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Revista de la Asociación Geológica Argentina

versión impresa ISSN 0004-4822

Rev. Asoc. Geol. Argent. vol.69 no.3 Buenos Aires set. 2012

 

ARTÍCULOS

Landslides in the Andes and the need to communicate on an interandean level on landslide mapping and research

 

Reginald L. Hermanns1, Patricio Valderrama2, Luis Fauqué3 , Ivanna M. Penna4,5, Sergio Sepúlveda6, Stella Moreiras5,7 and Bilberto Zavala Carrión2

1Norges geologiske undersøkelse, Leiv Eirikssons vei 39, NO 7491, Trondheim, Norway.reginald.hermanns@ngu.no
2Instituto Geológico Minero y Metalúrgico. Lima, Perú.
3Servicio Geológico Minero Argentino. Buenos Aires, Argentina.
4Departamento de Ciencias Geológicas. Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Buenos Aires, Argentina.
5Consejo Nacional de Investigaciones Científicas y Técnicas-(CONICET).
6Departamento de Geología, Universidad de Chile.
7IANIGLIA, CCT, Mendoza, Argentina.

 


Abstract

Landslides in the Andes are some of the highest natural threats to society with single events killing up to several thousand people. Landslide mapping and landslide research became a more widely spread discipline in geosciences in the Andean countries. However efforts today by far do not match the threat and both more investigations and more mapping activities are needed to support decision makers in land use planning. In this communication we discussed five key issues that we suggest to focus on in upcoming years: Impact of climatic change on landslides occurrence, landslides susceptibility and hazard maps, prediction of megalandslides, seismically triggered landslides, and temporal spatial distribution of mud and debris flows potential.

Keywords: Landslides; Hazard; Prediction; Triggering mechanisms; Climate change; Andes.

Resumen

Deslizamientos en los Andes y la necesidad de comunicar a nivel interandino sobre el mapeo y la investigación de deslizamientos.

Los deslizamientos en los Andes son unas de las mayores amenazas naturales a la sociedad, con eventos individuales que han causado la muerte de varios miles de personas. El mapeo e investigación de deslizamientos se convirtió en una disciplina ampliamente difundida en los países andinos. Sin embargo, los esfuerzos actuales no se corresponden aún con la amenaza, y más investigaciones y mapeo son necesarios para apoyar a los tomadores de decisiones en la planificación de usos del suelo. En esta comunicación se discuten cinco temas clave en los que se sugiere poner el foco en los próximos años: el impacto del cambio climático en la ocurrencia de deslizamientos, mapas de peligro y susceptibilidad a deslizamientos, predicción de megadeslizamientos, deslizamientos disparados sísmicamente, y la potencial distribución espacio-temporal de flujos de barro y detritos.

Palabras clave. Peligrosidad; Deslizamientos; Predicción; Génesis; Cambio climático; Andes.


 

Introduction

This communication is inspired by the discussions on the 1st, 2nd, and 3rd Symposium on landslides in the Andes (respectively in 2008, 2010, and 2012 at the geological congresses of Argentina, Peru, and Chile) and is published with 8 selected full papers that were presented on the 2nd Symposium on landslide in the Andes in Peru in 2010. It reviews briefly the impact of landslides on communities and discusses five key topics that require follow up investigations and mapping activities in the Andean region that ideally also should be in future discussed on Interandean platforms. The paper also includes an overview on landslides in the Sihuas valley (Peru), that follow large scale irrigation of coastal plateaus in arid environments. This example strongly highlights the impact of human activity on nature and points out the need of improving the understanding of landslide processes.

The Andes are the world's second highest mountain belt and the inter-Andean valleys are in parts densely populated. Several cities with more than 1 million inhabitants lie in areas with high relief contrasts. Destructive landslides in those large urban centres, some of them resulting in a large number of live losses, have been reported (Salcedo 2007; Hermanns et al. 2012a). However, landslides with a large loss of life also occurred in more remote areas of the Andes. For example failures of Nevado Huascarán in 1962 and 1970 in Peru resulted in the loss of live of thousand of persons (Evans et al. 2009). Even more persons lost their life in mudflows, following the eruption of Nevado del Ruiz in Colombia in 1985 (e.g. Montero Olarte 2007). In the Andean regions, natural conditions favouring landslides place additional strains on rural population, often affected by poverty - especially among the indigenous population - that is due to geographic isolation, high transport costs, and limited infrastructure. This leads to a situation that mortality risk due to landslides is among the highest on earth (UN Millennium Project, 2005).

