Abstract
Abstract. Aguascalientes valley, located in the central part of México, is affected for a land subsidence process triggered by groundwater withdrawal since the 1980's. Currently, the occurrence and the reactivation of surface faults and earth fissures due to differential subsidence, is the main concern for government bureaus dealing with urban planning, because of the damages that these terrain discontinuities are able to cause in constructions and infrastructure. The deformation and rupture process produces an active linear discontinuity with a variable width of influence, where the constructions and infrastructure are prone to get damage. Therefore, the determination of this width of influence is critical for urban planning and hazard determination. In this work, the results of a geophysical seismic survey carried out in six surface discontinuities located in the subsidence area of the Aguascalientes Valley, are presented and discussed. The study included acquisition of seismic tomography profiles measured perpendicularly to the discontinuities, in order to obtain the P-wave velocity sections. The resulted P-wave velocity models show a low-velocity anomaly within the fracture trace with widths from 50 to 100 m. This anomaly is interpreted as a mechanically disturbed zone due to the activity of the surface fault, this is, due to the presence of fracturing caused to the adjacent material by the differential subsidence. The results suggest that the material in the disturbed zone, is experiencing a increase in secondary porosity, caused by the deformation and rupture process, and the subsequent generation of small fissures and voids. The results of this study have practical implications because the methodology allows defining the influence zone of an active discontinuity, and therefore, to establish a restricted width along the surface discontinuity. The definition of this restraint zone is a first step to produce a subsidence hazard zoning including not only the discontinuity trace but its width of influence.
Highlights
The overexploitation of groundwater from granular aquifers causes a decrease in the elevation of the land surface
The results show that the seismic wave velocity decreases when it propagates through the active zone, which was interpreted as an effect of the area where the subsoil material has been weakened due to the fault activity, thereby decreasing its mechanical resistance
The obtained seismograms were analyzed to pick the firstbreak of each geophone associated with the arrival of direct seismic wave, and with them elaborating a domocrone, that is a time versus distance graph of the entire arrangement
Summary
The overexploitation of groundwater from granular aquifers causes a decrease in the elevation of the land surface This problem is known as land subsidence and is accompanied by differential settlements, which represent a threat to construction and infrastructure, since land subsidence can cause large economic losses. Land subsidence cases have been documented in different countries, for example in Mexico, Japan, the United States, China, Italy, Iran and Indonesia. Several cities in these countries have reported problems related to subsidence as damages to constructions and infrastructure due to terrain fractures, as well as floods due to changes in surface level (Holzer and Johnson, 1985; Galloway and Burbey, 2011). Terrain discontinuities can be differentiated in four types due to their origin and behavior: (a) Ground failures related to land subsidence, are those that did not exist before the process of groundwater extraction, and they were generated due to differential subsidence; (b) Ground failures related to reactivation of paleochannels, are those associated with the removal of fine material (silt, sand) due to subsurface flow, with the formation and collapse of cavities in the trajectory of old buried streams. (c) Tectonic fault and cracks reactivated due to differential subsidence associated to groundwater pumping, are those tectonic faults and cracks affected the aquifer sediments and have been reactivated due to the subsidence process; (d) faults and cracks affecting the aquifer sediments generated before the groundwater exploitation but remain inactive even though the subsidence process is occurring
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