Estimation the upper limit of prehistoric peak ground acceleration using the parameters of intact stalagmites and the mechanical properties of broken stalagmites in Domica cave, Slovakia

Suitably shaped (tall, slim and more or less cylindriform), vulnerable, intact stalagmites (STM) in Domica cave have been examined. Some of these STMs are suitable to estimate the upper limit for horizontal peak ground acceleration generated by paleoearthquake. This research is the continuation of our previous examination of STMs in Baradla cave, north-east Hungary. The density, the Young’s modulus and the tensile failure stress of broken STM samples have been measured in mechanical laboratory, whereas the natural frequency and the heights and diameters of intact STMs were determined by in situ observation. The value of horizontal ground acceleration resulting in failure and the theoretical natural frequency of STM were assessed by theoretical calculations. The age of the samples taken from the STM(2.26m) standing in show part of Domica cave has been determined by Multi Collector – Inductively Coupled Plasma Mass Spectrometry analysis (MC-ICPMS). The ag value (the upper limit for horizontal peak ground acceleration) needed to break STM(5m) in Čertova diera (Ördög-lik) hall coming from theoretical calculation is almost the same (~ 0.059 g) as in case of STM(5.1m) in Olimposz hall (~ 0.055 g) of Baradla cave. According to our measurements and theoretical calculations, in the last 2-5 kyears the geological structures close to Baradla and Domica caves have not generated paleoearthquakes, which would have produced horizontal ground acceleration larger than 0.061g. This value can be reached even in moderate size earthquakes. Our result has to be taken into account when calculating the seismic potential of faults near to Domica cave (e.g. Darnó, Plešivec (Pelsőc) and Rožňava (Rozsnyó) lines) Introduction It is very important to obtain an unbiased estimate of seismic hazard, because it is one of the most important factors controlling the costs of building construction. The seismic hazard of an area of interest is specified in terms of the horizontal ground acceleration which shall not be exceeded with a given probability within a certain time period. Estimating seismic hazard appropriately requires above all the information on the largest earthquake that has occurred in the past. The most of the large earthquakes occur at plate boundaries. However in territories with low or moderate seismic activity, as intraplate areas, the recurrence time of large earthquakes, belonging to the same source zone, can be as long as 10 000 years (Scholz [1990]). Therefore in territories with low or moderate seismic activity such information is usually not available since earthquake catalogues do not contain sufficiently long time period, as they are based characteristically on 1000 to 2000 years observational period. The lack of knowledge about these largest earthquakes is therefore usually balanced by assumptions about earthquake statistics and/or fault geometry. Those assumptions are difficult to quantify, rendering hazard estimation arbitrary and thus questionable. To obtain more reliable and realistic data regarding the frequency and magnitude of earthquakes, we can investigate paleoearthquakes, which occurred before historic times. The research of the relationship between earthquakes and the growth, tilting and breaking of speleothems is promising, and investigations of this kind have been initiated in recent times (Forti and Postpischl [1984, 1988]; Delaby [2001]; Cadorin et al. [2001]; Lacave et al. [2000, 2004]; Kagan et al. [2005], Becker et al. [2006]). The relationship between earthquakes and the growth and breaking of intact and vulnerable stalagmites have already been investigated earlier in Hungary (Szeidovitz et al. [2008, 2007, 2005]), and after that in Bulgaria (Paskaleva et al. [2008, 2006], Szeidovitz et al. [2008a], Gribovszki et al. [2008]). Slovakia is rather rich in dripstone caves, however similar investigations of intact and slim stalagmites for the aim of seismic hazard did not complete before our research. Recently, research cooperation between us and Austrian and Czech colleagues in similar stalagmite investigation began. Vulnerable stalagmites can be found in Baradla cave, Hungary, and in its continuation, Domica cave, Slovakia. These vulnerable stalagmites are well suited to the paleoseismic investigations; that is, they have the necessary large height/diameter ratio. Our investigations suggest that these stalagmites can be broken even at low horizontal acceleration (<0.6 m/s). These speleothems therefore can be used as indicators, whether or not large paleoearthquakes occurred in the surroundings of the investigated caves. The acceleration level (determined by our previous stalagmite investigation) for the territory of Baradla cave is lower than the PGA value determined by probabilistic seismic hazard calculation (Tóth et al. [2006]) for a much shorter period of time, and evidently, the expected PGA would be even greater for a 70 000-year interval. Aim and method of our research The aim of our research is: estimating the upper limit for horizontal peak ground acceleration generated by paleoearthquake. Specially shaped, (more or less cylindriform), intact, slim and vulnerable stalagmites were chosen for the estimation. The steps of our investigation are the following: ⎯ non-destructive determination of the natural frequencies and the dimensions of slim intact/unbroken stalagmites in situ; ⎯ laboratory measurements of the mechanical properties of broken stalagmites: