The destructive Kocaeli earthquake (magnitude 7.4) of August 17 1999 involved horizontal slip of up to 5 metres. Although it is possible to measure strains precisely using GPS arrays, many stations are needed to fully grasp strain patterns. Interferometric processing of before and after radar data (InSAR) presents an opportunity to examine seismic strains over very large areas. Displacements associated with the Kocaeli earthquake on the North Anatolian Fault, recorded by InSAR, extended for up to 60 km either side of the fault (Mayer, L. and Lu, Z. 2001. Elastic rebound following the Kocaeli earthquake, Turkey, recorded using synthetic aperture radar interferometry. Geology, v. 29, p. 495-498).
The fault runs parallel to the look direction of SAR beams from the ERS-2 satellite in its ascending orbits. This fortuitous geometry charted relative motions in a horizontal sense on either flank of the major strike-slip fault system, with a precision of about 3 cm. Interesting in its own right, the recorded strain helps understand how and where the elastic strain energy released by earthquakes was stored. The key to energy storage is the rebound pattern associated with strain release during earthquakes, to which the InSAR results are an approximation. This pattern depends theoretically on the displacement along the fault itself, the shear modulus of the rock involved and the depth to which faulting extends. In the case of Kocaeli, faulting penetrated to between 6 and 15 km below the surface. Because elastic strain builds up around active faults, it may be possible to use InSAR monitoring as a means of predicting the risk of future failures on dangerous faults, like the North Anatolian Fault. Earthquake records show that successive failure migrates westwards along the Fault, getting ever closer to Istanbul.