Involvement of Geological and Social Conditions in Complex Mass Movements. The Quilloturo Flood in Ecuador (2024)

1. Introduction

The impact of mass movements in the Andean mountain ranges and South America is determined not only by geological conditions and physical factors, but also by the interaction with social factors that shape disasters [1,2,3]. This comprehensive approach directly influences the vulnerability, response, and adaptation capacity of exposed communities [4]. The magnitude of the disaster, therefore, is explained by pre-existing conditions of social vulnerability, rather than solely by the intensity of the physical phenomenon [1]. This is influenced by population density, land use, a lack of knowledge about the territory, and institutional capacity for risk management [5,6]. In this context, a comprehensive landslide analysis requires a deep understanding of lithology, geological structures, geomechanical and hydrogeological conditions, and precipitation patterns. These parameters, along with socioeconomic conditions (including occupation zones and the level of social preparedness for hazards), shape the risk to which a community is exposed [2,5,7].

Mass movements pose a constant threat in countries with rugged geomorphology and topography, especially when the geological conditions of the terrain can exacerbate these processes. In the case of Ecuador, the interaction of natural and social factors complicates the assessment of these events and risk management [1,9]. Mass movements are gravitational displacements of the terrain (soil and/or rock) in areas with steep slopes, which can include landslides, flows, and rockfalls, constituting one of the most frequent natural hazards and one with the most significant social and economic impact in the Andean region [2,9,10,11]. The assessment of ground stability must consider geotechnical, geological, and geomorphological parameters (lithology, slope, geological structures), taking into account changes in infiltration due to precipitation. Additionally, land use and the influence of other triggering factors, such as seismicity, must also be considered [8,12,13].

The trigger for mass movements is frequently linked to rainfall (abnormal or torrential and seasonal), which is often intensified in relation to climate change or periodically occurring phenomena, such as the El Niño and La Niña phenomena in South America [14,15,16]. These physical factors are critical when the social dimension increases the vulnerability of exposed communities. The community's perception of risk is a fundamental point to be considered in the entire process, and their social conditions and preparedness play a role in the adoption of mitigation measures to address these events [5,6,7]. Elements such as lack of planning, population density, socioeconomic level, land use, lack of knowledge of geological activity, and institutional capacity in risk management increase the severity of the associated consequences [2,3].

This research uses the event that occurred in Quilloturo (Ecuador) in June 2024 as a case study, which exemplifies a complex mass movement event that occurred in a low-prepared area. Extraordinary or extreme rainfall in the days and hours leading up to the event not only caused slope instability but, combined with the social and human factors present in the area, resulted in a disaster with casualties and material losses [17]. This phenomenon involved a combination of geodynamic processes, including landslides, debris flows, rockfalls, displacements, and flash flooding. That, suddenly, impacted the natural environment and local populations, ultimately affecting social conditions [17]. This highlights the need to integrate interdisciplinary approaches in risk analysis and management [8].

In Ecuador, studies conducted in the Portoviejo (Manabí province) and Cuenca (Azuay province) areas show that the underestimation of risk, often due to a lack of awareness, contributes to the exposure of population settlements in potentially unstable areas. This is exacerbated by deficiencies in institutional communication and a lack of environmental education, which translates into a lack of understanding of mass movement processes [5,6].

The article aims to analyze the interrelationship between the physical and social conditions that contributed to the magnification of a complex mass movement. It is based on the application of a methodological framework that integrates geological, geotechnical, geomorphological, and social analyses. The goal is to obtain relevant technical evidence that can be incorporated into the formulation of municipal and local risk management policies, articulating the physical dimension of the hazard with social knowledge and vulnerabilities. This will help mitigate or reduce exposure and improve community resilience to future events [2,3,8].

2. Geographical Location and Geological Characteristics

2.1. Geographical Location and Geomorphology

The study area is located between the hamlets of La Merced and Quilloturo and is part of the rural parish of Río Verde (Baños de Agua Santa canton, Tungurahua province) in Ecuador (Figure 1). It is crossed by the Baños–Puyo road (E30), which is part of the state road network and constitutes one of the most important communication routes in the country, as it connects the Coast and the Central Cordillera with the Amazon.

The settlements are located in narrow, flat areas, measuring less than 300 meters in width and elongated in shape. They are bordered to the north by State E30 Road and to the south by ravines formed by intense erosion of the edges of the lava flow formations from the Tungurahua volcano (located further south in the study area). These settlements, known as hamlets, are located from west to east as La Merced, San Jorge, El Placer, and Quilloturo (Figure 1).

Geographically, it is located in the heart of the Cordillera Real of the Andes. Its area of influence is characterized by extremely steep terrain, with slopes exceeding 30°, corresponding to a fluvial valley with steep V-shaped vertical cuts. This valley is drained by the Pastaza River, which crosses the area from west to east, and several high-altitude (high-energy) rivers also drain into it. On the left bank, the Blanco and Verde rivers converge, along with the San Jorge, La Merced, and San Pedro streams, and other unnamed streams, such as those that run through the hillsides near the hamlets of El Placer and Quilloturo. The Chinchin Chico, Chinchin Grande, and San Pedro rivers are tributaries of the Pastaza River on its right bank [17].

