Geoscience Reference
In-Depth Information
Fig. 3.14
Fallen massive
stalagmite calcified to the
floor with new vertical
stalagmite
(E) Breakdown. The gallery of the cave between
70 and 140 m is almost completely filled with blocks
of large size. On the basis of morphological charac-
teristics (presence of calcite cover or series of small
stalagmites on the blocks) it can be assumed that the
breakdown
In the frames of bilateral scientific cooperation
between Bulgarian and Hungarian Academies of
Sciences, in 2007 was performed a study on the nat-
ural frequencies of stalagmites. The aim of the study
was to estimate the upper limit of prehistoric ground
acceleration which can break the thin and high poplar-
like stalagmites in the cave.
For the purpose of the study a stalagmite with height
of 237 cm and diameter between 8.3 and 13.7 cm was
selected. A three geophones SM6 with natural fre-
quency of 4.5 Hz was fixed with adhesive tape on dif-
ferent heights on the speleothem (Fig.
3.19
).
Measuring the natural frequency of a stalagmite
requires forced horizontal vibration by gentle hand
hitting. The horizontal acceleration was registered by
SIG SMACH SM-2 digitizer. The sampling rate of the
analog-digital converter was set at 256 Hz, whereas
the cutoff frequency of the anti-aliasing filter was
50 Hz. The power spectral density of the vibration
was determined by Fourier transform.
The natural frequency of stalagmite is 17.8 Hz
(Table
3.1
.). If the natural frequency is below 20 Hz
(the approximate upper limit of the frequency range
of the nearby earthquakes), then resonance can occur.
Five samples of broken stalagmites in Elata Cave
were collected for laboratory measurements of their
mechanical properties. The samples were found on
the
was
approximately
at
Upper
Pleisto-
cene—Lower Holocene age.
Fig. 3.15
Rose-diagram of the directions of 41 fallen and
recovered with new speleothem stalagmites from Elata Cave
ground
at
the
same
cave
passage
where
the
