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Journal of South American Earth Sciences 25 (2008) 64–73

The Abanico del Quindio alluvial fan, Armenia, Colombia:Active tectonics and earthquake hazard

Carlos A. Vargas a,*, Marco Nieto a, Hugo Monsalve b, Luis Montes a, Mireya Valdes a,b

a Departamento de Geociencias, Universidad Nacional de Colombia – Bogota, Colombiab Facultad de Ingenierıa – CEIFI, Grupo Quimbaya, Universidad del Quindio, Armenia, Colombia

Received 15 March 2005; accepted 30 June 2006

Abstract

The Abanico del Quindıo (AQ) fan, a volcaniclastic deposit from the Ruiz–Tolima volcanic complex (RTVC), Colombia, providesinsight into recent deformation in the Central Andes. The use of geological observations, geophysical measurements, and estimates offault-scarp ages constrain timing of recent tectonic activity. Gravity and magnetic analyses, along with geomorphologic cartography,allow the detection of lateral variations in basement distribution and at least three structural trends that cut the AQ: the Armeniafault (NNE), El Danubio fault (NNW), and Hojas Anchas fault (E–W). Recent deformation in the zone results from slip on theArmenia and El Danubio faults and suggests a maximum interval magnitude of 5.1 < Mw < 6.3, with ages ranging between2560 ± 480 yr B.P. and 4120 ± 780 yr B.P. Although no surface ruptures are associated with historical events on the fault segmentsin this zone, blind structures may have influenced the hypocentral distribution of events recorded after the Armenia Earthquake (Mw

6.2, 25-01-1999). Further geophysical studies are needed to understand the Romeral Fault System and assess the earthquake hazardfor the city of Armenia.� 2008 Published by Elsevier Ltd.

Keywords: Seismotectonic; Volcaniclastic fan; Fault-scarp ages; Armenia (Colombia)

1. Introduction

On 25 January 1999, at 18:19:16 UTC, an earthquake ofmagnitude Mw 6.2 occurred near the city of Armenia(Fig. 1) in the central Colombian Andes. The resultingdamage included 1185 fatalities, more than 4370 injuries,and 160,000 left homeless. Monsalve and Vargas (2002)locate the epicenter at 75.669�W, 4.465�N, with a focaldepth of 18.6 km, fault-plane solution U = 8�, d = 65�,and k = �21�, as well as a stress drop of 28.4 N/m2. Thehipocentral distribution of aftershocks defines a ruptureof approximately 10 km in length.

The complex geology of the Abanico del Quindio (AQ)fan dominates the epicentral zone. The AQ is a volcaniclas-

0895-9811/$ - see front matter � 2008 Published by Elsevier Ltd.

doi:10.1016/j.jsames.2006.06.001

* Corresponding author. Tel.: +57 1 3165000x16506; fax: +57 13165390.

E-mail address: cavargasj@unal.edu.co (C.A. Vargas).

tic fan of Quaternary age cut by several active faults of theRomeral Fault System (RFS). Movement on the Cordobafault, one branch of the RFS, likely caused the January1999 event. Future seismic events on the other faults ofthe RFS, including the Armenia, El Danubio, HojasAnchas, and La Isabela, present major potential hazardsfor the city of Armenia.

Paris and Romero (1994) and Chicangana and Vargas(2005) describe two behaviors of the RFS; from the Gulfof Guayaquil (�4�S) to the vicinity of Cali (�4�N), move-ment is right-lateral strike-slip, whereas farther north (�5–9�N), movement is left-lateral strike-slip. Since the UpperPleistocene, the kinematic behavior of the transition areahas remained poorly defined.

