The Vrancea seismic region is an area of concentrated intermediate-depth seismicity in the South-Eastern Carpathian bend in Romania. Despite the small seismogenic volume, the seismic energy release is unusually high compared to other subduction zones. The most recent moderate size earthquake of October 27, 2004 (Mw> = 5.8) occurred in an area relatively free of seismic activity in the preceding 15 years. The data, provided by various earthquake monitoring systems installed on the Romanian territory, represent the best quality instrumental information ever recorded in this area. The accurate JHD location of seismicity for 1996-2003 shows features close to a two-dimensional geometry, which are in very good agreement with the fault plane solution of the 2004 event. The hypocenters of the background seismicity are confined to an approximately vertical plane, NE-SW oriented, which practically coincides with the steeply/dipping NE/SW trending nodal plane. The source parameters, from spectral amplitude inversion and empirical Green’s function deconvolution, are consistent with the source scaling properties of Vrancea intermediate-depth earthquakes. They imply a very efficient high-frequency seismic radiation, caused by high stress drop at the source (~ 200 MPa).
The Vrancea region represents the most concentrated seismic active area in Europe, located at the South-Eastern Carpathian bend in Romania (Fig. 1). An unusual concentration of seismic energy is released at intermediate depths (60 – 180 km) in a narrow lithospheric body strongly pulled down into the asthenosphere (e.g., Sperner et al., 2006) by a process not fully elucidated yet. Recent tomographic images identify an extended high-velocity body beneath the Carpathian arc (Martin et al., 2004), while the earthquake distribution is constrained to a small region at the north-eastern border of this body. Accurate Joint Hypocenter Determination (JHD) locations (Bonjer et al., 2005) show that the earthquake zone has a nearly two-dimensional geometry.
According to damage rheology models (Lyakhovsy et al., 1993; 1997), a seismogenic system is characterized by a fast and effective healing process and develops specific geometrical patterns. The strong clustering of earthquakes in Vrancea and the continuous seismic energy release could be explained by such a system. For example, low internal friction and fast healing rate, describing systems with relatively short memory, lead to the development of highly localized, geometrically regular, fault systems. The associated seismicity patterns are compatible with the characteristic earthquake distribution and quasi-periodic temporal occurrence of large events. Conversely, high internal friction and slow healing rate, describing systems with longer memory, tend to lead to the development of disordered systems, characterized by power law like frequency-size statistics of earthquakes and random or clustered temporal distributions of large events.
Fig. 1. Vrancea region location (solid line rectangle). Telemetry network (red triangles), network of digital K2 accelerometers (blue diamonds), installed within the German-Romanian collaborative programme CRC461 (Bonjer et al., 2000) and real-time network (black stars). The real-time network is based on K2-stations (BMG, CRA, DRG, IAS, TLC, VRI), the Bucovina array, the CTBTO-IMS-station MLR, station Buzias, and the GEOFON station TIRR.
In agreement with the damage rheology model simulations, the Vrancea seismic source is characterized by a deficit of moderate-size shocks (magnitude around 6.5) relative to a linear Gutenberg-Richter distribution. Since the major event of March 4, 1977 (Mw = 7.4), two earthquakes with magnitude above 6.5 occurred, in 1986 (August 30, Mw = 7.1) and 1990 (May 30, Mw = 6.9), while only two medium-size earthquakes (5.5 ≤ Mw ≤ 6.5) occurred in the same time interval, in 1990 (May 31, Mw = 6.4) and in 2004 (October 27, Mw = 5.8). The seismic regime in Vrancea is significantly below the prediction of the Gutenberg-Richter law for medium-size earthquakes during the last 30 years. However, a time span of 30 years is rather short and the abundance of M>7 events may just be a short-term phenomenon and the "lack" of events near 6 might not be real in the long run.
The purpose of this report is to analyze the most recent event (27 October 2004). This event is extremely important because it is by far the best instrumented event ever recorded in Romania.
Pre- and post seismic regime
The October 2004 hypocenter is located at the north-eastern edge of the seismically active area (Fig. 2), as can be seen also on a vertical cross section oriented North-East, tangential to the bending of the Carpathian arc (Fig. 3). The earthquake occurred in a region of little activity during the last 25 years, possibly locked after the Mw 7.4 1977 shock, which affected the same depth domain (h ~ 100 km).
Fig. 2. JHD locations of 1996-2003 seismicity at the South-Eastern Carpathian arc bend (Bonjer et al., 2005). Coloured circles represent epicentres of Vrancea intermediate-depth earthquakes. Black crosses represent epicentres of crustal earthquakes. The epicentre of the October 27, 2004 event is represented by a larger yellow star. The smaller yellow star is the epicentre of the second largest earthquake in 2004 (September 27, Mw = 4.7), that occurred in the deeper segment of the Vrancea seismic zone.