Exposure of civilisation to natural hazards and especially landslides/floods is not new within the Andes. For instance, landslides have strongly impacted on settlements of the Chavin and Chimú cultures, which might have also contributed strongly to the decay of these cultural centres, Turner et al. (1999) and Pozorski and Pozorski (2003), respectively. Also economic growth is often hindered by landslides and entire industrial sectors can heavily be damaged. For example, due to the multiple landslides triggered by the Mw 6.2 earthquake on April 21st in 2007 that dropped into the Aisén fjord, southern Chile, and caused displacement waves with run-up heights up to 70 m, fish farming industry was nearly entirely destroyed (Naranjo et al. 2009).

The threat of landslides to society and economic growth is especially showcased with the largest clusters of multimillion-cubicmeter landslides that occurred within the past 12 years. One of these clusters of landslides of the Andes that occurred in the 21st century is within the La Paz valley and partly within the city of La Paz, Bolivia. (Hermanns et al. 2008). The latest of these landslides destroyed an urban area with around 5000 houses on February 26th in 2011 (Hermanns et al. 2012a). The urban centre of La Paz and the satellite town El Alto is among the densest populated area within the Andes but it lies only few km north of the rim of a gigantic landslide with a surface of 60 km2 that occurred not more than 11,000 years ago (Hermanns et al. 2012a). The other cluster of multimillion-cubic-meter landslides of the 21st century lies in southern Peru in the Sihuas valley (Hermanns et al. 2008; Fig. 1). First results of a mapping activity in the Sihuas valley are summarized in this communication as an example on the human impact on the special and temporal distribution of landslides.

Landslides in the Sihuas valley, Peru

The Sihuas valley is incised several 100 m in a coastal plateau at an elevation of ~1200 m altitude. The plateau lies within one of South American driest regions with only rare minor precipitation events separated by dry periods lasting several years. The Sihuas valley itself is green and fertile due to the Sihuas River draining the inner Andean region towards the Pacific. In the past 25 years the coastal plateau has been progressively converted into a large oasis through South America's largest irrigation project Majes - Sihuas. The irrigation started in 1981 and water started seeping out of the valley walls below the irrigation in 1996. Since 2002 six multimillion cubic meter slides have occurred from that slope (Fig. 1).


Figure 1: Satellite image showing landslide distribution of prehistoric and historic landslides in the Sihuas Valley, Southern Peru. Ages were reported by local farmers within the Sihuas valley. Note dense vegetation cover due to irrigation on the N side of the valley.

Within the Sihuas valley there are 3 prehistoric deposits with volumes between 15 x 106 m3 and more than 1 km3. These deposits have not been dated yet; however their age is expected to be older than the Holocene, because their toe areas are covered by massive deposits of laharic flows from the inner part of the Andes, that are not known for the Late Pleistocene / Holocene in that region. Within the past 8 years megalandslides occurred in the valley involving volumes up to ~30 x 106 m3 (Fig. 1). The rockslide deposits have destroyed several km2 of agricultural land within the valley. The scar areas of two of these slides lie on the southern slope and events occurred in the mid 20th century along a leaking water channel. The other six sourced from the northern slope south of the Majes irrigation project, where a large amount of water is seeping out of the valley walls (Fig. 2). Infiltration into the subsoil from the irrigation on top of the plateau is interpreted as the main factor triggering these recent rockslides. This is especially alarming as a second phase of the Majes - Sihuas irrigation project should start in a few years and also in other valleys in southern Peru (Vítor, Tambo de Cuesta) multimillion cubic meter landslides have been observed in relation with large scale irrigation projects. On human time scales the Sihuas valley has been settled and the valley walls had at least been stable in this area since the Late Moche civilization. This is evidenced by multiple graveyards that became exhumed in landslides in this valley (Fig. 3). Rests of cloth of mummies were AMS radio carbon dated to an age of 1270 ± 35 BP (TUa - 7860).


Figure 2: Oblique view of Sihuas valley with megarockslide covering cropland within the valley. Background showing coastal plateau and snow covered mountains of the high Andes. The plateau along the coastal desert is covered partially by oasis due to extensive irrigation. Note the multiple dark patches along the arid valley slopes indicating large quantities of water seeping out of the valley walls.


Figure 3: Mummy exposed by landslides from the N-slope of the Sihauas valley. Rests of cloth of the mummy was AMS radio carbon dated to an age of 1270 ± 35 BP, therefore the mummy belongs to the Late Moche civilization.

