Acta Geodyn. Geomater., Vol. 4, No. 1 (145), 87-98, 2007 MICRO-DEFORMATION MONITORING OF ACTIVE TECTONIC STRUCTURES IN W SLOVENIA Andrej GOSAR 1),2) *, Stanka ŠEBELA 3) , Blahoslav KOŠŤÁK 4) and Josef STEMBERK 4) 1) University in Ljubljana, Faculty of Natural Sciences and Engineering, Aškerčeva 12, 1000 Ljubljana, Slovenia 2) Environmental Agency of the Republic of Slovenia, Seismology and Geology Office, Dunajska 47, Ljubljana, Slovenia 3) Karst Research Institute ZRC SAZU, Titov trg 2, 6230 Postojna, Slovenia 4) Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, V Holešovičkách 41, 18209 Prague, Czech Republic *Corresponding author‘s e-mail: andrej.gosar@gov.si (Received February 2007, accepted March 2007) ABSTRACT Monitoring of tectonic movements along three active faults of Dinaric (NW-SE trending) fault system in W Slovenia using TM 71 extensiometers was set up in 2004. After two and a half years of measurements clear trends of displacement were recorded. The average left-lateral displacement along a crack in the inner fault zone of the Idrija fault in Učja valley was reading 0.38 mm/year. Short term (10 months) rates were even greater and reached the value of 0.54 mm/year. Since the Idrija fault is considered generally to be dextral strike-slipping, the observed left-lateral displacement can be explained by variations in principle stress. Raša fault monitoring site at the foot of Vremščica Mt. established an average reverse uplift of hanging wall (SW) block of 0.24 mm/year and left-lateral displacement of 0.16 mm/year. Short term (9 months) vertical displacements reached the value of 0.53 mm/year. The inclined displacement is in agreement with geological and seismological observations. In the Postojna cave system two instruments were installed at the fault which extends parallely to Predjama fault zone. The average vertical displacement rate at Postojna 1 site was 0.01 mm/year. Both devices recorded similar reaction which can be attributed to 12 July, 2004 (Mw=5.2) earthquake with an epicentre 70 km away from the measuring site. Since there were no other stronger earthquakes in the vicinity and time span of monitoring, no other correlations were established with earthquake activity. The observed displacement rates along all three monitored faults of up to 0.5 mm/year are consistent with the regional deformation rate in W Slovenia established from GPS measurements which is of the order of 2 mm/year. KEYWORDS: tectonic movements, active tectonics, monitoring, Dinaric fault system, Slovenia installed in November 2004 on Raša fault at the SE foot of Vremščica Mt. and on Idrija fault in Učja valley. The fifth instrument TM 71 was set up in 2006 on Kneža fault which is located between Idrija and Ravne fault (Fig. 1). TM 71 is a mechanical extensiometer designed for installation on narrow cracks (crack gauging) to monitor relative micro-displacements between both walls of the crack. It works on the principle of mechanical interference (Moire effect), and displacements are recorded by interference patterns of two optical grids (Košťák, 1991). The instrument provides three-dimensional results – displacement vector in two perpendicular planes (horizontal and vertical) and angular deviations (rotation). The sensitivity of the system is: 0.05-0.0125 mm in all three space co-ordinates, and 3.2 10 -4 rad in angular deviations (Stemberk et al., 2003). The main advantages of this purely mechanical instrument are: it completely avoids the use of electrical transmission means, it furnishes good performances under severe outdoor conditions, and has a long-term stability. 1. INTRODUCTION Slovenia is situated at the NE boundary of the Adriatic microplate, at the junction of three major geotectonic units: the Alps, the Dinarides and the Pannonian basin. Recent geodynamics of the region is controlled by CCW rotation of the Adriatic plate in relation to relatively stable European plate (Vrabec and Fodor, 2006), which results in Slovenia predominant N-S oriented principle stress orientation. On the south lies the undeformed Istrian platform is situated, which gradually evolves into the External Dinarides. For Western Slovenia moderate historical to recent seismicity is characteristic (Poljak et al., 2000). Within the COST 625 action (Stemberk et al., 2003) we set up deformation monitoring of three active faults in W Slovenia using four TM 71 extensiometers (Šebela et al., 2005). In the first half of 2004 two TM 71 instruments were installed in Postojna cave system on the Dinaric oriented (NW- SE) fault that is situated about 1 km north of Predjama fault. The third and fourth instruments were