Joel Hirales Rochin1
1Geologist Head of Research Projects and professor of the Area of Earth Sciences, Department of Civil Engineering, Technological Institute of La Paz, Boulevard Forjadores of Sudcalifornia. La Paz, Baja California Sur., Mexico. Joelhirales@itlp.edu.mx. ORCID ID: https://orcid.org/0000-0001-8802-0919.
Geology as a tool to identify areas of geological risk is useful to determine the close relationship between the geological space and the sustainable urban development of a city. From this interaction, you can respond to the growing demand for solutions of an environmental and urban nature.
At the national, regional and local level where the study area is located, there is a growing need to create new urban areas, but these are not linked to an adequate analysis of the geological environment and the knowledge of the main factors that control risk conditions and consequently, its impacts have manifested due to different factors.
The methodology to achieve the objectives was based on a characterization of the geological, hydrogeological and geomechanical conditions of one of the main urban settlements of the city of La Paz, capital of the state of Baja California Sur., Mexico and with it, a set of thematic maps using the Analytic Hierarchy Process (AHP) methodology and finally a risk susceptibility map related in local areas to flood events, debris flows, rock falls and landslides.
The results represent a first stage of a larger-scale project and with this it is possible to contribute new knowledge to be used in the most precise zoning of geographic risks, which will allow the state capital a sustainable growth of the population of the city. The improvement of current construction standards and the corresponding zoning to anticipate their development in an orderly manner. Finally, it is considered that this type of zoning of areas susceptible to risks (urban settlement scale) provides an analysis of the risk conditions where the citizen of any city can locate in their own community and know their own conditions of civil protection, all the opposite of most risk studies that offer large-scale results and only offer a broader view.
Key words: Geological risks, sustainable urban development, RS (Remote Sensors), GIS (Geographic Information Systems).
The hazards of natural origin that occur on our planet are always linked to its own geodynamics, which include geological and hydrogeological processes. Therefore, it is very important to know and apply the knowledge we have of it in order to prevent and mitigate its effects.
Geology and its environment make up the geological environment where there is a constant interaction with the human being. This interaction is in a constant dynamic and evolution, which leads to processes that affect both the natural and human environment. The first is formed by geology, biology, edaphology and the second is the anthropic, represented by cities, infrastructure, public works, populations, etc.
Both environments interact constantly and in the case of the study area a planned and sustainable relationship that allows a harmonious relationship between the two has not yet been established.
The search for this “empathy” between the environment and the human being has come to shape disciplines such as Urban Geology, which, in recent years, has gained relevance in Mexico, and is becoming an important development-planning tool urban current. The notions of danger, vulnerability and geological risk can establish a close relationship between changes in the geological environment and urban development [1-2].
Mexico is located in the Ring of Fire, in the tectonic plates of the Pacific, Cocos, North America and the Caribbean. Tectonic activity in this area is high, with an incidence of 80% in seismic and volcanic activity worldwide. In addition, the country is located within four of the six cyclone generating regions in the world, which influence the territory of Tehuantepec, the Eastern Caribbean Sea Region, the Campeche maritime platform and the Eastern Atlantic Region. In total, this geo-climatological location places 17 Mexican entities in areas of vulnerability and danger, where almost half of the Mexican population lives .
Therefore, the geological, hydrogeological and meteorological conditions of the environment that make up the city of La Paz, Baja California Sur, Mexico are determining factors in its urban geology, since these elements play an important role in potential risks (floods and landslides of land) in urban and suburban areas.
At the national, regional and local level where the study area is located, there is a growing need to create new urban areas, but these are not linked to an adequate analysis of the geological environment and knowledge of the main factors that control risk conditions and consequently its impacts have manifested due to different factors, such as:
(1) Global climate change, (2) Inadequate land use, (3) Ignorance of the geological environment and (4) Uncontrolled urban-population growth.
All this, when not being applied, has been reflected in cases of natural disasters that occurred in different populations of Mexico, examples of this are the cases in the populations of Minatitlán, Jalisco [4,5] or Meztitlán, Hidalgo. , Chapala, Jalisco , and Totomoxtla, Puebla  where floods and landslides have occurred with serious damage.
