GROUND SUBSIDENCE IN MEXICO CITY IMAGED BY INSAR, GPS AND GRAVIMETRY

Groundwater extraction in the basin of Mexico exceeds recharge, lowering the water table by 0.1-1.5 m/yr, reducing pore fluid pressure in the aquifer and overlying acquitard, and leading to compaction of lacustrine shales and surface subsidence. We describe the recent subsidence of Mexico City due to ground water withdrawal using a combination of Interferometric Synthetic Aperture Radar (InSAR) for high spatial resolution and Global Positioning System (GPS) data for calibration and improved temporal information. Interferometric SAR analysis using ERS-2 data indicate land subsidence in Mexico City between February 1996 and January 2000 at rates as high as 378 mm/yr in the eastern metropolitan area. The downtown region shows rates up to 115 mm/yr. GPS data suggests that these rates have been steady at least since 1995 and are due to steady declines in the water table. Available well data indicate consistent water level drops over the last decade, concurrent to the near-constant subsidence rate measured by both GPS and INSAR. Static groundwater level define an approximate linear relation between water level drop and surface subsidence: Correlation of the InSAR fringe pattern with mapped stratigraphic units and a gravimetric analysis indicates that maximum subsidence is primarily controlled by unconsolidated Quaternary lacustrine clays and silts in the shallow sub-surface.