Several studies in the 1990s estimated the contributionof the RFS to earthquake hazards in the central ColombiaAndes. These studies paid special attention to the populatedareas in the transition area, such as the city of Armenia

-82 -80 -78 -76 -74 -72 -70 -68 -66 -64Longitude (º)

-4

-2

0

2

4

6

8

10

12

14La

titud

e (º

)

W

C

E

Western rangeCentral rangeEastern range

Study Area

W

CE

Brazil

Peru

Ecuador

PanamaVenezuela

COLOMBIA

RFS

-76 -75.9 -75.8 -75.7 -75.6 -75.5 -75.4 -75.3 -75.2 -75.1Longitude (º)

4

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

Latit

ude

(º)

Pereira city

Armenia cityIbagué city

Tolima volcano

Nevado del Ruiz volcano

"Abanico del Quindio" alluvial fan - AQ

"Abanico de Ibagué" alluvial fan

Romeral Fault System - RFS"El Cestillal" trench

"Ibagué" trench2

2

Armenia earthquake (Mw=6.2, 1999)

-75.8 -75.77 -75.74 -75.71 -75.68 -75.65

Longitude (º)

4.43

4.46

4.49

4.52

4.55

4.58

4.61

Latit

ude

(º)

La Isabela fault

Armenia city

Calarcá

Montenegro

1

1

Armenia earthquake rupture zone

0 1 2 3

kilometers

0 2 4 6

kilometers

Fig. 1. Localization of study area. (Top left) Physiographic context associated with the Central Range (CR) and Romeral Fault System (RFS). (Top right)The RFS cuts the Abanico del Quindio (AQ) fan over the west flank of the CR. The Ibague fault cuts the Ibague fan over the east flank of the CR. Bothfans have similar genetic properties related to the Ruiz–Tolima volcanic complex (RTVC). (Bottom) Principal tectonic structures in the study area.

C.A. Vargas et al. / Journal of South American Earth Sciences 25 (2008) 64–73 65

(Monsalve et al., 1996; Villegas et al., 1997; Vargas andCarvajal, 1998). However, due to the lack of neotectonicand earthquake information, these studies underestimatedthe earthquake hazard for local structures. The January

1999 event demonstrated the need to review existinginformation and launched a series of analyses of the localearthquake hazards that incorporated new geological andgeophysical information (Bohorquez et al., 1999; Parra

66 C.A. Vargas et al. / Journal of South American Earth Sciences 25 (2008) 64–73

and Mejia, 2000; Monsalve et al., 2001; Ojeda and Haskov,2001; Ugalde et al., 2002; Monsalve et al., 2005; Vargaset al., 2005).

This paper briefly describes the AQ structure on thebasis of geological observations, geophysical measure-ments, and estimates of the fault-scarp ages related torecent neotectonic activity. Although historical events onthe fault segments in this zone show no surface ruptures,blind structures appear to influence the hypocentral distri-bution of events recorded after the Armenia earthquake.Similar examples of active tectonics related to blind struc-tures have demonstrated the need for exhaustive geologicaland geophysical studies to understand complex tectonicareas (Berberian et al., 2000; Telebian et al., 2004; Bennettet al., 2005; Parsons et al., 2006). This information can be

Fig. 2. Geological units around Armenia city. Geological m

used to reevaluate the local seismic hazard and understandthe RFS and its tectonic implications in the north Andeanregion.

2. Geological setting

The Cajamarca complex, a pre-Mesozoic, polymetamor-phic basement, underlies Armenia and the surroundingregion (Fig. 2). Several subduction-related volcanicsequences (Arquia and Quebradagrande complexes,Amaime Formation) cover the Cajamarca complex. Meso-zoic and Cenozoic plutons (Cordoba and Navarco Riverigneous complexes) intrude the Cajamarca; continentaldeposits (Penderisco, Cauca Superior, La Paila, and Zarzalformations) cap the sequence (Neuwerth et al., 2006). In

ap showing the general tendency of RFS to the NNE.

