Attenuation in Southeastern Carpathians area: Result of upper mantle inhomogeneity

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1 Tectonophysics 410 (2005) Attenuation in Southeastern Carpathians area: Result of upper mantle inhomogeneity M. Popa *, M. Radulian, B. Grecu, E. Popescu, A.O. Placinta National Institute for Earth Physics, Bucharest-Magurele, Romania Received 23 December 2003; received in revised form 20 July 2004; accepted 24 December 2004 Available online 23 September 2005 Abstract The systematic analysis of seismograms recorded on the Romanian territory using Vrancea intermediate-depth earthquakes shows a strong asymmetric pattern relative to the epicentral area: on one side, in the Transylvanian Basin and the Eastern Carpathians (approximately along the inner volcanic chain), the amplitudes are reduced by a factor of 20 on average and the high frequencies are attenuated, in contrast with the other side, in the foreland platform. This pattern is explained by a significant attenuation increase caused by a strong lateral variation of the structure in the upper mantle, immediately towards NW of the Vrancea seismic active volume. This region corresponds to the most recent volcanic activity in the Persani Mountains and with the low-velocity body adjacent toward NW to the high-velocity body subducted beneath Vrancea area as indicated by seismic tomography and heat flow results. The CALIXTO 99 tomography experiment, deployed for 6 months in 1999, provides the largest number of observations for Vrancea earthquakes ever recorded on the Romanian territory. We select data from 8 earthquakes generated in this time interval in the Vrancea nest, which were recorded with signal/noise ratio greater than 5 by at least 25 stations. All of them are small- to moderate-magnitude events (3.6VM w V4.2). The attenuation is much more important in the high-frequency range (N1 Hz), than at low frequencies. Since the large Vrancea earthquakes can radiate significant energy in the low-frequency range (b1 Hz), our results show that the seismic hazard level is much more uniform all over the Romanian territory in the low-frequency range than in the high-frequency range. D 2005 Elsevier B.V. All rights reserved. Keywords: Seismic wave attenuation; Vrancea region; Intermediate-depth earthquakes; Upper mantle lateral inhomogeneity 1. Introduction * Corresponding author. addresses: mihaela@infp.ro (M. Popa), mircea@infp.ro (M. Radulian). The Vrancea seismogenic area, located in a complex tectonic zone where the Southeastern Carpathians belt suddenly changes its direction from NW SE towards E W (Fig. 1), is one of the most puzzling seismic regions of intra-continental deformation. The present-day tectonic activity is characterized by a nest of intense intermediate-depth seismicity ( km) that is often interpreted as reflecting the late stage of an intra-continental collision between the western Eurasian continental margin and the intra /$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi: /j.tecto

2 236 M. Popa et al. / Tectonophysics 410 (2005) Fig. 1. Location of Vrancea zone and seismic stations on the Romanian territory. Vrancea epicentral area is represented as a dashed elliptical area. The CALIXTO 99 stations are plotted as stars and the telemetered stations are plotted as solid triangles. The diamonds are the digital accelerometer stations (University of Karlsruhe). Not all the stations represented in the figure could be used in this study (see the next figures). The regions A, B and T delimited by oblique lines schematically represent the regions of high-, low- and intermediate-attenuation regimes. Carpathian microplates. Other regions with similar intra-continental tectonics are Bucaramanga (Columbia Bolivia) and Hindu-Kush (Afghanistan). These peculiar regions may hold important clues to the late-stage development of active ocean continental or continental continental interaction in areas of relatively small spatial extent, when compared with the scale of global plate tectonics. The collision was driven by Miocene Pliocene subduction retreat with last pieces of the Tethyan Ocean successively subducted towards SW and W (Csontos, 1995; Mason et al., 1998; Stamplfli and Borel, 2002; Sperner et al., 2002). Subduction ceased in the northeastern part (~14 Ma ago) when the continental East European lithosphere entered the subduction zone, while in the southern part subduction is still active (Sperner et al., 2001). The tectonic setting in the SE Carpathians marks the final stage of subduction of presumably the eastern part of the Alpine Tethys and related collision of the Tisia Dacia block with the Moesian platform (Matenco and Bertotti, 2000). The collision was associated with the formation of a large thrust-and-fold belt composed of flysch sediments (Sandulescu, 1984) and the eruption of large volumes of calc-alkaline magma along the eastern edges of the Alpaca and Tisia Dacia microplates (Szabo et al., 1992; Csontos, 1995; Mason et al., 1998; Seghedi et al., 1998; Nemcok et al., 1998). Magmatic activity was post-collisional and migrated from NW to SE along the strike of the arc, accompanied by a general decrease in the volume of magma erupted within each volcanic center (Szakacs and Seghedi, 1995). The youngest volcanic activity in the eastern Transylvanian Basin (Harghita Persani area, located km NW of Vrancea) is characterized by calc-alkaline and alkali basaltic eruptions of magmas between 2.1 and 0.6 Ma ago (Szakács, 1993; Seghedi and Szakacs, 1994; Downes et al., 1995; Seghedi et al., 1998). To explain the time-lag between the end of the major crustal shortening and lithospheric subduction (around 10 Ma) and the onset of the calc-alkaline volcanism, different geodynamic models invoke roll-back, detachment and/or break-off of the subducted lithospheric slab (Csontos, 1995; Mason et al., 1998; Seghedi et al., 1998; Linzer et al., 1998), or delamination of the lower part of the lithospheric mantle from the lower plate (Gîrbacea, 1997; Gîrbacea and Frisch, 1998; Chalot-Prat and Girbacea, 2000).