The purpose of this research is the continuity of the zoning of geological hazards at the local level of the southeast part of the city, as this will determine the interaction between the geological environment and the urban environment. In this way an extension of the knowledge that has not been carried out before the current conditions of the settlement and urban growth of the city of La Paz in its southeastern portion would be achieved, when studying these geological risk factors. Therefore, the need for an update of geological-urban knowledge that allows the state capital for sustainable urban-population growth, improvement of current construction regulations and the corresponding zoning to anticipate its development in an orderly manner is visualized.
2.- Study zone
La Paz, capital of the State of Baja California Sur, is located in the southern part of the peninsula of Baja California, Mexico.
The predominant hydrogeological conditions of the southern region of the Baja California peninsula are classified as arid to semi-arid (average temperature of 30.37 ° C) [9,10] and this factor causes a high probability of incidence of tropical storms and hurricanes that develop in the Eastern Pacific, being in the last decades the average annual rainfall of 164.59 mm.
The city of La Paz is characterized by a morphology dominated by basins and mountains, the result of the interaction of magmatic-tectonic processes, which settle in Holocene deposits that correspond to alluvial material (which varies from sand to sandy gravel) of the currents active, where the thickness of the material varies and can reach a few meters in the main streams .Geologically, volcanic and volcanic rocks (sandstone and volcanic conglomerates, rhyolitic tuffs, andesitic lahars and lava flows) are represented by the Comondú Formation with an age between 30 and 12 Ma in the central portion [12-16]. Structurally the valley of La Paz is formed by a graben with north-south orientation, limited to the east by the La Paz fault, located on the slopes of the Las Cruces mountain range, and to the west by El Carrizal fault ., then the geological-structural conditions of the study area place it in an area with permanent microsismicity. The total population of the city of La Paz is 251,871 inhabitants .
The study area is located in the southeast portion of the city of La Paz, located between coordinates UTM 12 R 569000-575000 E and 2664000-2674000N (Figure 1) and an area of 85.96 km2.
Figure 1. – General location map of the study area.
The zoning of the risk consists of the delimitation and characterization of a space and / or area of the physical environment according to certain properties and constitutes one of the pillars of urban planning. The complexity of these interrelationships makes it difficult to estimate threats in an integrated manner and it is considered that the vision of threats independently is more in line with the sustainability scheme that is intended to be given to the planning of the territory and particularly to the zoning of the risk. Thus, an adequate zoning of the risk must contemplate the study of the specific risks that, due to the action of each of the identified natural hazards, occur in an area. From this procedure and with the support of several tools and guide classifications [19-20) it is possible to characterize risk situations (including their dimensions) (Figure2).
Figure 2.- Methodology applied to the study area.
The Hierarchy Process (AHP) method used in this paper is a mathematical method created to evaluate alternatives when several criteria are considered and is based on the principle that the experience and knowledge of the actors are as important as the data used in the process. The AHP development by Saaty (1970) , which consists of matrix analysis and involves value judgments. In this way, the matrix of preference over the selected criteria was generated, obtaining the weighting of the eight chosen variables. It was important the knowledge of the study area, the documentation and local studies generated to date, where the criteria of the specialists are taken up.
3.1. – Estimation and numerical assignment of slip factors.
The determination of the numerical allocation of the landslide risk assessment factor is a numerical system that depends on the relevant factors. By superimposing the elements or parameters that indicate geological and hydrological hazards, a zoning map of susceptibility can be developed, as well as delimiting the possible risk areas, granting each factor a specific weight and value, and analyzing the situations site by site, with the help of the various thematic maps .
The AHP uses comparisons between pairs of elements, building matrices from these comparisons, and using elements of matrix algebra to establish priorities among the elements of a level, with respect to an element of the immediately higher level.
In the study area, specific factors of the physical environment were identified and numerical values were assigned to the factors according to the degree of influence. The different classes within each causal factor were also assigned values according to their influence, to give a more precise assignment of each causal factor and their respective subclasses.
