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Monitoring tectonic movements in the Simitli Graben, SW Bulgaria
Engineering Geology 57 (2000) 179–192 www.elsevier.nl/locate/enggeo
Monitoring tectonic movements in the Simitli Graben, SW Bulgaria N.D. Dobrev a, *, B. Kosˇt’a´k b a Department of Engineering Geodynamics, Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Street, block 24, 1113 Sofia, Bulgaria b Institute of Rock Structure and Mechanics, Academy of Sciences of Czech Republic, V Holesˇovie`ka´ch 41, Prague 8 18209, Czech Republic Received 30 June 1999; accepted for publication 7 February 2000
Abstract Long-term in situ monitoring of slow tectonic movements has been applied to a seismoactive region of SW Bulgaria, within the epicentral zone of one of the strongest earthquakes in Europe (4 April 1904, M=7.8). The region has been found the most seismoactive in Bulgaria being of interest to many scientists. Three spatial extensometers were installed here in carefully selected sites to reflect fault movements on fissures. The extensometer TM-71 used here, enables three-dimensional detecting of even very slow movements with the accuracy of 0.01 mm and high stability over time. After 17 years of measuring, the rates of tectonic movements were established at all three monitoring points. Movements recorded at point B6 located in the seismoactive Kroupnik fault zone are of a relatively high rate. Locally, they show left-lateral strike–slips at rates of ca. 2.7 mm year−1, as well as thrusting with a mean rate of 1.9 mm year−1. Monitoring point K7 located in a fissure of the same zone on a steep slope affected by recent earthquakes has shown an uplift tendency of the block W of the Strouma Fault, with a result of gradual slope subsidence occurring from time to time. Monitoring point K5 located in a fissure of Strouma Fault zone became increasingly active during the last 2 years after 8 years of relative quiescence. Before that, only low left-lateral movements could be observed. Long-term fissure monitoring has shown quite a number of details interpretable to the dynamics of a broad region. Permanent shear displacements were found to develop after earthquakes. It was established that only a certain distinct part of local earthquakes provide such a displacement reaction at the monitoring points showing particular seismic connections. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Displacement; Extensometers; Faults; Monitoring; Southwest Bulgaria
1. Introduction Complex tectonic structure in SW Bulgaria with a composite fault mosaic, where the epicentral zone of one of the strongest earthquakes in Europe is located, has been studied by many scientists for years. It is the Simitli Graben in the Strouma * Corresponding author. Fax: +359-2-718148. E-mail address: [email protected] (N.D. Dobrev)
River section, which has been studied most intensively ( Fig. 1). Here, two large earthquakes occurred on 4 April 1904, 10:02 and 10:25 GMT, M=7.3 and 7.8, respectively (Christoskov and Grigorova, 1968; Shebalin et al., 1974). In spite of insufficient historical data, there are reports that at least two other strong earthquakes occurred in this region, one on 4 September 896 AD, and the other on 6 December 1866 AD ( Watzof, 1902; Shebalin et al., 1974). Presently, it is the most
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Fig. 1. Neotectonic sketch of Simitli Graben and neighbouring territories (according to Zagorcev, 1992b). Grabens: 1, Simitli; 2, Blagoevgrad; 3, Padesh; 4, Razlog; 5, Brezhani; 6, Sandanski; 7, Delchevo. Horsts: 8, Rila; 9, Pirin; 10, Kresna; 11, Vlahina; 12, Lissia; 13, Kachov Rid. Main faults: KF, Kroupnik Fault; SF, Strouma Faults; GF, Gradevo Fault; BF, Brezhani Fault.
seismoactive region in Bulgaria. Every month, ca. 10–20 earthquakes occur here. Approximately 30% of those earthquakes reach magnitude of ca. M=3. Continuous tectonic activity is evident in the region with a high distribution of large landslides along the main fault lines (Dobrev, 1997, 1999; Dobrev and Boykova, 1998). The landslides and the other slope processes seriously endanger the important railway and Sofia–Thessaloniki Road that pass through the graben. So far, they are the only transport connections between Bulgaria and Greece. Within the last few years, the road and the railway were destroyed several times by landslides, debris flows and rockfalls. The gas pipeline, which passes through a creeping landslide slope and the active Kroupnik fault, is also at risk. These problems highlight the need to evaluate the rates of tectonic movements in this area. The impact of tectonic movements on landslide distribution has been already analysed (Dobrev and Boykova, 1998). Many different studies of the fault dynamics in the Simitli Graben, and the adjacent Kresna Gorge have been carried out so far concerning geodynamic development (Hoernes, 1904; Zagorcev, 1969, 1992a,b; Moskovski and Georgiev, 1970; Vrablyanski, 1974, and others), as well as recent and present movements (Milev et al., 1984; Milev and Vrablyanski, 1988, and others). Geodetic measurements have recently been completed using the global positioning system searching for block movements. In 1982, three observation points were estab-
lished to monitor faults in the region using a new method ( Kosˇt’a´k and Avramova-Taceva, 1988). It is believed that the results have an importance reaching beyond the local interests. This presentation cannot be carried out without a short description of the local geological–tectonic situation, and some seismic data. It also shows that local and global geodynamical theories and models should be supplemented also by such observations in detail, which can quantify deformations and movements.
2. Geological–tectonic characteristics of the region The Simitli Graben is situated in SW Bulgaria along the Strouma River ( Fig. 1), at the crossing point of some significant fault structures: the Strouma fault zone ( local orientation 150–170°), the Kroupnik Fault (20–80°, general orientation 40°); and the Gradevo Fault (30–40°) (Boyadjiev, 1971; Zagorcev, 1975). A longer part of the graben has developed along the Kroupnik and Gradevo faults, while the Strouma fault zone is bed of the Strouma River (Fig. 2). Further to the south, the river follows the Strouma Fault into the Kresna Gorge cut into the Kresna Horst. The Simitli Graben originated during the late Miocene over Precambrian metamorphites and Palaeozoic and Upper Cretaceous granites. The graben is filled with Neogene slightly lithified sediments: sandstones; conglomerates; clays and coal — with a maximum thickness of 1500 m.
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Fig.2. Scheme of the more important faults in Simitli Graben with the localisation of the monitoring points in the geomorphological situation. The main faults are: 1, Kroupnik Fault; 2, Gradevo Fault; 3, Strouma faults; 4, Brezhani Fault.
Quaternary deposits are deluvial–proluvial at the foothills of flanked horsts as well as alluvial sands and gravels of the Strouma River terraces, accumulated in levels given by vertical movements. The southern border of the graben is defined by the seismoactive Kroupnik Fault (Fig. 2). To the west, it can be followed by a distance of ca. 20–30 km beyond the western boundary of the graben. Kachov Rid Horst is defined by the Kroupnik Fault (NW border) and the Brezhani Fault (SE border). It is closed between the Krusto and Kachov Chukar Peaks (Fig. 2). This horst divides the Simitli Graben and the Brezhani Graben. The Strouma fault zone crossing the Simitli Graben is a long and wide tectonic structure, built by a number of parallel fault fractures. It extends from the Chalkidiki Peninsula to the Pannonian Basin (Jaranoff, 1960; Zagorcev, 1969, 1975, 1992a,b; Moskovski and Georgiev, 1970; Boyadjiev, 1971; Vrablyanski, 1974). In the framework of the Simitli Graben, it is demonstrated by many slickensides in crushed and mylonised zones.