3 M. Popa et al. / Tectonophysics 410 (2005) The origin of the lithospheric material that is responsible for the intermediate-depth seismicity is still under debate. Fuchs et al. (1979) pioneered the idea of a paleosubducted oceanic slab beneath the crust. Based on results of global tomography, Wortel and Spakman (2000) invoked the subduction of a normal oceanic lithosphere that is presumed to have been detached from the continental lithosphere of Eastern Europe plate (beneath the northeast Carpathians), and may be in the process of breaking off from the Moesian platforms below Vrancea. The discrepancy of about 130 km between the Miocene suture position and hypocenters position was explained by the delamination of the continental lithosphere along a horizontal interface followed by rollback and dipping steeply down in the upper mantle (Gîrbacea and Frisch, 1998; Gvirtzman, 2002). This process postulates very shallow asthenosphere NW of the seismogenic region as well as upwelling asthenospheric material as source for the alkali-rich volcanism in the Persani mountains. At the same time, the presence of a seismic gap at shallow depth (around 50 km) might be explained by a zone of reduced viscosity. The gap is typical for break-off under very slow (b0.2 cm/yr) plate convergence velocity (Davies and von Blankenburg, 1995). Sperner et al. (2001) further developed this model, considering a last piece of formerly subducted lithosphere tearing off at present just beneath Vrancea. The low collision rate is typical for continental collision due to the buoyancy of the continental crust involved in subduction/underthrusting. Upper continental crust cannot participate directly in subduction and convergence can continue only when it is decoupled/delaminated from the mantle or transformed into higher-density materials. The most recent models suggest that Vrancea is the result of a low-rate continental convergence with a thermally young and highly buoyant lower plate and no active surface processes that cause a loss of coherence in the subducted slab and hence a gravitationally sinking stretched body (Cloetingh et al., 2004). The evolution from an active subduction to a passive sinking due to gravity is also in agreement with the maximum shear stress migration from the upper surface of the descending slab to its lower surface, as predicted by the numerical computation of the stress distribution in the Vrancea descending slab for a model with detachment scenario (Ismail-Zadeh, 2003). The predicted spatial patterns of the high-velocity dslabt and low-velocity asthenosphere in the upper mantle beneath Carpathians bending zone differ between models, and could be seismically determined by a combination of seismic tomography, seismic attenuation and S-wave splitting. High-resolution seismic tomography reveals important lateral inhomogeneities of the lithospheric mantle structure across the Vrancea region, in particular a high-velocity body and an adjacent low-velocity body situated toward the interior of the Carpathians arc. The earthquakes are generated inside the high-velocity body (at the lower surface of the descending slab), involving a significantly smaller volume as compared with the high-velocity body dimension (Martin et al., 2001). The tomography results support the model of lithospheric subduction accompanied by slab retreat and lithospheric delamination. However, the nature of the complex geodynamic processes (unusual foredeep basin, concentrated seismic activity and large-scale strain, recent unusual volcanism of the inner Carpathians, vertical motions and folding) involved in the recent evolution of the Carpathians Orogen system is still unknown and subject of numerous ongoing researches. The Vrancea earthquakes reach magnitudes as high as M w 7.7 with laterally strongly asymmetric damage, sharply decreasing towards the inner side of the Carpathians arc as compared with the outer side damage (e.g., Radu and Polonic, 1979; Mandrescu and Radulian, 1999). Unfortunately, the information both macroseismic and instrumental for the northwestern and northern parts of Romania is much more scarce than for the southeastern and southern parts, and therefore much less reliable. The recent seismological evidence suggests significant differences in the attenuation of the seismic Table 1 Intermediate-depth Vrancea earthquakes considered in this study Date Hour:min:sec Lat. (N8) Lon. (E8) h (km) M W 1999/06/20 00:09: /06/22 08:02: /06/29 20:04: /07/04 08:21: /07/13 13:10: /08/07 02:25: /09/14 23:48: /10/12 23:48:

