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Subsidence in the Avcilar district of Istanbul, Turkey measured by satellite radar interferometry 1992 – 1999

Samuray Akarvardar(1), Kurt Feigl(2) and Semih Ergintav(3)

(1) Observatoire Midi-Pyrenees, 14., av Edouard Belin, 31400 Toulouse, France
(2) University of Wisconsin-Madison, 1215 W Dayton St. Madison, WI 53706, United States
(3) TUBITAK MAM, PK. 21 Gebze, 41470 Kocaeli, Turkey

Abstract

Analysis of 14 synthetic aperture radar images acquired by the ERS-1 and ERS-2 satellites between 1992 and 1999 by interferometry (INSAR) reveals subsidence around the Avcilar area of Istanbul (Turkey). Using the General Inversion for Phase Technique, (GIPhT), we analyze a set of 24 interferometric pairs. The interferometric fringe patterns show a crescent shape. We interpret them as purely vertical subsidence at a secular rate. The maximum subsidence rate of 7 mm/yr occurs at a point located at latitude 40.98ºN and longitude 28.71ºE. A simple 4-parameter elastic Mogi model consisting of three infinitesimal spherical sinks at a depth of 2300 ± 1300 m deflating at 78 ± 34 thousand cubic meters per year describes subsidence signal to first order. The model also accounts for tropospheric effects by estimating one vertical phase gradient for each image acquisition epoch. The model fits the data with a cost of 0.21 cycles per datum for the 9288 phase measurements included in the inversion. For the complete data set, including 29,241 unmasked pixels in the 24 pairs, the cost is 0.19 cycles per datum. Both these fits are significantly better than the null hypothesis and the prior model with 95% confidence for 18 free parameters. The spatial distribution of the subsidence suggests that most of the subsidence occurs as compaction in a shallow layer of unconsolidated, partially saturated soil (Cukercesme formation) with a relatively weak lithology. This layer appears to deform easily in several processes, including compaction, landslide failure, and seismic wave amplification. It may also be susceptible to liquefaction during a large earthquake.

 

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry