PROPAGATION OF FRACTURING RELATED TO LAND SUBSIDENCE IN THE VALLEY OF QUERETARO, MEXICO

摘要

Land subsidence in the Valley of Queretaro is an active phenomenon that has been associated to extensive groundwater withdrawal. However, a fault-fracture system affecting the city of Queretaro is aligned with north-south striking regional faults suggesting that buried fault scarps determine the geometry and propagation of fractures. Likewise, fault and fractures can influence groundwater flow in the aquifer and acuitard layers. In order to correlate major fracturing to hidrogeological parameters we have analyzed a database that consists of stratigraphic and piezometric information of extraction wells. We have studied the near-surface geometry of fracturing on vertical profiles collected perpendicular to the main pattern of the fracture with Ground Penetrating Radar (GPR). Radargrams indicates that deformation is concentrated on a single narrow structure in the northern part of the valley, whilst deformation is distributed in a wider area and in at least three different structures in the southern part of the valley. We propose that differential deformation influences propagation and orientation of the fracture trace near the surface because of variations in vertical compaction of sediments. Variations in depth of physical and mechanical properties of sediments suggest that fracture is also controlled by the history of loads and the stratigraphic boundary between coarse and clayey sediments. Thus, the morphology of the near surface fracturing depends upon the spatial relation between regional fracturing and the depositional conditions of sediments.