4 238 M. Popa et al. / Tectonophysics 410 (2005) waves in the upper mantle beneath Vrancea region (Popa et al., 2003). Preliminary Q intrinsic estimations using P to S ratios (Popa et al., 2000) indicate also a similar asymmetry across the Carpathians arc. The same asymmetry is observed in the macroseismic field of the Vrancea strong events and is in agreement with the recent tomographic image, which indicates a low-velocity body adjacent toward NW to the highvelocity body subducted beneath Vrancea area (Martin et al., 2001) and heat flow measurements (Demetrescu and Andreescu, 1994). The low Q values in the upper mantle in the back-arc region could be related to some tectonic active processes, such as the most recent volcanic activity in the Persani Mountains, upwelling of the asthenosphere just behind the Vrancea seismogenic zone, or slab delamination. New instrumental data that have become available for the inner part of the Carpathians by means of the tomography experiment CALIXTO 99 and the extension of the digital accelerographs network towards NW within the German Romanian programme CRC461 (Wenzel et al., 1998; Bonjer et al., 2000; Bonjer and Rizescu, 2000; Bonjer et al., 2002a,b) are crucial in confirming the hypothesis of the lateral variation of the attenuation properties in the upper mantle. The main goal of this study is to check this hypothesis and its Fig. 2. Fault-plane solutions for the selected earthquakes obtained using first P-wave polarities. The degree of variability of the fault plane solutions is also indicated.

5 M. Popa et al. / Tectonophysics 410 (2005) consequences with an extended data set, applying the largest possible number of recording instruments and the best possible azimuthal coverage. To reach this aim, we select a set of 8 earthquakes occurred during the CALIXTO 99 experiment (Table 1), for which we had the largest number of observations per earthquake ever recorded in Romania for local events. All the fault plane solutions of the events are of reverse faulting type (which is typical for Vrancea subcrustal earthquakes), with significant differences in the nodal planes orientation (Fig. 2). At the same time, we reveal a new aspect of extreme significance for the seismic hazard assessment: the strong frequency dependence of the attenuation across the Carpathians. 2. Attenuation of seismic waves from Vrancea subcrustal foci The CALIXTO 99 experiment, deployed in the Vrancea region within the programme CRC461, consisted of 120 seismic stations (both short-period and broadband instruments) partly covering the inner side of the Carpathians arc, a region poorly monitorized by the Romanian national network (Fig. 1). During the experiment operation (May to November 1999) 77 local earthquakes with magnitude 3VM w V4.2 have been recorded. A preliminary analysis of the data obtained in the experiment (Popa et al., 2003) outlines the lateral variation of the seismic wave attenuation towards the Transylvanian Basin and Eastern Carpathians, observed both in P- and S-wave trains and their associated spectra. Examples of seismograms and associated Fourier spectra recorded toward NW and SE from Vrancea region are given in Fig. 3. Two pairs of stations situated approximately symmetrical relative to the epicenter are considered (see also Fig. 4): C07 (at 125 km to the west) and F10 (at 118 km to the east) with short-period instruments recordings for event of 1999/07/13, and C08 (at 135 km to the west) and F11 (at 108 km to the east) with broadband instruments recordings for event of 1999/06/29. The recordings characteristics, as shown in these examples, are typical for all the other data, independently of earthquake magnitude, location, focal mechanism and type of recording instruments. The maximum amplitude is by a factor of a few tens higher in the Carpathians foredeep than in the Transylvania area. At the same time, a strong decrease in the high-frequency relative to low-frequency values is apparent in the spectra of the stations toward NW. As a consequence of the strong cut-off of the high frequencies in the Transylvanian Basin the noise level becomes comparable with the signal at frequencies above 5 7 Hz. To better illustrate the results of this study, we define three areas, apparently with different attenuation properties, separated by (conventional) straight lines, like in Figs. 1 and 4. They are oriented NE SW, following the elongation of the Vrancea epicentral area in the same direction. In a vertical cross-section oriented SE NW (Fig. 5 after Cornea et al., 1981) the high-attenuation region (A) corresponds to a thinner lithosphere below Transylvania, while the low-attenuation region (B) corresponds to a much thicker lithosphere beneath the foreland platform region. A dtransition zonet (T) separates the two distinct regions. Certainly, the geometrical scheme as defined in our approach is not rigorous, but helps us in illustrating the results of our analysis. Only the stations that we use in this study are plotted in Fig. 4. Since our attention is focused on the lateral variation of seismic motion, essentially on an east west direction across the Vrancea region, we restrict our analysis to the stations located at latitudes greater than 44.58N. The selected stations have well-defined parameters for the instrument correction and recordings with a minimum signal/noise ratio 5 for at least one of the selected earthquakes. The velocity recordings are first corrected for instrument parameters and then rotated to a radial transverse coordinate system. Windows of about 5 s for P- and S-wave trains are selected. Noise windows before P and S-wave arrivals are also considered (examples are given in Fig. 3). A cosine tapering for ten points at the left and right edges of the signal and a baseline correction are applied in all cases. The maximum amplitude is for the considered windows and does not correspond necessarily to the first body wave arrival. Examples of distribution of the peak ground velocity (PGV) for the horizontal components of the earthquakes of and are given in Fig. 6. The values are in A/s and represent the maximum among the radial and transverse components. The region A is not so well covered with recording instruments and for many events the earthquake signal is below the noise. However, the available information is sufficient to show a systematic strong lateral variation