3. Fault dynamic observations Geodetic observations indicate an uplift of the Kresna Horst at the rate of ca. 2 mm year−1, and less in the Neogene grabens along the Strouma River — a rate of <2 mm year−1. Levelling benchmarks along the Strouma River between the towns of Simitli and Kresna ca. 14 km south of the fault crossing point have shown alternating signs of vertical movements within 30 mm between 1956 and 1992 (Milev, 1999). Geomorphological studies indicate that the Kroupnik Fault can be described generally as a normal fault with a left-lateral strike–slip component. Its fault trace is divided into sections determined by a system of transverse faults. Rates of vertical and horizontal components of fault movements may be different at each section along the Kroupnik Fault, closed between two neighbouring transverse faults. Strike–slip is left-lateral everywhere, but with variable rates. The vertical component of movements along the Kroupnik Fault is normal, but in two sections it is reverse (Melo and
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Gradishteto Hills). These reverse movements can be explained locally — the local stress field situation and the fault pattern is very complex. Landslides and erosional forms indicate strong movement along the full length of the fault section with the Shirinata–Shturbovska Mahala villages reaching the maximum between the Krusto Peak and the river. The left-lateral component of movements along the Kroupnik Fault can be confirmed by the way the Strouma River meanders in front of the entrance to the Kresna Gorge. The fault movements in the studied area are considered to be generally caused by NE–SW compression, and NW–SE extension (Zagorcev, 1992b; Matova et al., 1996), resulting in left-lateral movements on the Kroupnik Fault, and rightlateral slips on transverse faults. Shanov and Dobrev (1999) calculated the subhorizontal extension to be 281°/37°, as well as compression N11° for the Kroupnik Fault, with a reorientation in the eastern part of the graben to 20–45°/58– 65°NW (Dobrev, 1999). These results are based on slickenside investigations in four outcrops and explain the left-lateral strike–slip along the Kroupnik Fault. The kinds of movements along the Strouma Fault have also been discussed. Moskovski and Georgiev (1970) found right-lateral movement and thrusting, with an uplift in the western side. This is confirmed by the most of the previous investigations (Zagorcev, 1969, 1970, 1975; Vrablyanski, 1974, etc.). However, this is contrary to the fact that major tectonic planes in the gorge are inclined to NE, therefore thrusting should rather be in favour of an eastern side uplift. Milev (1999) concluded that uplifts at present read 0 to +3.4 mm year−1 for the western of Kresna Horst separated by the Strouma River, and +1.4 to +3.4 mm year−1 for the eastern block. Gradients may even change signs. Concerning horizontal movement, results fall to error limits, and the direction of horizontal movements cannot be defined in the observed network in the studied region. The outlined discussion shows the need for a more precise and detailed measurement to quantify present movements trends even in active fault
structures such as the epicentral zone of the Simitli Graben.
4. Monitoring of the fault structures A thorough investigation of local fault structures resulted in the selection of three suitable sites for monitoring and potential active tectonic movements. Point K5 ( Fig. 2) is situated in Kresna Gorge on one of the parallel fractures of the Strouma Fault. There, ca. 80 m above the left bank of the Strouma River, a huge block of granite is separated from a steep rock slope by a fault fracture zone 2.5 m wide filled with broken and crushed granite. Point K7 ( Fig. 2) represents a fissure of the Kroupnik fault zone strongly affected by the seismic events of 1904 (Grigorova and Palieva, 1968; Dobrev and Avramova-Tacheva, 1999), when a remarkable seismogravitational slope deformation originated ca. 700 m SE of the village of Kroupnik. Eight apparent steps in the slope dipping 30–35° NW are separated by a number of open fissures in the granite. This slope deformation represents a potential danger to Sofia–Thessaloniki Railway. The point K7 is situated in the centre of the slope deformation in a fissure 1 m wide, reaching to a considerable depth, as shown by steam coming out in the winter (Dobrev and AvramovaTacheva, 1999). Radon outburst has also been noticed here ( E. Avramova and B. Ranguelov, personal communication). The main fault of this seismoactive Kroupnik zone is believed to coincide with the fault slope. Point B6 (Fig. 2) is situated on the Kroupnik Fault near the village of Brezhani. Here, the fault comes out in the terrain as a continuous apparent step or scarp, which represents contact between the Precambrian amphibolites and the Neogene coarse-grained sediments of the Simitli Basin. A contact zone 3.4 m wide was opened here by a trench to install the instruments. At all three points a steel tube bridging the opposite bodies in contact was installed to monitor differential movements between the two sides. The tube was rigidly cemented into drill holes in the opposite rock walls in the form of two separate
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Fig. 3. A view of the gauge TM-71 (after Kosˇt’a´k, 1977).
holders leaving space for a gauge system. In the case of the point B6, a special arrangement was adopted to fix the tube holder into the incompetent sedimentary rock ( Fig. 3). The points were instrumented with TM71 crack gauges using a method described earlier (Avramova-Tacheva et al., 1984; Kosˇt’a´k and Avramova-Taceva, 1988). The moire´ extensometer ( Kosˇt’a´k, 1969, 1977, 1991) has is capable of detecting movements and microdisplacements in three dimensions (Fig. 4). It works on the principle of mechanical interference — moire´, which records displacement as a fringe pattern on superposed optical grids mechanically connected with the opposite walls or crack faces. Due to this principle, which completely avoids any electrical transmission, the gauge displays an extremely large longterm stability, and infallible performance under hard outdoor conditions. Practically, it means that values recorded over decades can be well compared. Results will then be provided as displacements on structural planes in millimetres, and time
trends derived as rates in millimetres per year under hard field conditions in a prolonged operation. Long-term trends of slow microdisplacements could be detected effectively with a final accuracy of 0.01 mm provided that measurements are taken for at least 3 years. It can discern seasonal as well as other environmental effects, that need to be recognised and compensated for in the data. Temperature effects in the system including holders are eliminated numerically, while such effects upon the rock are not eliminated in the data and are observed in climatic cyclic variations. The data were obtained in three Cartesian coordinates, calculated from recorded interference patterns. They represent differential movements according to Table 1. It is always: X, horizontal, across the contact; Y, horizontal slip; and Z, vertical displacement. The movements are relative between the two sides, presented in graphs as displacements of the lower block on the slope to the opposite one, although interpolation must consider the movement at both sides.
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Fig. 4. Cross-section at monitoring point B6 (after Avramova-Tacheva et al., 1984): 1, amphibolite; 2, grey-green tectonic clay, 0.7 m thick; 3, amphibolite breccia with clayey matrix; 4, grey-black tectonic clay, 0.25–0.30 m; 5, crushed granite blockage with clayeysandy matrix, 0.85 m; 6, granite blockage with sandy matrix; 7, soil stratum and deluvial clay; 8, technogenic embankment; 9, concrete; 10, former relief profile line.
Resulting movements in the three points with a monthly observation frequently, for more than one decade (between 1982 and 1999) are given in Figs. 5–7. 5. Observed movements 5.1. K5: Kresna Gorge–Strouma Fault (Fig. 5) X: quasi-sinusoidal character of movements with a mean amplitude 1.2 mm in seasonal variation; 1982–1993 very slow long run compression increase; since 1994 — gradual change to extension 0.3 mm year−1; 1986–1987 several sequences of sudden pulsation — seismic events: 15 May 1986, M=4.2, H=19 km, epicentre 9 km N of K5, and fore-
and aftershocks (Solakov and Simeonova, 1993); Y: less pronounced seasonal variation (within 0.3 mm) interrupted by a number of reversed slips (up to 0.5 mm); 1982–1988 no trend shown; since 1989 — gradual change to left-lateral slips 0.1–0.4 mm year−1; Z: 1982–May 1986 — subsidence 0.25 mm year−1 with several reversed slips; 15 May 1986–July 1986 — co-seismic slips 0.73–0.88 mm; July 1986–December 1987 — subsidence re-established, one reversed slip in summer 1987 coincident with X compression; 1988–1995 — no trend; since 1995–sudden increase of subsidence 1.26 mm year−1.
Table 1 Space orientation of the axes of the monitoring points in SW Bulgaria (after Avramova-Tacheva et al., 1984; Kosˇt’a´k and AvramovaTacheva, 1988) Monitoring point No.