The main characteristic of all the tributaries is the turbulent flow of their waters, due to the steep slopes, which gives them high energy, high erosive power, and transport capacity. These aspects enable the transport and/or erosion of supermetric rock blocks. Climate is characterized by heavy rainfall episodes (in intensity and duration) during the winter season from June to August [17].

2.2. Geological Knowledge

The Cordillera Real of the Andes, where the study area is located, is a mountain range composed mainly of metamorphic rocks arranged in north-south trending bands and covered, in some places, by rocks and surface deposits of volcanic materials and a sedimentary origin (Figure 2). Metamorphic rocks include graphite-garnet-muscovite and paragneiss schists (Agoyán Unit), gneissic granite (Tres Lagunas Unit), graphitic schist and gneiss (Cuyuja Unit), and marble and metasediments (Cerro Hermoso Unit). In addition, intrusive igneous rocks, such as granodiorites and deformed diorites of the Azafrán Unit, have been mapped. West of the town of Río Verde, a small calcic-magnetic skarn outcrops as a result of hydrothermal alteration associated with metamorphism, known as the Río Verde skarn [19].

Structurally, the area is traversed by regional faults trending NNE-SSW, including the main ones known as the Baños and Llanganates faults. The former is a right-lateral strike-slip fault, while the latter is a reverse fault with the hanging wall moving ESE [19]. Other important structures include faults that form the tectonic contacts between the Cerro Hermoso, Cuyuja, and Azafrán units, classified as reverse faults [19](Figure 2).

All the metamorphic rocks that outcrop on the left bank of the Pastaza River, from La Merced to the confluence of the Patate and Chambo Rivers, as well as those that outcrop on the right bank, from the Agoyán Dam to La Merced, exhibit very continuous, finely spaced, and in some parts folded schistosity or foliation planes related to metamorphic processes (Figure 3a). This geological structure has an NNE-SSW orientation with planes that are strongly inclined to subhorizontal (Figure 3b).

From the La Merced sector (San Jorge ravine) to Azafrán (3 km east of Río Verde, and already outside the study area), the rocks are traversed by mostly vertical, N-S trending planes (Figure 3a). These planes, called cleavage planes or shear joints, formed during deformation processes [20], are more discrete or not very evident, are spaced at millimeter intervals, and exhibit gentle undulations (Figure 3b). According to Litherland et al. [19], evidence of deformation decreases drastically to the east, where the aforementioned geological feature disappears. This geological structure varies from N-S/vertical to NE-SW, dipping to the SE and NW.

The rocks and surface deposits of volcanic origin are related to the recent activity of the Tungurahua volcano and are large lava flows emplaced along the Pastaza River channel. Sedimentary deposits are related to the erosion and transport of rock fragments and debris of various sizes by water and/or gravity, deposited on the slopes or beds and margins of the rivers and streams that cross the region. Several surface geological processes of an erosive nature also affect the rock massif of the Cordillera Real of the Andes (Figure 4a). Specifically, the La Merced-Quilloturo area (part of the Patate-Baños-Mera region) is characterized by the continuous presence of these erosive processes, which are also associated with mass movements of varying magnitude and type.

Most of the mass movements that have occurred in the area in recent years are small. However, due to the presence of mountainous areas with steep slopes, these processes possess high energy, which is why they form very rapid flows that may be channeled. This characteristic causes these processes to generate strong impacts, even leading to the destruction of mitigation works (Figure 4b).

3. Past and Background of Mass Movements in the Area

The study area is prone to various types of mass movements, including avalanches, debris flows, rockfalls, and combined or complex landslides, which have been identified for some time.

Rock and mud avalanches are the most frequent events that occur along the margins of ravines and in the slope cuts of communication routes, varying in magnitude, composition, and speed. These are material falls that can occur in dry or wet conditions, accompanied by blocks, debris, and soil that are activated in areas of steep slope, depositing in the lowest and most stable areas.

Debris flows, which are typically associated with the winter season (rainy period), occur along natural channels such as ravines and rivers. They can be slow or rapid depending on the saturation of the material and involve the movement of small particles up to medium-sized blocks.

Rockfalls are typically restricted to areas with steep slopes and the presence of a highly fractured and exposed rock mass. The size of the blocks (up to 18 m) varies depending on the fracturing state of the rock mass, the slope of the embankment, and the distance and mode of displacement (Figure 5a). In the case of free fall, the blocks may collide with each other (fragmentation) or come into contact with the slope surface (sliding and flow systems).

Complex or combined mass movements consist of various types of processes described above, which may originate with an initial typology and evolve or change throughout their development, as was the case with the event that occurred in the Quilloturo sector (in 2024).

