International Symposium on Strong Vrancea Earthquakes and Risk Mitigation
Oct. 4-6, 2007, Bucharest, Romania
SEISMICITY CHARACTERISTICS IN THE PARTICULAR CASE OF VRANCEA
(ROMANIA) INTERMEDIATE-DEPTH EARTHQUAKES
Maria Rogozea1, Bogdan Enescu1,2, Bazacliu Olivia1, Mihaela Popa1, Mircea Radulian11
ABSTRACT
The aim of the study is to investigate the behavior of some common seismicity parameters
and define seismicity pattern specific for the Vrancea seismogenic zone, located at the
South-Eastern Carpathians arc bend, in Romania. A persistent and unusually strong cluster
of earthquakes is generated here in a confined volume at intermediate depths (60 – 180 km).
The atypical geometrical configuration of the hypocenters, elongated along NE-SW direction
and close to a planar distribution, the persistence of the earthquake generation in time, rise a
lot of questions and debates in connection with this seismic area.
The seismicity characteristics are investigated on the basis of a set of Vrancea intermediatedepth events recorded by the seismic network of National Institute for Earth Physics
between January 1995 and May 2007.
The variation of the seismic activity in space and time, frequency-magnitude distribution,
deformation accumulation curve (Benioff’s curve) are analized.The study area, situated
between 60 and 180 km depth, is divided in two active segments, one centered around 90
km depth, other centered around 140 km depth. The evolution of seismic activity shows
alternative accelerating and decelerating deformation release in the upper segment and
lower segment, respectively. The different seismicity behavior in the two segments of the
seismic active volume and the apparent interconnection between them can be speculated to
predict the most probable future particular seismic hazard pattern. We identify the
characteristics of the preparation process of the strong subcrustal events originating in
Vrancea region and analyze how they can be incorporated in a time-dependent seismic
hazard assessment.
Keywords: Vrancea, seismicity, seismic cycle.
INTRODUCTION
Vrancea seismic zone is located in Romania at the South-Eastern Carpathians arc bend,
where at least three major tectonic units are in contact: the East-European plate, the IntraAlpine subplate and the Moesian subplate. The accumulation deformation is almost entirely
released by earthquake generation in the subcrustal domain (60≤h<180km). Here 2-5
shocks with magnitude Mw~7 occur per century producing a lot of casualties and severe
damage.
The purpose of this paper is to study the variation of the seismic activity in space and time,
frequency-magnitude distribution, deformation accumulation curve (Benioff’s curve), on
different time and space windows.
We use an homogeneous catalog of Vrancea intermediate-depth events recorded by the
seismic network of National Institute for Earth Physics between 1995 and 2007 (2205
events) and a routine catalog for a 70-year time interval (6589 events).
1
National Institute for Earth Physics, P.O. Box MG-2, 077125, Bucharest-Magurele, Romania Tel.:+4021
4050671, Fax:=4021 4050673
2
Research Center for Earthquake Prediction, Disaster Prevention Research Institute, Kyoto University,
Gokasho, Uji, Kyoto 611-0011, Japan
International Symposium on Strong Vrancea Earthquakes and Risk Mitigation
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DEAPTH VARIATION OF EARTHQUAKE SIZE DISTRIBUTION
Recent studies show that the seismic activity in the given region develops in space and time
as a consequence of the system geodynamic evolution and therefore, it can not be
considered of purely random type. Similarly, the scaling relations, such as frequencymagnitude distribution, can provide valide information as concerns the distribution and
evolution of the local stress field in correlation with the evolution of the geodynamic system.
We plot in Figure 1 the variation on depth of the b-slope of the frequency-magnitude
distribution for the considered data set (1 January 1995 – 31 May 2007). The slope was
determined by the maximum likelihood method (Aky, 1965; Utsu, 1965) since this method
provide more reliable estimations as compared with the least square regression method. As
the magnitude threshold for completeness we took Mw=2.5.
The variation of the b-slope of the frequency-magnitude distribution shows three anomalies:
a minimum at 80-110 km depth, where b value reaches the value of 0.6 (this is the zone
where the major earthquake in 1977 was produced), a relative maximum around 110 km
depth and the decrease in the bottom slab down to a value of 0.5 (around 140 km depth).
A general tendency of b value to decrease with depth, is in agreement with the increase of
the lithostatic stress and stress drop (Radulian and Popa, 1996). The anomalous low values
in the upper and lowerpart of the slab correlate well with the presumed two active segments
able to generate the largest Vrancea shocks.
Figure1. The depth variation of the b-slope
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M. Rogozea et al.
SEISMIC CYCLE ANALYSIS
In the last 100 years, several seismic cycles can be identified in the Vrancea subcrustal
domain. Each cycle is characterized by a shock larger than 6.5 and a relative lack of
earthquakes at intermediate magnitudes (5.5 to 6.5) as compared with the linear
Guttenberg-Richter frequency-magnitude distribution. Possibly, the major shocks are
generated by a percolation type process (Trifu and Radulian, 1991).
The succession of seismic cycles in the last 100 years is given in Fig. 2. The evolution of
seismic activity shows alternative accelerating and decelerating deformation release (e.g.,
Bowman and King, 2001; Bufe et al., 1994) in the upper segment and lower segment,
respectively, of the subducting lithosphere (Figure 3).
Figure 2. Benioff’s cumulative curve for the last 100 years in the Vrancea subducting zone.
Figure 3. Successive cumulative processes in Vrancea as revealed by Benioff’s curves.
International Symposium on Strong Vrancea Earthquakes and Risk Mitigation
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CONCLUSIONS
Analysis of seismicity outlines an inhomogeneous structure beneath Vrancea region with two
seismic active segments, relatively separated.
The behavior of the b slope of the frequency-magnitude distribution on depth is
characterized by two minimum values (0.5-0.6) around 90 km depth and 140 km depth, in
correlation with the two active segments. We attribute these minima with high stress regions,
able to generate major shocks.
The processes generating the large earthquakes in the lower and upper part of the Vrancea
seismic active body seem to differ significantly: Accelerated seismic release is characteristic
in the lower slab, while decelerated seismic release seems to control the deformation in the
upper slab. These differences seem to control the specific behavior of the seismic cycles in
the two segments considered separately and could be crucial elements in assessing time
dependent hazard for Vrancea seismic source.
ACKNOWLEDGMENTS
The work is a part of the MENER project “Integrated researches related to generation of
intra-continental earthquakes of intermediate depth in the Vrancea zone”(Contract no.
732/24.07.2006) - financed by the Ministry of Education and Research of Romania.
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