Rhodope Geodynamic Hazards, Late Alpine tectonics and Neotectonics International Conference - May 2001

The Simitli graben (70 km²) is situated in SW Bulgaria, about 100 km S from Sofia (Fig. 1).It is one of the numerous Tertiary grabens in this part of Bulgaria (Zagorcev 1992). The graben is filled by Neogene sediments: sandstones,conglomerates, clays and coal. The maximal Neogene thickness is about 1500 m in the central parts of the graben. The flanked mountains are built by metamorhic and magmatic rocks: amphibolites, gneisses, marbles and granites (Marinova Zagorchev 1990, 1993).

Fault pattern

The southern border of the graben is defined by the seismoactive Kroupnik fault. Its orientation reads about 60 near the village of Kroupnik and 20 - 40 to the East (Fig. 2).

The Strouma fault zone is a long and wide tectonic structure. It is characterized as a complicated fault system built by parallel fault lines with a direction of 160-180. On the territory of Bulgaria it is composed of several groups of smaller fault zones. In the region of the Simitli graben, the Strouma fault zone forms the contemporary bed of Strouma River(Fig. 3). It is manifested by many slickensides, crushed and mylonized zones.

These faults form a compound fault mosaic. This is predetermined by the geodynamic development of the region since the Miocene till now. The character of all the movements along the faults in the Simitli Graben is determined by its compression in the SE-NW direction causing an extension in N-E direction (Zagorcev,1992; Matova et al., 1996), sinistral movements along the Kroupnik fault and prevailing dextral movements along its transverse faults. The sinistral character of the movements along the Kroupnik fault are confirmed by the meander of the Strouma River in front of the entrance to the Kresna Gorge.

Seismogenic terrain deformations

As a result of April 4, 1904 earthquake, a vast territory was affected by seismogenic deformations. The most remarkable terrain deformations had been observed just in the Simitli graben (Hц rnes1904; Watzof 1905; Grigorova , Palieva 1968).
An important length of the Kroupnik fault was activated during the earthquakes, but the cutting happened exactly at the crossing zone of the Strouma and Kroupnik faults. The such formed surface rupture was long about 1.8 km and high about 2 m - according to the different authors and based on the information obtained by elder native residents. The realised shear movement characterises the Kroupnik fault as a sinistral oblique fault. The rupture crossed the Strouma river and due to this reason the river was stemmed. Nowadays, the height of the rupture is already between 8 and 15 m after the residual deformations (Fig. 4).

The gravitational exogenic phenomena were the most distributed there. The landslides triggered all over graben territory but the most serious deformations were formed in the slopes of the flanked horsts. A serious slope deformation (volume 4.5 million m³) occurred on the right side of the Strouma river, near the village of Kroupnik (Fig. 5). A semi-circular scar can be seen clearly yet. According to the Varnes' classification of slope movements (1978), the type of this deformation could be treated to a gravitational sagging.

The most serious slope deformation affects the mountain slope SW from the village of Polena (Dobrev, 1999a) (Fig. 6). Its volume could be estimated of hundreds thousand cubic meters. It is not clear wheter it is connected with the catastrophic April 4, 1904 Earthquake.

Fault movements monitoring

Movement registration in the Kroupnik epicentral zone was performed with the use of a special instrumentation developed in the Institute of Rock Structure and Mechanics, Prague, to monitor long-term deformational increments on cracks and discontinuities. It is used a moirй extensometer having a capacity to detect barely descernible movements in three dimensions. The technique was many times tested in slope stability investigations and research.
Results of the present long-term monitoring on the Kroupnik Fault movements are presented on Fig. 7 (Dobrev, 1999b; Dobrev ,Kostak, 2000). Established diagrams of functions v = ѓ(t) express displacements in three Carthesian coordinates, where not only movement trend, but seasonal temperature fluctuations, as well as some additional effects (impact of seismic events, rockfalling, irregular distribution of temperature fluctuations into the rock massif, etc.) are superposed. The registered movements show left-lateral strike slip (2.7 mm/a) and prevailing local thrusting.

Landslides

Landslides are wide spread in the area of Simitli graben and its flanked horst. They cover 4.2% of the graben area (Dobrev, 1999a). The most of the landslides are formed in the Neogene sediments as well as in the tectonic crushed and mylonized rocks on the flanked mountain slopes. More than 200 landslides are depicted here. The slow tectonic movements and the groundwater lable fluctuations are the main slope destabilizing factors. For this reason, many landslides arose along the main faults(Fig. 8).

Landslide Hazard Map of the Simitli graben was prepared basing on the available information for occurred old and recent landslides in this territory(Fig. 9) (Dobrev , Boykova 1998; Dobrev 1999a). For the present study, it was used the approach depicted by DeGraff and Canuti (1988). The so presented approach of assessment of the landslide hazard illustrates a good correlation between the tectonic movements and the landslide occurrences. This is very important for the determination of the factors of landslide activity. The methos solves a problem where the establishing of the tectonic movements as a factor of the slope instability is a difficult task, often impossible. In our case, the susceptibility to landsliding depends on the tectonic movements more than the other instability factors and conditions.

Debris flows

Debris flow phenomena affect the flanked frame of the graben, mainly its southern parts. The road Sofia-Thessaloniki is oftenly disrupted by debris flows in some places - one of them is shown in Fig.10. The last serious phenomena in this place occurred on October 4 and December 28, 1993, and on September 4 and 13, 1998.

Rock falls

Rockfalling occurs at the flanked frame as well. The main reasons are the high level of cracking of rocks and the water freezing within the winter period. Oftenly rock blocks disrupt the road and railway. Fig. 11 shows the Seismic station at the village of Kroupnik affected by fallen granite blocks (Dobrev 1999a).

Erosion

Erosional processes are wide-spread on the territory of the Simitli graben. They depend on the rate of vertical movements of the tectonic blocks that build the graben bedrock. Due to this reason the erosion is the most developed in the southern parts of the graben. Many gullies and dales in this locality are deeply grooved into Neogene and Quaternary sediments (Fig. 12). Prone to erosion are the Neogene sandstones (Simitli Formation) as well as the deluvial deposits that cover the slopes within the graben.

Man-made hazards

Man-made hazards are connected with:

1) Mining activity in Oranovo Coal Mine (landsliding, surface subsiding);
2) Groundwater table fluctuations in the urban areas;
3) Sliding in the tail near Poleto village (Fig. 13);
4) Slope phenomena along the road, railway and gas-main system (landslides, rockfalls, erosion).

Acknowledgements

The present study has been done under the financial support of the Ministry of Education and Science of Bulgaria (grant 5053).

References

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