Eastern Rhodopes) and Their Relation to Recent Seismic Activity

Geoarchaeology and Archaeomineralogy (Eds. R. I. Kostov, B. Gaydarska, M. Gurova). 2008. Proceedings of the International Conference, 29-30 October 2008 Sofia, Publishing House “St. Ivan Rilski”, Sofia, 295-298. FAULTING AND ROCK DEFORMATIONS ESTABLISHED AT THE RUINS OF PERPERIKON (EASTERN RHODOPES) AND THEIR RELATION TO RECENT SEISMIC ACTIVITY Nikolai Dobrev Geological institute, Bulgarian Academy of Sciences, 1113 Sofia; [email protected] ABSTRACT. The ancient town of Perperikon (Eastern Rhodopes) is situated just in the epicentre of the M4.5 February 20, 2006 earthquake. The focal mechanism shows a pure sinistral movement along the main fault with direction 135°, and a dextral slip of 55° along the transversal faults. The earthquake caused rockfalling at Perperikon. The ruins reveal contemporary displacements along the main fault directions determined by the focal mechanism. Some displacements could be connected to other seismic events that have occurred in the past, as well as to a slow tectonic creep. Seismic activity evidences should be found through more detail studies including use of permanent monitoring tools. Introduction field study have already been published (Glavcheva et al., 2006; On February 20, 2006, at 17:20 GMT an earthquake with Dobrev et al., 2007). During the field studies the researchers magnitude 4.5 was recorded by the National Seismological visited the ruins of the ancient town of Perperikon, located at 1.5 Centre of Bulgaria (NOTSSI, Geophysical Institute of Bulgar- km NW of Murgovo. Parts of the ruins were cleaned during the ian Academy of Sciences). The hypocenter was situated at archaeological studies performed in the recent years. the following coordinates: 41.69°N, 25.48°E, 12 km deep (Glavcheva et al., 2006). The location was in the Eastern Geological setting Rhodopes Mountain area, near the Murgovo village, about 20 In brief, the geology of the studied area is represented by Pre- km NE of the town of Kardzhali (Fig. 1). Several days after Cambrian, Paleogene, Neogene and Quaternary formations (Fig. the main shock, a research team from the Bulgarian 1). The research area is built of two Paleogene formations from Academy of Sciences visited this area (Glavcheva et al., volcanogenic-sedimentary complex. The Perperikon area is built 2006; Dobrev et al, 2007). The team established that the of greenish tuffs. Coal-bearing-sandstone Formation, mainly most affected area is around the Murgovo village and the sandy clays, sandstones, marls and coal layers, builds the area neighbouring villages of Chiflik (2 km SE of Murgovo) and around the Murgovo village and the slope below Perperikon. Gorna Krepost (2 km NW of Murgovo). The results from this Fig. 1. Geological map of the epicentral area (after Boyanov et al., 1990; 1992; Goranov et al., 1995,;Kozhoukharov et al., 1995). 1 – Quaternary, alluvial deposits; 2 – Paleogene-Neogene sediments; 3 – Paleogene, volcanic-sedimentary complex; 4 – Pre-Cambrian complex of metamorphic rocks; 5 – fault 295 Fig. 2. Map of earthquake epicenters recorded by NOTSSI within the period February-June 2006 (Dobrev et al., 2007) Seismicity 80º (Fig. 4). A folded stratum marks a small-size normal The earthquake of February 20, 2006 surprised many locals faulting. The second fault could be seen near this place. The but it was not a surprise for the scientists (Fig. 2). Previous direction is to 120-130°, with a probable strike-slip component researches provide information by Grigorova et al. (1978), and a small-size subsidence of the Southern side (Fig. 5). The Papazachos et al. (2000) and others. Another earthquake of topsoil layer is 15-20 cm thin at the Northern side, and 40-50 almost the same size as the magnitude 4.5 event of 2006 can cm at the Southern side. be found on the 28 August 1936 in the macroseismic bulletin by Kirof (1941). The two earthquakes occurred unexpectedly During the field examination of the seismic effects, few fault and without any foreshock activity. The difference is that the structures were established that were not published in the 2006 M4.5 event was followed by an earthquake sequence existing literature. Some of them can be seen at the ruins of (Glavcheva et al., 2006). 75 weak earthquakes were recorded the ancient town of Perperikon that are cleared in the last four months after the main shock. Their epicentres follow a years of excavations (Glavcheva et al., 2006, Dobrev et al., general direction of 130-140° (Fig. 2). 