+X, Widening of the horizontal zone
+Y, Horizontal shear movement in the fault (fissure) direction
+Z Vertical movement
Local fault orientation (°)
K5 B6 K7
SW block to 225° Neogene basin to 290° The northern block to 350°
SW block to 135° Neogene basin to 200° The northern block to 260°
Subsidence of the SW block Subsidence of the Neogene basin Subsidence of the northern block
150 20–40 80
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N.D. Dobrev, B. Kosˇt’a´k / Engineering Geology 57 (2000) 179–192 Table 2 General trends at point B6 Period
Component X
Y
Z
August 1982 – December 1983
ca. +0.9 mm year−1 extension of the fault zone
ca. +3.5 mm year−1 left-lateral movement
January 1984 – October 1985 November 1985 – December 1987 January 1988 – December 1992 January 1993 – April 1996 May 1996 – July 1998 August – Sep. 1998 October 1998 – February 1999 March 1999 – May 1999
−4.39 mm year−1 fault zone compression +1.07 mm year−1 fault zone extension +1.89 mm year−1 fault zone extension +2.93 mm year−1 fault zone extension +1.44 mm year−1 fault zone extension
+1.16 mm year−1 left-lateral movement +4.63 mm year−1 left-lateral movement +2.60 mm year−1 left-lateral movement +1.51 mm year−1 left-lateral movement +0.54 mm year−1 left-lateral movement −0.15 mm year−1 right-lateral movement +4.91 mm year−1 left-lateral movement +0.23 mm year−1 left-lateral movement
ca. −1.5 mm year−1 thrusting (uplifting Neogene basin) −9.28 mm year−1 thrusting −0.98 mm year−1 thrusting −1.92 mm year−1 thrusting −1.89 mm year−1 thrusting −0.34 mm year−1 thrusting +0.25 mm year−1 normal fault movement +1.94 mm year−1 normal fault movement +0.81 mm year−1 normal fault movement
fault zone extension ca. +1.8 mm year−1 fault zone extension
On 3 May 1999, the total displacements found at that point were as follows: X=1.68 mm, Y= 0.83 mm, and Z=3.00 mm. The vector of total displacement are u =3.54 mm. The maximal xyz total displacement u found at this point is xyz recorded on 29 March 1998, reaching 4.62 mm (X=0.86 mm, Y=0.55 mm, and Z=4.50 mm). 5.2. B6: Brezhani–Kroupnik Fault (Fig. 6) Total displacements found at this point between August 1982 and May 1999 are quite impressive: X=19.15 mm, Y=40.30 mm, and Z=−33.09 mm, and the vector of total displacement u =55.55 mm. xyz Very clear trends can be found in intervals separated by individual abrupt displacements. A seasonal effect of temperature is evident in the Xcomponent only, and eliminated in calculations using the least squares method (Dobrev and Avramova-Tacheva, 1997). The mean rates fluctuate ca. 1.9 mm year−1 at axis Z and 2.7 mm year−1 at axis Y. Results in nine characteristic periods are given in Table 2.
As far as the earthquakes are concerned, a characteristic development of movements has been observed in the last period ( Table 2). It may be seen that during the last 2 years the rate of leftlateral horizontal slips was gradually decreasing, and the movements were changing their character to right-lateral. This occurred in the short period of August–September 1998, while the vertical movements changed to normal faulting. On 26 October 1998, 10:00 GMT, a local earthquake M=3.5 occurred there ( Toteva et al., 1999). The co-seismic displacements were DX=4.4 mm, DY= 1.8 mm and DZ=2.7 mm. After this earthquake the rate of left-lateral strike–slip considerably increased, while the vertical movement returned to normal.
5.3. K7: Kroupnik–Kroupnik Fault (Fig. 7) X: seasonal variation (within 1 mm) recognised only after summer 1992; trend not clear till the beginning of 1987; since 1987 — a progressive trend to fissure opening (0.29 mm year−1);
Fig. 5. Diagram of displacements found at Point K5. Schematic block-diagram of the situation at the monitoring point K5 and the orientation of the axes in space included.
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Fig. 6. Diagram of displacements found at B6. Schematic block-diagram at Melo Hill and the orientation of the axes is included.
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Fig. 7. Diagram of the displacements found at K7. Schematic situation of monitoring point K7 and the orientation of the axes is included.
July 1985 — extreme displacement (fissure compression by 0.7 mm); Y: calm period interrupted by abrupt slips (0.2–0.5 mm month−1) over 6–18 months; July 1985 — extreme right-lateral slip by Y= −6.9 mm; August 1985–October 1989 — trend to leftlateral slip (+0.18 mm year−1); October 1989–February 1994 — trend reversed to right-lateral slip (−0.22 mm year−1); Z: calm period interrupted by abrupt slips (0.3– 0.8 mm) over 3–6 months; the slips are of subsidence in character until 1991; later reversed;
July 1985 — extreme vertical displacement Z=−6.0 mm (subsidence of the upper block in the fissure most probably). On 24 February 1994, the total displacements found at that point were as follows: X=1.26 mm, Y=−5.33 mm, and Z=−4.85 mm. The vector of total displacement is u =7.32 mm. xyz 6. Discussion Our three observation points represent three different situations considering the fault configuration, potential slope movements, and relation to the Simitli Graben. Two of them, K5 and K7, are located in hard rock, and observed displacements
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are of the same magnitudes, that is, within several millimetres per decade while the third is located in a contact between competent and incompetent rock, and observed displacements are within few millimetres per decade. All the three points are placed on tectonic fractures within a larger tectonic zones, and all are on slopes, which may be suspected of producing gravitational deformation interfering with the tectonic one. 6.1. Slope gravitational movements It is just the point B6 out of the three where the gravitation movements are most likely, and vertical movements show upslope displacements of the lower lying sediments, that is, movements opposite to gravitational ones. There is no doubt, therefore, that at least the movements observed at B6 are due to active tectonics. Contrary to that, it is the point K5, where a most stable situation appears to be, showing downslope movements of the lower block. Therefore, it is the active tectonics, which is obviously the primary factor of the region.
6.2. Effects
1. 2. 3. 4.
Four types of effects can be observed: long-time trends; short-time trends; abrupt displacements; and seasonal variations.
The last effect is connected mainly with the Xcoordinate, showing winter opening and summer closing of the contact, and is not an object of our discussion. Long-time trends and their progressive development are found mostly in the Y and Z components showing a slipping process along the joint. They are probably of primary importance, as they can indicate source of movements or orientation of active sources. Short-time trends will then indicate short-time deviations from general patterns in which the forces develop. They may reflect slip development in the depth as well as of the local friction conditions. They may be of interest if a typical and
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repeatedly uniform reaction is detected and correlated with tectonic events. Abrupt displacements are typical of seismic regions. They are mostly the after-effects of vibrations due to earthquakes being displaced to a new position. They will follow general slip trends, although exceptions to this rule are also frequent, depending on local conditions in the system of joints or even to orientation in respect to epicentre. They can represent a local stick-slip process triggered by distant forces. Generally, the observed reactions in active tectonic regions will represent a local response to distant powerful processes, thus reflecting positional redistribution in the joint system, or even a progressive change in the boundary conditions of large blocks. It is just the last objective, which is the most attractive and leading aspect for the selection of observation points on a given fault system. However, it calls for a long range observation. 6.3. Interpreted results In the interpretation it should be clear that movements must be always seen as relative to the two sides in contact, allowing for both sides to move. 6.3.1. Point B6 Interpretation of the negative vertical slips in the basin as real uplifts with respect to the core blocks of the Pirin and Rila Mts is unlikely. It is rather a contact pressure, which thrusts the marginal zone of sediments as a plough lifting it up slowly. However, the pressure comes from the mountainous side at an angle of N 11° (Shanov and Dobrev, 1999), resulting in a horizontal slip: left-lateral strike–slip in the point. The local positive vertical movements can be additionally confirmed by the deeply grooved gullies in this part of the graben. A local horizontal pressure reaches the point from SSE, indicating movement of the Kachov Rid Horst in the northern direction here. The basin is under constant pressure of the flank mountains, which is indicated by the X-component
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measured after 1988. An earlier extension period can be explained locally. Individual abrupt slips were investigated earlier ( Kosˇt’a´k and Avramova-Taceva, 1988, 1993), and in most cases may be interpreted as the aftereffects of known earthquakes. It is established that the contact reacts to events from far south. The Strouma Fault seems to be an effective lead to earthquake vibrations (Shanov and Dobrev, 1997). Short-time trends have recently been indicated and investigated (Shanov, 1993; Shanov and Dobrev, 1997). Normalised models of the deformation rate changes before strong earthquakes were derived. It has been established that the similar anomalies in the helium content of the thermal springs in the town of Simitli occur 4– 6 days before the earthquakes ( Ranguelov and Shanov, 1991). 6.3.2. Point K7 The point lies on the same Kroupnik Fault, separated from B6 by Strouma Fault. The main effect was the abrupt slip recorded in July 1985. In spite of the relatively low seismic activity of 1985, attempts were made to find a source of the displacement. However, only an earthquake of 17 September, Pehchevo, M=3.4 (Solakov and Simeonova, 1993), ca. 14 km W of K7 was felt in this area, that is, after the slip. If the idea of the source of Montenegro (M=5.2, May 1985), due to the distance is rejected, then the question of an earthquake source is hard to explain. It is remarkable, however, that the slip was not a subsidence of the outside slope block, but the slip was reverse. Moreover, it was connected with a horizontal slip, a right-lateral strike–slip by ca. 7 mm. Therefore, rather than a slope gravitational slip, a strike from the mountainous side was indicated, with considerably large pressure component from the east. For a long period after the slip until 1991, the Xcomponent had tendency to show release in pressure with a number of additional continuing slips in Y as well as continuing vertical slips in Z. After that the trend reverses. Generally, the situation has shown pressure conditions along the edge of the Kroupnik Fault with an uplift tendency of the block W of the Strouma Fault, resulting from time to time (e.g.
after 1991) in gradual subsidence in the slope deformation. 6.3.3. Point K5 Even after 17 years of observation, this point set in the Strouma Fault did not show any trend in X, which is contrary to the two points set on the Kroupnik Fault. This can be interpreted as a sign of normal compression across the fault. At the same time, the Z-component can be interpreted as an uplift in the eastern section of the Kresna Horst rather than subsidence in the slope. The Ycomponent exhibits an occasional tendency to smaller lateral strike–slips, left-lateral are prevalent. Such a slip is increasingly pronounced after 1992, while reversed to right-lateral in 1997 in an obvious connection with a sharp increase of vertical movement. After that, the tendency noticed before 1997 reappeared. Later developments will be of a special interest.
7. Conclusions 1. Data obtained after 17 years of observation in the three points of the investigated faults in SW Bulgaria provide consistent results, indicating fault boundary conditions in the investigated area. A following stress and movement configuration can be derived: $ Rigid rock of the mountainous blocks S of the Kroupnik Fault is under pressure. Principal compressive forces follow the rim of the Kroupnik Fault oriented close to E–W, stimulating pressure across the Strouma Fault. $ An overall tendency to uplifting in the mountains, confirmed also by levelling (Milev, 1999), creates an additional pressure against sedimentary fill of the Simitli Graben due to an upheaval and so even gravitational spreading in the mountainous mass. Such a pressure induced by flank hills of the mountains results in local uplifts in more ductile sedimentary strata along the Kroupnik Fault. $ It has been confirmed that the horizontal component of movements along the NE section of the Kroupnik Fault is a left-lateral strike–slip. The right-lateral strike–slip along the western section, which has been found at point K7, may
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be explained only as local movements, not representative for the fault. $ It is rather the fault structure sloping to E by ca. 80–85°, which is now effective in thrust slipping of the Strouma Fault. Presently, small left-lateral strike–slips, with occasional rightlateral breaks were noticed. $ Major movements point to SE as at the source pole of the movement activity, to the eastern section of the Kresna Horst. $ Movements in the contact line between the competent rock edge and the sedi-mentary fill of the Simitli Graben reach a mean rate of ca. 2 mm year−1 and horizontal long-run slips (NE section) with earthquake strikes up to 6 mm individually. $ It is the Strouma Fault structure which has been found as the main connector with the earthquake epicentres effecting the slip movements on the Kroupnik Fault. 2. A characteristic development of movements may be observed during the period of an earthquake, before and after. 3. Monitoring points of the fault movements can be successfully established on fault structures in tectonically active regions. They may provide data about boundary conditions and stress as well as movement configuration in the fault structure of the investigated region. The Kroupnik Fault monitoring program in SW Bulgaria started as an experiment. As such, it proved that even ground measurements in incompetent rock can be successfully organised. It is evident that in spite of some technical problems, it is just a contact between competent and incompetent rock, which may well provide valuable data, since ductility of bodies such as sediments provides freedom for a rigid block in contact to move. A thorough field investigation of the terrain prior to the decision about the placing of the points and their instrumentation are an integral part of any monitoring program. Detailed fault monitoring may allow recognition of data that are beyond the capacity of global observations. Acknowledgements This experimental field study was supported financially by the Ministry of Education and
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Sciences of Bulgaria (Grants Nos. NZ-23/91, NZ-635/96 and 5053/MON ), and Grant Agency of the Czech Republic, Grants Nos. 205/94/1769 and 205/97/0526. During the years of the research successful cooperation was carried out thanks to the joint efforts of scientific staffs of the Geological Institute of the Bulgarian Academy of Sciences in Sofia and the Institute of Rock Structure and Mechanics of the Academy of Sciences of the Czech Republic in Prague. The authors are grateful for the encouragement and suggestions by E. Avramova-Tacheva, I. Broutchev, S. Shanov, P. Petrov and D. Evstatiev as well as colleagues from the Geophysical Institute of BAS who assisted with implementing and supporting the present monitoring. All such support essential for the project is gratefully acknowledged.
References Avramova-Tacheva, E., Vrablyanski, B., Kosˇt’a´k, B., 1984. An attempt to detect recent movements along seismogenic faults. Review of the Bulgarian Geological Society, Bulgarian Academy of Sciences XLV (3), 276–288, (in Bulgarian). Boyadjiev, S., 1971. Strouma deep fault. In: Yovchev, Y. ( Ed.), Tectonical structure of Bulgaria. Technica, Sofia, pp. 431–435, (in Bulgarian). Christoskov, L., Grigorova, E., 1968. Energetic and space-time characteristics of the destructive earthquakes in Bulgaria after 1900. Bull. of Geophysical Institute, Bulg. Acad. Sci. 12, 79–107, (in Bulgarian). Dobrev, N., 1997. The landslides in the Simitli kettle. Eng. Geol. Hydrogeol. 24, 41–65, (in Bulgarian). Dobrev, N., 1999. Geological hazard processes in the Simitli graben. Ph.D. Thesis (in Bulgarian). Dobrev, N., Avramova-Tacheva, E., 1997. Analysis and prognostication of monitored rock deformations, Proc. IAEG Conference, Athens. Balkema, Rotterdam, pp. 613–618. Dobrev, N., Avramova-Tacheva, E. 1999. Seismogenic terrain deformations in the Simitli graben. Symposium ‘‘95 years after the earthquakes of 1904 in Kroupnik–Kresna region’’, Blagoevgrad, Bulgaria, 27–28 April, 1999, pp. 137–146 (in Bulgarian). Dobrev, N., Boykova, A. 1998. Landslide hazard assessment in tectonic active grabens. In: Proc. of the 8th Congress IAEG, Vancouver, Canada, pp. 2007–2112. Grigorova, E., Palieva, K., 1968. Macroseismic characterisation of disastrous earthquake on April 4, 1904. Bulletin of Geophysical Institute 12, 109–111, (in Bulgarian). Hoernes, R., 1904. Berichte u¨ber das Makedonische Erdbeben vom 4. april 1904. Mittelungen der Erdbeben-Kommission
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der Kaiserlichen Akademie der Wissenschaften in Wien, XXIV, 54. Jaranoff, D., 1960. La tectonique de la Bulgarie. Technica, Sofia, (in Bulgarian, summary in French). Kosˇt’a´k, B., 1969. A new device for in situ movement detection and measurement. Experimental Mechanics 9, 374–379. Kosˇt’a´k, B., 1977. Tercove Meridlo TM 71 a jeho uziti pro mereni velmi pomalych pohibu na poruchach a trhlinach. Inzenyrske stavby 5, 213–218, (in Czech). Kosˇt’a´k, B. 1991. Combined indicator using moire´ technique. Proc. 3rd Int. Symp. Field Measurements in Geomechanics, Oslo, Balkema, Rotterdam, pp. 53–60. Kosˇt’a´k, B., Avramova-Taceva, E., 1988. A method for contemporary displacement measurement on a tectonic fault. Journal of Geodynamics 10, 115–125. Kosˇt’a´k, B., Avramova-Taceva, E., 1993. Recent rock mass movements investigated in the territories of Bulgaria and Czechoslovakia. Acta Montana, ASCR, Praha, Series A 4 (90), 7–24. Matova, M., Shanov, S., Nikolov, G., Kurtev, K., 1996. Tectonic factors for the seismicity of SW Bulgaria. In: Proc. of the XV Congress of Carpatho–Balkan Geological Association, Symposium on the Seismicity of the Carpatho–Balkan region, Athens, 17–20 September, 1995, pp. 30–35. Milev, G., 1999. Generalization of the results from the geodetic measurements in the region of the Strouma fault zone — the crossing point of the Strouma fault and the Kroupnik fault. Symposium ’95 years after the earthquakes of 1904 in Kroupnik–Kresna region’, Blagoevgrad, Bulgaria, 27–28 April, 1999, pp. 155–166, (in Bulgarian). Milev, G., Vrablyanski, B., 1988. Possibilities for the study of recent crustal movements in the Struma fault zone, on the basis of neotectonic and geodetic data and their relations to earthquakes. Journal of Geodynamics 9, 265–277. Milev, G., Vrablyanski, B., Simeonova, R., 1984. Recent movements in the region of the Kresna gorge and the Krupnik fault, Problems of the Geology of SW Bulgaria. Technica, Sofia, pp. 72–79, (in Bulgarian). Moskovski, S., Georgiev, A. 1970. On the Structure of the Kresna Gorge Region. An. Sofia Univ., Geology, pp. 95-112 (in Bulgarian). Ranguelov, B., Shanov, S. 1991. Helium content and extensometer measurements in a seismic active zone in Southwestern Bulgaria. In: Proc. of 1st General Conference of the Balkan Physical Union, 26–28 September, 1991, Thessaloniki, Greece, pp. 508–510.
Shanov, S., 1993. Medium-time earthquake prediction based on tectonic fault zone displacement data. Acta Montana, ASCR, Praha, Series A 4 (90), 52–62. Shanov, S.B., Dobrev, N.D., 1997. Impact of the seismic processes on the movements along the Kroupnik fault zone (SW Bulgaria). Comptes rendus de l’Acade´mie bulgare des Sciences 6, 95–98. Shanov, S., Dobrev, N., 1999. Reconstruction of the tectonic stress field in the epicentral zone of the Kroupnik earthquake (04.04.1904). Symposium ‘95 years after the earthquakes of 1904 in Kroupnik–Kresna region’, Blagoevgrad, Bulgaria, 27–28 April, 1999, pp. 117–124 (in Bulgarian). Shebalin, N.V.K, Ka´rnik, V., Hadzievski, D. ( Eds.), Catalogue of Earthquakes. UNDP/UNESCO Survey of the Seismicity of the Balkan Region. Parts I and II 1974. Printing Office of the University Cyril and Methodius, Skopje. Solakov, D., Simeonova, S. ( Eds.), Bulgaria Catalogue of Earthquakes 1993. Geoph. Inst. Bulg. Acad. Sci., 39 pp. Toteva, T., Rizhikova, S., Ranguelov, B. 1999. Recent seismicity of the Kresna earthquake zone. Symposium ‘‘95 years after the earthquakes of 1904 in Kroupnik–Kresna region’’, Blagoevgrad, Bulgaria, 27–28 April, 1999, pp. 65–76 (in Bulgarian). Vrablyanski, B., 1974. Neotectonic investigations in the Simitli graben and its frame. Bull. Geol. Inst. of Bulg. Acad. of Sci., Ser. Geotect. 23, 195–220, (in Bulgarian). Watzof, S., 1902. Tremblements de terre en Bulgarie observe´s pendant le XIX-e`me sie`cle Institut me´te´orologique central. Imprimerie de l’Etat, Sofia, (in Bulgarian, summary in French). Zagorcev, I., 1969. The Struma deep fault during the Late Alpine orogenic stage. Acta Geologica Academiae Scientarium Hungaricae 13, 437–441. Zagorcev, I., 1970. On the tectonic movements in a part of South-west Bulgaria. Bull. of the Geophysical Institute, Bulg. Acad. of Sci., Series Geotectonics 19, 141–152, (in Bulgarian). Zagorcev, I., 1975. On the Struma fault-bundle and fault-block structure of the Vlahina block. Geotectonics, Tectonophysics and Geodynamics 3, 3–17, (in Bulgarian). Zagorcev, I.S., 1992a. Neotectonics of the central parts of the Balkan Peninsula basic features and concepts. Geologische Rundschau, Stuttgart 81 (3), 635–654. Zagorcev, I.S., 1992b. Neotectonic development of the Struma ( Kraisˇtid) Lineament, Southwest Bulgaria and Northern Greece. Geological Magazine 129 (2), 197–222.
Monitoring tectonic movements in the Simitli Graben, SW Bulgaria N.D. Dobrev a, *, B. Kosˇt’a´k b a Department of Engineering Geodynamics, Geological Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Street, block 24, 1113 Sofia, Bulgaria b Institute of Rock Structure and Mechanics, Academy of Sciences of Czech Republic, V Holesˇovie`ka´ch 41, Prague 8 18209, Czech Republic Received 30 June 1999; accepted for publication 7 February 2000
Abstract Long-term in situ monitoring of slow tectonic movements has been applied to a seismoactive region of SW Bulgaria, within the epicentral zone of one of the strongest earthquakes in Europe (4 April 1904, M=7.8). The region has been found the most seismoactive in Bulgaria being of interest to many scientists. Three spatial extensometers were installed here in carefully selected sites to reflect fault movements on fissures. The extensometer TM-71 used here, enables three-dimensional detecting of even very slow movements with the accuracy of 0.01 mm and high stability over time. After 17 years of measuring, the rates of tectonic movements were established at all three monitoring points. Movements recorded at point B6 located in the seismoactive Kroupnik fault zone are of a relatively high rate. Locally, they show left-lateral strike–slips at rates of ca. 2.7 mm year−1, as well as thrusting with a mean rate of 1.9 mm year−1. Monitoring point K7 located in a fissure of the same zone on a steep slope affected by recent earthquakes has shown an uplift tendency of the block W of the Strouma Fault, with a result of gradual slope subsidence occurring from time to time. Monitoring point K5 located in a fissure of Strouma Fault zone became increasingly active during the last 2 years after 8 years of relative quiescence. Before that, only low left-lateral movements could be observed. Long-term fissure monitoring has shown quite a number of details interpretable to the dynamics of a broad region. Permanent shear displacements were found to develop after earthquakes. It was established that only a certain distinct part of local earthquakes provide such a displacement reaction at the monitoring points showing particular seismic connections. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Displacement; Extensometers; Faults; Monitoring; Southwest Bulgaria
1. Introduction Complex tectonic structure in SW Bulgaria with a composite fault mosaic, where the epicentral zone of one of the strongest earthquakes in Europe is located, has been studied by many scientists for years. It is the Simitli Graben in the Strouma * Corresponding author. Fax: +359-2-718148. E-mail address: [email protected] (N.D. Dobrev)
River section, which has been studied most intensively ( Fig. 1). Here, two large earthquakes occurred on 4 April 1904, 10:02 and 10:25 GMT, M=7.3 and 7.8, respectively (Christoskov and Grigorova, 1968; Shebalin et al., 1974). In spite of insufficient historical data, there are reports that at least two other strong earthquakes occurred in this region, one on 4 September 896 AD, and the other on 6 December 1866 AD ( Watzof, 1902; Shebalin et al., 1974). Presently, it is the most
0013-7952/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 1 3 -7 9 5 2 ( 0 0 ) 0 0 02 7 - 2
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Fig. 1. Neotectonic sketch of Simitli Graben and neighbouring territories (according to Zagorcev, 1992b). Grabens: 1, Simitli; 2, Blagoevgrad; 3, Padesh; 4, Razlog; 5, Brezhani; 6, Sandanski; 7, Delchevo. Horsts: 8, Rila; 9, Pirin; 10, Kresna; 11, Vlahina; 12, Lissia; 13, Kachov Rid. Main faults: KF, Kroupnik Fault; SF, Strouma Faults; GF, Gradevo Fault; BF, Brezhani Fault.
seismoactive region in Bulgaria. Every month, ca. 10–20 earthquakes occur here. Approximately 30% of those earthquakes reach magnitude of ca. M=3. Continuous tectonic activity is evident in the region with a high distribution of large landslides along the main fault lines (Dobrev, 1997, 1999; Dobrev and Boykova, 1998). The landslides and the other slope processes seriously endanger the important railway and Sofia–Thessaloniki Road that pass through the graben. So far, they are the only transport connections between Bulgaria and Greece. Within the last few years, the road and the railway were destroyed several times by landslides, debris flows and rockfalls. The gas pipeline, which passes through a creeping landslide slope and the active Kroupnik fault, is also at risk. These problems highlight the need to evaluate the rates of tectonic movements in this area. The impact of tectonic movements on landslide distribution has been already analysed (Dobrev and Boykova, 1998). Many different studies of the fault dynamics in the Simitli Graben, and the adjacent Kresna Gorge have been carried out so far concerning geodynamic development (Hoernes, 1904; Zagorcev, 1969, 1992a,b; Moskovski and Georgiev, 1970; Vrablyanski, 1974, and others), as well as recent and present movements (Milev et al., 1984; Milev and Vrablyanski, 1988, and others). Geodetic measurements have recently been completed using the global positioning system searching for block movements. In 1982, three observation points were estab-
lished to monitor faults in the region using a new method ( Kosˇt’a´k and Avramova-Taceva, 1988). It is believed that the results have an importance reaching beyond the local interests. This presentation cannot be carried out without a short description of the local geological–tectonic situation, and some seismic data. It also shows that local and global geodynamical theories and models should be supplemented also by such observations in detail, which can quantify deformations and movements.