Relevant factors for the zoning mapping of susceptibility to landslide risks include lithology, slope, elevation, use and land cover, urban road density, line density, drainage density and precipitation. The maximum numerical estimate of the sliding risk evolution factor for different categories is determined based on their estimated importance to cause instability. The important factors responsible for the geo-hazards (landslide & floods) area were assigned numerical values (range) on a scale of 1 to 5 in order of importance (Figure 3).
Figure 3.-Table of values assigned to the conditioning factors for areas susceptible to geo-hazards.
This part of the methodology is the basis for giving weight (wi) to each parameter and defining its relative importance to induce landslides (Ri). These weighted factor maps were superimposed using a multivariate criteria analysis to prepare a risk susceptibility map (MSR) for the present study area .
3.3. Mapping risk vulnerability
The risk vulnerability map is based on a zoning of landslide risk (rock removal, debris flow and falling), which was prepared when calculating the landslide potential index and classifying the slip potential index in several lands susceptible to landslide, as low, medium, high and very high.
The slip potential index is defined as:
Slip potential index (SPI):
(SPI) = Σn i = 1 (wi * Ri) (1)
Where wi denotes the weight for the factor i and Ri denotes the classification of the class of the factor i. In this study, the total number of factors (n) is 8, where the class classification varies from 1 to 5.
The landslide model was created and the weighting and classification are assigned to each category. Depending on the issues and their impacts, different areas were delineated. The total estimated zoning of landslide risk (ZLR) was calculated as follows:
Value ZLR = L + SL+ ELE + SL/U-SL + DD+ URD + SFD + RF
Where, Value of ZLR = Sum of ratings of all the causative factors, ZLR = Lithology + Slope+ Elevation + Soil/Use Soil + Drainage Density+ Urban Road Density + Structural Density + Rainfall.
The different thematic layers were reclassified using the Jenks method, which is based on Jenks’ natural break algorithm, thereby minimizing the internal variability of the classes and maximizing the differences between classes. In addition, the Latent Semantic Indexing (ISL- Latent Semantic Indexing) was used to prepare the risk susceptibility map (RSM), whereby all the layers of the map were superimposed and validated using the landslide incidence points collected during fieldwork.
Based on this methodology and through the use of the ArcGIS software, several thematic maps based on and related to the digital elevation model (DME) were generated: General geological map, elevations, slope, aspect, drainage, land / vegetation use, precipitation and as a final product, a susceptibility map.
4.-Results and conclusions
The results and the methodology of this research work was the generation of maps of vulnerability (susceptibility) in geological risks that for the first time are focused on different geological issues of the urban and suburban area, which can be used as a basis for the future urban-sustainable development of the city of La Paz. These areas include urban development zones where streams and stream flooding areas are currently located, as well as hill slopes, with processes susceptible to landslide.
It also highlights the proposed methodology, the possibility of evaluating the conditions of susceptibility at the state and regional level, complementing this work with statistical methods, as has been done by other authors [24-28].
Due to its climate, topography, type of soils and slopes, the Valley of the City of La Paz, BCS, Mexico, have a surface hydrology based on runoffs that drain through small channels (streams) to the Bay of La Paz. Low velocity runoff generates significant areas of flooding in the suburban areas of the NE-SE portion, which extend and cross, sometimes causing severe damage to the population (Figure 4).
Figure 4. Map shows the spatial-vectorial analysis map (buffer) showing areas vulnerability to flood.
The urban geology of the study area is characterized by an isolated sequence of outcropping rocks in its SW portion, which are mainly constituted by volcanic and vulcanosedimentary rocks. This lithology is grouped in the Comondú Formation recognized within the area in which its geomechanical condition and anthropic action stand out. In an annexed way, families of structural data were analyzed where it was possible to distinguish a series of alignments and irregularities in the drainage pattern a series of failures with a normal component that cuts off the top of the lithological sequence. Which have a scale of about 10 m. of length with dominant course as possible failures or fracture.
In the form of diaclases in a system of three main orientations (Figure 5). Considering this, they were grouped into several data sets (Faults N10°E/28°SE, Fracture 1: N20°W/50°SW, Fracture2:S45°E/85°NE and Fracture3: S70°E/85°SE) and were represented on stereographic diagrams and kinematic maps to select the areas of greatest susceptibility.
