External geodynamic processes, also called exogenous, are represented by processes occurring at the earth's surface, which influence the evolution and modelling of the landscape (Liu et al., 2020; Mas-pla & Bach, 2008; J. Medina, 1991), destroying rock formations raised by internal geodynamics and dragging debris to lower places (Fan et al., 2018; Tarbuck & Lutgens, 2005). These types of events frequently occur in mountainous areas, caused mainly by intense rainfall, cycles of freezing and thawing of glaciers (Oliver-Smith, 2014; Vit Vilímek et al., 2013), by earthquakes (Balzano et al., 2019; Iverson, 2000; Laimer, 2017; Serey et al., 2019), climate change, the occurrence of the El Niño - Southern Oscillation (ENSO) event and glacial activity (Menacho Agama, 2018; Valderrama Murillo et al., 2016; Vít Vilímek et al., 2014). Depending on the atmospheric agent that originates it and the factors that intervene in its development, they can be classified into mass gravitational movements, water flows (flood), snow avalanches, wind, littoral, coastal and glacial erosion (Lario & Bardají, 2017; J. Medina, 1991).
The ENSO is a complex cyclical event that affects atmospheric and oceanic processes in the tropical Pacific region, occurring every 2 to 7 years, whose relations are insufficiently understood (Vit Vilímek et al., 2013). It presents a considerable irregularity in amplitude, duration, evolution and spatial structure (Chen et al., 2019); their effects extend throughout the globe for more than a year (Strahler, 1989). Its occurrence leads to a series of disturbances in the dry climates of the western slope of the Andes. In average years, the center of convection and precipitation is usually over the western Pacific, but during ENSO, the location moves to the eastern Pacific (Chen et al., 2019). This event causes the formation of convective clouds and heavy rainfall over the desert coast (Oliver-Smith, 2014) and the Peruvian maritime yunga, without vegetation and soils with little protection against water erosion, producing extreme events such as floods and mass gravitational movements. In Peru it is known as the occasional absence of the cold Humboldt sea current, a product of the weakening of the South Pacific anticyclone that interrupts the outcrop of deep waters and increases the temperature of surface waters.
Various mass movements occur due to earthquakes, and they can be verified in different places around the world. An example is the case of Quetta and its surroundings (Pakistan), that during the 7.7 ° earthquake on the Richter scale, which occurred on May 31st, 1935, it triggered various external geodynamic events, causing the death of more than 30,000 people; and likewise, what happened after the Kashmir earthquake in October 2005 triggered 1,293 mass movements in 174 locations in Balakot, Pakistan with at least 100,000 dead and significant material losses (Khattak et al., 2010; Rahman et al., 2014). Large earthquakes such as Chi-Chi and Wenchuan generated a variety of mass landslides with a deposit of materials in the called "glacial lake outburst flood - GLOF" (Emmer, 2017; Huang & Li, 2014; Lin et al., 2007); ten of thousands of landslides were triggered by the Gorkha and Dolakha earthquakes in Nepal (Kargel et al., 2016; Martha et al., 2017).
In regions such as the Himalayas, mass movements are mainly caused by earthquakes and heavy rainfall, the combination of which accelerates the frequency of such events (Rahman et al., 2014). They even increase the risk of overflowing glacial lakes and the natural formation of temporary dams due to the obstruction of watercourses, which, when collapsing, cause destructive flows (Rahman et al., 2011).
Keefer (1984) proposes a typical maximum distance from the epicentre of the earthquake for the occurrence of mass movements. It can vary according to the earthquake's magnitude, presenting an area of 0 km² for a 4 ° earthquake and approximately 500,000 km² when the earthquake is 9.2 ° on the Richter scale. For example, in Peru on May 31st, 1970, there was an earthquake of magnitude 7.9 º; its epicentre was 44 km southwest of Chimbote. However, the most significant damage occurred in the Santa River valley, causing the death of 10,000 people in Huaraz city and an avalanche due to the breaking of a block of glacier ice that buried Yungay city (Emmer, 2017) and caused the death of 22,000 people (Castaños & Lomnitz, 2012).
