Geoscience Reference
In-Depth Information
(Gephard and Forsyth
1984
; Michael
1987
). The
assumption of these methods is that the orientations of
the principal stress axes are constant within the vol-
ume, which is being analyzed. Such dynamic inter-
pretation of focal mechanism data is not always
appropriate (Scholz
2002
). For some areas, limited
number of earthquakes has fault-plane solutions, for
weaker earthquakes (M \ 4) it could be expected
determination of the focal mechanism only in the case
of sufficient records from seismic stations around the
epicentral area.
For the karst areas, the nearest earthquakes with
determined fault-plane solutions can give useful
information about the tendency of spatial orientation
of the active (with water stream) galleries. These
galleries, by their definition, have to be oriented
approximately at along the same direction as the axis
P (the compression) or perpendicularly to the axes
T (tectonic extension).
Fig. 2.20
Example of stereographic projection (lower hemi-
sphere) with recorded first brakes of type ''contraction'' C and
''dilatancy'' D by the seismic stations from different azimuths
around the earthquake focus (Georgiev and Shanov
1991
—
Reproduced by permission of Bulgarian Geophysical Journal).
The
nodal
planes
and
the
position
of
P
and
T
axes
are
determined
2.2.5
Time Sequence
of the Reconstructed Stress Fields
the parameters of the focus and the processes in it are
calculated, sufficiently, for the tectonic studies accu-
racy through these records. The regions of positive
and negative first breaks (first arrival) of longitudinal
waves enter on circle in Wolf' Projection, and both
possible nodal planes are defined (Fig.
2.20
).
The movement occurred along one of the nodal
planes. The direction of tectonic compression (P) and
tectonic tension (T) are situated at the two opposite
bisectors of the angles defined by the nodal planes.
The crossing of the nodal planes defines the inter-
mediate axis B. These three axes are practically
indirectly related to the principal axes of tectonic
stresses within the rocks r
1
(P), r
2
(B) and r
3
(T).
The interpretation of focal mechanisms for tectonic
analysis has to make the distinction between the fault-
plane and the auxiliary plane. The ambiguity of this
interpretation is that it cannot be made from the focal
mechanism itself and it must be based on comparison
with the local geological structures. The main prob-
lem is connected with the synonymous determination
of the nodal plane, along which the movement is
performed, and it could be realized through additional
tectonic and geophysical information only. The other
uncertainty arises in attempting to relate the P and
T axes with r
1
and r
3
axes of the tectonic stress field.
Various
When working in karst areas, any collected informa-
tion concerning the reconstructions of the stress fields
can be of importance for ranging the tectonic impacts
consecutively in the geological time scale, and to make
the conclusions for the relationship between tectonics
and karst evolution. Normally, a number of paleotec-
tonic stress fields can be reconstructed. The first of
evident bench marks for ranging them in the time is the
age of the concerned karstified rocks. The first impact
forming fractures had been obligatory manifested after
the formation and lithification of the rocks, not earlier.
The next steps are to find reconstructions of the stress
field in the area in younger sediments than these con-
taining the karst system. These sediments can provide
information about the stress fields after their formation,
but the deformations obligatory have impacted the
older rock sequences. The last step is to compare the
reconstructions of the stress field with the fault-plane
solutions from nearest earthquakes, if available. These
solutions give immediate information closely corre-
sponding to today stress conditions and they can help
the correct interpretation. The appropriate arrangement
in time of the deformations can be successful only
when using all available geological, geomorphologi-
cal,
inversion
schemes
have
been
developed
hydrogeological,
geophysical,
and
geodetic