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7 M. Popa et al. / Tectonophysics 410 (2005) Fig. 3. Comparative representation of the seismograms typical for a short-period back-arc station (C km epicentral distance) and a short-period fore-arc station (F km epicentral distance) and a broadband back-arc station (C km epicentral distance) and a broadband fore-arc station (F km epicentral distance) for the event of 1999/07/13, and 1999/06/29, respectively: (a) velocity as a function of time; (b) Fourier amplitude spectra. Selection of the noise and body wave windows is also exemplified. One horizontal component (N S) was not properly working for the station C08. For the other stations the horizontal components are rotated to radial and transversal components.

8 242 M. Popa et al. / Tectonophysics 410 (2005) Fig. 4. Investigated area and the seismic stations (belonging to CALIXTO 99 and K2 networks) used in this study (only the stations in Fig. 1 located North relative to 458N latitude and with at least one record with signal/ noise ratio greater than 5). The short-period and broadband stations are plotted with solid circles and stars, respectively. in attenuation of the seismic wave radiation across the Vrancea region. The observed asymmetry can be caused in principle by source, propagation and site effects. To test the effect of the focal mechanism, we corrected the waveforms for source radiation pattern using the fault plane solutions of Fig. 2. When several solutions were equally possible, we simply selected an daveraget solution. The implied variability does not affect significantly the level of the corrections. At the same time, Fig. 5. WNW ESE seismotectonic vertical cross-section across South-Eastern Carpathians (after Cornea et al., 1981 with modifications). The borehole sites in Transylvanian Basin and Carpathians Foredeep subsequently used for the local site response estimation are also represented.