Some precursory seismicity could be recognized post-event, but it was neither strong nor well defined to allow a pre-event recognition (Fig. 4). However, a slight increase in seismic activity (earthquakes per month) can be observed just prior to the October 2004 event that followed a relatively continuous activity decrease, noticed over a larger time scale. The same pattern, but even less clear, possibly occurred before the previous larger shock of April 28, 1999 (Mw = 5.3). It is difficult, however, to claim that this kind of pattern of seismicity represents a precursor, and how the possible anomaly might be related to the size of the following earthquake. A similar relative intensification of activity has been also reported both in the crustal domain (in front of the Carpathians Arc), and in the lower depth domain (below 130 km).
Fig. 3. Hypocentre distribution of the earthquakes of figure 2 on two perpendicular, vertical cross sections across the Vrancea area for earthquakes between 1996 and 2003. The October 27, 2004 and September 2004 events are represented by yellow stars.
Fig. 4. Seismic activity (all detected earthquakes/month) in the Vrancea intermediate-depth source for a 10-year (1995 – 2004) time interval (Oncescu et al., 1999/NIEP updated). Arrows indicate the time occurrence of the largest events recorded during this time interval: April 28, 1999 (Mw = 5.3) and October 27, 2004 (Mw = 5.8). The value corresponding to October 2004 is plotted with a bold symbol. The continuous line is smoothing the observations with a running moving window (with a 5 points window length).
The aftershock activity was poor. We identified only 12 aftershocks during the first month after October 27. The largest aftershock (Mw = 3.2) occurred at about 10 days after the main shock. All other events had Mw < 3. The weak aftershock activity for Vrancea shocks with magnitude below about 6.5 is in opposition to the case of the major shocks and seems to be an important feature of the rupture process in this region. One possible explanation could be a percolation-type source process, which triggers only the largest characteristic earthquakes of the focal region (Trifu and Radulian, 1989).
The fault plane solutions obtained from the P-wave polarity inversion, as well as that obtained from inversion of spectral amplitude of local recordings (using the algorithm of Ebel and Bonjer, 1990 as implemented by Oncescu and Rizescu, 1998), agree very well with the moment tensor solutions obtained from global and regional data (Fig. 5). The seismic moment value obtained from local data is 5.4 × 1017 Nm (Mw=5.76), which agrees well with the USGS estimate of 7.0 x 1017 Nm (Mw = 5.83).
The vertical nodal plane, oriented NE-SW, is probably the rupture plane since it corresponds well with the geometry of the seismicity pattern (Fig. 3). The trend agrees also with the rupture planes for the major Vrancea events (Müller et al, 1978, Oncescu and Trifu, 1987, Trifu et al., 1992), which follow the same NE-SW elongation of the seismic active zone. However, none of the previous largest events had such a near-vertical NE-SW oriented rupture plane.
Fig. 5. The fault plane solutions of the main shock obtained using global data and local recordings: (a) from polarities and (b) from spectral amplitudes (Radulian and Popescu, 2004). ETH - Swiss Seismological Service, Institute of Geophysics, ETH, Hoenggerberg, Zurich, Switzerland; INGV - MEDNET network - Istituto Nazionale di Geofisica e Vulcanologia - Mediterranean Very Broadband Seismological Network; HARV - Department of Earth and Planetary Sciences, Harvard University; USGS - United States Department of the Interior Geological Survey.
We applied the empirical Green’s function deconvolution technique to
estimate the rupture complexity and duration. We looked for events with
hypocentres as close as possible to the target event. Due to the rather
unusual position of the main shock hypocentre, located in a relative gap
of seismicity (Figures 2 and 3), the probability to find co-located
earthquakes was low. We finally selected four earthquakes (Table 1)
located within a radius of 13 km around the main shock hypocentre.
Table 1. Earthquakes selected for the empirical Green’s function deconvolution. The event of 27/10/2004 is the main event. δ represents the distance between the epicentres of the main shock and the empirical Green’s function events.
Fig. 6. Example of empirical Green’s function deconvolution results for Vrâncioaia (VRI) station, located in the Vrancea epicentral area. A: P-wave (velocity) seismograms of the main event and the empirical Green’s function events; B: apparent source time functions from deconvolution, using three of the four possible empirical Green’s functions.
