GEOLOGIJA 50/1, 45–54, Ljubljana 2007
Monitoring of micro-deformations along Idrija and Rasa faults
in W Slovenia
Opazovanje mikro-deformacij ob Idrijskem in Baškem prelomu v zahodni Sloveniji
Andrej GOSAR1,2
1Environmental Agency of the Republic of Slovenia, Seismology and Geology Office, Dunajska 47,
Ljubljana, e-mail: andrej.gosar®gov.si 2University of Ljubljana, Faculty of Natural Sciences and Engineering, A{ker~eva 12, Ljubljana
Key words: tectonic movements, active tectonics, monitoring, Dinaric fault system, Idrija fault, Ra{a fault, Slovenia
Kljuène besede: tektonski premiki, aktivna tektonika, monitoring, Dinarski prelomni sistem, Idrijski prelom, Ra{ki prelom, Slovenija
Abstract
Monitoring of tectonic movements along two active faults of Dinaric (NW-SE trending) fault system in W Slovenia using TM 71 extensiometers was set up in 2004. After two years ob measurements some clear trends of displacement were developed. The average left-lateral displacement along a crack in the inner fault zone of the Idrija fault in U~ja valley is 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 as dextral strike-slip, is the observed left-lateral displacement explained by local permutation of principle stress axis. In the Ra{a fault monitoring site at the foot of Vrem{~ica Mt. at Ko{ana the average reverse uplift of hanging wall (SW) block of 0.24 mm/years and left-lateral displacement of 0.16 mm/year were established. Short term (9 months) vertical displacements reached the value of 0.53 mm/year. The oblique sense of displacement is in agreement with geological and seismological observations. Since there were no stronger earthquakes in the vicinity and time span of monitoring, no correlations were established with seismic activity. The observed displacement rates along 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.
Izvleèek
V letu 2004 smo v zahodni Sloveniji pri~eli z opazovanjem tektonskih premikov ob dveh aktivnih prelomih Dinarskega prelomnega sistema z mehanskimi ekstenziometri TM 71. V dveh letih opazovanja so se razvili jasno izra`eni premiki, ki omogo~ajo prvo interpretacijo. Na razpoki v notranji coni Idrijskega preloma v dolini U~je smo izmerili levo horizontalno zmikanje s povpre~no hitrostjo 0,38 mm/leto. Hitrost premikov v kraj{em obdobju (10 mesecev) je celo ve~ja in dose`e 0,54 mm/leto. Ker je Idrijski prelom sicer desno-zmi~en, pojasnjujemo opazovan levi zmik z lokalno permutacijo glavne napetostne osi. Na razpoki v coni Ra{kega preloma v vzno`ju Vrem{~ice pri Ko{ani smo izmerili reverzno dvigovanje krovninskega bloka (SW) s povpre~no hitrostjo 0,24 mm/leto in levo horizontalno zmikanje s hitrostjo 0,16 mm/leto. Hitrost vertikalnih premikov v kraj{em obdobju (9 mesecev) dose`e 0,53 mm/leto. Opazovan po{evni zmik se ujema z geolo{kimi in seizmolo{kimi podatki. V obdobju meritev nismo ugotovili nobene korelacije med premiki in potresno aktivnostjo, vendar v bli`ini opazovanih prelomov v tem ~asu tudi ni bilo mo~nej{ih potresov. Opazovane hitrosti premikov ob obeh prelomih, ki so do 0,5 mm/leto, so pri~akovane glede na regionalno stopnjo deformacij, ugotovljeno z GPS meritvami, ki je reda velikosti 2 mm/leto.
46
Andrej Gosar
Introduction
Within the COST 625 action (Stemberk et al., 2003) we set up deformation monitoring of three active faults in W Slovenia using five TM 71 extensiometers. In the first half of 2004 two TM 71 instruments were installed in Postojna cave system on the Di-naric oriented (NW–SE) fault that is situated about 1 km north from Predjama fault ([ebela et al., 2005). The third and fourth instruments were installed in November 2004 on Ra{a fault at the SE foot of Vrem-{~ica Mt. and on Idrija fault in U~ja valley (Fig. 1). The fifth instrument TM 71 was set up in 2006 on Kne`a fault which is located between Idrija and Ravne fault. After two
years ob measurements some clear trends of displacement were developed.
Tectonics of W Slovenia
Slovenia is situated at the NE part the Adria microplate, whose northern margin (Southern Alps-Dinarides) is highly deformed and backthrusted onto the central, less deformed part of the Adria microplate (Poljak et al., 2000). Collision of European and African plates results in predominantly N–S oriented recent principal stress direction in the region of Slovenia. This resulted in a system of conjugate strike-slip faults. In W Slovenia a right-lateral NW–SE oriented
Figure 1.