9 M. Popa et al. / Tectonophysics 410 (2005) the waveforms were corrected for geometrical spreading. The correction factor is of the form ATðR=hÞ=F rad ð1þ where A is the observed amplitude, R is the hypocentral distance, h is the focal depth and F rad is the source radiation factor. Since we are interested only in relative values, the geometrical spreading is considered taking the epicenter point (R =h) as reference. In each case, we consider as radiation pattern the largest value among the factors computed for SV and SH waves. Generally, F SV rad NF SH rad, due to the predominance of the reverse faulting mechanism. To avoid division by zero, whenever the F rad coefficient was below 0.1, this was set 0.1 (for S waves, this case was only occasionally encountered). This may be reasonable, since we are looking for maximum velocity amplitudes not exactly for the first phase, but for a selected window (containing secondary phases as well), as exemplified in Fig. 3. To check the source influence, we compare in Fig. 6 the PGV distributions corrected for instrument on one side and corrected for instrument (Fig. 6a), source radiation pattern and geometrical spreading (Fig. 6b) on the other side. The associated source radiation pattern is also shown (Fig. 6c). Although an influence of the source radiation pattern is evident in some cases (1999/06/20, 1999/09/14 and 1999/10/12), this is only a secondary effect as compared with the general strong NW SE contrast. The particular events we considered in our analysis are characterized by more S-wave radiation toward W than E in many cases, just in disagreement with the observations. For example, in six cases (1999/06/29, 1999/ 07/04, 1999/07/13, 1999/08/07, 1999/09/14 and 1999/ 10/12) one maximum of S-wave radiation is located in Transylvanian Basin, but no corresponding effect is visible in the observed PGV distribution. The major difference in amplitude level of the recorded motion between the foredeep area and Transylvanian Basin is equally observed for the P waves. At any rate, the sharp decrease to NW of both P- and S- wave amplitudes cannot be explained by source radiation pattern which rather is in favor of an opposite configuration of the asymmetry. Therefore, we have to reject the possibility that source radiation pattern is the cause of the observed configuration of the PGV values. To test the contribution of site effects, we selected a few locations in the Transylvanian Basin (Filitelnic, Cristur and Ocna de Sus) to compare with stations in the foredeep region (Petresti and Umbraresti), specified in Fig. 5. The parameters of the geological structure of the upper sedimentary layers are adopted on the basis of borehole data (Mandrescu, personal communication). The differences outlined in the site responses computed for one-dimensional structures approximating the upper sedimentary layers (of thickness: 4.5 km Filitelnic, 2.0 km Cristur, 2.4 km Ocna de Sus, 10.6 km Petresti and 8.1 Umbraresti) down to the bedrock layer as concerns the amplitude level and frequency content (Fig. 7) cannot account for the foremost attenuation effect we observed on the NW SE direction. Since the analyzed structural data are representative for the intra-carpathians and foreland areas, we conclude that the site response properties cannot explain this firstorder effect and have to consider a larger scale effect, related to the tectonic and geological contrast at the contact between the stable Paleozoic Mesozoic lithosphere of the East-European/Scythian/Moesian foreland units and the younger, Alpine lithosphere, in the East Carpathians/Transylvania back-arc area (Fig. 5). The amplitudes are in all cases lower in region A than in region B by a factor of 20 in average. For particular cases the attenuation factor can be as high as 100 or more. Smaller amplitude values in the vicinity of the epicenters can be observed, as a typical effect for Vrancea intermediate-depth events (e.g., Radulian et al., 2000). The variability of the amplitude values (corrected for instrument, radiation pattern and geometrical spreading) is shown in Fig. 8. We used clusters of stations in foredeep area and Transylvanian Basin, respectively. The difference in amplitude for a given earthquake size can be as large as 5 times, but is an order of magnitude smaller than the difference systematically observed across the Vrancea region. The shift between the amplitudes in the foredeep and Transylvania sites is above the amplitude errors (Fig. 8). 3. Attenuation frequency dependence The attenuation effect is mainly present at high frequencies. Low-pass filtered seismograms (1 Hz upper frequency) show significantly less lateral variation (Fig. 9), the amplitudes being comparable all over the study area. This is also visible in the filtered seismograms (Fig. 10).