The collection of information on historical landslides has relied heavily on the verbal testimonies of the EcoMinga Foundation's park ranger team (NGO, Network for the Protection of Threatened Forests, https://ecominga.org/. Accessed on 25 August 2025, EcoMinga hereafter) as one of its most reliable sources. Furthermore, working with the community through focus groups, which included representatives and residents of the affected areas, allowed for the identification of landslides that have impacted the villages since the mid-20th century, as well as establishing the characteristics and parameters used by residents to describe, identify, and characterize them.

According to community memory, approximately 70 or 80 years ago, a flow-type event, locally known as an alluvial flood, occurred in the same area as the Quilloturo event, but on a much larger scale, covering the entire surface of what is now the Quilloturo community and even overflowing the Pastaza River. There are no written records or direct sources to verify whether there were fatalities or destroyed houses. This is an example of community knowledge transmitted orally from one generation to the next, reflecting the experiences and understanding that result from living within a specific territory [21]. Despite the lack of documentary evidence, a possible connection is suggested between this event and the one that destroyed Ecuador's first hydroelectric plant, located in Río Verde, in 1945 [22].

Regarding the occurrence of recent events, documentary evidence is available, including technical reports from the National Risk Management Secretariat (SNGR) and digital press records. For example, the Situation Report – Rainy Season Ecuador No. 25 [23] from the National Risk Management Secretariat (SNGR, the Spanish abbreviation) indicates that on April 28, 2018, several landslides occurred, affecting the Baños-Puyo road (E30). Specifically, the landslide in the El Placer sector swept away a bus, resulting in two deaths and 28 injuries [24] (Figure 5b). According to information from residents, the landslide was caused by a collapse resulting from water damming, which triggered a debris flow from the hillside onto the road.

Another similar and significant event occurred in 2022 following a period of heavy rains that also dammed up in a ravine, generating a mudflow (affecting a house with three people and a heavy vehicle) [26] (Figure 6a,b). The community identifies this event as the Los Chivos mudslide, making it a local reference point for debris flows, their warning signs, and the damage they can cause.

The most recent one, the Quilloturo mudslide, occurred on June 16, 2024, and is one of the most impactful events recorded to date in this area (Figure 5a), serving as the starting point of this investigation and resulting in a total of 14 victims being inventoried.

4. Methodology and Field Data Acquisition

4.1. Geological Field Data Acquisition Methodology

The geological characterization was based on a process of analysis and evaluation to validate the available geological information and publications, including those generated by Litherland et al. [19]. This documentation was complemented by new cartographic and descriptive data on the rocks and, above all, the geological structures of the study area. For the documentation of historical mass movements, data were obtained from different sources and fieldwork for delineation, in addition to verbal testimonies obtained by the support team in the social component (EcoMinga).

The recognition and identification of critical zones that were included in the inventory and diagnosis of mass movements comprised structural mapping associated with the lithologies, with sampling (of soils and rocks) for subsequent laboratory testing. Petrographic analyses (macroscopic and microscopic) were applied to the rocks, and granulometric and triaxial tests were performed on the soils. The results obtained enabled us to complement the existing information on geology, structure, and mass movements gathered in previous phases.

4.2. Social Knowledge and Methodology in the Analysis of Mass Movements

The community information gathering focused on collecting and analyzing collective memory and local perceptions of mass movements that occurred on the slopes between La Merced and Quilloturo.

The methodological approach combined qualitative and participatory techniques, aiming to integrate the community’s empirical knowledge with the technical and geospatial analyses performed. The adoption of participatory methodologies is justified by the need to incorporate the population’s perception of risk and perspective for a comprehensive vulnerability analysis [4].

For the design of the interview and focus group guides (Figure 7a), four thematic axes were defined to guide the data collection:

a.

Temporal and spatial characterization of the hamlets. Composed of an analysis of the history and development of the communities, economic activities, migration processes, and population growth, and finally, the identification of milestones that modified the social and territorial dynamics.

b.

Characterization of mass movements. This involved analyzing the location and names of drainage systems (ravines) according to local memory, describing land uses on hillsides (agriculture, grazing), and the relationship between the intensity and duration of rainfall and the occurrence of events.

c.

Analysis of precipitation and the event of June 16, 2024. This included comparing recorded rainfall with previous years, identifying and describing the mass movements documented by the community, and recognizing natural “premonitory” factors observed by the inhabitants.

d.

Reconstruction of the sequence of events of June 2024. Based on the accounts of the chronology of the events, the reported impacts, and the assessment of the perceived magnitude.

At the same time, semi-structured interviews were conducted with residents of La Merced and El Placer (Figure 7b), focusing on:

• Identifying the characteristics of the hillsides, ravines, and springs through the creation of interactive maps.
• Describing mass movement events, their relationship to rainfall, and their impacts at the community level.
• Documenting historical aspects of the communities, their livelihoods, and social and economic transformations.

Thus, a focus group was established with 25 residents of Quilloturo, where participatory methodologies were employed to recover collective memory. The activities included:

• Group creation and development of interactive maps (Figure 8), where geographical features, ravines, and past events were located.
• b. Plenary session for comparison and validation, which allowed for the exchange of information between groups and the prioritization of events and rainfall using a subjective scale of 1 to 5, where 1 represents a minor event and five a major event, such as the one that occurred on June 16, 2024.
• c. Collective construction of a timeline to identify relevant milestones from the formation of the hamlet to the 2024 event.

All participants were adult residents of the hamlets of El Placer, Quilloturo, and La Merced, who voluntarily consented to participate and provide information for research and scientific dissemination purposes.

The activities were carried out in two field campaigns, as detailed below:

• First campaign: unstructured interviews with two eyewitnesses (Darwin Recalde and Carlos Freire) of the events of June 15 and 16, 2024, complemented by visits to the impact zone in Quilloturo. This activity enabled the reconstruction of the temporal and spatial sequence of rainfall and landslides, as well as their primary impacts.
• Second campaign: semi-structured interviews and the creation of community maps with three residents of El Placer and three of La Merced, focusing on local history, the affected areas, risk perception, and community strategies in response to the events.
• Focus group: conducted in Quilloturo with 25 residents affected by the June 16 (2024) event. Participatory exercises (talking maps, timelines, event prioritization) were used to analyze the perception of threat, rainfall intensity, and factors associated with landslides (Figure 7a,b and Figure 8).

The information obtained through interviews, maps, and the focus group was compared with the results of technical analyses, field observations, and document review. This process enabled the linking of local knowledge with geological, rainfall, and geospatial evidence, providing a comprehensive understanding of the environmental and social dynamics associated with the mass movements that occurred in June 2024.

5. Results

5.1. Local Geology and Implications for Mass Movements

In general, it has been observed that in areas with steeper slopes (vertical to sub-vertical), most of the rock outcrops are fresh and resistant to the unconfined compression test (Smith's hammer). In contrast, in areas with gentler slopes (inclined to sub-horizontal), the rocks are weathered to highly altered, with residual soil development that can reach a thickness of up to 3 m.

The metamorphic basement rocks outcrop on slopes greater than 40°, occupying the study area from the entrance to the El Puyal sector (right bank of the La Merced ravine) to approximately 200 meters west of the San Jorge River. They also outcrop on the slopes of the La Merced-Puyal, La Merced-El Corazón, and Río Verde-Llanganates roads (the latter outside the study area), as well as on some riverbanks and streambeds, such as those of La Merced and San Jorge. The sequence consists of quartz-feldspathic schist, quartz-biotite schist with calcite, chlorite schist, and muscovite-biotite schist. Litherland et al. [19] also report the presence of gneisses.

Intrusive igneous rocks outcrop in the eastern part of the study area, along the Baños-Puyo road (E30), from the San Jorge River to the tunnel between Quilloturo and Río Verde. Outcrops are also observed in the channels and margins of the Los Chivos and Quilloturo ravines. The outcropping rocks are fresh and composed of minerals with no (or very weak) orientation, containing quartz, slightly altered plagioclase, and biotite. In thin sections, the quartz shows deformation. In the outcrops on the road slope between Quilloturo and the San Jorge ravine, as well as in the drainage channels between Quilloturo and San Jorge, the mineral components are moderately to strongly oriented or aligned, forming bands of sheared rocks parallel to the cleavage and geological faults.

One observable characteristic is that rock alteration is more intense on the northeastern edge of the study area, specifically in the headwaters of the Quilloturo and Los Chivos ravines, which form part of the mountains separating the Pastaza and Río Verde river basins. In some outcrops, the rock mass exhibits strong evidence of stratified or layered sedimentary structures. It is therefore considered to be of volcano-sedimentary or volcaniclastic origin, with clear indications of recrystallization (Figure 9a).

The existence of stratified rocks (in thick layers) of volcaniclastic origin is corroborated by the composition and structures of the tabular blocks (≤ 3 m in diameter) of fine-grained rocks present in the deposit formed during the mass movement that affected the Quilloturo area in 2024 (Figure 5a).

The current surface deposits in the study area include colluvial, colluvial-alluvial, alluvial, and alluvial fan deposits (Figure 9b). The colluvial deposits located in El Puyal or on the slopes north of the La Merced-Quilloturo road are covered by a layer of topsoil up to 1 m thick. Colluvial deposits are found in several areas (located NNW of Quilloturo and El Placer). In general, these deposits consist of meter-sized fragments of deformed granodiorites, metamorphic rocks, and altered rocks (basement rocks) with a sandy to silty matrix and variable thickness (on the order of 3 m). The colluvial deposit in the middle basin of the Quilloturo ravine is part of a remnant of the deposit mobilized during previous events (June 16, 2024) (Figure 9b).