The Peruvian Andes are vulnerable to erosive processes due to: their high relief, steep slopes, shallow soil depth, geological instability and climate variability, which intensified and amplified the effects they generate in their environment, having significant implications on the health of ecosystems and people's well-being (Frey et al., 2018; Guevara, 2017; Mergili et al., 2015; Oliver-Smith, 2014; Tacconi Stefanelli et al., 2018). Precisely, Ancash department, with highly rugged physiography, has been affected by natural disasters due to orogenic activity, climate change and growing demographic pressure (Vit Vilímek et al., 2013).
According to Bonnot (1984), Huaraz province presents different geological structures, which are predisposed to mass movements, generated mainly by the different climates that characterize each of the five levels of altitude of the study area, geographically distributed on the slope western Cordillera Blanca and both slopes of the Cordillera Negra. Other triggering causes include human intervention (Hegglin & Huggel, 2008), such as modifications in stress conditions due to discharges or overloads, alterations in surface drainage, changes in infiltration processes, decrease in vegetation cover (Casamitjana & Carl, 2019), or the rapid urbanization process (Castaños & Lomnitz, 2012; Chacón et al., 2005; Meléndez de la Cruz, 2018; Suarez, 1998).
In Peru, it is necessary to identify the most relevant factors in mass movement processes in the Andes, which allows improving risk management and reducing the impact on people. For that, this work aims to determine the relationships between the external geodynamic events that have occurred in Huaraz province and the geomorphological complexity of its territory characterized by the different levels of altitude, its physiography, and the types of vegetation cover that protect its surface.
1.1. Study Area
The Huaraz province is part of the Áncash department, located between latitudes 9 ° 20'48 "S - 9 ° 48'3" S; and longitudes 77 ° 15'56 "W - 78 ° 2'31" W (Figure 1), whose territory of 250,697.88 ha is on the Cordillera Negra to the west and the Cordillera Blanca to the east.
It presents a rugged relief, where many geomorphological processes reduce energy imbalances due to differences in elevation existing over small distances (Mergili et al., 2015). It has an altitudinal variation from 525 m a.s.l., at the confluence of the Remate and Sensen Grande streams in the Pampas Grande district, to 6.131 m a.s.l., on the summit of the Rurichinchay mountain, Independencia district. Therefore, Huaraz province is distributed in 5 of the 8 altitude levels or “regions” proposed by Pulgar Vidal (2014), that was described below:
Maritime Yunga Region
It is constituted by the sector of the western slope of the Andes Mountains that extends between 500 and 2,300 m a.s.l. (Pulgar Vidal, 2014). It has the ecoregions of subtropical Coastal Desert above 525 m a.s.l., and Desert Scrub-Dry Forest above 1,000 m a.s.l. (Britto, 2017). It is characterized by being prone to debris flows during summer rainfall.
In this region, the Pariacoto conventional meteorological station (CMS) is at 1,312 m a.s.l., with an average annual temperature of 19.4°C and an average annual rainfall of 115 mm. According to Thornthwaite's classification, it has an arid climate, with zero excess water, corresponding to the second mega-thermal, and a low thermal efficiency concentration in summer (E d B2´ a´). Likewise, the CMS Chacchan, located in the Pariacoto district at an elevation of 2,266 m a.s.l. (near to the border with the Quechua region) shows average annual precipitation values of 225.4 mm, which according to the Köppen classification, would be Desert (BW) with scant rainfall concentrated in December to March.
According to the national vegetation cover map, this area is the cardonal (cacti) with 13,111 ha, riparian forests with 199 ha and shrubby scrub with 23,725 ha.
refers to temperate lands, optimal for human life, rich in agricultural soil (Pulgar Vidal, 2014), included in the Andean zone between 2,300 m a.s.l. and 3,500 m a.s.l. characterized by an undulating relief, being able to present steep sections, and where the inter-Andean valleys of greater fertility are found, such as the Santa River (Britto, 2017), the most densely populated sector of the Andes. This territory has the ecoregions of Mesoandine, Montane Rain Forest, Northwest Montane Rain Forest, Seasonal Dry Forest and Desert Scrub - Dry Forest (Britto, 2017).
