or some of their subsidi-
aries, now re-inverted as normal (at least transitorily), are
not exactly placed on the Chilenia-Cuyania suture but are
related to Cuyo basin Triassic extension.
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