Inventory of large landslides along the Central Western Andes (ca. 15°–20° S): Landslide distribution patterns and insights on controlling factors

Assessing risk of potential natural catastrophes in cities remains challenging, in particular as we need to elaborate quantitative criteria for exposure and vulnerability. Statistical and probabilistic methods have been applied to Arequipa, one of the most vulnerable Latin America cities. The second largest city of Peru is highly exposed to natural hazards: earthquakes, eruptions from the historically active El Misti volcano, rain-triggered flash floods and mass flows together with landslides from the Río Chili canyon walls. First, we propose a long-term probabilistic multi-hazard assessment for the Misti composite cone located 17 km from Arequipa. Second, we examine criteria for delineating areas prone to mass flow hazards and characterizing multiple sources of vulnerability forthe city. Third, a statistical methodology to better estimate damage probability for buildings is proposed

Aportes de la teledetección en la gestión de riesgo de desastre: detección de movimientos en masa en el valle del Colca – Arequipa, series temporales de 40 años

La ocurrencia de movimientos en masa provocó parte de las pérdidas económicas y humanas más catastróficas del Perú. La teledetección ha demostrado su capacidad para detectar y caracterizar movimientos en masa en un área amplia, en particular mediante el uso de algoritmos de detección de cambios para la identificación de movimientos rápidos. Por lo tanto, la detección de movimientos en masa se convirtió en una herramienta importante para contribuir a la gestión del riesgo de desastres a escala nacional. No existen técnicas comunes para detectar movimientos de diferente velocidad simultáneamente. En este estudio enfocamos nuestra atención en la detección de movimientos en masa de velocidad rápida y lenta a través de series temporales de 40 años, como resultado de la generación de Modelos de Elevación Digital (DEM) en el valle del Colca - Sur de Perú

What are the main factors that trigger the giant-landslides in the Peruvian western Andes? The Aricota giant-landslide case study

The central part of western Andes concentrates giant paleo-landslides that mobilized large volumes of rock, well preserved due to hyper arid climate of this region (Audin and Bechir, 2006; Pinto et al., 2008; Crosta et al., 2014; Crosta et al., 2015; Crosta et al., 2017; Mather et al., 2014; Zerathe et al., 2017) and bring us the opportunity to study the sequence of ancient events. The main goal of this study is to identifiy the role of climate and tectonics phenomena’s on the triggering of giant-landslides in Peruvian western Andes. We realized a multidisciplinary study on the Aricota landslide by using high-resolution DEM derived from Pléiades satellite image (Lacroix et al, 2016. Zerathe et al, 2016.) and coupled with cosmogenic nuclide methodology, We realize dating on the main body of landslides and main scarps in order to obtain chronological constraints of the gravitational destabilization events. We identified two events: (1) a first event with an estimated volume of ~2 km3 that event generated an impressive dam in the main valley and formed a lake approximately 6 km long upstream; and (2) a second event of rocks avalanche with an estimated volume of ~0.3 km3. 10Be dating (17 samples) suggest ages of 17.9 ± 0.7 ka and 12.1 ± 0.2 ka (weighted mean and 1 sigma uncertainty of the weighted mean) for these two events. Comparing those mean ages with local paleoclimatic reconstructions, we noticed that the failures match well with two important wet events of the Altiplano: Heinrich 1 (14 - 18 ka) and Younger Dryas (11 - 13 ka), which correspond to the formation of the Tauca and Copiasa lakes respectively (Placzek et al., 2013). This suggests that the climate probably played an important role in the triggers of landslides. However, as several neotectonic faults also affect this area (in the 50 km of radius, e.g. Incapuquio fault or Purgatorio fault), we can’t rule out a possible seismic trigger or even an earthquake/climate coupling. On the western flank of the Andes, the Aricota landslide is one of the few dating events. Although there are numerous recognized paleo-landslides. So, other new landslides ages, should also allow us to know more about the origin of these phenomena in this arid region

Using 10Be cosmogenic surface exposure dating to determine the evolution of the Purgatorio active fault in the Andean forearc, southern Peru