There is the Cajamarquilla CMS in La Libertad district at 3,286 m a.s.l., located in the Casma river basin whose data, after being processed, shows an average annual temperature of 12.8 °C and an average annual rainfall of 495 mm. Thornthwaite's classification presents a dry to sub-humid climate, with moderate excess of water in summer, corresponding to the first microthermal, and a low concentration of thermal efficiency in summer (C1 s B1’ a’). As there was no data from a station located in Huaraz province within the Santa River Basin, it was decided to work with the Recuay automatic meteorological station (AMS), which is located close to the study area, at the height of 3,431 m a.s.l., that shows an average annual temperature of 15.1 ° C and an average annual rainfall of 831 mm. It is a sub-humid to a humid climate, with a slight water deficit in any season, corresponding to the second mega-thermal, and a low concentration of thermal efficiency in summer (C2 r B2´ a´). Likewise, the Anta CMS located at an elevation of 2,760 m a.s.l., shows average annual precipitation values of 643 mm, which according to the Köppen classification, corresponds to a Temperate climate with dry winter (Cw). According to the national vegetation cover map, in this sector, there are 30,543 ha of predominantly shrubby scrub on the western slope of the Cordillera Negra, 33 ha of Meso-Andean relict forest, 54 ha of high Andean grassland, 135 ha of forest plantation and 22,673 ha of predominant agricultural plantations in the Santa River basin.
Suni or Jalca Region
refers to the cold highlands, with undulating relief, sometimes steep and rainier than the previous ones (Pulgar Vidal, 2014); it includes the terrain located between 3,500 m a.s.l., and the 4,000 m a.s.l. It is place to the jalca, desert puna and wet-dry puna ecoregions (Britto, 2017). According to the Peruvian vegetation cover map, this area has 7 ha with little and no vegetation, 308 ha of bofedal (high Andean wetlands), 153 ha of high Andean relict forest, 13,587 ha of shrubby scrub, 16,456 ha of Andean grassland, 104 ha of forest plantations and 15,083 ha of agricultural plantations. In areas with arid and cold climates, there is a greater vulnerability to mass movements due to low levels of chemical weathering, low soil and scarce vegetation, presenting rocks exposed by a rapid tectonic uplift (Mergili et al., 2015).
The Pira CMS, located in Pira district in the Casma river basin, at an elevation of 3,720 m a.s.l., shows average annual precipitation values of 686 mm, which according to the Köppen classification, would have a Temperate climate with dry winter (Cw) or cold and dry winter Borealis with periodic rain in summer (Dw).
Puna Region: denomination given to the cold and extensive high Andean plateaus, of tame hills, rugged yellowish grasslands, deserted and wild, rich in pastures, habitat of wild South American camelids (Pulgar Vidal, 2014), it comprises the territory between 4,000 m a.s.l., and the 4,800 m a.s.l. According to Pulgar Vidal (2014), in places where the mountain range does not exceed 5,000 m a.s.l. constitutes the top of the summits, with a soft to abrupt topography by sharp slopes and springs of streams, housing the ecoregions of desert puna and wet-dry puna (Britto, 2017), there is the storage of water contained in wetlands and for a significant presence of lagoons. It presents the following plant covers: 14,253 ha with scarce and no vegetation, 861 ha of wetlands, 1,519 ha of high Andean relict forest, 238 ha of shrubby scrub, 71,289 ha of Andean grassland and 13 ha of forest plantation.
As we did not have data from a station in Huaraz province on this altitude level, we decided to use the Milpo CMS, located in the Catac district, Recuay province. It is close to the study area, at an elevation of 4,400 m a.s.l., which shows average annual precipitation of 1,141.3 mm, which according to Koppen's classification, it would be Boreal in cold and dry winter with periodic rain in summer (Dw).