2. Geological–tectonic characteristics of the region The Simitli Graben is situated in SW Bulgaria along the Strouma River ( Fig. 1), at the crossing point of some significant fault structures: the Strouma fault zone ( local orientation 150–170°), the Kroupnik Fault (20–80°, general orientation 40°); and the Gradevo Fault (30–40°) (Boyadjiev, 1971; Zagorcev, 1975). A longer part of the graben has developed along the Kroupnik and Gradevo faults, while the Strouma fault zone is bed of the Strouma River (Fig. 2). Further to the south, the river follows the Strouma Fault into the Kresna Gorge cut into the Kresna Horst. The Simitli Graben originated during the late Miocene over Precambrian metamorphites and Palaeozoic and Upper Cretaceous granites. The graben is filled with Neogene slightly lithified sediments: sandstones; conglomerates; clays and coal — with a maximum thickness of 1500 m.
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Fig.2. Scheme of the more important faults in Simitli Graben with the localisation of the monitoring points in the geomorphological situation. The main faults are: 1, Kroupnik Fault; 2, Gradevo Fault; 3, Strouma faults; 4, Brezhani Fault.
Quaternary deposits are deluvial–proluvial at the foothills of flanked horsts as well as alluvial sands and gravels of the Strouma River terraces, accumulated in levels given by vertical movements. The southern border of the graben is defined by the seismoactive Kroupnik Fault (Fig. 2). To the west, it can be followed by a distance of ca. 20–30 km beyond the western boundary of the graben. Kachov Rid Horst is defined by the Kroupnik Fault (NW border) and the Brezhani Fault (SE border). It is closed between the Krusto and Kachov Chukar Peaks (Fig. 2). This horst divides the Simitli Graben and the Brezhani Graben. The Strouma fault zone crossing the Simitli Graben is a long and wide tectonic structure, built by a number of parallel fault fractures. It extends from the Chalkidiki Peninsula to the Pannonian Basin (Jaranoff, 1960; Zagorcev, 1969, 1975, 1992a,b; Moskovski and Georgiev, 1970; Boyadjiev, 1971; Vrablyanski, 1974). In the framework of the Simitli Graben, it is demonstrated by many slickensides in crushed and mylonised zones.
3. Fault dynamic observations Geodetic observations indicate an uplift of the Kresna Horst at the rate of ca. 2 mm year−1, and less in the Neogene grabens along the Strouma River — a rate of <2 mm year−1. Levelling benchmarks along the Strouma River between the towns of Simitli and Kresna ca. 14 km south of the fault crossing point have shown alternating signs of vertical movements within 30 mm between 1956 and 1992 (Milev, 1999). Geomorphological studies indicate that the Kroupnik Fault can be described generally as a normal fault with a left-lateral strike–slip component. Its fault trace is divided into sections determined by a system of transverse faults. Rates of vertical and horizontal components of fault movements may be different at each section along the Kroupnik Fault, closed between two neighbouring transverse faults. Strike–slip is left-lateral everywhere, but with variable rates. The vertical component of movements along the Kroupnik Fault is normal, but in two sections it is reverse (Melo and
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Gradishteto Hills). These reverse movements can be explained locally — the local stress field situation and the fault pattern is very complex. Landslides and erosional forms indicate strong movement along the full length of the fault section with the Shirinata–Shturbovska Mahala villages reaching the maximum between the Krusto Peak and the river. The left-lateral component of movements along the Kroupnik Fault can be confirmed by the way the Strouma River meanders in front of the entrance to the Kresna Gorge. The fault movements in the studied area are considered to be generally caused by NE–SW compression, and NW–SE extension (Zagorcev, 1992b; Matova et al., 1996), resulting in left-lateral movements on the Kroupnik Fault, and rightlateral slips on transverse faults. Shanov and Dobrev (1999) calculated the subhorizontal extension to be 281°/37°, as well as compression N11° for the Kroupnik Fault, with a reorientation in the eastern part of the graben to 20–45°/58– 65°NW (Dobrev, 1999). These results are based on slickenside investigations in four outcrops and explain the left-lateral strike–slip along the Kroupnik Fault. The kinds of movements along the Strouma Fault have also been discussed. Moskovski and Georgiev (1970) found right-lateral movement and thrusting, with an uplift in the western side. This is confirmed by the most of the previous investigations (Zagorcev, 1969, 1970, 1975; Vrablyanski, 1974, etc.). However, this is contrary to the fact that major tectonic planes in the gorge are inclined to NE, therefore thrusting should rather be in favour of an eastern side uplift. Milev (1999) concluded that uplifts at present read 0 to +3.4 mm year−1 for the western of Kresna Horst separated by the Strouma River, and +1.4 to +3.4 mm year−1 for the eastern block. Gradients may even change signs. Concerning horizontal movement, results fall to error limits, and the direction of horizontal movements cannot be defined in the observed network in the studied region. The outlined discussion shows the need for a more precise and detailed measurement to quantify present movements trends even in active fault
structures such as the epicentral zone of the Simitli Graben.
4. Monitoring of the fault structures A thorough investigation of local fault structures resulted in the selection of three suitable sites for monitoring and potential active tectonic movements. Point K5 ( Fig. 2) is situated in Kresna Gorge on one of the parallel fractures of the Strouma Fault. There, ca. 80 m above the left bank of the Strouma River, a huge block of granite is separated from a steep rock slope by a fault fracture zone 2.5 m wide filled with broken and crushed granite. Point K7 ( Fig. 2) represents a fissure of the Kroupnik fault zone strongly affected by the seismic events of 1904 (Grigorova and Palieva, 1968; Dobrev and Avramova-Tacheva, 1999), when a remarkable seismogravitational slope deformation originated ca. 700 m SE of the village of Kroupnik. Eight apparent steps in the slope dipping 30–35° NW are separated by a number of open fissures in the granite. This slope deformation represents a potential danger to Sofia–Thessaloniki Railway. The point K7 is situated in the centre of the slope deformation in a fissure 1 m wide, reaching to a considerable depth, as shown by steam coming out in the winter (Dobrev and AvramovaTacheva, 1999). Radon outburst has also been noticed here ( E. Avramova and B. Ranguelov, personal communication). The main fault of this seismoactive Kroupnik zone is believed to coincide with the fault slope. Point B6 (Fig. 2) is situated on the Kroupnik Fault near the village of Brezhani. Here, the fault comes out in the terrain as a continuous apparent step or scarp, which represents contact between the Precambrian amphibolites and the Neogene coarse-grained sediments of the Simitli Basin. A contact zone 3.4 m wide was opened here by a trench to install the instruments. At all three points a steel tube bridging the opposite bodies in contact was installed to monitor differential movements between the two sides. The tube was rigidly cemented into drill holes in the opposite rock walls in the form of two separate
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Fig. 3. A view of the gauge TM-71 (after Kosˇt’a´k, 1977).
holders leaving space for a gauge system. In the case of the point B6, a special arrangement was adopted to fix the tube holder into the incompetent sedimentary rock ( Fig. 3). The points were instrumented with TM71 crack gauges using a method described earlier (Avramova-Tacheva et al., 1984; Kosˇt’a´k and Avramova-Taceva, 1988). The moire´ extensometer ( Kosˇt’a´k, 1969, 1977, 1991) has is capable of detecting movements and microdisplacements in three dimensions (Fig. 4). It works on the principle of mechanical interference — moire´, which records displacement as a fringe pattern on superposed optical grids mechanically connected with the opposite walls or crack faces. Due to this principle, which completely avoids any electrical transmission, the gauge displays an extremely large longterm stability, and infallible performance under hard outdoor conditions. Practically, it means that values recorded over decades can be well compared. Results will then be provided as displacements on structural planes in millimetres, and time
trends derived as rates in millimetres per year under hard field conditions in a prolonged operation. Long-term trends of slow microdisplacements could be detected effectively with a final accuracy of 0.01 mm provided that measurements are taken for at least 3 years. It can discern seasonal as well as other environmental effects, that need to be recognised and compensated for in the data. Temperature effects in the system including holders are eliminated numerically, while such effects upon the rock are not eliminated in the data and are observed in climatic cyclic variations. The data were obtained in three Cartesian coordinates, calculated from recorded interference patterns. They represent differential movements according to Table 1. It is always: X, horizontal, across the contact; Y, horizontal slip; and Z, vertical displacement. The movements are relative between the two sides, presented in graphs as displacements of the lower block on the slope to the opposite one, although interpolation must consider the movement at both sides.
