Estimating the upper limit of prehistoric peak ground acceleration using an in situ, intact and vulnerable stalagmite from Plavecká priepast cave (Detrekői-zsomboly), Little Carpathians, Slovakia—first results

Abstract

Earthquakes hit urban centres in Europe infrequently, but occasionally with disastrous effects. Obtaining an unbiased view of seismic hazard (and risk) is therefore very important. In principle, the best way to test probabilistic seismic hazard assessments (PSHAs) is to compare them with observations that are entirely independent of the procedure used to produce PSHA models. Arguably, the most valuable information in this context should be information on long-term hazard, namely maximum intensities (or magnitudes) occurring over time intervals that are at least as long as a seismic cycle. The new observations can provide information of maximum intensity (or magnitude) for long timescale as an input data for PSHA studies as well. Long-term information can be gained from intact stalagmites in natural caves. These formations survived all earthquakes that have occurred over thousands of years, depending on the age of the stalagmite. Their 'survival' requires that the horizontal ground acceleration (HGA) has never exceeded a certain critical value within that time period. Here, we present such a stalagmite-based case study from the Little Carpathians of Slovakia. A specially shaped, intact and vulnerable stalagmite in the Plavecká priepast cave was examined in 2013. This stalagmite is suitable for estimating the upper limit of horizontal peak ground acceleration generated by prehistoric earthquakes. The critical HGA values as a function of time going back into the past determined from the stalagmite that we investigated are presented. on the location of the Plavecká priepast cave is consistent with the critical HGA value provided by the stalagmite we investigated.The approach used in this study yields significant new constraints on the seismic hazard, as tectonic structures close to Plavecká priepast cave did not generate strong earthquakes in the last few thousand years. The results of this study are highly relevant given that the two capitals, Vienna and Bratislava, are located within 40 and 70 km of the cave, respectively.

Numerical Modeling of Stalagmite Vibrations

“…where a 0 = 0.559; a 1 = 3.507; a 2 = 9.820; a 3 = 19.244; a 4 = 31.808; D is the average diameter, H is the height of stalagmite, q is the mass density of the stalagmite, E is the dynamic Young's modulus (see also Bednárik 2009;Gribovszki et al 2017). The latter values are given in Table 4, and they show indeed a rather good correspondence with the numerical values in the far right column of Table 3.…”

Characterizing Stalagmites’ Eigenfrequencies by Combining In Situ Vibration Measurements and Finite Element Modeling Based on 3D Scans

“…In contrast, the presence of intact speleothems indicates that, considering their growth, no earthquake-driven ground motions were sufficiently large to break them. Hence, knowing the speleothem's characteristics allows estimating an upper limit on past horizontal ground motions that did not result in the failure of studied speleothems [12][13][14][15][16][17][18][19][20]. This information provides an important view in the past seismicity in the area where the cave is located.…”

“…The natural frequency and damping factor of the speleothems are fundamental parameters in the study of their response to seismic shaking. With an eigenfrequency in the range of moderate local (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) or larger remote earthquakes (0.1-10 Hz), earthquake ground motions can cause resonance of the speleothem. Their failure can occur at a lower acceleration than predicted by static methods.…”

“…Their failure can occur at a lower acceleration than predicted by static methods. The in situ study of the natural frequency of speleothems has been done with a laser interferometer [23,24] or with a geophone attached to the stalagmite [13][14][15][16][17]19]. In those experiments, the stalagmite was slightly excited using a finger or a rubber hammer.…”

Eigenoscillations and Stability of Rocking Stones: The Case Study of “The Hus Pulpit” in The Central Bohemian Pluton

“…There are, however, cases when the lowest eigenfrequencies of the studied bodies close to collapse or breaking can be only several hertz or lower, falling thus into a typical seismic-frequency band and, therefore, resonances may occur. The examples are, e.g., natural rock columns in the Chiricahua Mountains, Arizona, where the heights of columns may be limited by a fundamental vibrational mode that matches the seismic shaking of large and moderate prehistoric earthquakes occurred nearby (Hall 1996), the Minaret of Jam in Afghanistan with fundamental period longer than one second (Menon et al 2004) or speleothems (Lacave et al 2004;Becker et al 2006), where eigenfrequencies can be measured in situ, calculated theoretically and modelled numerically by finite element method (Gribovszki et al 2017(Gribovszki et al , 2018. Precariously balanced rocks represent another class of bodies as they are not fixed to their bedrocks and thus their wobbling is possible.…”