Colluvial-alluvial sediments have been identified in the La Merced and San Pedro ravines, consisting of angular to rounded rock blocks. In the La Merced ravine, the fragments are approximately 4 m in diameter, with smaller or meter-sized blocks predominating. In the San Pedro ravine, however, blocks ≤ 1 m in diameter predominate, while in the Quilloturo ravine, angular, sub-angular, and sub-rounded meter-sized blocks are found (within the riverbed).

The alluvial sediments are confined to the Pastaza River course and adjacent to the current channel. Deposits with rounded clasts (volcanic and metamorphic rocks) up to 1 m in diameter have been identified on the San Pedro - El Placer road, suggesting that the Pastaza riverbed was about 3 meters above the level of the San Pedro - El Placer road (hanging terraces).

Finally, alluvial fans, related to flow-type mass movements, were identified in the Los Chivos and Quilloturo ravines. These deposits consist of fragments of altered granodioritic rocks (≤ 9 m in diameter), with fragments ≤ 3 m predominating, within a silty-sandy matrix.

Regarding geological structures, the rocks are traversed by various discontinuities, such as geological faults, shear cleavage, schistosity foliation, and fractures and joints (Figure 3a,b).

The geological faults outcrop along the road between La Merced and Río Verde. Another structure is exposed north of Río Verde. In general, these structures comprise zones of varying thickness (ranging from a few centimeters to several decimeters) where the rocks exhibit intense ductile deformation. The rocks tend to be mylonite. The strike and dip of these structures vary from N-S to NE-SW and from vertical to moderately inclined. The fault in the San Pedro ravine is a reverse fault with a dextral horizontal component. The fault plane strikes NE-SW, with the hanging wall moving eastward. In this outcrop, a fault plane with striations dipping 20° to the south occurs, indicating movement with both horizontal and vertical components.

Another important feature is the presence of shear-cleavage planes. These structures primarily affect the intrusive igneous rocks classified as deformed diorites and granodiorites, forming bands or belts with closely spaced, nearly parallel, and highly undulating planes (Figure 10a). The shear zones constitute complex systems of hundreds of highly continuous planes trending N-S and NE-SW. The dip of the planes varies from vertical to moderately inclined.

The intersection between shear belts and transverse structures, primarily joints orthogonal to the shear planes, enables the formation of the granodiorite or deformed diorite megablocks (Figure 10b) that form part of the colluvial or alluvial deposits on the slopes or valleys of the Río Verde-Quilloturo-Río San Jorge area.

Foliation is another important structure restricted to the westernmost part of the study area, which is composed of metamorphic rocks. The strike and dip of the schistosity planes are highly variable, although most planes trend north-south. These planes are steeply inclined (with dips greater than 70°).

Joints are another structure that affects all the rocks, with joints perpendicular to the shear planes predominating. The joints are spaced approximately 50 cm apart from each other. The strike of the main set tends to be ENE-WSW, dipping sharply to the NNW (planes with inclinations greater than 45°), that is, on a counter-slope.

5.2. Field Verification and Inventory of Mass Movements

An important task of the fieldwork has been the execution of an inventory of mass movements. Figure 11 graphically represents the location of mass movements observed through aerial photography and verified in the field through direct observation. These analyzed landslides are listed in Table 1, which shows the event code (Code Type of slide), the area it occupies, and its dimensions, direction, and inclination parameters. The status in which each event was cataloged is also indicated [10,11].

5.3. Field Distribution of Fallen Boulders and Typology

Rockfalls occur in areas with steep slopes, where the rock mass exhibits a high degree of fracturing. The detached material accumulates at the base of the slopes, and the size of the boulders depends on the degree of fracturing, the slope's inclination, the distance traveled, and the type of displacement, which can occur through free fall or by bouncing off the surface.

These processes are frequent along the cuts of the main road between La Merced and Río Verde, where boulders of different sizes are observed, with diameters reaching up to 9 meters (Figure 12). During surveys conducted along State E30 Road, metric boulders were identified on the road's margins, with diverse origins: some resulting from recent falls and others derived from material transported by landslides or flows. Their presence at the site is mainly due to the difficulty of removing them with machinery during cleaning operations. These blocks correspond to local lithologies and tend to concentrate in areas naturally prone to their formation and accumulation.

A random inventory of boulders located along or in the immediate vicinity of the road was conducted (Figure 13). A total of 85 boulders were recorded, with maximum diameters of 9 m and minimum diameters of 0.6 m (see the Figure 5a and Figure 12). The identified shapes included spherical, roller, disc, and foliated, with roller and disc morphologies predominating, primarily associated with deformed metamorphic and intrusive rocks. In contrast, the spherical shapes, more frequent in the El Placer and Quilloturo sectors, correspond to granodiorite-type intrusive rocks and volcano sedimentary rocks. Only 4% of the clasts exhibited a foliated morphology.

Regarding roundness, most clasts are subangular to subrounded, while only 7% exhibit angular shapes, primarily associated with recent falls.

According to the distribution observed in Figure 13, the largest blocks are concentrated in the hamlets of La Merced, San Jorge, and Quilloturo, with the latter showing the greatest accumulation. These blocks are associated with the June 16 (2024) event and are located mainly in the central-western part of the deposit.

