Gravimetry has the potential to provide important data, in combination with GPS, for detecting vertical surface motions and subsurface mass changes. Here, we focus on the first results of joint gravity and GPS studies in order to understand better the vertical component of the postseismic deformations of the 1999 earthquakes along the western North Anatolian Fault. We investigate the relationship between gravity changes and GPS motions during the period 2003–2005. The changes in this period constitute a snapshot of the nonlinear movements that were not studied before in the Marmara Region. The first observations evaluated here demonstrate that the joint analysis of GPS and gravity data help to constrain the 3D postseismic deformations and hence expand our knowledge of the geophysical process in the Marmara Region. We identify what appear to be different crustal properties in the western and eastern parts of the region. Furthermore, the GPS results indicate that the western extension of the 1999 İzmit rupture area presently has low strain accumulation. To the extent that this behaviour continues through the earthquake cycle, it reduces the moment release of the expected future earthquake in the eastern Marmara seismic gap. In contrast, the western part of Marmara region has important strain loading. While our results are not sufficiently accurate for detailed interpretation, the observed strain accumulation implies the potential for a significant earthquake in the western Marmara region. Generally, possible fault creep extending west of the İzmit fault break following the İzmit earthquake is very important to understand the future seismic hazard in the Marmara region because it reduces the amount of strain accumulation during the earthquake cycle which will either delay the onset of future events or produce smaller future earthquakes.

The post seismic time dependent motions of 1999 Izmit and Düzce earthquake sequences in Marmara region ,Turkey, provide the opportunity to improve significantly our understanding of the mechanical behaviour of the western part of North Anatolian Fault. Therefore, an international cooperative project have been started to determine the long-term potential post seismic crustal deformations along the tectonic fault using GPS and gravity data sets in this region. We focus into the first results of joint gravity and GPS studies to understand 3-dimensions post seismic deformations of 1999 earthquakes along the North Anatolian Fault between the 2003 and 2005 years in the Marmara Region. The GPS data set following the 1999 earthquakes and the gravity observations were analyzed to detect of the accurate 3-dimension post seismic deformations and to determine gravity changes in time and space dependent on vertical crustal movements. The first results indicate that the gravity changes and some of the GPS time series show, strongly, seasonal unmodeled behaviours, and vertical motion from GPS field and gravity results shows good correlation. Furthermore, they show that height, gravity and potential changes between the western and eastern part of the network are very different. The western part shows significant strain loading, which is documented by horizontal displacements and potential increases.