RMZ – Materials and Geoenvironment, Vol. 59, No. 2/3, pp. 257–266, 2012 257 Characteristics of the Predjama fault near Postojna, SW Slovenia Značilnosti Predjamskega preloma pri Postojni, JZ Slovenija sTanka šeBela¹, * ¹ZRC SAZU Karst Research Institute, Titov trg 2, SI-6230 Postojna, Slovenia *Corresponding author. E-mail: [email protected] Received: August 31, 2012 Accepted: October 10, 2012 Abstract: The course of the Dinaric oriented (NW-SE) Predjama fault in the area of the northern Pivka valley and through Javorniki mountain is not well determined. The position of the Predjama fault through the Pivka flysch valley is mostly determined hypothetically. In the area of Javorniki hills the fault is divided into several parallel strands and accompanied by mostly cross-Dinaric (NE–SW) oriented faults. Pred- jama fault in the area of Postojna was probably not the seismogenic source of Postojna MLV = 3.7 earthquake in 2010. Micro-tectonic de- formations detected by TM 71 extensiometers in Postojna cave sys- tem on the other hand show minor ongoing aseismic displacements in northern NW-SE oriented fault, which is parallel to the Predjama fault. Izvleček: Potek dinarsko usmerjenega (SZ–JV) Predjamskega preloma na območju severnega dela Pivške kotline in čez Javornike ni dobro ugo- tovljen. Položaj Predjamskega preloma skozi flišno Pivško kotlino je večinoma določen hipotetično. Na območju Javornikov je prelom razdeljen na več ločenih vzporednih prelomov, ki jih spremljajo pred- vsem prečno dinarsko (SV–JZ) usmerjeni prelomi. Predjamski prelom na ozemlju Postojne verjetno ni bil izvir potresa MLV = 3.7 v Postojni leta 2010. Mikrotektonske deformacije, ugotovljene z ekstenziometri TM 71 v Postojnskem jamskem sistemu, pa kažejo manjše tektonske aseizmične premike na severnem SZ–JV usmerjenem prelomu, ki je vzporeden Predjamskemu. Review paper 258 šeBela, s. Key words: Predjama fault, Postojna, Slovenia Ključne besede: Predjamski prelom, Postojna, Slovenija INTRODUCTION the NW–SE orientation south of Pos- tojna. However, the subsequently de- Recent tectonic activity of Slovenia is rived fault plane solutions of the event driven by northward movement of the (ŽiVčić et al., 2011) did not confirm Adria microplate and its counter-clock- this idea. Therefore we review the ex- wise rotation (e.g. VraBec & FoDor, isting geological literature with respect 2006). The NW–SE trending Predjama to the position of Predjama fault in the fault is generally not considered as im- northern part of the Pivka valley. In portant as the Raša or the Idrija fault. this area, the precise location of the It is treated as a continuation of Avče fault zone was mostly infered, result- fault (Janež et al., 1997). Its position ing in mismatch from 200 m to 1500 m east from Col, through Trnovski Gozd between the existing interpretations of is clearly visible. The continuation of the Predjama fault location. The most the fault to northern Pivka valley be- unclear is the SE-ward continuation of tween Predjama and Postojna and fur- the Predjama fault towards Javorniki ther into Javorniki hills is less clear. hills, where the fault is complicated by NE–SW oriented faults and is prob- The January 15, 2010 MLV = 3.7 earth- ably separated into several segments. quake in the area of Postojna town called attention to the fact that recent- ly not only the area around Pivka or PREDJAMA FAULT GEOMETRY IN THE VI- Snežnik Mountain is seismically ac- CINITY OF POSTOJNA tive, but also the northern part of the Pivka valley at Postojna can generate A compilation of structural data, dis- strong motions. The January 1, 1926 M cussed in this section, is shown on the = 5.6 Cerknica earthquake was strong- geological map in Figure 1. ly felt inside the Postojna cave system, with reported stalagmite collapse in- On the Postojna sheet of the 1 : 100 000 side the cave (ŠeBela, 2010). Basic geological map of SFRJ (Bus- ER et al., 1967) the Predjama fault is Preliminary geological interpreta- terminated about 500 m west from tion of the 2010 Postojna earthquake Predjama and does not continue attributed the event to slip on the re- through the northern part of Pivka gional Predjama fault, which runs in valley and through Javorniki hills. RMZ-M&G 2012, 59 Characteristics of the Predjama fault near Postojna, SW Slovenia 259 Figure 1. The basic structural-geological map of the Postojna area and position of Predjama fault. 