5.4. Complex Landslides Characterization

Complex landslides are mass movements that cannot be classified into a single type, as they combine several landslide characteristics, such as rotational, translational, rockfall, and flow, among others, or they begin as one type and transform into another as they progress. These processes can also involve a combination of materials and exhibit varied triggering mechanisms, as was the case with the event that occurred in the Quilloturo sector in 2024.

According to field observations from the point located on the right bank of the Quilloturo ravine (UTM coordinates: 799,383E - 9,844,925S), it has been deduced that the mass movement process began as a sequence of several small debris avalanches on both banks of the ravine. These avalanche events were triggered by heavy rainfall and culminated in a larger debris flow that followed the ravine's course. According to evidence and accounts from residents, the main debris flow gained speed and energy due to the steep slope of the ravine. Upon reaching the bottom of the ravine and colliding with the right bank, this already destructive flow incorporated the colluvial deposits along its path, splitting into two parts: one flowing south and southwest of the ravine, and the other along the natural channel. Both debris flows reached the E30 Road and the houses located on the former alluvial fan of this ravine, causing loss of life and property damage.

In a parallel or slightly subsequent process, and according to accounts from residents, boulders dislodged by erosion from the flow crossed the right bank of the ravine. As they traveled downstream, their large size caused them to collide with each other, breaking them into smaller blocks and clasts. Some of this material, due to the reduced water level and the collisions, was propelled through the air, similar to projectiles, toward the town of Quilloturo, exceeding the height of the debris flow. This process struck structures, homes, and even people, causing some deaths. Verbal reports from residents indicated that these ballistic clasts left marks on a column of a structure (UTM coordinates 799,573E, 9,844,612S), as well as on broken roofs and windows. Also observed were remnants of centimeter-sized rock clasts on the roads and on the roof of some houses located in the southern part of the town of Quilloturo.

Figure 14 shows the surface area of each of the processes described above. As can be seen, the left side of the map represents the deposition zone of the flow from the right bank of the ravine (alluvium with boulders area), while on the right side, the flow was more like a densified flow (fewer or smaller boulders). The ejected boulders (ballistic area), as can be seen, reached the populated area almost simultaneously with the flows.

The displaced and accumulated material in the deposition zone consists of deformed and altered blocks of diorite, silicified sedimentary rocks of varying diameters (up to 9 m), trees, trash, and mud.

The total travel distance of the event in the Quilloturo sector, from the starting point (where several avalanches occurred), considering the different slope breaks with gradients between 60° and 70°, to the base of the deposit, is approximately 900 m.

Based on the data obtained in the field and plotted on the corresponding map, the deposit is estimated to have an approximate area of 39000 m² and a thickness of 2.5 m, which allows us to establish a volume of 97500 m³, classifying it as a medium-magnitude event [10,11].

5.5. Social Perception of the Landslide Process

The information presented in this document is based on primary sources, obtained from the accounts of residents in the three communities analyzed. This survey allowed us to recognize differences in the origin and development of each of them.

According to the collected testimonies, the community of La Merced began as a small settlement with a school. With the construction of the Baños de Agua Santa–Puyo road in the 1970s, several families relocated to the area and built new homes, thereby boosting its growth. In the early 2000s, the expansion of basic services consolidated the settlement. The construction and expansion of the road, along with the San Francisco Hydroelectric Plant, were key factors in its development and integration with neighboring communities.

In the case of the El Placer community, it originated between the 1930s and 1940s on large tracts of swampy land owned by a few proprietors, which contained a small number of houses. Its development was consolidated around 1970 with the construction of a public-school serving children from El Placer, Quilloturo, and the former hamlet of Nueva Libertad. Subsequent growth was rapid and unplanned, resulting in a dense distribution of houses. It currently has more than 600 inhabitants, whose number has increased mainly due to family ties among the original settlers, and the gradual implementation of basic services has facilitated its consolidation.

The Quilloturo community consolidated starting in the 1960s with the establishment of the first houses on large tracts of land, initially owned by a few proprietors. Its growth was linked to the development of El Placer, and from 2015 to 2016 onward, the expansion of basic services further contributed to its consolidation. It was initially considered part of El Placer, and since 2019, it has been recognized as an independent hamlet.

5.5.1. Community Perception and Classification of Historical Mass Movements and Associated Precipitation

Considering the historical mass movements identified in the study area and described in the previous sections, the community was asked to categorize the events by size, using a scale from 1 (small) to 5 (large). The Quilloturo mudslide of June 16, 2024, was used as the largest reference point and classified as a 5.

Based on these guidelines, the community assigned a value of 3 to the Los Chivos ravine mudslide (2022) and a value of 1 to the 2018 mudflow (Figure 2). Furthermore, it was proposed to categorize the level of precipitation associated with the identified events using a scale from 1 (light rain or drizzle) to 5 (heavy rain due to duration and intensity), with level 5 corresponding to the Quilloturo mudslide (2024) as a reference.

