Land Subsidence, Associated Hazards and the Role of Natural Resources Development (Proceedings of EISOLS 2010, Querétaro, Mexico, 17–22 October 2010). IAHS Publ. 339, 2010.

Copyright © 2010 IAHS Press

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Zoning map of ground failure risk due to land subsidence of San Luis Potosí, Mexico JESÚS PACHECO-MARTÍNEZ1, JORGE ARZATE-FLORES2, RUBÉN LÓPEZ-DONCEL3, RAFAEL BARBOZA-GUDIÑO3, JOSÉ LUIS MATA-SEGURA3, ANTONIO DEL-ROSAL-PARDO1 & JORGE ARANDA-GÓMEZ2

1 Centro de Ciencias del Diseño y de la Construcción de la Universidad Autónoma de Aguacalientes, Av. Universidad #940, Ciudad Universitaria, CP 20131, Aguascalientes, Ags, Mexico geochuy@gmail.com

2 Centro de Geociencias de la Universidad Nacional Autónoma de México, Mexico 3 Instituto de Geología de la Universidad Autónoma de San Luis Potosí, Mexico

Abstract We present and analyse geologic, hydrogeologic, topographic and geophysical evidence which ties the formation of the active, aseismic faults in the valley fill with: (1) land subsidence triggered by groundwater withdrawal, and (2) buried topographic features, probably controlled by the geological structures under the valley fill deposits that contain the aquifers. Based on the collected data and the geologic interpretation of geophysical data, we propose a ground failure risk map where we show zones with different probabilities of ground failure. The risk map and attached recommendations are intended to be integrated to local building regulations in the municipalities of San Luis Potosi (SLP) and neighbouring Soledad de Graciano Sánchez (SGS). Key words San Luís Potosí, Mexico; zoning risk; earth fissure; surface fault; subsidence INTRODUCTION

San Luis Potosí (SLP) is a medium size city (110 km2 in area, with approx. one million inhabitants) located in the southern part of the Chihuahua desert, 360 km northwest of México City. The main sources of water for the population and industry in San Luis Potosí are a small dam and groundwater, which is pumped out from two granular aquifers in the valley fill, and from one fractured-controlled aquifer in the bedrock. Of the water used in the city in the 1960s, 60% came from the dam. Currently only 8% comes from that source. Water level decline in the aquifers in 1972 was estimated at 0.9 m/year, while in 1990 it was 1.3 m/year (Carrillo-Rivera et al., 2002). Earth fissures related to land subsidence were first reported in the 1990s in SLP (López-Doncel et al., 2006a). However, we believe that land subsidence in the region started before the actual damage to constructions was noticed. GENERAL GEOLOGY OF SLP VALLEY

The study area is located at the northern end of a local graben known as Villa de Reyes Graben. To the east of the city of SLP there are hills where there are exposed Cretaceous marine rocks. The phenomenon of subsidence occurs within the sedimentary fill that forms the valley of SLP (Fig. 1), which represents a trough containing a substantial column composed of alluvial fills that are reworked with Quaternary volcanic products, as well as with general continental sediments with thicknesses ranging from 50 to 500 m in the bedrock. The bedrock consists of rhyolites that are of Oligocene age, from about 26–31 million years (Aguirre-Hernández, 1992; Martínez-Ruiz, 1997). The structural frame exposed in the mountains that surround the valley is also located in the bedrock that underlies the sediments inside the valley.

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Fig. 1 General geology of SLP valley with known cracks until 2009.

In the superficial part of the valley fill there are exposed gravel and sand deposits of Quaternary age. Sedimentological works that describe the lithology, as well as the grain size of the Quaternary sequences that fill the valley of San Luis Potosi, are scarce. These are only limited to brief descriptions of the “cuts”, which are taken during the drilling of wells, and these descriptions normally use descriptive terms that do not give clues about its origin or mechanism of deposition. However, these descriptions have been used as a tool to interpret and reconstruct the outline of the bedrock that is found under the Quaternary fill (Aguirre Hernandez, 1992). The thickness of the Quaternary sequences is very variable and it is logically influenced by the contour of the bedrock, this tends to change from 50 to >500 m (Aguirre Hernandez, 1992; Martínez-Ruiz 1997). SUBSIDENCE AND CRACKS IN THE CONURBATED AREA OF SLP-SGS

The VSLP is subjected to a process of gradual subsidence; it is associated with the vertical deformation of the granular fill due to the lowering of the groundwater level. In addition, because this collapse is generated in large areas and due to the magnitude that is a few millimetres per year, it is imperceptible in short periods of time. However, a problem that is associated with the sinking and that is mainly evident in the construction effects, refers to the surface fracturing and faulting that are associated with the differential subsidence; i.e. when the sinking occurs with different magnitudes in an area that is relatively small. In the city of SLP the generation of fracturing and faulting zones in the granular fill has been mainly observed since the 1990s by the damage caused in buildings, as well as in the urban infrastructure (López-Doncel et al., 2006a). Subsidence is now widespread and there are two surface fault systems that are composed of four major faults: (1) a system with N–S direction, and (2) a system with a close direction to E–W (López-Doncel et al., 2006b). By 2009, only in the suburbs of SLP-SGS have around 41 km of fracturing and faulting areas been quantified. Although there is no information about the area affected by the subsidence in the valley of SLP, taking into consideration the areas where the fractures have been developed as well as the location of the greatest thickness of the alluvial fill and the lacustrine deposits in the graben of San

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Luis Pososí-Villa de Reyes, an area of approx. 1800 km2 with the potential to develop and generate subsidence and fracturing zones is estimated. Currently there is no register of affected properties, and it makes it more difficult to estimate the economic implications that are derived from the affected buildings, but according with the length of the cracks that are located within the urban area of the city of SLP and SGS, it is estimated that at least 2000 buildings, including mostly dwellings of different types, have been directly damaged to different degrees by the fractures that range from imperceptible damage at first sight, to damage that puts at risk the structural stability of the construction itself. MECHANISM OF CRACKING GENERATION IN THE VALLEY OF SLP

The cracks of the SLP valley seem to follow two different mechanisms of generation. Some cracks are located in areas with gravimetric anomalies and they are aligned with the regional geological structures, so that they could be similar to those observed in other valleys in the centre of the country (Garduno-Monrroy et al., 2001; Pacheco et al., 2006; Pacheco & Arzate, 2007), they are associated with soil compaction, over-exploitation of aquifers, and they are in combination with the existence of bedrock with irregular topography that is lying beneath the sedimentary fillings where the aquifer is found (Fig. 2). This mechanism of generation of the fractures is used by Jachens & Holzer (1982) to explain the origin of the fractures in Arizona, USA.

Fig. 2 Formation of surface faults and fractures associated with a differential compaction of granular fill, which forms the aquifer of the valley of SLP (modified from Carpenter, 1999).

It has also been observed that other fractures are neither aligned with regional geological structures, nor with gravity anomalies. The mechanism of these cracks appears to be related to the formation of holes by “tubing” and their collapse is on the centre line of former streams. The mechanism of formation of this type of fracture is not still fully understood. It presumably starts with the “undermining” or “tubing” of paleochannels, which are the result of a fine drag and the consequent formation of voids that can collapse (Fig. 3). The fracturing that is generated is aligned with fluvial deposits that are buried by younger fillings. This mechanism has been suggested by Suarez-Placencia et al. (2005) to explain the fracturing in the valley of Nextipac, Jalisco, Mexico. The existing cracks in the suburbs of SLP-SGS with the configuration of the bedrock are shown in Fig. 4. The bedrock was determined from gravimetric data and lithologic logs of wells.

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Fig. 3 Formation of fractures that are associated with paleochannels. Cross section of a paleochannel it is interspersed with more recent fillings of different mechanical properties. The formation of holes is generated by the dragged material by water flow in the longitudinal direction of the paleochannel. The process ends with the collapse of the roof and formation of cracks and faults.

Fig. 4 Configuration of the bedrock as well as cracks in the valley of SLP.

However, the level of groundwater in urban areas shows a spatial relationship between the area of greatest drop and the area that has been affected the most by cracks (Fig. 5); additionally Fig. 4 shows that this zone is located on a topographic irregularity of the bedrock.

ZONE OF RISK BY FRACTURES IN THE VSLP The probability of occurrence of new fractures that are related to subsidence in the valley of SLP can be associated with the presence of three factors: a rocky floor with irregular topography, a

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granular aquifer system that is capable of being drained, and a decreasing level of groundwater. The first two factors are the scenario that is potentially dangerous, and the third factor is the unchaining element or the trigger of the phenomenon. The observations in the valley of SLP suggest that there are two main factors involved with the fractures that are associated with the paleochannels; one is the presence of ancient stream fills (a potentially dangerous scenario) and the other is a trigger factor that seems to be a subsurface current that carries particles that generate floor holes that erode and collapse. In order to develop the zoning map, it was made a crossroad of spatial occurrence of the gravimetric anomaly maps, of gradient of the gravimetric anomaly, and a map of the existing cracks, the bedrock configuration and the geology map. Figure 6 shows the danger map of fracturing in the study area.

Fig. 5 Levels of groundwater (1998) and cracks in the VSLP.

Fig. 6 Zoning map of risk associated to cracks generation in SLP valley.

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López-Doncel, R., Mata-Segura, J. L., Cruz-Marquez, J., Arzáte-Flores J. & Pacheco-Martínez, J. (2006a) Riesgo geológico para el patrimonio histórico. Ejemplos del centro histórico de la Ciudad de SLP. Boletín de la Sociedad Geológica Mexicana 58(2), 259–263.

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