Monitoring of microdisplacements in Golyamata Tsepnatina cave, Madara plateau, NE Bulgaria

Abstract

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 Bolgarija Edini zgodnjesrednjeveski skalni relief v Evropi, Madarski konjenik, je vkljucen v Unescovo svetovno dediscino. 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 proucevali tudi premike razpok za robom visoke planote, ki so odvisni od dinamike robnih delov visoke planote in oblikujejo kamninske bloke. Monitoring vkljucuje uporabo 3D-ekstenzometra model TM-71, ki je namescen v jami. Dodatno vkljucuje tudi pet merilnikov premikov, ki so namesceni 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-zmicno komponento v podlagi visoke planote. Ugotovili smo vplive bližnjih in oddaljenih potresov z epicentri v SV Bolgariji (Provadija), Romuniji (Vrancea) in Turciji (Izmit). Grafi ugotovljenih premikov kažejo dolgotrajne strižne procese vzdolž kamninskih blokov, ki oblikujejo razpoke. Kljucne besede: deformacije kamnin, seizmicni ucinki, monitoring, mikropremiki, Madara visoka planota, Bolgarija.