Based on this scale, the community characterized seasonal rainfall as follows:

• Level 1: very light rainfall, no flows related to.
• Level 2: light rainfall, related to the 2018 flow.
• Level 3: moderate rainfall linked to the Los Chivos ravine event (2022), of shorter duration and lower intensity, with peaks between 08:00–11:00 and 13:00–14:00.
• Level 4: normal winter rainfall, prolonged from night until midday, typical of June, July, and August.
• Level 5: heavy rain, similar or associated with the Quilloturo event (2024), beginning on June 15 around 8:00 PM and reaching maximum intensity between 2:00 AM and 6:00 AM the following day.

According to testimonies, small flows or mudslides were recorded every winter in the ravines of Quilloturo and Los Chivos, with maximum sizes of level 2.

5.5.2. Community-Based Early Warning Indicators for Mass Movements

During participatory work with communities, several empirical indicators were identified that residents recognize as precursor signs of a possible mass movement, specifically those locally known as mudslides. These factors, derived from local observation and direct experience, should be considered key elements in individual and collective decision-making for timely evacuation.

Among the leading indicators identified were:

• Sudden drying of the ravine: in the Los Chivos ravine mudslide (2022), the flow stopped a day before the event, and in the Quilloturo ravine mudslide (2024), the water stopped flowing a few minutes before the mudslide.
• Increased sediment content: the water flows down like a stream, with a greater amount of mud and fine materials.
• Increased stream flow: visibly larger or more turbulent flow.
• Noises or roars in the headwaters: unusual sounds, similar to thunder or rockfalls, recorded days or hours before the event; for example, in Los Chivos (2022), roars were heard a week before, and in Quilloturo (2024) for several minutes before the mudslide.
• Sound of rocks rolling within the stream.
• Persistent rain: moderate-intensity rainfall (levels 2 to 3, according to the local scale) lasting for more than an hour.

6. Discussion

Geological and social factors are conditions that influence complex mass movements, so both must be considered as a single, multidimensional problem in risk assessment. This requires a comprehensive understanding of the factors involved, including both natural and anthropogenic ones. Geological factors, such as lithology, geological structures, material arrangement, slopes, geomechanical properties of the terrain, and hydrogeological conditions, control slope stability [10,11,29]. Social factors can intervene, amplify, and exacerbate these events. These complex mass movements, characterized by the presence of compound failure mechanisms or combined events, are more likely to develop in areas where geological parameters exhibit greater heterogeneity [30].

Mass movements occurring in the Cordillera Real and the Andes, for example, those that occurred in the study area of La Merced and Quilloturo (2024), are typically avalanches and debris flows. However, authors such as Riemer et al. [31] and Correa [32] identified other types of processes in regions of the Ecuadorian Andes where volcanic and volcano-sedimentary materials are not predominant. According to [31], their studies conducted in a section of the Paute River basin, specifically in the towns of Ingapata, La Letra, and San Pablo (Azuay Province), revealed that the metamorphic rocks of the Ecuadorian Andes exhibit evidence of deep-seated mass movements. Correa [32] mentions that the predominant mass wasting phenomena on the Loja-Zamora Road (southern Ecuador), which crosses the southern zone of the metamorphic belt of the Cordillera Real, include rotational and translational landslides, debris flows, and rockfalls. All of them have a unique type of rupture and/or movement.

The origin and composition of deposits formed by mass movements are directly related to the lithology and structural context of the rock masses. Thus, the deposits formed by mass movements between the towns of La Merced and Quilloturo consist mainly of fragments of deformed granodiorite and, to a lesser extent, recrystallized rocks, with diameters of less than or equal to 8 meters, set in a sandy-silty matrix. Meanwhile, along the Loja-Zamora Road, a region composed of metamorphic rocks, the deposits formed by mass movements consist of gneiss fragments in a clayey-silty matrix in the eastern area and soil or residual soil formed by the weathering of phyllites, slates, and schists, as well as quartzite fragments in the western area, which eventually form colluvial deposits [32].

Considering the scale of the study carried out and the new cartographic data provided in the research, it is established that the presence of surface deposits, specifically colluvial type and undifferentiated residual soil and the incorporation of strong vegetation soil, are essential factors to include in any modeling that intends to explain the origin, development and evolution of the complex processes that occur on slopes similar to those of the La Merced-Quilloturo sector, where the fundamental factor is water (precipitation) and social factors.

Social conditions are another necessary element of analysis for determining the risk associated with complex mass movements. Vulnerability factors such as population density, economic level, community preparedness and knowledge of hazards, institutional capacity for response and territorial planning, and land use and urbanization negatively influence the severity of landslide impacts [9,33]. Areas with informal settlements (typical of Ecuador and South America) tend to have greater exposure and less adaptive capacity, which exacerbates the consequences of disasters [34]. Furthermore, changes in land use resulting from urban expansion, deforestation, and infrastructure development often intensify slope instability by altering natural drainage patterns and disrupting the integrity of the slope [35].