The apparent source time function for well constrained, stable deconvolutions consists of a simple pulse suggesting a simple rupture process at the focus. We found no systematic variation of the pulse width and amplitude with azimuth and, therefore, neglect, in a first approximation, source directivity effects. Also, we use the entire set of deconvolved source time functions (for different main/empirical Green’s function pairs and different stations) to estimate the source duration by a simple averaging procedure.
Subsequently, the source radius r and stress drop Δσ are computed following Boatwright (1980):
and Brune (1970):
With τ1/2 the pulse half width (τ1/2 = τ/2), v the rupture velocity, α the P-wave velocity at the focus and θ the angle between the normal to the fault and emergent seismic ray, we obtained:
which are in good agreement with values from spectral amplitude inversion, respectively,
The high stress drop is a common feature for all Vrancea earthquakes, and particularly for the larger shocks (Oncescu, 1989; Oncescu et al., 1998; Gusev et al., 2002). This feature, together with the deep focus, implies an efficient high-frequency radiation, as visible in the recorded seismograms even at relatively large distances.
The effects of the earthquake
The earthquake was felt within 200 km distance from the epicentre. It caused no serious injuries.
The distribution of the maximum horizontal acceleration (PGA) is derived from data recorded by the joint free-field K2 seismic network of CRC461 and National Institute for Earth Physics (Bonjer et al., 2000; Rizescu et al., 2002) (Fig. 7). 40 K2 accelerometers properly recorded the earthquake. The ground motion is linearly interpolated between stations. The PGA distribution is extending essentially in a NE-SW direction with two maxima located symmetrically relative to the epicentre, which seems typical for Vrancea intermediate-depth earthquakes (Mândrescu et al., 1988; Mândrescu and Radulian, 1999; Sokolov et al., 2004). Although the NE extension is more pronounced, the highest seismic radiation is noticed towards SW, in the Ploiesti area (the maximum measured acceleration was 266 cm/s2 at station SEC, located about 100 km SW from the epicentre). The northern part of the 50 cm/s2 contour line is not well constrained by data, but was fixed through predicted values at the map border. These boundary values were calculated using the attenuation law of Moldovan et al. (2000).
Fig. 7. The PGA distribution for the 27 October 2004 earthquake (Bonjer et al., 2005). White triangles show the K2 stations, which recorded the event. The epicentre is marked by a white star.
A moderate size earthquake (Mw = 5.8) occurred in the Vrancea seismic region on October 27, 2004 in the depth range of about 100 km that was affected by the last major shock of March 4, 1977 (Mw = 7.4). The event was felt over a large area from Bulgaria to the Republic of Moldova, but without noticeable damage and human injuries. The maximum observed intensity was VI.
The investigated earthquake belongs to the few medium-sized events (5.5 < Mw < 6.5), which are characterized by a deficit in aftershock generation compared to the major earthquakes (Mw ≥ 6.5). This relative scarcity could be explained by a seismic regime implying characteristic earthquakes, or eventually by a percolation type generation process (Trifu and Radulian, 1989).
The high precision JHD locations show a double vertical seismogenic zone. One zone is oriented parallel to the vertical nodal plane from the focal mechanisms suggesting this plane to represent the rupture plane. The source parameters (Mo = 5.4 × 1017 Nm, r = 830 – 982 m, Δσ = 169 – 178 MPa) were estimated using spectral amplitude inversion and empirical Green’s function deconvolution. They are consistent with the source scaling properties of Vrancea intermediate-depth earthquakes, i.e. they show an efficient high-frequency seismic radiation, due to the high stress drop at the source (~ 200 MPa).
Because of the significant improvement of earthquake monitoring on the Romanian territory in the last years, the event of October 2004 was the best ever recorded stronger Vrancea event. The resulting seismological database provides a fundamental and indispensable tool for stimulating future research work in this intriguing seismotectonic area.
This study benefited from the data recorded by the K2 network installed in the framework of the Collaborative Research Center 461 ‘Strong Earthquakes’ of Karlsruhe University in collaboration with the National Institute for Earth Physics, Bucharest-Magurele (Bonjer et al., 2000). Partly, these data are a contribution of the National Institute for Earth Physics of Bucharest to the EC-project MEREDIAN-2 (Mediterranean-European Rapid Earthquake Data Information and Archiving Network – contract EVR1-CT-2000-40007).
We thank George Chircea, Viorel Pirvu and Werner Scherer as well as Olivia Bazacliu, Mioara Pompilian and Rainer Plokarz for their indispensable efforts in collecting and processing the earthquake data.
The maps and depth profiles were plotted with the Generic Mapping Tool (Wessel and Smith, 1991).
B. Sperner provided the topographic image of figure 2.
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