Active transpressive deformation has been occurring along the Andean hyperarid forearc for the last 3 Myrs but many of these faults are still not described even if able to produce large damaging earthquakes. Active faulting along the northern part of the Arica Bend can be recognized due to the presence of well-preserved and sharp fault scarps indicating recent surface slip. During the Mio-Pliocene, deposition within the forearc continental basins resulted in the formation of vast fan deposits and conglomerates of the Moquegua Formation, which can be considered as bedrock in this exposure study (~45-4 Ma; Tosdal et al., 1984; Sebrier et al., 1988a; Roperch et al., 2006). The typical vertical Purgatorio fault scarps offset both the Moquegua bedrock and several younger geomorphic features associated with <300kyrs climatic and 400 years old volcanic extreme events. This study focus on quantifying slip rate variations in time along a 5-meters high vertical fault scarp to understand how the fault is evolving. These results are achieved via surface exposure dating of the sampled seismically broken cobbolds of the Moquegua formation outcroping vertically along the fault scarp. These samples are well-suited to the application of in situ produced cosmogenic radionuclides for surface exposure dating, as the hyperarid region has extremely low erosion rates. We sampled the scarp away from any significant drainage so as to avoid possibly disturbed areas. The sampling did involve extracting quarzite conglomeratic material along the bedrock scarp and on the upper surrounding crests. The aim has been to measure Berylium-20 TCN (Terrestrial in situ Cosmogenic Nuclides) concentrations to determine exposure age as a function of height on the scarp. This has been successfully employed on one scarp in Italy based on Chlorine-36 TCN (Palumbo et al., 2004). However, slow faults behaviour remains unclear and more contributions are needed. Quaternary activity of the Purgatorio fault system was evidenced by Hall et al. (2008). They highlighted a vertical offset of about ~100 m for a pediment surface intercepted by the fault, and dated at ~280 ka. Considering that the pediment surface is horizontal, this would gave a maximum of ~0.3 mm/yr of vertical deformation since 280 ka. Our new data provide evidences of constant activity of the fault during the Holocene with a mean vertical motion of 2 ± 1 mm/ yr. These news results strengthen the idea that the Andean forearc is still submitted to contratile deformation, bring additional knowledge on the structural model of the area, and raise the question of the local seismological hazard

Toward the feldspar alternative for cosmogenic 10Be applications

The possibility of quantifying surface processes in mafic or volcanic environment using the potentialities offered by the in situ-produced cosmogenic nuclides, and more specifically by the in situ-produced 10Be, is often hampered by the rarity of quartz minerals in the available lithologies. As an alternative to overcome this difficulty, we explore in this work the possibility of relying on feldspar minerals rather that on quartz to perform in situ-produced 10Be measurements in such environments. Our strategy was to cross-calibrate the total production rate of 10Be in feldspar (P10fsp) against the total production rate of 3He in pyroxene (P3px) by measuring 3He and 10Be in cogenetic pyroxene (3Hepx) and feldspar (10Befsp). The samples were collected from eight ignimbritic boulders, exposed from ca 120 to 600 ka at elevations ranging from 800 to 2500 m, along the preserved rock-avalanche deposits of the giant Caquilluco landslide (18°S, 70°W), Southern Peru. Along with data recently published by Blard et al. (2013a) at a close latitude (22°S) but higher elevation (ca. 4000 m), the samples yield a remarkably tight cluster of 3Hepx - 10Befsp total production ratios whose weighted-mean is 35.6 ± 0.5 (1s). The obtained weighted-mean 3Hepx - 10Befsp total production ratio combined with the local 3Hepy total production rate in the high tropical Andes published by Martin et al. (2017) allows to establish a total SLHL 10Be in situ-production rate in feldspar mineral (P10fsp) of 3.57 ± 0.21 at.g-1.yr-1 (scaled for the LSD scaling scheme, the ERA40 atm model and the VDM of Lifton, 2016). Despite the large elevation range covered by the whole dataset (800–4300 m), no significant variation of the 3Hepx - 10Befsp total production ratios in pyroxene and feldspar was evidenced. As an attempt to investigate the effect of the chemical composition of feldspar on the total 10Be production rate, major and trace element concentrations of the studied feldspar samples were analyzed. Unfortunately, giving the low compositional variability of our dataset, this issue is still pending

Monitoreo GPS y satelital del deslizamiento de Maca (Colca)