C.A. Vargas et al. / Journal of South American Earth Sciences 25 (2008) 64–73 67

general, these rocks show a complex distribution of blockscovered by the AQ, alluvial deposits, and recent volcanicdeposits deformed by activity on the RFS (Maya and Gon-zalez, 1995; Pardo and Moreno, 2001). Espinosa (in press)describes several geomorphologic units within the AQ andsuggests they correspond to at least 14 volcaniclastic fansfed by the Ruiz–Tolima volcanic complex (RTVC;Fig. 3). Espinosa (in press) interprets different ages for eachfan and suggests a variable AQ thickness reaching up toseveral hundreds of meters.

The RFS extends along the western boundary of theCentral Range of Colombia and marks the limit betweentwo lithologic domains: continental to the east and oceanicto the west. In the AQ, the RFS is characterized by numer-ous fault strands spread between the San Jeronimo fault inthe east and the Silvia-Pijao fault in the west. Minorbranches of the RFS cross the AQ near Armenia and dis-play at least three structural orientations: NNE for theArmenia fault, NNW for the El Danubio fault, and E–Wfor the Hojas Anchas and La Isabela faults. Althoughmicrotectonic and seismological analyses indicate left-lateral strike-slip with an inverse component and dip tothe east for the Armenia fault, field surveys suggest at leastthree relay dips from south to north. The NNW- and E–W-trending faults are normal with dip to the west and south,respectively.

The relationship between existing faults and seismicactivity in the area remains unclear; however, hundredsof superficial events have occurred since the Armeniaearthquake measuring 0.1 6ML 6 3.0 (Fig. 4). TheObservatorio Sismologico del Quindio (OSQ) has recordedvarious earthquakes felt in Armenia and believes theyoccurred along the Armenia fault.

3. Data and analysis method

Four hundred fifty-eight (458) gravity stations and 259magnetic stations were occupied at varying intervals (100–400 m) along roads in the region. The instrumentation con-sisted of one Worden Gravimeter Master with a precisionof ±0.01 mgal and two Geometrics proton magnetometers(G-656). Stations D-QN2, D-QN15, D-QN23, and D-QN35 of the GPS Quindıo’s Network (IGAC, 2000) pro-vided absolute gravity values. Loops (29 in all) were closedat least every two hours. Precision GPS using the WGS84datum provided coordinates at each station.

Instrument drift, latitude, free air, Bouguer, terrain, andearth tide corrections were applied to the raw gravity data.Theoretical gravity was derived from the equation associ-ated with the international datum of Hayford (IGSN71);terrain effects were corrected by the Hammer (1939)method using DTMs with scales of 1:25,000 and1:100,000 (intermediate and distant effects, respectively)and field observations. Longman (1959) equations wereused for the earth tide corrections.

Diurnal variation, cultural noise elimination, and IGRF(International Geomagnetic Reference Field) corrections

were applied to the raw magnetic data. Filtering separatedregional and residual anomalies (both gravimetric andmagnetic), which were then mapped by triangulation withlinear interpolation. With methods described by Talwani(1959), GMSYS software was used to produce geologicalmodels based on the Bouguer gravity and magneticanomalies.

A geomorphologic analysis of aerial photos of the studyarea permitted the definition of neotectonic structures asso-ciated with the Armenia, El Danubio, and Hojas Anchasfaults. This study followed Hanks (2000) by applying thediffusion equation under transport-limited conditions. Eq.(1) is a linear, second-order differential equation thatrelates the change in elevation H to time t using the propor-tionality constant j and landscape curvature x (distancealong the profile):

oHot¼ j

o2Hox2

ð1Þ

where diffusivity [L2T�1] varies with climate, scarp material,and scarp height. The slope aspect (Hanks et al., 1984) isconsidered constant over the timescales investigated.Hanks (2000) expresses Eq. (1) to reduce the scarp slopeas follows:

ouox

����x¼0

� b ¼ affiffiffiffiffiffiffipjtp � tan hs � tan hf : ð2Þ

In this reduction, hs ¼ tan�1ð@u@x

��x¼0Þ represents the

maximum slope-scarp angle, hf = tan�1b is the fan slopeangle, and a is the effective half-vertical displacement.The field survey in the study area used several control sta-tions along these faults to measure the effective displace-ment (2a) and the angles hf and hs. This triplet data,along with the 14C data reported by other authors, enablesthe calibration of the constant j for the area and the deter-mination of scarp-fault ages. Rupture lengths along thefaults produced estimates of the possible magnitudes ofseismic events.

