Geoscience Reference
In-Depth Information
The team of Lacave explored two caves in Vercors,
France: Choranche Cave and Antre de Vénus Cave.
The measurement of the natural frequency was made
using Polytech OFV 3001 high-resolution laser inter-
ferometer. At the top of the studied speleothem was
stuck a piece of reflecting tape to ensure good reflec-
tion of the laser ray. The speleothems were excited
with a light hit by rubber stick or with light breath for
the soda straws. In Choranche Cave were measured 12
speleothems (stalagmites, stalactites, and soda straws)
and 8 speleothems were measured in Antre de Vénus
Cave—5 stalactites, 2 soda straws, and one stalactone.
The study showed that most of the speleothems were
not a subject of seismic motion because their natural
frequencies were higher than the range of seismic
excitation.
The thin poplar-like stalagmites are among the
most attractive and rare speleothems. Szeidovitz et al.
(
2005
) were the first who used especially these spe-
leothems as paleoseismic indicators. Their studies on
Baradla Cave in Hungary show that the high 5.1 m
stalagmite in Olimposz Hall of the cave could be
broken at a horizontal acceleration of 1.14-0.34 m/s
2
.
The diameter of the stalagmite is 7-10 cm and dating
samples from the base and the top of the stalagmite
are taken (Fig.
3.7
).
Even a higher value hardly exceeds the 7 levels
(1 m/s
2
) of the MSK-64 intensity scale. Regarding the
fact that the stalagmite is older than 100,000 years,
they supposed that no strong seismic event occurred
in the examined area.
The methodology of the Hungarian seismologists
is based on the equation of Cadorin et al. (1998). They
calculate the self-frequency of a stalagmite with the
following formula:
Fig. 3.6
Inclined stalagmite line (sm 1) with new vertical
stalagmite (sm 2) where a and b are places for dating samples
(Quinif 1997)
Samples for absolute dating to determine the time
interval in which the paleoseismic event happened are
selected from certain areas of deformed speleothems
(Figs.
3.5
and
3.6
).
3.2.2
Measurement of Natural
Frequencies and Horizontal
Ground Acceleration
of Speleothems
Generally, the method presents the inverse approach
to the morphological and statistical analyses of
deformed speleothems. This complex technique is
based on the hypothesis that the availability of non-
broken speleothems in caves could be interpreted as
absence
of
strong
seismic
events
during
historic
times.
To our knowledge, this type of research was first
applied in field studies in France and Hungary by the
teams of Dr. Corinne Lacave from Résonance Ingé-
niours-Conseils SA (Geneve, Switzerland) and Prof.
Gyozo Szeidovitz from the Geodetic and Geophysical
Research Institute of Hungarian Academy of Sciences
(Lacave et al.
2000
,
2003
,
2004
; Szeidovitz et al.
2005
,
2008
). Important in theoretical aspect are the
laboratory tests of stalagmites rupturing at the
University of Liege (Cadorin et al.
2000
,
2001
).
Theoretical modeling of the mechanical behavior of
speleotems during seismic event was performed by
the Saclay Mechanics and Technology Department of
CEA, France (Gilli et al.
1998
).
1
3ED
2
=
16qH
4
f
¼
p
ð
3
:
1
Þ
P
and ground acceleration resulting in fracture with the
following:
a
g
¼
rr
u
2qH
2
ð
3
:
2
Þ
The abbreviations are as follows: H is the length of
the stalagmite (m), D is its diameter, r is its radius
(m), q is the density of the stalagmite (kg/m
3
), r
u
is
the breaking tension (Pa), and E is the Young mod-
ulus (Pa). If resonance phenomena occurs as well, the
