Mass Movement Processes at the Motozintla Basin, Chiapas, Southern Mexico

GEOFÍSICA INTERNACIONAL (2012) 51-2: 169-186 ORIGINAL PAPER Mass movement processes at the Motozintla Basin, Chiapas, Southern Mexico Juan Manuel Sánchez-Núñez, José Luis Macías*, José Juan Zamorano-Orozco, Ricardo Saucedo, José Ramón Torres and David Novelo Received: April 27, 2011; accepted: October 13, 2011; published on line: March 30, 2012 Resumen Abstract Uno de los principales desafíos que presentan las Prediction of prone areas for mass movement zonas montañosas es la identificación y predicción processes (MMP) is one of the major challenges de áreas susceptibles a procesos de remoción in mountainous areas. This is the case of the en masa (PRM). Éste es el caso de la cuenca de Motozintla basin that is located along the Motagua- Motozintla, localizada en el sureste de México, sobre Polochic fault system in the Chiapas State la traza del sistema de fallas Polochic-Motagua en southeastern Mexico. The area is dominated by el estado de Chiapas. En la zona predominan rocas Permian to Tertiary rocks extremely affected del Pérmico y Terciario, severamente afectadas by faults, weathering and intense erosion. por fallas, fracturas y un intenso intemperismo y Motozintla has been seriously disrupted by two erosión. Motozintla ha sido gravemente afectado hydrometeorological phenomena in 1998 and 2005. por dos fenómenos hidrometeorológicos en 1998 In this paper, geological and structural mapping y 2005. Para determinar las zonas peligrosas, (1:30 000), photointerpretation and GIS techniques propensas a sufrir procesos gravitacionales de were applied to determine the hazard areas that remoción de masas en la cuenca, se realizó un may go off by mass movement processes driven mapeo geológico-estructural (1:30,000) mediante by gravity in the basin. The inventory shows 88 fotointerpretación y técnicas de SIG. El inventario mapable mass movement processes ocurred during obtenido mostró 88 procesos cartografiables de the past 25 ky represented by debris avalanches, remoción en masa, representados por avalanchas, rock falls, slides and debris flows. Debris flows are caídas de roca, deslizamientos y flujos de the most dangerous phenomena that may directly escombros. Los procesos más peligrosos para la impact the city of Motozintla posing at risk 23,755 ciudad de Motozintla son los flujos de escombros, people. Future mass movement processes may ya que éstos la han impactado negativamente y happen mainly in the NW part of the basin where ponen en riesgo a 23,755 habitantes. La zona highly altered rocks of the Chiapas massif and noroeste de la cuenca es considerada como la más Todos Santos Formation are exposed. vulnerable a sufrir procesos de remoción en masa, debido a que afloran rocas sumamente alteradas Key words: Mass movement processes, geology, del Macizo de Chiapas y la Formación Todos Santos. Motozintla, Chiapas, Mexico. Palabras clave: Procesos de remoción en masa, geología, Motozintla, Chiapas, México. J. M. Sánchez-Núñez J. J. Zamorano-Orozco Posgrado en Ciencias de la Tierra Instituto de Geografía Instituto de Geofísica Universidad Nacional Autónoma de México Universidad Nacional Autónoma de México Ciudad Universitaria Ciudad Universitaria Delegación Coyoacán, 04510 Delegación Coyoacán, 04510 México D.F., MéxicoTel. 5623 0222 ext. 45462 México D.F., México E-mail: [email protected] CIIEMAD-IPN *Corresponding author: [email protected] R. Saucedo J. L. Macías J. R. Torres Instituto de Geofísica, Campus Morelia Instituto de Geología Universidad Nacional Autónoma de México Universidad Autónoma de San Luis Potosí 58190 Morelia, Michoacán Álvaro Obregón 64, Centro SLP, México D. Novelo Tel. 826 2330 al 39 Instituto de Geofísica E-mail: [email protected] Universidad Nacional Autónoma de México [email protected] Ciudad Universitaria Delegación Coyoacán, 04510 México D.F., México Tel. 5622 41 24 ext. 19 E-mail: [email protected] 169 J. M. Sánchez-Núñez et al. Introduction volume of the displaced material (Lugo-Hubp, 1989; De Pedraza, 1996; Gutiérrez-Elorza, 2008). During 1998 and 2005, two hydrometeorological events, tropical storm Earl in the Atlantic and Slide is a downslope slow movement of soil, Hurricane Stan in the Pacific, respectively, affec- rock or debris driven by gravity along a surface of ted Chiapas in southern Mexico. Both events weakness above an undisturbed substrate. This poured uncommon rainfall that exceeded the material is displaced along a planar, undulating or local annual records causing thousands of land- curved surface of rupture. The slide morphology slides and several floods at the flood plains in consists of an upper scarp or crown commonly Chiapas. As a result, in the state of Chiapas with slump blocks and a toe. The slide may be 2,200 homes were destroyed and at least 40,000 