Stripe of normal mechanisms for crustal earthquakes with M ≤ 3.5 flanking the western side of the thrust front zone in the Andes backarc

doi:10.4236/ns.2013.58A1003

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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: [email protected]

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

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

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

S. Nacif et al. / Natural Science 5 (2013) 18-26

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

S. Nacif et al. / Natural Science 5 (2013) 18-26

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:

Grade

minute

grade

minute km km Strike

Dip

grade Rake

02/

11/

21:

24:

50.81

−32

48.64

−68

56.84 29.5 7.3 258.7

75.1

15.8

84.8

108.7 TH VW

02/

11/

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

02/

12/

01:

03:

04.97

−32

35.26

−69

8.65 15.8 5.5 254.9

28.6

54.4

−58.7

−126 NO W

02/

12/

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

02/

12/

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

02/

12/

04:

17:

27.70

−33

13.96

−68

42.18 19.8 3.8 338.2

236.9

66.1

26.3

153.6 ST W

02/

12/

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

02/

12/

02:

13:

44.88

−33

9.84

−69

4.60 26.3 1.9 205

−90

−90 NO VW

02/

12/

01:

42:

15.72

−32

50.22

−69

8.13 29.5 5.1 239.2

0.8

80.3

−74.8

−147 NO VW

02/

12/

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

02/

12/

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

02/

12/

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

02/

12/

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

02/

12/

04:

28:

07.65

−33

4.61

−68

58.36 24.6 2.9 309.3

167.1

48.4

62.8

117.2 TH VW

03/

01/

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

03/

01/

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/

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/

00:

44:

26.05

−31

45.33

−69

34.23 10.0 15.3 220

−90

−90 NO W

03/

01/

08:

20:

58.99

−33

25.53

−68

54.69 21.3 3.6 285.9

195.8

89.9

140 ST R

S. Nacif et al. / Natural Science 5 (2013) 18-26 23

Continued

03/

01/

21:

08:

36.92

−32

52.69

−68

37.93 26.0 2.9 28.3

150

80.3

−74.8

−147 NO VW

03/

01/

16:

07:

00.98

−33

14.79

−68

59.32 28.0 3.0 30

210

90 TH VW

03/

01/

09:

25:

14.72

−32

58.59

−69

5.08 33.3 4.5 18

271.7

74.8

22.2

131.8 TS VW

23*

03/

01/

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

03/

01/

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

03/

01/

05:

34:

06.35

−32

48.91

−68

42.95 29.9 2.7 35

215

90 TH W

03/

01/

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

03/

01/

00:

57:

18.85

−32

49.54

−69

14.79 34.9 5.0 175.9

328.7

11.2

−84.9

−116.7 NO VW

03/

01/

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

03/

01/

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

03/

01/

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

03/

02/

00:

51:

57.52

−33

20.59

−68

44.40 25.3 5.3 333.7

180.9

11.2

63.3

95.1 TH W

03/

02/

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

03/

02/

08:

46:

50.07

−32

51.09

−69

14.51 17.5 4.5 23

284.2

58.7

32.4

163.5 ST R

34*

03/

02/

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

S. Nacif et al. / Natural Science 5 (2013) 18-26

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

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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