RAPID N-S EXTENSION IN THE MYGDONIAN GRABEN (NORTHERN GREECE) DEDUCEDFROM REPEATED GEODETIC SURVEYS

In November 1994, a geodetic network situated on the Mygdonian graben, 30 km NE of Thessaloniki, Greece, was remeasured using the GPS techni que. This network was established in 1979, in the epicentral area of t he 1978 earthquake (Ms = 6.5), and was measured using triangulation te chniques seven times between 1979 and 1989. The comparison between 197 9 and 1994 data shows that this part of the Mygdonian graben experienc ed about 8 cm of N-S horizontal extension. Most of the extension is co ncentrated in a narrow (one or two kilometers) E-W zone, located on th e southern boundary of the graben. Furthermore, the analysis of repeat ed triangulation surveys suggests a rate of extension of 5.7 +/- 1.3 m m/y, that remained constant with time between 1979 and 1994. This exte nsion could result from long-term postseismic relaxation processes, or from continous aseismic slip within the graben.