Micro-deformation
monitoring sites
equipped with TM 71
extensiometers on Idrija
fault in U~ja valley (1)
and on Ra{a fault at
the foot of Vrem{~ica
Mountain (2).
Slika 1.
Lokacije opazovanj
mikro-deformacij
s TM 71 ekstenziometri
na Idrijskem prelomu v
dolini U~je (1)
in na Ra{kem prelomu
v vzno`ju Vrem{~ice (2).
Monitoring of micro-deformations along Idrija and Ra{a faults in W Slovenia
47
strike-slip faults prevail and in E Slovenia a left-lateral NE–SW oriented strike-slip faults. In addition there are several W–E oriented reverse faults and north verging thrusts.
Adria’s major aseismic outcrop is the Istria peninsula. In northern Slovenia we observe a significant and sharp (few mm/ year) dextral (and transpressive) gradient in GPS velocities along the Sava fault and Periadriatic zone, suggesting that lateral extrusion in the NE Alps is still active and being driven by the CCW rotation of Adria (Weber et al., 2006). In External Dinarides GPS observations showed N- to NNE-di-rected movements in the range from 0.5 to 2 mm/year (Vrabec et al., 2006).
The basic structural characteristic of the External Dinarides is a dense pattern of faults in a NW–SE direction, in addition to the thrusts with the south-westward direction of thrusting (Placer, 1981). In External Dinarides strike-slip and transpression displacement along NW–SE trending faults is present.
Seismicity of W Slovenia
The territory of Slovenia can be considered as one of moderate seismicity. No surface rupture related to an earthquake has been detected so far in Slovenia. In External Dinarides we find moderate historical and recent seismicity (Fig. 2). Data from the
4S"N-
Figure 2.
Seismicity map
of Western Slovenia
(EARS catalogue for the
years 567–2004).
Slika 2.
Karta seizmi~nosti
zahodne Slovenije
(katalog ARSO
za obdobje 567–2004).
14'E
48
Andrej Gosar
last 20 years show that most earthquakes in SW Slovenia are situated along Ra{a and Idrija faults, delineated in NW–SE direction (Michelini et al., 1998). Focal mechanisms for the most earthquakes indicate right-lateral or reverse faults (Poljak et al., 2000). The strongest earthquake ever recorded in the Alps-Dinarides junction was the 1511 western Slovenia earthquake (M = 6.8). The exact location and mechanism of this event are still debated (Fitzko et al., 2005).
Although strike-slip and thrust-type dominate, there are also a few earthquakes with normal-type faulting. From the fault plane solutions it is evident that the governing stress in the region runs approximately in a N–S direction (Poljak et al., 2000).
The region of NW Slovenia undergoes a recent increase in seismic activity with two damaging earthquakes in the Upper So~a valley. The 12 April 1998 (Mw = 5.6) and 12 July 2004 (Mw = 5.2) earthquakes occurred on the NW–SE trending near-vertical Ravne fault in the Krn mountains at 7–9 km depth (Zupan~i~ et al., 2001). The focal mechanisms of both earthquakes show almost pure dextral strike-slip.
Monitoring of micro-deformations with TM 71 extensiometers
Within the COST 625 action five locations for TM 71 measurements of active faults in Slovenia were selected. Monitoring started in Postojna cave system on February 2004 ([ebela et al., 2005). In November 2004 two TM 71 instruments were installed on Idrija fault (U~ja) and on the Ra{a fault (Vrem{~ica) (Fig. 1). The fifth instrument in Slovenia was installed on Kne`a fault in November 2006 (Gosar et al., 2007).
TM 71 extensiometer
TM 71 is a mechanical extensiometer (Fig. 3) designed for installation on a narrow cracks (crack gauge) 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 (Ko{t’ák, 1991). The instrument provides three-dimensional results – displacement vector in two perpendicular planes (horizontal and vertical)  and  angular  deviations  (rotati-
Figure 3. TM 71 instrument for monitoring micro-displacements in three-directions.
Slika 3. In{trument TM 71 za meritve mikro-premikov v treh smereh.
on). The sensitivity of the system is: 0.05– 0.0125 mm in all three space co-ordinates, and 3.2 10-4 in angular deviations (Stem-berk 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. TM 71 instrument was developed at the Institute of rocks structures and mechanics of the Czech academy of science (Ko{t’ák, 1991).