El deslizamiento de Maca, ubicado a las orillas del río Colca en la región Arequipa, afecta directamente a un pueblo de 800 habitantes, a la carretera usada por el turismo (160 000 visitantes en 2009), y a los andenes pre-incaicos [Fidel y Zavala, 1994]. En febrero y Julio 2013, sismos superficiales (Mw 5.4 y 5.7), con epicentros localizados a 10-15 km de la zona de estudio, destruyeron un parte del pueblo y reactivaron el movimiento. Después de los sismos de febrero, el gobierno regional de Arequipa pidió al INGEMMET una evaluación de la seguridad física de la zona [Zavala et al., 2013], la cual concluye que el pueblo es inhabitable. Siguiendo este primer trabajo, presentamos en este estudio los resultados del monitoreo geodésico de la zona, realizados entre 2011 y 2014, con el objetivo de mejorar la caracterización del deslizamiento y de sus detonantes. Se realizaron varias mediciones geodésicas, que comprenden: (1) 11 campañas de mediciones GPS entre noviembre 2011 y diciembre 2013, (2) la instalación de un GPS permanente en diciembre del 2012, (3) 3 adquisiciones de imágenes ópticas del satélite Pleiades. A partir de estas imágenes satelitales, se construyó un Modelo de Elevación de Terreno preciso (2 m de resolución, precisión de 70 cm). Este modelo muestra el gran impacto del rio en la dinámica del deslizamiento. En particular, el análisis del modelo de elevación muestra que el movimiento tiene como origen el desvío del rio Colca por el deslizamiento de Lari al frente de Maca. Usando la correlación óptica de las imágenes satelitales, se calculó el campo de velocidad de deslizamiento para la zona de Maca, este campo de velocidades, permitió delimitar con precisión la zona de deslizamiento rapido (velocidad del movimiento >50 cm/año), que cubre una superficie de 0.6 km². Finalmente, el monitoreo GPS muestra el control principal de la lluvia en el movimiento. Este monitoreo demuestra que el movimiento de Maca es una zona muy interesante para estudiar los mecanismos de deformación de los deslizamientos asociados a la lluvia y los sismos

Succesive desetabilization of a dome complex constructed on an extinct, hydrothermally altered volcano: The Tutupaca Volcano case study (Southern Perú)

The Tutupaca volcanic complex (17°01' S, 70°21' W) is located to the south of Peru, and belongs to the Central volcanic Zone of the Andes. Tutupaca is composed of an old, hydrothermally altered and highly eroded basal edifice, and two younger twin peaks, located to the northern part of the complex (the Western and Eastern Tutupaca; Samaniego et al., 2015). The youngest Eastern edifice of Tutupaca is composed by at least 7 coalescing lava domes (named Dome I to VII by Manrique, 2013) and its associated deposits, among which are block-and-ash flow and debris avalanche deposits. We identifiedtwo debris avalanche deposits associated with this edifice. An older deposit (Azufre debris avalanche) was channelized in the valleys located to the E and SE of the volcano, reaching up to 3.5 km from its source region. This DAD occurred soon after the emplacement of the first Eastern Tutupaca domes (I, II,III) and its age was recently estimated by exposure dating at 6-8 ka BP. The younger deposit (Paipatja debris avalanche) outcrops immediately to the NE of the amphitheater and was associated with a large PDC deposits that was radiocarbon dated at 218 ± 14 a BP (Samaniego et al., 2015; Valderrama et al., 2016). Both debris avalanche deposits have two different sub-units: (1) the main subunit, hereafter called hydrothermal-altered blocks-rich debris avalanche deposit (HA-DAD) that is a whitish-yellow volcanic breccia with heterolithological and heterometric blocks, and (2) dome-rich debris avalanche (DR-DAD) sub unit, composed by non-altered dome blocks. In proximal areas, the DR-DAD overlaps the HA-DAD; whereas, in distal areas, these two units are mixed forming a hummocky and/or ridged topografphy. In addition to the similar facies of these DAD, we propose that the triggering mechanism for these debris avalanches was similar in both cases. The ascent of a dacitic magma, coupled with the fact that the Tutupaca dome complex was constructed on top of an older, hydrothermally-altered volcanic edifice, induced the destabilisation of the edifices, producing the debris avalanche and its related pyroclastic density currents

Active tectonics around the Cusco City, Perú: Record of earthquakes in the last 14,000 years, from paleoseismological data

The city of Cusco in Peru was hit by several strong earthquakes in historical times (Silgado, 1978), but the seismogenic source of these earthquakes are yet unknown. Cusco is surrounded by geological faults with evidence of Quaternary tectonic activity (Sébrier et al., 1985; Cabrera, 1988; Mercier et al., 1992; Benavente et al., 2013), but there is not enough data to establish a history of fault reactivations and the seismogenic potential of each one these structures. The Tambomachay Fault is the closest to the city of Cusco (~4 km). The NW sector has a well-preserved morphology. In this sector, we observe 14,000-year-old lateral moraines dated from cosmogenic nuclide 10Be, that show normal type fault displacements. This make this sector a prime target for paleoseismology studies. Our paleoseismological results with the ages obtained from dating C14, suggest that the Tambomachay Fault generated at least four seismic events, with surface ruptures, in the last 14,000 years. Using empirical relationships, we conclude that the Tambomachay Fault is capable of generating earthquakes with magnitudes greater than 6.7Mw, putting at high risk the inhabitants of the city and the archaeological remains declared as world heritage by the UNESCO. In addition, the last seismic event, dated between 856-988 cal AD, coincides with the abandonment of the citadel of Pikillacta by the Wari culture (McEwan, 2015), pre-Inca culture located to the south of the city of Cusco and adjacent to Tambomachay Fault