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Fig. 4. Cross-section at monitoring point B6 (after Avramova-Tacheva et al., 1984): 1, amphibolite; 2, grey-green tectonic clay, 0.7 m thick; 3, amphibolite breccia with clayey matrix; 4, grey-black tectonic clay, 0.25–0.30 m; 5, crushed granite blockage with clayeysandy matrix, 0.85 m; 6, granite blockage with sandy matrix; 7, soil stratum and deluvial clay; 8, technogenic embankment; 9, concrete; 10, former relief profile line.
Resulting movements in the three points with a monthly observation frequently, for more than one decade (between 1982 and 1999) are given in Figs. 5–7. 5. Observed movements 5.1. K5: Kresna Gorge–Strouma Fault (Fig. 5) X: quasi-sinusoidal character of movements with a mean amplitude 1.2 mm in seasonal variation; 1982–1993 very slow long run compression increase; since 1994 — gradual change to extension 0.3 mm year−1; 1986–1987 several sequences of sudden pulsation — seismic events: 15 May 1986, M=4.2, H=19 km, epicentre 9 km N of K5, and fore-
and aftershocks (Solakov and Simeonova, 1993); Y: less pronounced seasonal variation (within 0.3 mm) interrupted by a number of reversed slips (up to 0.5 mm); 1982–1988 no trend shown; since 1989 — gradual change to left-lateral slips 0.1–0.4 mm year−1; Z: 1982–May 1986 — subsidence 0.25 mm year−1 with several reversed slips; 15 May 1986–July 1986 — co-seismic slips 0.73–0.88 mm; July 1986–December 1987 — subsidence re-established, one reversed slip in summer 1987 coincident with X compression; 1988–1995 — no trend; since 1995–sudden increase of subsidence 1.26 mm year−1.
Table 1 Space orientation of the axes of the monitoring points in SW Bulgaria (after Avramova-Tacheva et al., 1984; Kosˇt’a´k and AvramovaTacheva, 1988) Monitoring point No.
+X, Widening of the horizontal zone
+Y, Horizontal shear movement in the fault (fissure) direction
+Z Vertical movement
Local fault orientation (°)
K5 B6 K7
SW block to 225° Neogene basin to 290° The northern block to 350°
SW block to 135° Neogene basin to 200° The northern block to 260°
Subsidence of the SW block Subsidence of the Neogene basin Subsidence of the northern block
150 20–40 80
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Component X
Y
Z
August 1982 – December 1983
ca. +0.9 mm year−1 extension of the fault zone
ca. +3.5 mm year−1 left-lateral movement
January 1984 – October 1985 November 1985 – December 1987 January 1988 – December 1992 January 1993 – April 1996 May 1996 – July 1998 August – Sep. 1998 October 1998 – February 1999 March 1999 – May 1999
−4.39 mm year−1 fault zone compression +1.07 mm year−1 fault zone extension +1.89 mm year−1 fault zone extension +2.93 mm year−1 fault zone extension +1.44 mm year−1 fault zone extension
+1.16 mm year−1 left-lateral movement +4.63 mm year−1 left-lateral movement +2.60 mm year−1 left-lateral movement +1.51 mm year−1 left-lateral movement +0.54 mm year−1 left-lateral movement −0.15 mm year−1 right-lateral movement +4.91 mm year−1 left-lateral movement +0.23 mm year−1 left-lateral movement
ca. −1.5 mm year−1 thrusting (uplifting Neogene basin) −9.28 mm year−1 thrusting −0.98 mm year−1 thrusting −1.92 mm year−1 thrusting −1.89 mm year−1 thrusting −0.34 mm year−1 thrusting +0.25 mm year−1 normal fault movement +1.94 mm year−1 normal fault movement +0.81 mm year−1 normal fault movement
fault zone extension ca. +1.8 mm year−1 fault zone extension
On 3 May 1999, the total displacements found at that point were as follows: X=1.68 mm, Y= 0.83 mm, and Z=3.00 mm. The vector of total displacement are u =3.54 mm. The maximal xyz total displacement u found at this point is xyz recorded on 29 March 1998, reaching 4.62 mm (X=0.86 mm, Y=0.55 mm, and Z=4.50 mm). 5.2. B6: Brezhani–Kroupnik Fault (Fig. 6) Total displacements found at this point between August 1982 and May 1999 are quite impressive: X=19.15 mm, Y=40.30 mm, and Z=−33.09 mm, and the vector of total displacement u =55.55 mm. xyz Very clear trends can be found in intervals separated by individual abrupt displacements. A seasonal effect of temperature is evident in the Xcomponent only, and eliminated in calculations using the least squares method (Dobrev and Avramova-Tacheva, 1997). The mean rates fluctuate ca. 1.9 mm year−1 at axis Z and 2.7 mm year−1 at axis Y. Results in nine characteristic periods are given in Table 2.
As far as the earthquakes are concerned, a characteristic development of movements has been observed in the last period ( Table 2). It may be seen that during the last 2 years the rate of leftlateral horizontal slips was gradually decreasing, and the movements were changing their character to right-lateral. This occurred in the short period of August–September 1998, while the vertical movements changed to normal faulting. On 26 October 1998, 10:00 GMT, a local earthquake M=3.5 occurred there ( Toteva et al., 1999). The co-seismic displacements were DX=4.4 mm, DY= 1.8 mm and DZ=2.7 mm. After this earthquake the rate of left-lateral strike–slip considerably increased, while the vertical movement returned to normal.
5.3. K7: Kroupnik–Kroupnik Fault (Fig. 7) X: seasonal variation (within 1 mm) recognised only after summer 1992; trend not clear till the beginning of 1987; since 1987 — a progressive trend to fissure opening (0.29 mm year−1);
Fig. 5. Diagram of displacements found at Point K5. Schematic block-diagram of the situation at the monitoring point K5 and the orientation of the axes in space included.
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Fig. 6. Diagram of displacements found at B6. Schematic block-diagram at Melo Hill and the orientation of the axes is included.
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Fig. 7. Diagram of the displacements found at K7. Schematic situation of monitoring point K7 and the orientation of the axes is included.
July 1985 — extreme displacement (fissure compression by 0.7 mm); Y: calm period interrupted by abrupt slips (0.2–0.5 mm month−1) over 6–18 months; July 1985 — extreme right-lateral slip by Y= −6.9 mm; August 1985–October 1989 — trend to leftlateral slip (+0.18 mm year−1); October 1989–February 1994 — trend reversed to right-lateral slip (−0.22 mm year−1); Z: calm period interrupted by abrupt slips (0.3– 0.8 mm) over 3–6 months; the slips are of subsidence in character until 1991; later reversed;
July 1985 — extreme vertical displacement Z=−6.0 mm (subsidence of the upper block in the fissure most probably). On 24 February 1994, the total displacements found at that point were as follows: X=1.26 mm, Y=−5.33 mm, and Z=−4.85 mm. The vector of total displacement is u =7.32 mm. xyz 6. Discussion Our three observation points represent three different situations considering the fault configuration, potential slope movements, and relation to the Simitli Graben. Two of them, K5 and K7, are located in hard rock, and observed displacements
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are of the same magnitudes, that is, within several millimetres per decade while the third is located in a contact between competent and incompetent rock, and observed displacements are within few millimetres per decade. All the three points are placed on tectonic fractures within a larger tectonic zones, and all are on slopes, which may be suspected of producing gravitational deformation interfering with the tectonic one. 6.1. Slope gravitational movements It is just the point B6 out of the three where the gravitation movements are most likely, and vertical movements show upslope displacements of the lower lying sediments, that is, movements opposite to gravitational ones. There is no doubt, therefore, that at least the movements observed at B6 are due to active tectonics. Contrary to that, it is the point K5, where a most stable situation appears to be, showing downslope movements of the lower block. Therefore, it is the active tectonics, which is obviously the primary factor of the region.