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11 M. Popa et al. / Tectonophysics 410 (2005) Fig. 8. Test of the variability of the PGV values. Stations A07, A08, E03, E04, E06, E11, F08 and K28 are considered for the Carpathians foredeep area, and stations B02, C05, C07, S06, S07 and S08 for the Transylvanian Basin. All the PGV values (S wave), corrected for instrument, source radiation pattern and geometrical spreading which are available for the earthquakes in Table 1 are plotted. Dashed lines represent the least square approximations. Fig. 7. Examples of local effects (SV wave) typical for sites in the Transylvanian Basin (top) and Carpathians Foredeep basin (bottom). For the location of sites, see Fig. 5. The site response spectra are computed for horizontal sedimentary layers down to the bedrock layer. The maximum of the amplification factor (around 4) is obtained close to 0.3 Hz in all cases. Since the 1 Hz geophones strongly suppress frequency below 1 Hz, we have to treat carefully the short period recordings, mostly in the frequency range below 1 Hz (even if they are corrected for instrument response). In Fig. 10 we compare the filtered seismograms for the same stations pairs as in Fig. 3: the amplitude difference between the stations in the foredeep area and the intra-carpathian basin is reduced by a factor of 10 both for short-period (C07/F10) and broadband (C08/F11) instruments as compared with the unfiltered case of Fig. 3. This strong frequency dependence of the seismic wave attenuation across the Carpathians indicates a high level of hazard at low frequencies all over Romanian territory (Radulian et al., 2000). Our results are based exclusively on small magnitude events and this could rise questions when referring to attenuation on the frequency range below 1 Hz. Further investigation is required when data from larger Vrancea earthquakes will be available to conclude on this important subject. 4. Conclusions This study, based on the waveform analysis of small-magnitude Vrancea earthquakes, reveals strong lateral variations ascribed to the upper mantle structure inhomogeneity across the Vrancea region. A low-velo- Fig. 6. Distribution of the peak ground velocity (in A/s) for the earthquakes of 20 and 29 June 1999 which present significant differences in focal mechanism (see Fig. 2). The solid diamond is the epicenter. Only the stations with sufficiently high signal/ noise ratio are plotted. The values corrected only for the instrument response (diagram (a)) are presented for comparison with the values corrected for instrument response, source radiation pattern and geometrical spreading (diagram (b)). The corresponding radiation patterns for the largest S-wave horizontal amplitude are also plotted in diagram (c). They are practically not correlated with the observed PGV distributions. The representation in gray intensities is adjusted in each case to comprise a similar scale length.

12 246 M. Popa et al. / Tectonophysics 410 (2005) Fig. 9. Distribution of the peak ground velocity (in A/s) for the earthquakes of 20 and 29 June 1999 after the seismograms are corrected for instrument response, source radiation pattern and geometrical spreading and low-pass filtered with cutoff at 1 Hz. The solid diamond is the epicenter. Only the stations with sufficiently high signal/noise ratio are plotted. The representation in gray intensities is adjusted in each case to comprise a similar scale length. In comparison with diagram (b) of Fig. 6, the E W asymmetry in the filtered distribution is significantly reduced. city attenuating body seems to be located NW of the active seismic zone, acting like an effective filter mainly for the high-frequency radiation of the seismic waves propagating in this direction. Tentatively, it could be explained by an ascending flow of subduction-induced convection in the mantle wedge. This type of anomaly corresponds with a low-velocity anomaly outlined by the teleseismic tomography results (Bijwaard et al., 1998; Wortel and Spakman, 2000; Martin et al., 2001) and heat flow measurements (Demetrescu and Andreescu, 1994). Certainly, the source and local site properties are other important factors for the pattern of seismic ground motion distribution, but our analysis shows that the source and site effects play a secondary role. They cannot explain the systematic trends in amplitudes and spectral shapes over extended areas, like A, B and T (Fig. 1). The variation of the seismic motion features when passing from the foreland platform structure to the Transylvanian basin structure in the back-arc side is systematically observed regardless of earthquake magnitude, location or fault plane solution and must be related to a first-order anomaly in order to explain the distortion of the seismic wave radiation from the Vrancea intermediate-depth source. This type of attenuation anomaly is commonly noticed in many active subduction areas (e.g., Kadinsky-Cade et al., 1981; Chiu et al., 1985; Tsumura et al., 2000) and is usually related to back-arc volcanic activity. The tectonic interpretation for the Vrancea area is not straightforward due to the complicated intra-continental post-

13 M. Popa et al. / Tectonophysics 410 (2005) Fig. 10. Comparative representation of the seismograms typical for a short-period and broadband back-arc station (C07 and C08, respectively) and a short-period and broadband fore-arc station (F10 and F11, respectively) recorded for the events of 1999/07/13 and 1999/06/29. The velocities are low-pass filtered with cutoff at 1 Hz.

