Barajas-Nigoche L.D.,Servicio Geologico Mexicano SGM |
Carreon-Freyre D.C.,National Autonomous University of Mexico |
Mata-Segura J.L.,Autonomous University of San Luis Potosi |
Rivera-Leon A.,Servicio Geologico Mexicano SGM |
Cafaggi-Felix F.,Servicio Geologico Mexicano SGM
IAHS-AISH Publication | Year: 2010
In San Luis Potosí (SLP) City, fractures associated with land subsidence affect the urban structure and, in some cases historical value buildings. Early studies documented ground fracturing in this city and related it to groundwater withdrawal from the beginning of the last decade. Most of these studies were focused to detect fractures using geophysical gravimetric studies that detect major discontinuities in the subsoil. In this study we report mapping of near surface sequences and the interpretation of lithological records available from water wells, trenches, quarries, and drilling. Our results show that fine grained units are concentrated in two zones located north and east of the main SLP urban area. The sand deposits are predominantly distributed in the central part of the city and conglomeratic deposits are distributed near their fluvial sources. A new map of ground fracturing was achieved by integrating fractures related to the distribution of soils and sediments by the use of high precision geophysical tools, such as ground penetrating radar (GPR) and surface wave seismicity. The map suggests that most of fractures are localized in the central-east part of SLP City, ranging from some metres to 7 km in length. We noticed that the fractures with a N-S strike, that in some cases present lateral displacement, are related to the distribution of Tertiary continental deposits (Halcones Conglomerate). The longest fracture, named Aeropuerto Fault, has the same N-S orientation as fractures that present right lateral displacement with stratigraphic structures and related folding located in the south of the City, and has affected a church which displays evidence of structural torsion. Finally, a correlation between groundwater piezometric levels and stratigraphic variations in SLP City suggest that ground fracturing in granular media is closely related to high hydraulic gradients, mainly in the border zone of the Halcones Conglomerate. It is important to note that in semi-desert areas of central-north Mexico, the piezometric levels vary from 130 to 170 m depth. According to our interpretation, the generation of fractures in SLP City may be related to major regional discontinuities in the sedimentary basin, but their propagation depends on the stratigraphic variations and hydraulic behaviour of the near surface sequences. Copyright © 2010 IAHS Press.