• GPS;
  • San Gabriel Valley;
  • groundwater level changes;
  • hydrologic deformations

[1] Changes in groundwater levels cause water-bearing basins to deform. Here we provide a detailed history of horizontal surface displacements in the San Gabriel Valley, California, which we show are highly correlated with the water level changes measured at a nearby well: correlation coefficient of 0.96 ± 0.01. We use the surface response from a one year period during which water level change in the nearby well was over 16 m, to monitor the deformations over a 14 year period from 1998 to 2011. The water level changes lead surface deformation by 21 days, but the significance of the offset is only weakly supported (p-value = 0.129). The projection method can be used as a real-time monitoring or detection system for deformation caused by a variety of mechanisms such as fluid injection and removal (groundwater development, oil production, carbon sequestration), and by seismic and volcanic activity.