Idrija fault
The best morphologically expressed fault in the region of W Slovenia is the Idrija fault, which is clearly visible in topography and in aerial or satellite images. It extends from the Italian border near Bovec to the Croatia in Gorski Kotar (N of Rijeka), having a total length of more than 120 km (Fig. 1). The strongest historical earthquake in the region, the »Idrija« earthquake in 1511 with estimated magnitude of 6.8 and max. intensity X (Fig. 2), is usually related to this fault (Ribari~, 1979), but its exact location and relation to the faults in the region is still not clear (Fitzko et al., 2005). The second strongest known event with magnitude 5.6 happened in 1926 at the SE end of the Idrija fault. However, recent se-ismicity in the vicinity of this fault is rather low (Poljak et al., 2000).
No terrestrial geodetic measurements of tectonic movements along Idrija fault were performed so far. In 1977 they established the geodetic network across the fault in Ka-
Monitoring of micro-deformations along Idrija and Ra{a faults in W Slovenia
49
nomljica valley near Idrija and performed the first measurements, but later the measurements were never repeated (Kogoj, 1997).
In November 2004 we installed the TM 71 device in the NW part of Idrija fault, where good exposure of the main fault zone was found in the U~ja valley near Bovec. The whole fault zone is here more than 1 km wide and was divided in outer and inner
fault zone by ^ar & Pi{ljar (1993). The TM-71 instrument is installed on a prominent crack in the central part of the inner fault zone (Figs. 4 and 5) which cut 50 m high wall of a canyon.
In first 10 months of measurements a clear trend of horizontal displacements y = + 0.54 mm/year was developed (Fig. 6). It was followed by an outlying measurement at the beginning of 2006. It was most
Figure 4. Left: the crack in the inner zone of the Idrija fault in the U~ja valley. The arrow shows the
location of TM 71 instrument. Right: The exposure of the fault plane 50 m from the crack shown in the
left figure with clear striations indicating subhorizontal movements.
Slika 4. Levo: razpoka v notranji prelomni coni Idrijskega preloma v dolini U~je. Pu{~ica ka`e lokacijo
in{trumenta TM 71. Desno: izdanek prelomne ploskve 50 m stran od razpoke na levi sliki z izrazitimi
drsami, ki ka`ejo na subhorizontalne premike.
50
Andrej Gosar
[mm] displacement 0.8
Idrija fault
			^J^	
				
				40.38 mm/yr |
				
				
j 40.S4IT1	n/yr 1/7 v*^			Z
				
\				
years
|pi/200] angular deviation 0 06
2004.5                  ?no^o
years
Figure 5. The sketch of TM 71 instalation on the
crack in the Idrija fault zone with indication
of observed displacement.
Slika 5. Skica namestitve TM 71 na razpoki
v Idrijski prelomni coni z ozna~enim
ugotovljenim premikom.
probably caused by a mechanical impact on the instrument (fallen rock or ice), because later the same trend continued. The average displacement rate for the first two years of measurements is + 0.38 mm/year. In the same period no clear trend in angular deviation was developed in both planes.  Relative  movement  between  blocks shows left-lateral horizontal displacement (Fig. 5). This is unexpected result, because Idrija fault is considered as a dextral strike slip fault (^ar & Pi{ljar, 1993). There are clear geological evidences of dextral displacement for the geological history (Placer, 1982), but for recent times no direct proofs are available. Therefore the dextral strike slip movement was mainly inferred from the orientation of principal stress axis, which has N–S direction and from the fault plane solutions of two stronger earthquakes  that  occurred  on parallel Ravne fault. Observed left-lateral displacement should be therefore explained by local permutation of stress direction inside very complex fault zone (Kav~i~, 2006).
In any case longer observations and detailed tectonic mapping of complex Idrija fault cross-section in the U~ja valley are necessary to understand the relations between different cracks observed in outer and inner fault zone. X-axis (opening or closing the crack) shows clear seasonal effects, which are in good corre-Figure 6. Displacements and angular
deviations recorded with TM 71 at
the monitoring site U~ja on the Idrija
fault. +x represents closing of crack,
+y horizontal left-lateral slip,
+z downslope slip of SW block.
Slika 6. Premiki in kotne deformacije
izmerjeni s TM 71 na lokaciji
opazovanja U~ja na Idrijskem
prelomu. +x predstavlja stiskanje
razpoke, +y levo horizontalno zmikanje, +z spu{~anje SW bloka.
Monitoring of micro-deformations along Idrija and Ra{a faults in W Slovenia
51
lation with recorded temperatures. On the other hand no trend was developed yet on z-axes.