Janca or Cordillera Region: It includes the snow-capped peaks of the Andes, which present a frigid climate with normally solid rainfall (Pulgar Vidal, 2014). It is made up of the entire surface located above 4,800 m a.s.l., where the highest peaks of the Andes are found. Starting at 5,200 m a.s.l. it is covered by glaciers or rocky outcrops devoid of soil and vegetation, denoted by an extremely rugged physiography (Britto, 2017), predominantly glacial and physical erosion that causes avalanches and rock slides (Ingemmet, 1989). In the past, its glaciers formed the moraines that dammed the reservoirs that currently constitute the water reserves, maintaining the provision of streams (by runoff) and aquifers (by infiltration) in the basin. On the other hand, according to the Map of Vegetal Coverage of Peru (2015) in this level of altitude, the following units are presented: high Andean areas with little or no vegetation (8,747 ha), Andean grassland (51 ha) and area covered by glaciers (11,935 ha).
The Huaraz province is part of the Santa River basin that includes the western slope of the Cordillera Blanca and the eastern slope of the Cordillera Negra, as well as the basins of the Casma River, Culebras River, and the Inter-basin 1375959 (denomination established by the National Water Authority) locally called Río Seco, located on the western slope of the Cordillera Negra.
Huaraz is the second most populated province in the department, housing 163,936 inhabitants that represent 15.1% of the population of Ancash (INEI, 2018), who live in 1,264 populated centers. These populated centers are distributed in the 4 hydrographic basins as follows: 593 towns (47%) are in the Santa River basin; the Casma river basin contains 505 populations (40%); the Culebras river basin has 164 localities (13%), and the Rio Seco Inter-basin has only 2 populated centers (0.2%).
Most of its urban areas show a high risk of natural disasters of glacial origin (Ribeiro de Figueiredo, 2017). The best example is Huaraz city, which concentrates most of the province's urban population (Gobierno Regional de Áncash, 2014). It is a risk because the Quillcay River crosses it. According to Valderrama et al. (2013), it is located on an alluvial fan, where frequently happen floods, such as the 1941 catastrophe (Véliz, 1974), produced by snow avalanches or landslides that occur at the headwaters of the glacial valleys of the rivers that descend from the Cordillera Blanca such as Cojup, Llaca, Quilcayhuanca and Shallap (Indeci, 2003a).
Ferro (2009) indicates that after the 1941 disaster, Huaraz city reoriented its expansion towards the south with rapid urbanization and the appearance of human settlements such as Villón Bajo, Bellavista, Nueva Florida, Shancayan, Patay, Los Olivos, Vista Alegre, Rosas Pampa and Tacllan. According to Indeci (2003b), they present a high vulnerability due to their exposure to intense geological phenomena and limited resilience. Precisely one of the most vulnerable areas is the Río Seco sector, located to the south of the city, where the danger is the probable occurrence of flood due to extraordinary rains since its dejection cone has been occupied by some of the aforementioned human settlements (Silva Lindo et al., 2017).
According to Valderrama et al. (2013), the lithology of the province shows that it is mainly made up of volcanic, intrusive and sedimentary rocks, where the soil and expansion areas of the city of Huaraz are mainly made up of intrusive rocks -granites and granodiorites- (Bonnot, 1984), predominant in the Cordillera Blanca batholith. On the other hand, the Cordillera Negra is composed mainly of volcanic rocks -riolites and andesites- (Bonnot, 1984); it presents a less resistant soil and is susceptible to erosion.
Likewise, the Huaraz province presents modern and ancient faults. One of them is the active fault of the Cordillera Blanca, which is formed by a system of normal-type faults, which may be related to earthquakes of great magnitude (Indeci, 2003b).
According to the DesInventar database, a total of 46 seismic movements were registered in the period 1970-2012, in Ancash department, the years with the highest occurrences were: 1970 with 14 records; 1985 with 6 earthquakes; and 1972, 1977, 1987, 1996 with 3 events each. The earthquakes with the highest magnitude were: May 31, 1970 (M = 7.9 °), December 9, 1970 (M = 7.5 °), January 25, 1988 (M = 5.2 °, in Huaraz district) and 18 April 1996 (M = 5.6 °, Independencia district).