Bouguer anomalies were calculated along six profilesthat cross the most relevant faults of the area; the Bougueranomaly data were terrain corrected using digital models toscale 1:25,000 to 1:100,000. Magnetic anomalies, correctedby diurnal variation with an IGRF of 32000 nT were calcu-lated along three profiles. Geomorphologic triplet data(2a,hf,hs) were measured on eight locations along theArmenia (5) and El Danubio (3) fault-scarps. Lack offault-scarp exposures prevented measured triplet dataalong the Hojas Anchas fault.

The study used 14C data reported by Lalinde et al.(2004) for the El Cestillal fault (one paleoseismologicaltrench near to Pereira city) and data reported by Osorioet al. (2005) for the Ibague alluvial fan and Ibague fault(another paleoseismological trench near the city of Ibague).The Ibague fan falls on the east flank of the Central Rangebut shows a similar evolution of the fault-scarp process

Fig. 3. Geomorphological units in the AQ fan suggesting at least 14 volcaniclastic minor fans emplaced from the volcanic activity of the RTVC. AfterEspinosa (in press).

68 C.A. Vargas et al. / Journal of South American Earth Sciences 25 (2008) 64–73

-75.9 -75.8 -75.7 -75.6 -75.5Longitude (º)

4.3

4.4

4.5

4.6

Latit

ude

(º)

M 0 to 1 1.01 to 2 2.01 to 3

L

0 3 6 9

kilometers

Fig. 4. Epicentral distribution of the superficial events associated with local tectonic activity around the city of Armenia in 2004.

C.A. Vargas et al. / Journal of South American Earth Sciences 25 (2008) 64–73 69

(Fig. 1). Similar environmental conditions, such as climate,vegetation, erosion, source rocks, and superficial deposits,support this hypothesis. In both cases, the ash-volcanicdeposits covering the alluvial fans correspond to at leastsix eruptive events from RTVC. In the AQ fan, the ElCestillal fault cuts the volcanic ash layer of 2630 ± 80 yrB.P with an average effective displacement of 2a = 1.4 m.The Ibague fault cuts the recent volcanoclastic depositsof 1900 ± 80 yr B.P. with an average effective displacementof 2a = 1.5 m.

4. Results and discussion

Bouguer and magnetic anomaly maps were produced byapplying a triangular lineal interpolation to all metered sta-tions (Fig. 5a and c). The Bouguer anomaly map identifiesseveral positive gravity anomalies along the Armenia faultand four negative anomalies west of this structure thatcould be related to the Circacia, Quindıo River, HojasAnchas–Armenia, and La Florida fans (labeled 1–4 inFig. 5b). The magnetic anomaly map (Fig. 5c) shows twozones associated with Cretaceous basement rocks inthe east and AQ deposits in the west. The boundarybetween these zones may reflect a blind section of the

Cordoba fault to the north, which could be related to theEl Danubio fault. The system associated with the Armenia,Silvia-Pijao, Hojas Anchas, and La Isabela faults generatesa differential horizontal displacement that segments theCordoba fault (Fig. 5d). In this scenario, the Armenia faultcould be the last structure near Armenia with recentmovement.

Using GMSYS and outcrop observations, six densityand two magnetic susceptibility profiles were produced toestimate the possible distribution of some geological struc-tures in the area. These profiles used rock densities andmagnetic susceptibilities from samples taken in the field.They also allowed estimations of the thickness of the AQfan (Fig. 6a). The Quindio River fan area may locallyexceed 800 m thickness south of Armenia, whereas theArmenia–Hojas Anchas and La Florida fans appear tobe 500 m thick. The anomaly profile in the north produceda calculation of 550 m for the thickness of the Filandia fan(Fig. 6b). The convergence of the El Danubio, HojasAnchas, and La Isabela faults with the Armenia faultexplains these fan thicknesses. The AQ fan thins as it nearsthe city of Armenia. The 30 m thick volcanic ash layerswithin the fan reveal evidence of recent activity along theArmenia fault (Fig. 6c).

-30

-20

-10

0

10

20

30

40

50

60

70

-75.8 -75.77 -75.74 -75.71 -75.68 -75.65Longitude (º)

4.43

4.46

4.49

4.52

4.55

4.58

4.61

Latit

ude

(º)

mgal

0 2 6

kilometers

-30

-20

-10

0

10

20

30

40

50

60

70

-75.8 -75.77 -75.74 -75.71 -75.68 -75.65Longitude (º)

4.43

4.46

4.49

4.52

4.55

4.58

4.61

Latit

ude

(º)

mgal

1

24

3

0 2 6

kilometers

4

4

-400

-350

-300

-250

-200

-150

-100

-50

0

50

100

150

-75.8 -75.77 -75.74 -75.71 -75.68 -75.65Longitude (º)

4.43

4.46

4.49

4.52

4.55

4.58

4.61

Latit

ude

(º)

nT

0 2 6

kilometers

-400

-350

-300

-250

-200

-150

-100

-50

0

50

100

150

-75.8 -75.77 -75.74 -75.71 -75.68 -75.65Longitude (º)

4.43

4.46

4.49

4.52

4.55

4.58

4.61

Latit

ude

(º)

nT

0 2 6

kilometers

4

4

Fig. 5. Gravimetric and magnetometric results obtained in this study. (a) Simple Bouguer gravity anomalies in mGals with 458 station locations (crosssymbols). (b) Four negative anomalies related to Circacia (1), Quindıo River (2), Hojas Anchas–Armenia (3), and La Florida (4) fans. (c) Magnetic anomaly innTesla with 259 observations (cross symbols). (d) Two important anomalies associated with Cretaceous basement rocks in the east and the presence of the AQin the west. The limit between these zones could be a prolongation of the Cordoba fault to the north. The thick black lines are inferred blind structures.

70 C.A. Vargas et al. / Journal of South American Earth Sciences 25 (2008) 64–73

Using the empirical relation of magnitude versus faultlength from Wells and Coppersmith (1994), the earth-quake strength for the fault segments surrounding Arme-nia was estimated as 5.4 6MW 6 5.8, with the exceptionof the Armenia fault. Paris (2003) estimates a magnitudeof MW 6.3 for the Armenia fault, increasing the range of

the maximum magnitude interval to 5.4 6MW 6 6.3.This interval, consistent with the observed paleo-lique-faction structures within the volcanic ash deposits(Fig. 6d), suggests the presence of seismic sourceslocated at distances of less than 25 km (Ambraseys,1988).

-75.79 -75.76 -75.73 -75.7 -75.67 -75.644.43

4.46

4.49

4.52

4.55

4.58

4.61

4.64

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

Thickness (m)

-75.8 -75.77 -75.74 -75.71 -75.68 -75.654.43

4.46

4.49

4.52

4.55

4.58

4.61

0 2 6

kilometers

Longitude (°)

Long

itude

(°)

6

5

4

3

2

1

0 msnm

-1

-2

Anomaly Magnetic

Bouguer Gravity Anomaly

80

30

-20

-70

-120

-170 = Observed = Calculated

= Observed = Calculated

23191511

7

-13

-5

0 2 6 148 10 12 16

Scale 1:100000Distance (Km)

Dep

th (K

m)

Cauca Almaguer Fault Armenia Fault

Silvia - Pijao Fault

(mGal)

(nTesla)

Field 32092 nTInclination 29.85Declination -4.68

4

4

Fig. 6. Thickness map of AQ and evidence of active tectonics in the ashy-volcanic deposits that cover the AQ. (a) Thickness of AQ based on gravity andmagnetic profiles. (b) Simultaneous inversion of several gravity and magnetic measurements (black dots) in a profile with W–E direction (approximately),confirming the presence of the Armenia fault and significant thickness of the AQ: 0.0065 emu and 2.35 g/cm3 to the Tertiarian continental formations and0.0055 emu and 2.45 g/cm3 to the Quebradagrande complex. (c) Evidence of paleo-liquefaction in ashy-volcanic deposits near Armenia fault (star symbolin a). (d) Magnitude versus distance for paleo-liquefaction events, compiled from Ambraseys (1988), suggesting possible presence of seismic sourceslocated near Armenia (lined area).

C.A. Vargas et al. / Journal of South American Earth Sciences 25 (2008) 64–73 71

Calculations of the constant j for the Ibague and ElCestillal faults yield values of j = 1.8 ± 0.3 and1.2 ± 0.2 m2/ka, respectively (see Eqs. (1) and (2). Con-stants on this order of magnitude characterize other cli-

mates and regions (e.g., Nash, 1984; Hanks and Wallace,1985; Pierce and Colman, 1986; Niviere et al., 1998; Niviereand Marquis, 2000; Oemisch, 2004). The weathering in thisregion of the Andes, with semi-humid tropical conditions,

1 5 10 50 100 500

Epicentral distance (km)

5

6

7

8

9

Mom

ent M

agni

tude

2

Fig. 6 (continued)

72 C.A. Vargas et al. / Journal of South American Earth Sciences 25 (2008) 64–73

generates a thick regolith with no significant transport,which explains the low values for the constant. Vegetationcover in the area slows local erosion processes, so faultscarps remain in profile for a long time. Assuming a meanvalue of j = 1.6 ± 0.3 m2/ka for the region around Arme-nia, estimates of the ages of the El Danubio, Hojas Anchas,Armenia, and Isabela fault scarps can be established.

Estimates suggest ages of 2560 ± 480 yr B.P for the lastmovement on the Armenia fault and 4120 ± 780 yr B.P forthe last movement on the El Danubio fault. These valuesagree with the interpretation of the magnetic data, whichimplies displacement of the El Danubio–Cordoba fault sys-tem prior to that of the Armenia fault (Fig. 5d). This inter-pretation also indicates that blind structures throughoutthe region influenced the hypocentral distribution of eventsrecorded after the 1999 earthquake, despite the lack of sur-

face ruptures associated with historical events along the ElDanubio–Cordoba fault system. In summary, mapped sur-face ruptures do not define active tectonism within the AQ.More geophysical surveys, including reflection seismic,gravity, magnetics, and magneto-tellurics, could delineatesubsurface structures in the region to assess future earth-quake hazards.

5. Conclusions

Geological observations and geophysical measurementscontribute to estimations of fault-scarp ages associatedwith neotectonic activity in the AQ fan. Movements alongthe Armenia and El Danubio–Cordoba faults, ranging inage from 2560 ± 480 yr B.P. to 4120 ± 780 yr B.P., indicatemagnitudes in the interval of 5.1 < Mw < 6.3.

C.A. Vargas et al. / Journal of South American Earth Sciences 25 (2008) 64–73 73

However, the structural configuration in the zone andthe hypocentral distribution of aftershocks of the ArmeniaEarthquake (Mw 6.2, 25-01-1999) likely reflects structuresthat cannot be detected with standard geological methods.

Acknowledgments

We gratefully acknowledge the help provided by themembers of the Geophysics Group of the UniversidadNacional de Colombia, INGEOMINAS, and the Univers-idad del Quindıo by means of OSQ. The authors also thankGerman Camacho, Marisol Gomez, and Armanado Espin-osa for their help in the field, and Alfonso Lopez-Reina,Clara Ines Alvarez, and Adriana Duque their administra-tive and logistical help. This work has been partiallyfinanced by COLCIENCIAS and ‘‘Corporacion Autono-ma del Quindıo – CRQ’’ funds (Project 110113-14722,Contract CRQ055–2003).

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