Ra{a fault
The second important fault located approximately 25 km SW from the Idrija fault  is  the  Ra{a  fault  (Placer,   1981), which can be clearly traced in a length of 50 km from Anhovo (N of Nova Gorica) in the So~a valley to the Sne`nik thrust at Ilirska Bistrica (Fig. 1). Main features related to this fault are: almost straight valley of the Ra{a river between Kobdilj and [tor-je and its clear expression in topography around Vrem{~ica Mountain. A cross-section of this fault is well exposed near Seno`e~e where     the     highway crosses the fault trace. Otherwise    there    are only  few  good  exposures of the fault. The seismicity in the vicinity of the Ra{a fault is concentrated mainly in the Sne`nik Mt. area at its SE termination (Fig. 2). Hypocenters of the earthquakes in this area  define  a  steeply NE dipping fault plane (Michelini  et al., 1998).
We explored the whole trace of the Ra{a fault to find a suitable location for installation of TM 71 extensio-meter. In spite of its clear expression in the topography we recognized that there are very few good exposures. The best location was found at the foot of Vrem{~ica Mt., on its SE side, near Ko{ana (Fig. 7). There are two abandoned quarries in the Upper Cretaceous limestone situated exactly at the fault trace. There is a plan to put the upper quarry again in operation, but  the  lower one  is
abandoned for more than 15 years and will remain closed. Therefore, it provides a suitable place for crack gauge measurements. The contact between Upper Cretaceous limestone to the NE and Palaeocene Kozina limestone to the SE (Buser et al., 1967) is exposed in this quarry, separated by the main fault zone, which is approximately 10 m wide. Therefore, it was not possible to install the instrument in the main fault plane, but we selected a parallel crack (Figs. 7 and 8) in the exposed wall of the quarry built of Cretaceous limestone. The measurements started in November 2004.
In two years of observations the average reverse uplift of hanging wall (SW) block
Figure 7. The quarry at the foot of Vrem{~ica Mt. near Ko{ana on Ra{a fault zone with location of TM 71 instrument.
Slika 7. Kamnolom v vzno`ju Vrem{~ice pri Ko{ani v coni Ra{kega preloma z lokacijo in{trumenta TM 71.
52
Andrej Gosar
Figure 8. The sketch of TM 71 instalation on the crack in the Ra{a fault zone with indication of observed displacement. Horizontal plane (left) and vertical plane (right).
Slika 8. Skica namestitve TM 71 na razpoki v Ra{ki prelomni coni z ozna~enim ugotovljenim premikom. Vodoravna ravnina (desno) in navpi~na ravnina (desno).
of – 0.24 mm/years and left-lateral displacement of + 0.16 mm/year were established (Figs. 8 and 9). Short term vertical displacements reached the value of – 0.53 mm/year (Fig. 9). The oblique sense of displacement is in agreement with geological and seis-mological observations. Focal mechanisms of some stronger earthquakes in the active zone of Sne`nik Mt., SE of the monitoring site, indicate strike slip and reverse character. X-axis (opening or closing the crack) shows clear seasonal effects, which are in good correlation with recorded temperatures. No trend in angular deviation was developed in both planes so far.
Conclusions
The observed displacement rates of up to 0.5 mm/year at both monitored active faults are consistent with the regional GPS deformation rate in W Slovenia established from GPS measurements which is of the order of 2 mm/year (Vrabec et al., 2006). Since there were no stronger earthquakes in the vicinity and time span of monitoring, no correlations were established with seismic activity.
In addition to described monitoring sites in W Slovenia another one was set up in November 2006 at Kne`a fault near Tolmin (Zadlaz-^adrg).
Acknowledgments
The installation of TM-71 instruments in Slovenia was realised with the support of COST 626 action 3D monitoring of active tectonic structures. The author is grateful to Josef Stemberk and Blahoslav Ko{t’ ák for their cooperation in realisation of monitoring in Slovenia, to Toma{ Nydl, Lubomir Petro and Josef Hok for their help during installation of instruments and to Mojca Kav~i~ for taking regular readings.
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Monitoring of micro-deformations along Idrija and Ra{a faults in W Slovenia
53
Figure 9. Displacements and angular deviations recorded with TM 71 at
the monitoring site Vrem{~ica on the Ra{a fault. +x represents closing of
crack, +y horizontal left-lateral slip, +z downslope slip of SW block.
Slika 9. Premiki in kotne
deformacije izmerjeni s TM 71
na lokaciji Vrem{~ica na Ra{kem
prelomu. +x predstavlja stiskanje
razpoke, +y levo horizontalno zmikanje, +z spu{~anje SW bloka.
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