simple, multiple or successive, depending on the people were found homeless. One of the most disintegration of the material, type, and number damaged towns was Motozintla, founded in the of surfaces (Dikau et al., 1996). The slide deposit bottom of V-shape valleys at the confluence of shows transverse and radial cracks indicating the the rivers Xelajú Grande, Allende and La Mina. direction of movement. Based on these features, Motozintla is located between the Chiapas landslides are classified either as rotational or mountainous region and the Pacific coastal plain translational. The rotational slides have circular to that makes it an important trading point in the spoon scarp shapes, and the translational slides State. The 1998 and 2005 hydrometeorological have a nearly flat or low angle substrate terrain. phenomena provoked hundreds of landslides The latter, may occur in rock, debris or soil in in the Motozintla basin from the upper parts coherent or fragmented blocks (Hutchinson, of the basin, transporting loose debris downhill 1988). The common landslide triggers are rain- loading the rivers and eventually flooding the fall, earthquakes, and human activities that are town of Motozintla and other small communities. intensified by discontinuities in the substrate as Afterwards, authorities relocated affected people joint, fracture or fault intersections, bed tilts, in unsafe areas within the same basin due to the and deposits with different properties. lack of territorial planning and poor knowledge of mass wasting processes. Unfortunately, up to this Flows refer to the continuous downslope study the Motozintla basin does not have a good movement of suspended material in water geologic map neither it has a mass movement or fluidized air, these may move quickly or processes inventory. At first glance, the geology slowly, in dry or wet conditions, depending on and geomorphology of the basin suggested that water saturation. Particles in stream flows have the 1998 and 2005 events, were just a small relative motions of the total mass in which example of past larger processes occurred and they are transported. Some flows may contain there was no hint of their relationship with the local significant amounts of fine material that form the substrate, size, recurrence, and anthropogenic matrix. Mass movement develops large internal factors. In order to contribute to the reduction deformation and a great number of shear of hazards posed by hydrometereological events, surfaces (Gutiérrez-Elorza, 2008). we began a systematic geological mapping of the Motozintla and Chimalapa basins with the Complex processes begin as one type of reconnaissance of mass movement processes by movement and transform downslope to a using topographic data sets, aerial photographs, different one. For instance, slides frequently and satellite imagery processed with a GIS- transform to flows downslope. They form narrow, software. With this information we were able to elongated landforms adapting and moving along recognize the zones that may be subject to future river courses at considerable speed. events and past mass movements occurred in the Motozintla basin during the past 25 ky. Avalanches are large masses of rock that move downslope and therefore, individual particles Terminology do not have significant relative motions among each other (Dikau et al., 1996). Avalanches In this paper the following terms are used: have deeper failure surfaces affecting different Mass movement, is a downslope movement of rocks. The deposit shows jigsaw-fit blocks material generated either by natural or anthropic where stratification, fractures and faults may be phenomena that induce slope instability preserved. (Alcántara-Ayala, 2000). Fall, is a free movement of rocks, debris or soil that detach from the Methods substrate. This movement causes that loose fragments roll and bounce down reducing their The geologic and geographic information of the size, depending on the materials nature and area was collected and synthesized in a data resistance. The failure surface is steep and has base. To produce digital elevation models we used a special orientation. Falls may deposit materials the 1994 topographic maps scale (1:50,000) of downslope depending, on the paleorelief and the Instituto Nacional de Estadística, Geografía e 170 VOLUME 51 NUMBER 2 GEOFÍSICA INTERNACIONAL Informática (INEGI, 1994). With these models, we Gutiérrez (road 190). To the south, it connects prepared several thematic maps (elevation, slope, to Arriaga, Tonalá, Pijijiapan, Mapastepec and grayscale, and drainage). The photointerpretation Huixtla towns (federal highway 200). of the area was performed with six sets of aerial photographs of INEGI (1:75,000; 1:37,500), Tectonic Setting orthophotos and

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