6.2. Effects
1. 2. 3. 4.
Four types of effects can be observed: long-time trends; short-time trends; abrupt displacements; and seasonal variations.
The last effect is connected mainly with the Xcoordinate, showing winter opening and summer closing of the contact, and is not an object of our discussion. Long-time trends and their progressive development are found mostly in the Y and Z components showing a slipping process along the joint. They are probably of primary importance, as they can indicate source of movements or orientation of active sources. Short-time trends will then indicate short-time deviations from general patterns in which the forces develop. They may reflect slip development in the depth as well as of the local friction conditions. They may be of interest if a typical and
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repeatedly uniform reaction is detected and correlated with tectonic events. Abrupt displacements are typical of seismic regions. They are mostly the after-effects of vibrations due to earthquakes being displaced to a new position. They will follow general slip trends, although exceptions to this rule are also frequent, depending on local conditions in the system of joints or even to orientation in respect to epicentre. They can represent a local stick-slip process triggered by distant forces. Generally, the observed reactions in active tectonic regions will represent a local response to distant powerful processes, thus reflecting positional redistribution in the joint system, or even a progressive change in the boundary conditions of large blocks. It is just the last objective, which is the most attractive and leading aspect for the selection of observation points on a given fault system. However, it calls for a long range observation. 6.3. Interpreted results In the interpretation it should be clear that movements must be always seen as relative to the two sides in contact, allowing for both sides to move. 6.3.1. Point B6 Interpretation of the negative vertical slips in the basin as real uplifts with respect to the core blocks of the Pirin and Rila Mts is unlikely. It is rather a contact pressure, which thrusts the marginal zone of sediments as a plough lifting it up slowly. However, the pressure comes from the mountainous side at an angle of N 11° (Shanov and Dobrev, 1999), resulting in a horizontal slip: left-lateral strike–slip in the point. The local positive vertical movements can be additionally confirmed by the deeply grooved gullies in this part of the graben. A local horizontal pressure reaches the point from SSE, indicating movement of the Kachov Rid Horst in the northern direction here. The basin is under constant pressure of the flank mountains, which is indicated by the X-component
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measured after 1988. An earlier extension period can be explained locally. Individual abrupt slips were investigated earlier ( Kosˇt’a´k and Avramova-Taceva, 1988, 1993), and in most cases may be interpreted as the aftereffects of known earthquakes. It is established that the contact reacts to events from far south. The Strouma Fault seems to be an effective lead to earthquake vibrations (Shanov and Dobrev, 1997). Short-time trends have recently been indicated and investigated (Shanov, 1993; Shanov and Dobrev, 1997). Normalised models of the deformation rate changes before strong earthquakes were derived. It has been established that the similar anomalies in the helium content of the thermal springs in the town of Simitli occur 4– 6 days before the earthquakes ( Ranguelov and Shanov, 1991). 6.3.2. Point K7 The point lies on the same Kroupnik Fault, separated from B6 by Strouma Fault. The main effect was the abrupt slip recorded in July 1985. In spite of the relatively low seismic activity of 1985, attempts were made to find a source of the displacement. However, only an earthquake of 17 September, Pehchevo, M=3.4 (Solakov and Simeonova, 1993), ca. 14 km W of K7 was felt in this area, that is, after the slip. If the idea of the source of Montenegro (M=5.2, May 1985), due to the distance is rejected, then the question of an earthquake source is hard to explain. It is remarkable, however, that the slip was not a subsidence of the outside slope block, but the slip was reverse. Moreover, it was connected with a horizontal slip, a right-lateral strike–slip by ca. 7 mm. Therefore, rather than a slope gravitational slip, a strike from the mountainous side was indicated, with considerably large pressure component from the east. For a long period after the slip until 1991, the Xcomponent had tendency to show release in pressure with a number of additional continuing slips in Y as well as continuing vertical slips in Z. After that the trend reverses. Generally, the situation has shown pressure conditions along the edge of the Kroupnik Fault with an uplift tendency of the block W of the Strouma Fault, resulting from time to time (e.g.
after 1991) in gradual subsidence in the slope deformation. 6.3.3. Point K5 Even after 17 years of observation, this point set in the Strouma Fault did not show any trend in X, which is contrary to the two points set on the Kroupnik Fault. This can be interpreted as a sign of normal compression across the fault. At the same time, the Z-component can be interpreted as an uplift in the eastern section of the Kresna Horst rather than subsidence in the slope. The Ycomponent exhibits an occasional tendency to smaller lateral strike–slips, left-lateral are prevalent. Such a slip is increasingly pronounced after 1992, while reversed to right-lateral in 1997 in an obvious connection with a sharp increase of vertical movement. After that, the tendency noticed before 1997 reappeared. Later developments will be of a special interest.
7. Conclusions 1. Data obtained after 17 years of observation in the three points of the investigated faults in SW Bulgaria provide consistent results, indicating fault boundary conditions in the investigated area. A following stress and movement configuration can be derived: $ Rigid rock of the mountainous blocks S of the Kroupnik Fault is under pressure. Principal compressive forces follow the rim of the Kroupnik Fault oriented close to E–W, stimulating pressure across the Strouma Fault. $ An overall tendency to uplifting in the mountains, confirmed also by levelling (Milev, 1999), creates an additional pressure against sedimentary fill of the Simitli Graben due to an upheaval and so even gravitational spreading in the mountainous mass. Such a pressure induced by flank hills of the mountains results in local uplifts in more ductile sedimentary strata along the Kroupnik Fault. $ It has been confirmed that the horizontal component of movements along the NE section of the Kroupnik Fault is a left-lateral strike–slip. The right-lateral strike–slip along the western section, which has been found at point K7, may
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be explained only as local movements, not representative for the fault. $ It is rather the fault structure sloping to E by ca. 80–85°, which is now effective in thrust slipping of the Strouma Fault. Presently, small left-lateral strike–slips, with occasional rightlateral breaks were noticed. $ Major movements point to SE as at the source pole of the movement activity, to the eastern section of the Kresna Horst. $ Movements in the contact line between the competent rock edge and the sedi-mentary fill of the Simitli Graben reach a mean rate of ca. 2 mm year−1 and horizontal long-run slips (NE section) with earthquake strikes up to 6 mm individually. $ It is the Strouma Fault structure which has been found as the main connector with the earthquake epicentres effecting the slip movements on the Kroupnik Fault. 2. A characteristic development of movements may be observed during the period of an earthquake, before and after. 3. Monitoring points of the fault movements can be successfully established on fault structures in tectonically active regions. They may provide data about boundary conditions and stress as well as movement configuration in the fault structure of the investigated region. The Kroupnik Fault monitoring program in SW Bulgaria started as an experiment. As such, it proved that even ground measurements in incompetent rock can be successfully organised. It is evident that in spite of some technical problems, it is just a contact between competent and incompetent rock, which may well provide valuable data, since ductility of bodies such as sediments provides freedom for a rigid block in contact to move. A thorough field investigation of the terrain prior to the decision about the placing of the points and their instrumentation are an integral part of any monitoring program. Detailed fault monitoring may allow recognition of data that are beyond the capacity of global observations. Acknowledgements This experimental field study was supported financially by the Ministry of Education and
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Sciences of Bulgaria (Grants Nos. NZ-23/91, NZ-635/96 and 5053/MON ), and Grant Agency of the Czech Republic, Grants Nos. 205/94/1769 and 205/97/0526. During the years of the research successful cooperation was carried out thanks to the joint efforts of scientific staffs of the Geological Institute of the Bulgarian Academy of Sciences in Sofia and the Institute of Rock Structure and Mechanics of the Academy of Sciences of the Czech Republic in Prague. The authors are grateful for the encouragement and suggestions by E. Avramova-Tacheva, I. Broutchev, S. Shanov, P. Petrov and D. Evstatiev as well as colleagues from the Geophysical Institute of BAS who assisted with implementing and supporting the present monitoring. All such support essential for the project is gratefully acknowledged.
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