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Uporaba metode datiranja površinske izpostavljenosti na primeru podora Veliki vrh
Mrak, I.; Merchel, S.; Benedetti, L.; Braucher, R.; Bourlès, D.; Finkel, R. C.; Reitner, J. M.;
O podoru v Velikem vrhu (Košuta, Karavanke) ni zanesljivih zgodovinskih zapisov, vendar pa le ti obstajajo o podoru na Dobraču (25.1.1348), ki je od Velikega vrha oddaljen 46 km [1]. Podor je povzročil potres in naša hipoteza je bila, da je tudi podor v Velikem vrhu posledica istega dogodka. Tako smo s pomočjo metode datiranja površinske izpostavljenosti [2] analizirali vzorce matične kamnine v steni Velikega vrha ter vzorce s površine podornih blokov. Ugotavljali smo vsebnost 36Cl, ki se je začel tvoriti po podoru. Na podlagi poznavanja števila atomov 36Cl na gram Ca na leto izpostavljenosti, čas dogodka (podor) izračunamo iz koncentracij 36Cl izmerjenih s pomočjo pospeševalnika (AMS). Prvi rezultati kažejo, da sta se podora na Dobraču in Velikem vrhu zgodila istočasno, natančne analize podatkov pa še potekajo.

Zahvala: Del raziskave sofinancira program CRONUS-EU (Marie-Curie Action 6. okvirni program #511927).

Literatura: [1] C. Hammerl, Historical earthquake research – methods used as a basis for the hazard assessment applied to the earthquake of 1348 in Villach (Austria), Proceedings of the Third International Symposium on Historical Earthquakes in Europe (Prague 1991). [2] J. C. Gosse, F. M. Phillips, Terrestrial in situ cosmogenic nuclides: theory and application, Quaternary Science Reviews, 20 (2001) 1475.

The rockfall Veliki vrh is located in the valley Pod Košuto (Geben stream watershed Karavanke Mountains, Slovenia). The highest point of the researched area is Veliki vrh (2086 m); the lowest is the settlement Plaz (650 m). Among the geomorphologic processes nowadays the linear denudation and erosion prevails (the most common inclination of the surface is between 21-32° in 33-55°).
Between the settlement of Plaz and the Zajemen farm immense amounts of rockfall debris are present in different sizes, from big blocks (up to 10 x 10 m) to granule. The smaller grain sizes (fine sand, coarse silt…) are missing and the grain edges are sharp. The lithology of the material is the same as the one forming the Košuta ridge - Triassic Dachstein limestone and reefy limestone.
There are no reliable historical data about the rockfall event beside the oral heritage in form of a fairy tale describing the catastrophic falling of rocks over the settlement in the valley, killing many people and forcing the survivors to establish a new settlement further downstream. However, there are numerous written records about a historic rockfall taking place about 46 km away at Dobratsch, Carinthia on 25th January 1348 [1]. That rockfall was induced by an earthquake with the epicenter situated at Friuli (~74 km distance to Veliki vrh). There are no other records of natural hazards within historical times for the area, thus, it seems very likely that the same earthquake triggered both rockfalls. To test this hypothesis, we have applied the surface exposure dating method [2] on samples taken from the fresh bedrock and big boulders originating from the Veliki vrh rockfall.
As the long-lived radionuclide 36Cl is the product of nuclear reactions induced by the high-energy cosmic ray particles in a Ca-rich rock, its concentration can be used as a dating tool. The material has been previously shielded and production of 36Cl started as recently as the rockfall took place. Then, freshly produced surfaces – bedrock and boulders – have been exposed to cosmic rays launching the clock. As the so-called production-rate, i.e. how many atoms per gram Ca per year exposure, can be calculated for a certain environment, a precise time for the rockfall can be deduced from the 36Cl concentrations measured by accelerator mass spectrometry (AMS).
Preliminary results suggest a simultaneous timing of both rockfalls: Dobratsch and Veliki vrh. Detailed data analysis is in preparation.
Keywords: accelerator mass spectrometry; terrestrial cosmogenic nuclides (TCN); cosmogenic nuclide exposure dating
  • Poster
    1. Trienalni Posvet, Naravne Nesrece v Sloveniji (NNS '08 - 1st Symposium Natural Disasters in Slovenia), 11.12.2008, Ljubljani, Ljubeljana, Sloveniji , Slowenien

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