Ground deformation in an area later damaged by an earthquake: monitoring the Avcilar district of Istanbul, Turkey, by satellite radar interferometry 1992-1999

The Avcilar district of Istanbul was severely damaged by the M 7.4 Izmit (Kocaeli) earthquake of 1999. The same area underwent ground subsidence before the earthquake, as revealed by geodetic monitoring. Analysis of 14 synthetic aperture radar images acquired by the ERS-1 and ERS-2 satellites between 1992 and 1999 by interferometry (InSAR) measures the rate of subsidence. Using the General Inversion for Phase Technique (GIPhT), we analyse a set of 12 interferometric pairs. The interferometric fringe patterns show a rounded triangular shape that we interpret as secular subsidence at a constant rate. The maximum subsidence rate of 6 mm per year occurs at a point located at latitude 40.98 degrees N and longitude 28.71 degrees E. A simple four-parameter elastic Mogi model, consisting of three infinitesimal spherical sinks at a depth of 2.4 +/- 0.4 km deflating at 78 000 +/- 16 000 cubic metres per year, describes subsidence signal to first order. The model also accounts for tropospheric effects by estimating a vertical phase gradient and an additive offset for each image acquisition epoch. The model fits the data with a cost of 0.18 cycles per datum for the 4644 phase measurements included in the inversion. This fit is significantly better than either the null hypothesis or the initial model with 95 per cent confidence for 32 free parameters. The association of ground deformation with earthquake damage may be interpreted in terms of weak, compressible material in shallow subsurface layers.