The search for better living conditions, such as access to education for their children, led the inhabitants of the present-day hamlets of La Merced, Quilloturo, and El Placer to settle on narrow strips of flat land located at the foot of slopes showing signs of instability. As young communities, they lack experience and knowledge related to the functioning of the territory they have inhabited for less than a century, which has generated a false sense of security. Due to the lack of, or limited and nonexistent, application of policies regulating land use and the proper development of these hamlets, they have become disorganized and highly vulnerable to various types of natural and human-caused hazards.

According to García Acosta [36], the social construction of risk is related to the production and reproduction of conditions that determine the magnitude of the effects of a hazard. Therefore, in the case of the June 2024 disaster in Quilloturo, the natural dynamics of the slopes were not the primary cause of the human and material losses. Instead, it was the decision-making process involved in occupying and developing settlements in areas with a high degree of exposure to mass movements. This situation has been exacerbated in recent years by changes in climate patterns, evidenced by unusual rainfall reported by residents, which has increased the frequency of landslides and, consequently, the number of victims and material damage.

The increasing impacts of mass movements have led to a shift in residents' perceptions of safety in these territories, as well as a growing awareness of the need to adopt measures to mitigate or prevent future disasters. This process has also brought renewed value to the memories of one of the first generations to inhabit the area—approximately 70 years ago—who recounted an event with similar characteristics. However, of greater magnitude than the one that occurred in Quilloturo in 2024. This experience highlights the importance of preserving and transmitting traditional and community knowledge about the behavior of the natural environment, which, based on observation and experience, can serve as early warning and prevention tools (Zuñiga, 2024).

The social research conducted in this study, with the active participation of the community and especially the affected individuals, demonstrated that the collective memory of the inhabitants is a key tool for understanding the processes that have shaped the risk landscape [37]. In the case of Quilloturo, this memory allowed for the identification of the factors and dynamics that make complex mass movements a multidimensional threat, capable of generating mudflows, rock fragment impacts, and displacements of large volumes of material in just a few seconds. These aspects must be considered in the development of hazard maps and in preparedness, prevention, and mitigation measures, such as planning evacuation routes and identifying areas of lower risk.

Therefore, in risk contexts such as those of Quilloturo, El Placer, and La Merced, management must integrate both technical criteria and the perceptions and active participation of the communities, promoting their empowerment and training in self-protection [35]. As in other Latin American communities, the experience of the inhabitants of Quilloturo demonstrates that the community's memory is a dynamic and adaptive knowledge, which is nourished by the events of the territory and has the capacity to learn and transform itself [38].

7. Conclusions

On June 15, 2024, a complex mass movement affected the community of Quilloturo, resulting in significant human and material losses. The phenomenon evolved from a series of avalanches to a debris flow, and due to the drainage and channel configuration, it generated rock fragmentation that acted as projectiles, impacting the inhabited area. The material from the mass movement came from various sources, primarily from a colluvial deposit located at the top of the slope north of Quilloturo.

Unlike other recent disasters in Ecuador associated with mass movements, such as the Alausí landslide (2023) [39] and the La Gasca debris flows (2022 and 2024) [40], the Quilloturo event did not present any anthropogenic factors as triggers or contributing factors. In this case, geological factors defined its dynamics, highlighting the incorporation of metric blocks as one of the most determining elements in its energy and impact.

Torrential rains are the primary trigger for landslides in Latin America [41], a situation exacerbated by extreme weather events associated with climate change, which alter rainfall patterns by reducing their duration but increasing their intensity. In the case of the Quilloturo landslide, it was determined that it was triggered by anomalous rainfall for the winter season, characterized by its high intensity over a period of less than 12 hours, according to community testimonies. This precipitation pattern simultaneously affected several provinces in central Ecuador (Chimborazo, Tungurahua, Napo, and Pastaza), generating floods, fluvial erosion, and multiple landslides.

According to Fernández [42], one of the main challenges of housing development in rural areas is the high degree of informality, which results in precarious housing located in areas vulnerable to natural hazards. In Quilloturo, land occupation lacks planning and is driven by economic opportunities stemming from strategic infrastructure such as the E30 Road and hydroelectric plants. These dynamics persist thanks to the area's scenic and natural value, which fosters community-based enterprises through its tourist attractions. However, given the lack of control over the growth of the settlements and the resistance of their inhabitants to relocation, it becomes essential to promote community-based preparedness and prevention initiatives that strengthen local resilience. It is worth noting that the presence of metric blocks limits the implementation of structural mitigation measures, such as slope stabilization.

Finally, the design of local risk management plans must consider that these are recent communities, less than a century old, that lack accumulated knowledge about the behavior of their territory. This situation is exacerbated by the limited value placed on experiences transmitted orally by the founding generations. Therefore, it is essential to integrate community knowledge as early warning indicators and predictive tools for future mass movements, promoting the development of early warning systems based on local observation and collective self-protection measures.