Liviu Constantinescu and D. Enescu
Abstract
The present work represents a contribution to the study of the mechanism
of the Carpathian earthquakes, since it is correcting several former results,
drawing new solutions and additional information, sinthetizing the existing
data, discussing their seismotectonical implications and integrating the seismologic
results in the general geophysical complex to which they belong.
The solutions shwon in the figures 1–20 were obtained for 20 Carpathian
earthquakes (15 intermediate and 5 normal ones – Table 1). The interpretation
of the data which determined these solutions (Table 2) shows the drawing of
the following conclusions:
1) The possible orientation of the fault plane shows, for the greater part
of the Vrancea earthquakes, the trend towards parallelism of the direction
of
this plane with the tangent at the bend of the Carpathians are or at the northern
branch of the Eastern Carpathians; the trend towards transversality against
the Carpathian arc is evident only in some cases.
2) As for the character of the faulting and the type of the earthquakes, the
results (Table 3) emphasize the prevalence of the dip-slip and of the reverse
faulting, respectively the prevalence of the compressional stress.
3) The preferential orientation of the null vector follows directions parallel
with those of the Carpathian arc and of the folding axis (Fig. 25), the transversal
orientation being not precluded anyhow. It can be considered that the prevalent
system of null vectors – parallel with the chief geomorphological lines-reflects
the recent, more active tectonics, whilst the subordinate system – marked
by transversality – is a probable expression of older, only sporadically
activated tectonics.
4) The investigation of the strain pattern of the foci reveals three kinds
of earthquakes: earthquakes with a compressional, nearly horizontal strain,
with
a NW-SE trend (Fig. 26), earthquakes also with compressional strain but having
a NE-SW strike (Fig. 27), and the earthquakes with compressional, nearly vertical
strain (Fig. 28). The drawing of Fig. 29 emphasizes the transversality of the
compressional strains against the Carpathian arc, the under-Carpathian folds
and the chief strike of the isolines of static anomalies, in most cases.
The nodal characteristic of the Vrancea Region is manifest both seismotectonically
and gravimetrically, evincing two directions perpendicularly disposed against
each order as far as the deformation of the earth crust and of the underground
masses is concerned; the principal one being parallel to the surface features
and the second one transversely disposed against the latter.
5) The study of the Carpathian earthquakes from the point of view of the theoretical
source equivalent with the focus has been performed both according to the assumption
of the source I and of the source II (Table 4). There has been noticed a trend
towards equivalence of the focus with source I and the normal earthquakes (plane
a as fault plane), and with source II at the intermediate earthquakes.
6) The graphic representation, determined by the removal of the ambiguity (Fig.
31) shows for the intermediate earthquakes a general tendency of orientation
of the fault plane according to directions parallel with the corresponding
tangents to the Carpathian arc, the slope of this plane being towards the outer
part of the Carpathians. The removal of the ambiguity does not corroborate
with the parallelism of the direction of the fault plane and of the line of
the epicentrum of some earthquakes (no. 7, 8 and 13), which had led to postulating
a tectonic line. The orientations established for the fault planes of normal
earthquakes indicate two tectonic lines: one with a NE-SW strike in connection
with east-western subsidence of the basement of the area (earthquakes no. 16,
17 and 19), and one with a NW-SE strike, the possible extension of one of the
tectonic lines of Dobrudja.
On the whole, the seismo-tectonic implications of the obtained solutions are
consistent with the interpretation of the gravity and magnetic anomalies: the
spatial distribution of the underground masses in ruptured blocks shows a prevalent
parallelism and a sporadic transversality against the surface characteristics.