TS16 – Deformation Measurements and Analysis I I. DOUKAS, A. FOTIOU, I. M. IFADIS, K. KATSAMBALOS, K. LAKAKIS, N. PETRIDOU - CHRYSOHOIDOU, C. PIKRIDAS, D. ROSSIKOPOULOS, P. SAVVAIDIS, K. TOKMAKIDIS and I. N. TZIAVOS TS16.5 Displacement field estimation from GPS measurements in the Volvi area FIG Working Week 2004 Athens, Greece, May 22-27, 2004 Displacement field estimation from GPS measurements in the Volvi area I. DOUKAS. A. FOTIOU. I. M. IFADIS. K. KATSAMBALOS. K. LAKAKIS. N. PETRIDOU - CHRYSOHOIDOU. C. PIKRIDAS. D. ROSSIKOPOULOS. P. SAVVAIDIS. K. TOKMAKIDIS and I. N. TZIAVOS Key words: GPS, Volvi area, Land displacements, Field velocity. SUMMARY A deformation study of the earth’s crust in the seismic zone of Volvi in Northern Greece is presented. Five GPS campaigns from 1994 to 2003 were carried out for the estimation of ground displacements and velocities. Emphasis is given on the multi-epoch analysis and the problem of the reference frame definition. Furthermore an attempt is made for the interpretation and evaluation of the computed displacements. INTRODUCTION The estimation of displacements and deformation parameters of the land is very important in Greece due to its great seismic activity. In May and June of 1978 in the Volvi area a sequence of earthquakes with largest magnitude of Ms=6.5 has occurred. A series of aftershocks with magnitudes up to Ms=5 were followed, seriously affecting the city of Thessaloniki. As a result 45 people were killed and 220 were injured. A total of 9480 buildings were severely damaged and approximately 91000 buildings suffered moderate to minor damage. The area of Volvi is located in the northern part of Greece, about 40 Km NE from the city of Thessaloniki (Mygdonian graben). Its seismic activity is evidenced by several destructive earthquakes that occurred in historical time (Papazachos and Papazachou, 1989). In 1979, one year after the last strong earthquakes a geodetic network consisting of sixteen pillars (see Figure 1, showing the complete network) was established in the area (Vlachos, 1980). Since then, the network has been re-measured several times, using classical geodetic techniques. In November 1994 a new survey of the volvi network was carried out using solely GPS techniques for the first time (Savvaidis et. al., 1997). The GPS surveys were repeated in 1995-1996 (Fotiou et. al., 2003), 1997 (Savvaidis and Ifadis, 2000) and more recently 2003. Various analysis techniques for plane crustal deformations concerning the Volvi area have been discussed in (Dermanis et.al., 1981). In the present study, all five GPS campaigns are being analyzing for the detection of possible spatial displacements and for the estimation of the corresponding velocity field. 1. GPS CAMPAIGNS AND DATA PROCESSING