Discussion

Examples given in the introduction and the case of the Sihuas valley in Peru makes evident that landslides have been a threat to civilization in the past, at present and that it will be a threat also in future. In this light the "Symposium on Landslides in the Andes" was given birth during the XVII Argentinean Geological Congress in Jujuy, Argentina in October 2008. This symposium was initiated to round up activities started within the Multi Andean Project: Geosciences for Andean Communities that brought together the Geological Surveys of the Andean countries to work in collaboration on landslides, volcanic hazard and earthquake hazard (a summary is given in Hermanns et al. 2011). However it was also organized to open communication among the geological surveys to a broader public mainly including research institutes and universities. The topic of landslides within the Andes was chosen as this topic was one of the three topics within the MAP:GAC project that received largest attention by the Geological surveys (PMA:GCA 2007, 2008, 2009). Selected papers from the first symposium were published one year after the symposium in the Revista de la Asociación Geológica Argentina (65/4). The second Symposium was held during the XV Peruvian Geological Congress in October 2010. During the 2nd symposium 14 talks and 1 poster were presented on landslide problematic in Argentina, Bolivia, Chile, Peru, and Norway. In this issue eight selected papers of these contributions are published as full papers. These papers resemble important issues of landslide research in the Andes that has been focus of investigation in the past but that require much more in depth study and extended mapping activities in order to give useful tools to planning institutes in order to reduce the negative impact of landslides in the Andean region: Impact of climatic change on landslides, landslides susceptibility and hazard maps, prediction of megalandslides, seismically triggered landslides, as well as mud and debris flows.

The impact of climate variability and climate change has selectively been the focus of research projects. Namely studies by Trauth and collaborators have focussed on the impact of climate change in past millennia on the distribution of rock avalanches in NW Argentina (e.g. Bookhagen et al. 2001, Trauth et al. 2002, Hermanns and Schellenberger 2008). Moreiras (2005) and Moreiras et al. (2012) showed the impact of climatic variability namely the effect of El Niño/ Southern Oscillation on the temporal distribution of landslides in the Mendoza valley. Clague et al. (2012) give a global review on the impact of climate change on hazardous processes in high mountains. Also the loss of glacial ice is understood for tropical mountain belts in the Andes (Francou et al. 2000; Thompson et al. 2006) its impact on distribution of landslides in the Andes remains poorly understood until today. The best understood processes of landslide conditioning by climate change in the Andes is the process of glacial calving into mountain lakes, that causes displacement waves overflowing and often breaching moraine dammed lakes resulting in debris flood, debris flow disasters (Reynolds 1992; Vilimek et al. 2005). Among those studies Valderrama and Vilca (2012) contributed to the understanding of the run-out behaviour of such processes with a paper on a recent case from April 11th, 2010 in the Cordillera Blanca.

In general, the vast amount of the scientific community has come to the conclusion that global climate will warm over the next century (IPCC 2007). This also includes the Andean region. However, different to other regions in the world (SafeLand deliverable 3.8, 2012) no research has yet been carried out in the Andes how future climatic scenarios will impact on distribution of landslides. Mapping out landslide prone areas is one essential tool for development planning. This is carried out in a stepwise approach (e.g., Lacasse and Nadim 2009).

On a regional level susceptibility to form landslides are mapped based on physical conditions that change based on geology, climate and landslide type. In a second step hazard zones are mapped in areas where consequences may be severe. This is done normally on a smaller scale that gives in addition to the aerial distribution of landslide impact also information on the frequency and magnitude (volume, intensity) of events. Landslide risk maps also include consequences, which finally allow prioritising areas for mitigation measures for landslides. Except for inventory maps, systematic mapping has still been scarce in the Andean region. Well known examples are susceptibility maps that have been produced on various scales in the Mendoza river catchment (Moreiras 2006a; Rosas et al. 2009). Ground-breaking advancement of landslide mapping is carried out at the Geological Survey of Peru (INGEMMET) with putting landslide mapping on a national scale. Villacorta et al. (2012) present the first nation-wide landslide susceptibility map produced within the Andean region. The effectiveness of this tool is demonstrated by Zavala et al. (2012) with a subset of the national dataset. The massive landslide of Rodeopampa, Cajamarca from February 22nd, 2010, falls in the highest susceptibility class and mitigation measures might have been taken if these maps would have been available prior to the event.

A different approach is required when assessing the likelihood on a slope scale, where a single landslide (often a large landslide) can have severe consequences. At this scale frequency analyses is impossible and the likelihood of failure in future has entirely based on geologic parameters and displacement rates. Within the Andes detailed mapping of slope conditions of deforming slopes in order to assess the likelihood of failure have yet not been carried out. Differently in Norway, where mapping for future failure scenarios is carried out systematically (Hermanns et al. 2012b, 2013). One example is the study by Saintot et al. (2012) from the Romsdalen valley, Norway, where several rockslopes are mapped in detail and displacements are determined to give likelihood ranges for future failures scenarios. In the Andes deposits of large rock slope failures have been mapped systematically on different scales in order to understand conditions under which they formed (Hermanns and Strecker, 1999; Fauque and Tschilinguirian 2002; González Díaz et al. 2006; Moreiras 2006b, Penna et al. 2011). However out of those analyses no predictive tools have been developed yet.