2007). The focal mechanism of the main shock was determined by the National Institute of Geophysics and Volcanology, Rome, Italy (Fig. 3). The solution gives almost a pure horizontal slip with a left component 135° and a right slip in direction 55°. The analysis of the tectonic stresses shows compression in direction NW-SE. The coincidence of direction 135° with the direction of the Perperek River and some relief features is obvious. The transversal system 55° coincides with some tributaries of the Perperek River. We concluded that the main faults that were activated during the earthquake followed the directions of 55° and 135° due to following reasons: distribution of earthquake epicentres (Fig. 2); relief features; intensity and detonation effects. Ground deformations established at Perperikon The ancient town of Perperikon was studied two times after the February 20, 2006 Earthquake. Detailed studies of the fault pattern, the active faults and the palaeoseismological trenching have not been performed at this area so far. During our studies, two types of ground deformations were established at the Perperikon area: 1 – faulting (co-seismic displacements after stronger ancient events or tectonic creep), and 2 – rock deformation effects (rockfalling). Fig. 3. Focal mechanisms of 2006, 20 February Earthquake determined by Faulting Kandilli Observatory, Istanbul (KAN) and the National Institute of Geo- Possible faults were established into the Quaternary deposits physics and Volcanology, Rome (INGV). This data was obtained from the at two places on the scarps of the cart-road to Perperikon (Fig. European Mediterranean Seismic Centre, Strassbourg, France (EMSC) 4-5). The first one marks a rupture in a probable direction 45- 296 Fig. 4. The first possible fault found at the cart-road to Perperikon (black Fig. 6. Faulting found at the Eastern part of the “throne room” (Glavcheva dashed line); the white dashed line shows a folded and disrupted stratum et al., 2006) Fig. 7. Fault displacements of rock blocks at the “throne room” (Dobrev et al., 2007) Fig. 5. The second possible fault found at the cart-road to Perperikon (black dashed line); the white dashed line marks the topsoil layer The most significant concentration of possible fault displacements were established near the “throne room” (Dobrev et al., 2007). Two of them were found at the “entrance” of the “throne-room” showing a subsidence of the Eastern side (about 15-20 cm) and unclear value of strike-slip component (Fig. 6, left side). The strike-slip displacements were established at different parts of the “throne room” locality. The cracks following direction 120-140° have left-lateral displacements (Fig. 7), and on the contrary, the transversal crack system (40-60°) have right-lateral slip. The left-lateral slips show usual displacements Fig. 8. Dextral slip of 8 cm marked by two elements carved into the tuffs of 20-40 cm. The only dextral slips show smaller values than the sinistral ones. Such example is shown on Fig. 8. The transversal displacement is 8 cm. It is marked by two sides of an ancient through-like positive form carved into the rock by the ancient citizens of Perperikon. There are examples of other cracks following the direction of the main fault with/without displacements at other parts of the ancient town (Fig. 9). The present study does not cover the whole area of Perperikon, i.e. only the peripheral parts of the excavated area of the ruins of the ancient town. Fig. 9. Fresh cracks following a direction of 135°, NW part of Perperikon 297 Rockfalling ment and Waters, and the National Agency of Civil Defence, The earthquake tremor provoked a single rockfalling at Kardzhali. Perperikon (Fig. 10). A rock block with volume near 1 m 3 collapsed not causing any significant damages. This block originated from an unstable rock outcrop located at the References Northern periphery of the Perperikon hill. The unstable block Azañón, J. M., A. Azor, G. Booth-Rea, F. Torcal. 2004. Active shifted on an old crack surface inclined about 45° to the SE. small-scale faults affecting the Alhambra palaces (SE Traces from other recent or past co-seismic phenomena Spain). – J. Quaternary Science , 19 , 3, 219-227. (rockfalls, rocktopples and landslides) were not established at Boyanov, I., D. Kozhoukharov, A. Goranov, E. Kozhoukharova, the ruins of Perperikon. M. Ruseva, J. Jilyafova. 1992. Geological Map of Bulgaria, Scale 1:100000. Sheet Haskovo . CGMR, Sofia. Boyanov, I., D. Kozhoukharov, A. Goranov, Y. Yanev, J. Jilya- fova, M. Ruseva. 1990. Geological Map of Bulgaria, Scale 1:100000. Sheet Iskra . CGMR, Sofia. Dobrev, N., R. Glavcheva, B. Ranguelov, S. Dimitrova, K. Hadjiiski. 2007. Analysis of the seismicity and the engineering-geological conditions of the epicetral zone of 2006, February 20 Earthquake, Murgovo Village, Kardzhali District. – Rev. Bulg. Geol. Soc., 68 , 1-3, 121-130 (in Bulgarian). Dobrev, N., R. Varbanov, M. Matova, G. Frangov, G. Drakatos, B. Kostak. 2005. First steps for monitoring on possibly active fault zone in East Rhodopes, Bulgaria. – Acta geodynamica et geomaterialia , Acad. Sci. Czech Rep., Prague, 2, 1(137), 7-12. Fig. 10. Collapsed rock block at Perperikon (Glavcheva et al., 2006) Glavcheva R., N. Dobrev, K. Hadjiiski, S. Dimitrova, B. Ran- guelov. 2006. The 2006 seismic activation in the Eastern Discussion Rhodopes (Mts) – Kurdjali area and some generalization The relation between the focal mechanism of the February on geological features in the region.

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