Employment of a Permanent Monitoring GPS Network at the Seismic Area of Volvi, Greece

Employment of a Permanent Monitoring GPS Network at the Seismic Area of Volvi, Greece P. D. Savvaidis, I. M. Ifadis Aristotle University of Thessaloniki, Department of Civil Engineering, Laboratory of Geodesy, GR-54006 Thessaloniki

The present-day active geodynamics of the Northern Aegean is governed by : GREECE - ΕΛΛΑΣ • the western termination of the North Anatolian fault, that accommodates lateral motion of Anatolia to the west • an N-S extension that could result from the spreading of the Aegean region over the subducted oceanic Mediterranean lithosphere

The Mygdonian graben in the northern part of the Aegean region is one of the active structures that accommodate the N-S extension. The formation of the graben is recent and active, as demonstrated by the important historical seismicity on this structure.

In 1978, severe earthquakes damaged an extensive area of Northern Greece including the city of Thessaloniki. • 45 people killed • 220 injured • 9,480 buildings collapsed • 91,000 buildings with moderate to minor damages

The main shocks came from the seismic zone of Volvi, NE of the city of Thessaloniki.

The main shock (Ms = 6.5) occurred on June 20, 1978 • It was preceded by several foreshocks the largest of which occurred on May 23, 1978 (Ms = 5.8) and June 19, 1978 (Ms = 5.2) The epicentres were located in the region between the Langada and Volvi lakes

Three particular • fault zones were observed after the earthquakes: The first family of cracks followed the southern boundary of the graben, striking roughly N80 from Loutra Volvi in the east to Stivos, Nikomedino and Yerakarou in the west. The other two fault zones, striking N130 and N110 respectively, cut the Mygdonian valley from Stivos in the SSE towards the Langada lake in the WNW near Skolari and as far as Analipsis

A 16 station geodetic control network was established in 1979 using classical geodetic techniques • The measurements were repeated in 1980, 1981, 1982, 1983, 1989, and 1990

In 1994, a new survey of the Volvi network was carried out using GPS technology European Project «EURO-SEISTEST: Volvi – Thessaloniki, a European Test Site for Engineering Seismology, Earthquake Engineering and Seismology».

In 1997 the GPS network was further extended with the addition of six more stations and re-measured, now covering the area from the outskirts of the city of Thessaloniki to the east, near to the boundary of the Mygdonian graben. EURO-SEISTEST

Comparison of epochs 1979 and 1994: 8 cm of N – S extension corresponding to a horizontal deformation rate of more than 5 mm/year Comparison of epochs 1994 and 1997: Indication of a N - S motion of the order of 1.5 cm

The results of the terrestrial and GPS campaigns suggest that a motion has been continuous with time since 1978 • This observed displacement field could be explained by different approaches (e.g. long term postseismic relaxation on the 1978 rupture plane, or continuous aseismic motion on the fault etc.), but in all cases more measurement data are needed

In 1999, the decision was taken for the establishment of a permanent monitoring GPS network in the area under investigation. This network, the VOLVI Array would consist of a limited number of permanent GPS receivers continuously monitoring data in locations, which have shown evidence of motion

STEP 1 Establishment and initialisation of a Continuous Reference GPS Station in the area of Thessaloniki · STEP 2 Establishment of two permanent GPS stations at the north and south of the area between the lakes · STEP 3 Addition of two more permanent GPS stations in the area

STEP 1 • A Continuous Reference GPS Station started to operate at the facilities of the Laboratory of Geodesy in autumn 1999 • Raw data are collected at hourly intervals and stored for future computations • RTCM corrections are being transmitted through a UHF radio network for Real Time Kinematic GPS applications covering the area of greater Thessaloniki

A preliminary estimation of the current ITRF co-ordinates of station "THES" was done using data from selected IGS sites in the surrounding region • The ITRF coordinates of THES • (upd. 3/2000): • X = 4463666.6966 ± 0.008 m • Y = 1890982.2682 ± 0.006 m • Z = 4131186.0218 ± 0.007 m

STEP 2  • Profitis Wide Area Computer Network  ISDN • Station (THES)  • Stivos

STEP 3 Evangelismos Profitis Stivos Yerakarou

SUMMARY • The Volvi GPS array will consist of five permanent monitoring GPS receivers, one of them being the Continuous Reference GPS Station THES in operation at the Laboratory of Geodesy • At present, another GPS receiver is in experimental operation at Stivos and the wide area computer network is being tested for optimum performance • It is expected that in a few months the array will be operational and the measurements will be used for the computation of ground deform-ation in the area of Volvi

Employment of a Permanent Monitoring GPS Network at the Seismic Area of Volvi, Greece