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

Special shaped (high, 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 method of our investigation is the same as before: 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 height 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 a STM(2.26m) standing in show-cave part of Domica cave have been determined by Multi Collector – Inductively Coupled Plasma Mass Spectrometry analysis (MC-ICPMS). The ag value (upper limit for horizontal peak ground acceleration needs to break STM(5m) in Ördög-lik Hall) coming from theoretical calculation is almost the same (~ 0.059g) as it is in case of STM(5.1m) in Olimposz Hall (~ 0.055g). On the grounds of our measurements and theoretical calculations, we can state that the geological structures close to Baradla and Domica caves did not excite such paleoearthquakes in the last 2-5 kyears, which would have produced horizontal ground acceleration larger than 0.061g. This value can arise even in case of 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-, Pelsőcand Rozsnyó lines) Introduction Why is it important to estimate the upper limit of horizontal ground acceleration for a given area and for a few 10 kyears in the past? It is very important to obtain an unbiased view of seismic hazard, which is one of the most important factors controlling costs of building construction. Estimating seismic hazard appropriately requires above all, information on the largest earthquake that can occur. The most of the large earthquakes occurred 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 enough long time period, as they are based characteristically on 1000 to 2000 years observational period. Knowledge about these largest earthquakes is therefore usually replaced 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 the continuation of it, at the 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 at the surrounding 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. For the estimation special shaped, (more or less cylindriform), intact, slim and vulnerable stalagmites were chosen. The steps of our investigation are the following: ⎯ determining non-destructively the natural frequencies and the dimensions of slim intact/unbroken stalagmites in situ in the cave; ⎯ measuring in laboratory the mechanical properties of broken stalagmites: