Karst regions are very vulnerable with respect to natural and human-activity related hazards. Some of them with beautiful caves were proposed to be of national value. The examination of broken and slim intact speleothems in Bulgarian caves (Varteshkata and Elata caves, western part of Balkan Mountain Range, North-West Bulgaria) allows estimating an upper limit for horizontal peak ground acceleration (PGA) generated by paleo-earthquakes.The density, the Young’s modulus and the tensile failure stress of the samples originating from broken speleothems have been measured in a laboratory.The fundamental frequency and damping of speleothems have been measured in cavity by in situ observations.The value of the upper limit horizontal ground acceleration resulting in failure and the natural frequency of speleothems were assessed by theoretical calculations using mechanical parameters — the density, the Young’s modulus and the tensile failure stress — of the samples originating from a broken speleothem from Elata cave.The ages of the samples taken from the investigated stalagmites have been determined by alpha spectrometry.The pure elastic behaviour in analytical modeling and calculations have been used.According to our modeling results the investigated speleothem has not been excited by a horizontal acceleration higher than 0.144 g in case of Varteshkata cave, and 0.326 g in case of Elata cave during the last few thousand years. These results can serve to improve the present seismic risk policy for karst regions, too.

We studied the Nivyanin fault segment from the Devene fault system in NW Bulgaria, using shallow electrical resistivity profiling. The measured profile was located on the Iskar floodplain near Chomakovtsi village and crossed a lineament that connects the northernmost exposures of Upper Cretaceous limestone and Miocene conglomerate on elevated surfaces in which the Iskar valley was incised. The results suggest that the studied fault segment was a strike-slip fault that was active after the deposition during the Miocene. The fault zone is broad over 100 m and consists of sub-vertical fractures and breccia. Vertical fault displacement did not affect the Quaternary fluvial deposit.

The only early medieval rock bas-relief in Europe Madara Horseman is included in UNESCO World Heritage List. This article presents the results from an in situ monitoring of microdisplacements across fissure forming the Golyamata Tsepnatina Cave located at the edge of Madara Plateau above the Horseman, NE Bulgaria. Additionally, we studied movements along cracks behind the plateau edge, which are related to the dynamics of peripheral plateau parts and shaped the rock slices. Monitoring involves the use of a 3D extensometer model TM-71 installed in the cave as well as five sets of pin marks installed across the cracks behind the plateau edge. The results obtained since 1990 indicate formation of a new rock slice from the cliff due to the gravitational extension of rock massif. The established horizontal microdisplacements of the rock slice to SSE imply the existence of a possible active fault with left-lateral component of movements at the base of the plateau. We established influences from near and distant earthquakes with epicenters in NE Bulgaria (Provadia), Romania (Vrancea), and Turkey (Izmit). The graphs of established movements mark continuously ongoing process of shear processes along the slice forming cracks.Key words: rock deformations, seismic effects, monitoring, microdisplacements, Madara Plateau, Bulgaria.Spremljanje mikropremikov v jami Goljamata Cepnatina, Visoka planota Madara, SV BolgarijaEdini zgodnjesrednjeveški skalni relief v Evropi, Madarski konjenik, je vključen v Unescovo svetovno dediščino. Ta prispevek obravnava rezultate in situ meritev mikropremikov v razpoki, v kateri je oblikovana jama Goljamata cepnatina, ki se nahaja na robu visoke planote Madara nad Madarskim konjenikom v SV Bolgariji. Dodatno smo proučevali tudi premike razpok za robom visoke planote, ki so odvisni od dinamike robnih delov visoke planote in oblikujejo kamninske bloke. Monitoring vključuje uporabo 3D-ekstenzometra model TM-71, ki je nameščen v jami. Dodatno vključuje tudi pet merilnikov premikov, ki so nameščeni v razpokah za robom visoke planote. Rezultati, ki smo jih pridobili od leta 1990, kažejo nastajanje novih kamninskih blokov iz klifa zaradi gravitacijskega razpiranja kamninske mase. Ugotovljeni vodoravni mikropremiki kamninskih blokov v smeri proti JJV, kažejo, da je možen aktiven prelom z levo-zmično komponento v podlagi visoke planote. Ugotovili smo vplive bližnjih in oddaljenih potresov z epicentri v SV Bolgariji (Provadija), Romuniji (Vrancea) in Turčiji (Izmit). Grafi ugotovljenih premikov kažejo dolgotrajne strižne procese vzdolž kamninskih blokov, ki oblikujejo razpoke.Ključne besede: deformacije kamnin, seizmični učinki, monitoring, mikropremiki, Madara visoka planota, Bolgarija.