GPR assessment of creep-fault-induced damage in urban areas

Urban areas built on old lacustrine basins in central Mexico show subsidence related to creep-fault processes. These processes are highly dynamic and enhanced by aquifer water extraction and El Nino events. The affected cities are Morelia, Salamanca, Silao, Celaya, Aguascalientes, and Queretaro, with a global population of around 2.5 million, and considerable industrial assets. The first three cities show NE-SW creep-fault trends, while in the remaining the trend is N-S. The subsidence rate varies: 2-3 cm/year at Salamanca, 4-6 cm/year at Morelia and Queretaro, and 6-8 cm/year at Celaya. In order to preliminarily evaluate the effects of the creep-faults, we performed a series of non-destructive ground penetrating radar surveys in the city of Morelia, that complement concurrent geological studies of the area. We report herein results in three locations where the surface expression of the faults is well defined, including substantial damage to homes and buildings. One of the objectives was to determine the length of the disturbance, perpendicular to the faults' trends. Using 50 Mhz antennas we reached an exploration depth of around 10 m in sandstone-bearing lacustrine deposits and lava flows. In most cases the faults show low-angle dips that vary with depth in the main fault. Additional, synthetic and antithetic faults are developed in the disturbed area of the main fault, which we define as the influence zone of the main disturbance. We find that this zone varies from 15 to 50 m on both sides of the fault, although it tends to be larger on the down thrown block. Faulting appears to be controlled by geological formations within the first 10-15 m in depth.