1- town or village, 2- river and its flow direc- tion, 3- Javorniška Koliševka, 4- hill with above sea level in metres, 5- TM 71 extensiometer in Postojna cave system, 6- ground-plan of Postojna cave system, 7- fault/hypothetic fault, 8- thrust fault, 9- dextral horizontal strike-slip movement, 10- sinistral horizontal strike-slip movement, 11- anticline, 12- syncline, 13- Eocene flysch rocks, 14- discordant contact between flysch and limestone. The eastward continuation of the Pred- vorniki was first documented by Plac- jama fault, from Avče fault into Ja- ER (1981 and 1996). RMZ-M&G 2012, 59 260 šeBela, s. On the sketch of geological struc- The investigations of Janež et al. (1997) ture of Postojna the Predjama fault and Rižnar (1997) agree with Placer’s is marked as a hypothetic subverti- (1996) interpretation of the position of cal fault (Placer, 1996). The fault is Predjama fault. According to Janež et visible in the highway road-cut close al. (1997) the fault runs about 200 m to the Postojna railway station about northward of the position shown on 190 m north from the railway bridge. the map of Buser (1989). They trace The inner fault zone is 18 m wide. Its the Predjama fault along the southern northeastern fault plane has orienta- edge of Sovič hill (677 m) near the tion of 25/70, with slickensides dip- town of Postojna and argue that the ping 295/12. In the Hrušica area the morphologicaly well-expressed scarp Predjama fault shows dextral horizon- dividing Upper Cretaceous limestone tal movement. In the vicinity of Pos- from Eocene flysch, which runs NW- tojna the NE block of Predjama fault ward from Postojna towards Predjama, moved towards right and slightly up is controlled by a fault zone parallel with respect to the SW block. The to the Predjama fault. Further to the apparent displacement on Predjama NW, west of the town of Col, the Pred- fault near Postojna is insignificant jama fault continues into the Avče fault (Placer, 1996). (Janež et al., 1997). There, the trace of fault is much better visible both in The morphologically well-expressed morphological and geological sense contact between Upper Cretaceous than in the northern Pivka valley and limestone and Eocene flysch is, ac- eastward in the southern parts of Ja- cording to Placer (1996), a fault, vorniki hills. which is running nearly parallel to the southern Predjama fault. This fault is The Bukovje fault, which is situ- geologically better determined in the ated north from Predjama fault near field than the principal Predjama fault, Bukovje was determined by čar & which runs through the Pivka flysch šeBela (2001). The fault is displacing valley. a morphologically well expressed ap- prox. N–S oriented sinistral strike-slip On the 1 : 500 000 geological map of fault, which runs north from Gorenje Slovenia (Buser, 1989) the Predjama towards Hrušica. The Bukovje fault fault runs about 1 km south of the shows insignificant dextral horizontal Nanoščica stream, and continues along displacement (čar & šeBela, 2001). the southern edge of Javorniki hills towards the Babno Polje where it ap- The structural-tectonic map of Slove- proaches the Idrija fault zone. nia (PolJak, 2000) traces the Predjama RMZ-M&G 2012, 59 Characteristics of the Predjama fault near Postojna, SW Slovenia 261 user fault similarly to B et al. (1967). 6 2010, MLV = 2.4, constrain a nearly The fault is traced in the area NW from vertical fault plane, which is either a Postojna, where it separates Hrušica SSE trending (N165° E) dextral strike- structure from Nanos structure, where- slip fault, or a WSW trending sinistral as the eastward fault continuation in strike-slip fault (ŽiVčić et al., 2011). the Eocene flysch of the northern Pos- Seismological data therefore suggest tojna valley and at the southern foot of that the 2010 Postojna earthquake was Javorniki hills is left undefined. not related to slip on the Predjama fault, but to some so far unidentified The detailed structural map of the area geological structure. between Postojna, Planina and Cerkni- ca (čar & GosPoDarič, 1984) was a Faults in suitable SSE-NNW orienta- major contribution towards under- tion were mapped by čar & šeBela standing the tectonic structure of the (1997) around the Pivka and Črna area. After almost 30 years this is still Jama, but were at the time not inter- the most important structural-geologi- preted as active or potentially ac- cal map of the region. Unfortunately, tive. Fissures and faults-orientation the Predjama fault zone is outside the data from the Postojna cave system mapped territory. (ŠeBela, 1998), presented in Figure 2, also demonstrate a prominent cluster of structures with NNW–SSE orienta- ACTIVE TECTONICS AND SEISMICITY tion, which is nearly as abundant as the NEAR POSTOJNA one in the Dinaric NW-SE orientation. The major geological structures in the On January 15, 2010 at 14:20 (UTC) cave, mostly fault zones, have a Dinar- an earthquake occurred in the area of ic NW–SE orientation (šeBela, 2012). Postojna town. Its local magnitude Significant fissured to broken zones was 3.7 and the highest intensity V (in sense of čar & GosPoDarič, 1984) EMS-98. The earthquake was felt in are mostly SSE–NNW orientated and the territory of SW Slovenia and also exhibit no slickensides. Significant in Jesenice, Velenje and Celje (Jesenko displacements in the Postojna cave et al., 2011).