The lithological conditions and their characterization are related to other factors (hydrogeological, hydro meteorological, and geomechanical) that generate the movements of removal of masses and blocks of rock.
Based on the above conditions, fall processes, landslides and / or rock flows were recognized. Of these two sets, only 6 debris flow and 35 rockfalls (high-temperature volcanic clastic deposits), in the upper and middle parts of the topography, without soil identification in these processes were identified in the area. Both occur in events mainly in falling movements, such as simple translational and block sliding that end up as slow movement flows according to the classification of Dikau (1996) . Being the failure mechanism and detachment mode controlled by geological and geotechnical factors. The volumes of rocky material removed in each removal process range from the order of meters to hundreds of cubic meters and the individual block runs reach 50 tons.
The movements were generated from the combination of geological factors (Slope, lithology and geological structure), hydrogeological (drainage density) and geotechnical (mechanical behavior of materials). The mechanism of failure and mode of detachment were controlled by geological and geotechnical factors.
This aspect of the results coincides with the research carried out by Momeni (2016),  Ahmed (2014),  when affirming that in landslides and fallen rocks, slope and appearance play an elementary role in the flow of material since The slope provides speed and appearance indicate the direction of that slope.
These phenomena coincide in the location of urban settlements of this type that are established at the foot of the existing topography. Therefore, it is considered that it is convenient to carry out more in-depth studies on the aspects that generate these landslide processes.
The distribution of these events were located within the slopes (27° to 40°), which is limited by the main streams: El Cajoncito and Los Bledales. The streams crosses the northeast and southeast portion of the city of La Paz and in turn coincides with some important urban settlements (San Pablo Guelatao, La Escondida, Lazaro Cardenas, Francisco Villa, 20 de Noviembre, Roma, Flores Magon, Costa Azul, La Rinconada, Agua Escondida, El Cardonal) since they are established in the very near margins of the hills and streams (Figure 5). This geohydrology and hydrometeorological condition denotes the poor urban planning and the high risk for its population, being settled in areas susceptible to flooding and landslides (debris flow and rockfalls).
Figure 5.-A, B, C, D, E,) Panoramic showing areas susceptible to flood.
The results obtained based on this geotechnical characterization (Geological Resistance Index (GSI)) turn out to be very homogeneous and close in parameters, since it is a volcanic and vulcanosedimentary lithology with a similar genesis in processes and evolution time.
The geomechanical behavior of the lithology is based on the observations at the outcrops level where the structure of the lithology formed by well-defined rock segments in two to three directions is appreciated, which constitutes normal failure and fractures in diaclases (Figure 6).
The conditions of the discontinuities are very good to good (range 70 to 80) since the surfaces are weatherproof (Figure 6) .
Figure 6.- Estimation of the Geological Resistance Index, GSI, based on a Geological Description of the Rock Massif (Marinos & Hoek, 2000).
The processes of removal of rock masses and blocks of rocks are delimited in the upper and middle part of the topography (100 to 50 meters above sea level) with 90% of events represented only by a single lithology: Riodacite. These events are basically simple block and / or translational landslides, which has a high influence by the factors of structural geology, erosion and gravity as sliding mechanisms.
Figure 7.-Table that shows the distribution of risk susceptibility.
The current research presented a methodological model for the evaluation, zoning of the susceptibility of geo-risks according to the characteristics of the study area.
It also highlights the proposed methodology, the possibility of evaluating the conditions of susceptibility at the state and regional level, complementing this work with statistical methods and the generation of an inventory of landslides (mass and fallen movements), which will allow comparing their results and coherence in the distribution of the occurrence of all present and future landslides, rockfalls or any geological risk.
In addition, the percentage and geographical distribution of risk susceptibility in this Northeast and Southeast portion of the city is clearly observed (Figure 7-8).
The methodological proposal also complements the results obtained with a new map of susceptibility of geological risks (Figure 8), which did not exist until today and that allows defining urban areas of urban development susceptible to these phenomena at urban settlement scale (1: 50,000).
Figure 8. Map showing the vulnerability by urban settlement in the northeast-southeast area.
The author thanks the Technological Institute of La Paz for the logistical support provided.
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