Datación de depósitos coluviales utilizando nucleídos cosmogénicos (10Be) - Cronología del deslizamiento Aricota

Los nucleídos cosmogénicos producidos in situ (10Be, 3He, 26Al) son ampliamente utilizados para fechar eventos geomorfológicos que ocurrieron los últimos millones de años (e.g. Gosse y Phillips, 2001). La aplicación más común de este método es el de fechar superficies glaciales expuestas (Bromley et al., 2009). Los nucleídos cosmogénicos también se utilizan para fechar escarpes de fallas e identificar sus reactivaciones (Benavente et al., in prep), eventos volcánicos (Samaniego et al., 2016) y el origen y cronología de grandes deslizamientos al datar sus depósitos o escarpes (Zerathe et al., 2014). Los nucleídos cosmogénicos permiten determinar la edad de estos eventos y su recurrencia. Asimismo nos permite proponer una cronología y discutir el origen o posible detonante (sismotectónico y/o climático). A comparación de otros métodos, los nucleídos cosmogénicos tienen una clara ventaja, porque la datación se realiza directamente en la superficie de rocas que quedaron expuestas luego de ocurrido el evento. La alta topografía de los Andes Centrales, la geodinámica regional y sus abruptas pendientes permiten el desarrollo de grandes deslizamientos. Uno de los ejemplos más importantes es el de Aricota, ubicado en la Cordillera Occidental del sur del Perú, cuya masa deslizada represo el río Curibaya, formando una laguna de 6 km de largo (Figura 1). Sin embargo, no se conoce el origen, detonante ni el tiempo en que se generó. Por consiguiente, los objetivos de este estudio, fueron: (1) cartografiar la estructura de la masa deslizada en base a datos de campo e imágenes pleiades (0.5 m de resolución); (2) Proponer una cronología de los diferentes eventos utilizando dataciones con 10Be. Geológicamente, en la zona de estudio afloran rocas volcánicas de la formación Samanape, rocas volcánicas de la formación Tarata, tobas de la formación Huilacollo, ignimbritas de la formación Huaylillas, depósitos cuaternarios y cuerpos intrusivos cretáceos del Batolito de la Costa. Asimismo, se evidencian fallas con orientación noroeste-sureste

Toward the feldspar alternative for cosmogenic 10Be applications in mafic environment

In situ-produced 10Be is one of the most commonly used TCN in quantitative geomorphology due to the fact that its production rate is relatively well constrained in the ubiquitous quartz mineral whose integrity minimizes the possibility of contamination by meteoric 10Be. Easily decontaminated from meteoric 10Be, it is in addition reliably measured using the Accelerator Mass Spectrometry technique for which its detection limit is lower than 104 at.g-1. However, volcanic or mafic areas are generally quartz free, which hamper the routine use of 10Be. In the case of a quartz poor lithology, an alternative possibility is to rely on 10Be - feldspars. Two preliminary studies (Kober et al., 2005 and Blard et al., 2013a) already provided promising results, demonstrating that (1) the decontamination protocol classically applied to quartz (Brown et al., 1991) efficiently removes all the meteoric 10Be contamination from the feldspar grains and (2) the total production rate of 10Be in feldspar is 8 to 10 % lower than that in quartz. However, only two samples were analyzed in both studies. In order to better constrain the 10Be in situ-production rate within feldspars, the number of samples analyzed needs to be increased. In this study, we developed a new chemical protocol for the 10Be extraction from feldspar matrices, and to cross-calibrate the total 10Be in situ-production rate in feldspar (P10fsp) against the total 3He production rate in pyroxene (P3px). The cosmogenic 3He and 10Be concentrations were measured, respectively, in pyroxene and feldspar extracted from eight samples of ignimbrite boulders from a giant landslide located between 800 and 2500 m in the high central Andes of Southern Peru. This area is ideally located, since two studies have already determined the local total 3He production rate in pyroxene on the nearby Altiplano (Blard et al., 2013b; Delunel et al., 2016)