14 248 M. Popa et al. / Tectonophysics 410 (2005) collisional system, with an anomalous high-velocity seismic body and atypical volcanism. Another fundamental outcome of the present study refers to the strong frequency dependence of the seismic wave attenuation. The attenuation affects mostly the high frequencies (N1 Hz). The numerical synthesis of the ground motion, taking into account the source and the wave propagation path indicates no significant attenuation of the peak ground parameters towards NW and N from the Vrancea epicenter (Radulian et al., 2000) if we consider the low frequency domain (b1 Hz). This result apparently contradicts the historicallybased intensity maps, which indicate no significant damage in the western and northwestern parts of the Romanian territory. Now we propose a key to explain the discrepancy between computation and observations: the frequency-dependent attenuation and vulnerability. The seismic motion amplitudes in low-frequency range are roughly the same in the intraand extra-carpathians regions (regions A and B, respectively). They affect mainly the high-tall buildings, which are almost absent in Transylvania or in the Carpathians mountain. The effect of the low frequencies is much more important in the extra-carpathians area, mainly in the city of Bucharest, with the greatest number of tall and old buildings (the predominant frequency of the building is in the range of s). In contrast, the predominant buildings of one to three storeys (affected by high frequencies) are commonly spread everywhere, but the extreme attenuation towards NW and N from Vrancea consistently explains the difference in damage between the intra-arc and extra-arc regions. Therefore, for seismic hazard assessment in the intra-carpathians area, it is important to take into account the difference in attenuation properties in correlation with vulnerability analysis. Acknowledgements We benefited from data provided by CALIXTO 99 experiment and K2 network within CRC461 programme of University of Karlsruhe (Germany) (Wenzel et al., 1998; Bonjer et al., 2000; Bonjer and Rizescu, 2000; Bonjer et al., 2002b). The work was achieved with the contribution of the Romanian Ministry of Education, Research and Youth, which financed the projects from CERES Program (Contract no. 33/ , Contract no. 36/ ) and MENER Program (Contract no. 90/ ). We appreciate very much the critical reviews and valuable recommendations by the two anonymous reviewers and editorial comments by S. Cloetingh which improved significantly the quality of the paper. References Bijwaard, H., Spakman, W., Engdahl, E.R., Closing the gap between regional and global travel time tomography. J. Geophys. Res., Bonjer, K.-P., Rizescu, M., Data release of the Vrancea K2 Seismic Network. Six CD s with evt-files and KMI v1-,v2-, v3-files. Karlsruhe-Bucharest, July 15, Bonjer, K.-P., Oncescu, M.C., Rizescu, M., Enescu, D., Driad, L., Radulian, M., Ionescu, C., Moldoveanu, T., Source- and site-parameters of the April 28, 1999 intermediate depth Vrancea earthquake: first results from the new K2 Network in Romania. The XXVII General Assembly of the European Seismological Commission, Lisbon, Portugal, Book of Abstracts and Papers, SSA-2-13-O, p. 53. Bonjer, K.-P., Rizescu, M. and Grecu, B., 2002a. Data Release of the Vrancea K2 Seismic Network. Two CD s (evt-files). Bucharest-Karlsruhe, October 24, Bonjer, K.-P., Rizescu, M., Grecu, B., Sokolov, V., Radulian, M., Dinu, C., 2002b. Ground motion pattern of large and moderate intermediate depth Vrancea earthquakes: first steps towards the generation of regional shakemaps in Romania. Conference bearthquake Loss Estimation and Risk ReductionQ, Bucharest Oct. 2002, Book of Abstracts, p. 54. Chalot-Prat, F., Girbacea, R., Partial delamination of continental mantle lithosphere, uplift-related crustal mantle decoupling, volcanism and basin formations: a new model for the Pliocene Quaternary evolution of the southern East Carpathians, Romania. Tectonophysics 327, Chiu, J.-M., Isacks, B.L., Cardwell, R.K., Propagation of high-frequency seismic waves inside the subducted lithosphere from intermediate-depth earthquakes recorded in the Vanuatu arc. J. Geophys. Res. 90, Cloetingh, S.A.P.L., Burov, E., Matenco, L., Toussaint, G., Bertotti, G., Andriessen, P.A.M., Wortel, M.J.R., Spakman, W., Thermo-mechanical controls on the mode of continental collision in the SE Carpathians (Romania). Earth Planet. Sci. Lett. 218, Cornea, I., Radulescu, F., Pompilian, A., Sova, A., Deep seismic soundings in Romania. Pure Appl. Geophys. 119, Csontos, L., Tertiary tectonic evolution of the Intra-Carpathian area: a review. Acta Vulcanol. 7 (2), Davies, H.J., von Blankenburg, F., Slab breakoff: a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens. Earth Planet. Sci. Lett. 129,

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