Speleothems as paleoseismic indicators: examples from Bulgaria
by
Konstantin Kostov
Geological Institute of Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

The broken and deformed speleothems in the caves are accepted by many researchers as a good tool for paleoseismic reconstruction. The "sismothems" (speleothems potentially broken by a seismic event) can be dated (U/Th, C14, etc.) and offer possibilities to obtain the age of the paleoearthquake that caused the collapsing. The study of the prehistoric seismicity on the base of speleoinformation (destructed, fractured speleothems, breakdowns, etc.) is a new type of research in Bulgaria. Deformations of speleothems with possible coseismic origin are established during the last years in few caves in the Balkan range and Rhodope Mts.

The Lepenitsa Cave is a spring cave with total length of 1525 m that is situated in the Western Rhodope Mts., South Bulgaria and located in one of the seismically active regions in the country. The detailed geomorphologic study of the cave and the statistical analysis of the geometry and orientation of the deformed speleothems may prove the coseismic origin of the deformations. In the first fossil level were measured the directions of 68 broken stalagmites or stalagmite's pieces and a rose diagram was made. Mostly of the studied stalagmites were candle shape ones, with average length of 40 cm and secondarily recovered with thick sinter deposits. In some cases recent small active stalagmites are developed on the broken ones. The selected samples were fallen on flat and stable horizontal floor which exclude their secondary disposition.

The diagram shows clearly expressed preferred orientation - the maxima generally correspond to the direction to the town of Velingrad - the epicenter of the strongest known earthquakes in the area until now (1905, Ms = 4.2 and 1977, Ms = 5.3). These preferred directions are different from the direction of the palaeoflow of the cave's sediments - an indication that the fall of the stalagmites is due to the earthquakes. An eventually dating of the sismothems in the future can estimate the age of the seismic event (or events) that caused the deformations in the Lepenitsa Cave.

The Shepran Cave is situated in Central Rhodope Mts. According to the local people (oral information by the speleologist Dr. V. Pashovski), during the very strong Chirpan earthquake in 1928 (Ms=7.0), two men from the village were in the cave and witnessed for breakdowns. The Shepran Cave is about 55 km to the south-west of the epicenter of the earthquake.

During the visit in the spring of 2001, we established a lot of deformations of speleothems, mainly fallen stalagmites. A lot of the studied samples were recovered with new speleothems.

The result of statistical analysis for 46 samples does not give well individualized directions. Several factors can explain this fact: irregularities on the floor or inversion on the way of the stalagmites. Some samples were without head resulting incertitude for the direction. In spite of this, one of the maxima corresponds to the epicenter of the Chirpan earthquake.

The Troana Cave is located in the Central Fore-Balkan, North Bulgaria. According to its attractive speleothems, well preserved from negative human impact, the Troana Cave is one of the famous caves in Bulgaria. From seismological point of view, the cave is situated in one of the seismically strongest regions of the country -- during the last century three strong seismic events occurred there with magnitude up to M=7.0 (Gorna Oryahovitza earthquake, 1913). In the parts up to the first sump of the cave were established deformations of speleothems with possible coseismic origin: fallen stalagmites, displaced stalactones and broken sodastrows.

There is no doubt that the future paleoseismic investigations of the Bulgarian caves will lead to new results about the seismic history of Balkan peninsula. An absolute dating of the speleothems in the future could give us a valuable information about the evolution of the sesmic activity of Bulgarian lands.

Key words: karst, cave, speleothems, paleoseismology, Bulgaria

Date received: February 28, 2002

Copyright © 2002 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # caiq-35.