(ABSTRACT)
The paper is mainly aimed to demonstrate, based on a statistical analysis
of the seismic events, that the elongated shape of the epicenter region within
Vrancea active seismic area is at least partly due to a space-time migration
of the intermediate-depth seismicity developed within a smaller, more isometric
active seismic zone moving towards SW.
Basic idea in developing the carried out research was that an eventual migration
of the seismic activity should be the result of two components: (i) a random
occurrence of the earthquakes within an inhomogeneous seismic body, and (ii)
a systematic (deterministic) trend generated by an assumed displacement of
sub-crustal masses under the influence of convective currents in the upper
mantle. To avoid, or at least to attenuate the stochastic component, seismic
activity within Vrancea area was integrated along time intervals between major
seismic events (of distinct moment magnitude): 1940 (Mw = 7.7), 1977 (Mw =
7.4), 1986 (Mw=7.1), and 1990 (Mw = 6.9). Among them, several sub-intervals,
related to the solar activity cycle were inserted, where the case. Both simple
and weighted averages have been used for determining the centers of the seismic
activity within each interval.
Statistics has been performed on a data set provided by the ROMPLUS catalogue
(Oncescu et al, 1998), which has been completed with more recent events as
provided by the web-site of the National Institute for Earth Physics. To study
the intermediate-depth seismicity, earthquake hypocenters located beneath 60
km depth were considered only. Besides, several deep events located far outside
Vrancea geotectonic area were also excluded. All in all, about 2200 seismic
events were taken into consideration, which represent a significant statistical
database.
Following the above-mentioned analysis an obvious migration of the intermediate-depth
seismicity in the Vrancea area was pointed out. Generally, the seismic activity
seems to move southwestward, by gently dipping, except for the moment around
the major earthquake of March 4, 1977, when the trend reversed. The catastrophic
earthquake of March 4, 1977 marked an important collapsing and a reversal in
the general migration trend. After the event, depth and sense of migration
came back on the general trend, to reverse again starting with 1990.
Key words: Vrancea, intermediate-depth seismicity, statistics, geodynamics,
space-time migration