That landslide damming is a major threat to communities in the Andes is known since 1914 when a large landslide dam breached (Groeber 1916) resulting in at least 175 casualties (González Díaz et al. 2001) along the 1000 km long stretch between the Andes and the Atlantic Ocean due to lack of any fast communication methods. The large impact of landslide dams and related failures on the development of entire regions became also evident by the La Josefina event, Ecuador, 1993. The direct costs due to the dam formation and failure added up to 1% of the gross domestic product of Ecuador in that year (Zevallos et al. 1996). Penna et al. (2012) focused with a study on a landslide dam with multiple phases of formation and failure in the Patagonian Andes. However, Fauque et al. (2005) have been so far the only ones in the Andean region to estimate the rate of formation and failure of such dams as a predictive tool for land use planning.

Earthquakes as a trigger mechanism for large landslides have been known due to the deadly consequences since the Ancash earthquake 1946 (Heim 1949) that triggered 5 rock avalanches and the Chimbote earthquake that triggered failure of Nevado Huascarán in 1970 (Evans et al. 2009) in the Cordillera Blanca of Peru. However earthquakes also trigger thousands of small landslides as experienced by the Chimbote earthquake (Plafker an Ericson 1978). Latest examples are the Mw 6.2 earthquake in 2007 in the Aisén fjord of southern Chile and Pisco earthquake in 2007 in Peru. Following both events systematic mapping of landslides has been carried out by Sepúlveda et al. (2010) and Zavala et al. (2009), respectively. The failure of one of the rock avalanches triggered by the 2007 Aisén fjord earthquake has been investigated in large detail by Oppikofer et al. (2012) using high-resolution digital surface model created from terrestrial laser scanning. Such detailed studies allow separating between deposits of primary failures due to the earthquake shock and secondary failures due to toe erosion of slopes by landslides. Based on historic observations it was lately discussed that megathrust-earthquakes along the subduction zone along the Pacific rim of South America rather produce widespread landsliding while shallow crustal earthquakes although of minor magnitude produce rather regional more concentrated but more voluminous landslides (Sepúlveda et al. 2012; Hermanns and Longva 2012). Systematic mapping of landslides triggered by earthquake in future can test those hypotheses.

Mud flows/debris flows are further serious threats to society in most part of the Andes. Even the driest parts of the Andes with average annual precipitation below 10 mm/year can be affected by debris flows during seldom stronger precipitation events, usually related to El Niño climatic events (Sepúlveda et al. 2006). For example on June 18th, 1991, 91 persons lost their life in Antofagasta (Chile) in a precipitation event of 14,2 mm / 3 hr (Hauser 1997). Such precipitations in other parts of the Andes rarely would cause any mass movements. During a magnitude 7.7 earthquake with an epicentre close to that town only two persons lost their life in 1995 indicating the large threat of water-saturated landslides in this region. This is further highlighted by both the most deadly landslide events in the past century. One event is related to exceptional high rainfall along the cost of Venezuela in 1999 (Salcedo 2007) the other to the melt of the glacier covering Nevado de Ruiz in Colombia during an eruption (Montero Olarte 2007). However, such events can threat most communities within the Andean valleys as showcased with an example of a mud flow in Termas de Reyes, Province of Jujuy, Argentina from January 12th, 2010 that caused 87 injured persons and severe material damage in a recreational area by González et al. (2012). In this area similar events have occurred in the past as documented by historic records and deposits of previous events. Such high hazard areas could easily be recognized during systematic mapping in future and managed using appropriate land use regulations.

Conclusion

Landslide mapping and investigation became a more important issue for geoscientists in the past two decades in the Andean region. The Symposium on landslides in the Andes became an important platform for exchange. We hope that this Symposium can be carried out also in future during geological congresses in order to make knowledge transfer easy.

We discussed here the impact of landslides in the Andes on society and pointed to five key mapping and research areas in which we see a large potential of development that can impact on decision making processes that will allow reducing the landslide threat within the Andes.

Acknowledgement

We want to thank the organizers namely Jose Macharé Ordoñez, Victor Carlotto Caillaux, and Lionel Fidel Smoll of the 15th Peruvian geological Congress to host the 2nd Symposium on landslides in the Andes. The International Centre of Geohazards supported the Symposium in Peru by providing travel allowances to invited speakers and supported editing through time for Reginald Hermanns and travel for Ivanna Penna. This is ICG contribution 410. The publication of selected papers in this volume of the Revista de la Asociación Geológica Argentina was made possible due to the support by Victor Ramos, Andres Folguera, and Pablo J. Pazos.

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Recibido: 11 de febrero, 2012
Aceptado: 23 de abril, 2012